ORNL-3593.txt

ORNL-3593
UC-25 — Metals, Ceramics, and Materials
TID-4500 (27th ed.)

WASTER

MECHANICAL PROPERTIES OF SOME REFRACTORY
METALS AND THEIR ALLOYS
H. E. McCoy, Jr.

R. L. Stephenson
I RiWeie; -Jdrs

OAK RIDGE NATIONAL LABORATORY
operated by

UNION CARBIDE CORPORATION
for the

U.S. ATOMIC ENERGY COMMISSION

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[t

[

.

H. E. MeCoy, Jr.

Contract No. W-7405-eng-26

METALS AND CERAMICS DIVISION

MECHANICAL PROPERTIES OF SOME REFRACTORY
METALS AND THEIR ALLOYS

R. L. Stephenson
J. R. Weir, Jr.

APRIL 1964

OAK RTNGE NATIONAL LABORATORY
Ozk Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U. S. ATOMIC ENERGY COMMISSION

R

@

ORNL-3593
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iii

CONTENTS

Strengthening Mechanisms in High-Temperature Materials ........
Solid-Solution Strengthening ...... it nneanns
Interstitial and Dispersion Strengthening ................

The Mechanical Properties of Nb, Mo, Ta, and W ................
Niobium-Base Alloys ...................................;..
Molybdenum-Base Al OYS ... vivrerrnecasrsncensennsnsonsss
Tantalum-Base AlloysS i iieitretnetietenerenesssesennennnsaa
Tungsten-Base AlLlOYS .iiviieieeerrnnneanecssoaanannansanas

Internal Ericéion Studies of Refractory-Metal Systefis .........

Effects of Irradiation on Refractory Metals ..............c....
)« B .1
Molybdenum ....ieeiiiieiiieneeeeetessenesnannsionnnsosssnes

B =0 v - V0 X

33

37
MECHANICAL PROPERTIES OF SOME REFRACTORY
METAIS AND THEIR ALLOYS

H. E. McCoy, Jr. R. L. Stephenson
J. R. Weir, Jr:

ABSTRACT

A critical evaluation has been made of the available
mechanical property data for Nb-, Mo-, Ta-, and W-base
alloys. It was found that insufficient data are available
to allow the design and construction of complex engineering
systems involving these materials. A general evaluation
of the potential service temperatures for Nb-, Mo-, Ta-,
and W-base alloys was made on the basis that conventional
alloys have been used up to two thirds of their absolute
melting point. Strengthening mechanisms that have been
used to achieve high operating temperatures for conventional
alloys and that could be applied to refractory alloys are
discussed.

A review of the literature on the effects of irradi-
ation on the mechanical properties of niobium, molybdenum,

tantalum, and tungsten has been made. It has been found

‘that the existing data on this topic are rather scant.

The date in gecneral show that the ductility of molybdenum,
tantalum, and tungsten is reduced after irradiation at ambient
temperatures. The yield and ultimate‘strengths are increased
slightly by irradiation. High-temperature tube-burst tests
show that the rupture life of the Nb—1% Zr alloy is not

'drastically influenced by irradiation.
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INTRODUCTION

Conventional high-temperature alloys, sucfi as the stainless steels
and nickel-base alloys, have constantly been improved. These materials
have and will continue to be invaluasble structural materials in the nu-
clear field. However, proposed future nuclear systems require materials
that will operate satisfactorily at temperatures in excess of the melting
points of the nickel- and iron-base alloys. A scan of the periodic chart,
melting points of the elements, and availability and subsequent costs
reveals only four candidate materials: niobium, molybdenum, tantalum, and
tungsten. In considering the potential of these metals and their alloys,
the physical property data in Table 1 are useful. The data on nickel and
iron are tabulated for comparison. The values for one half and two thirds
of the absolute melting point are significant because they indicate, re-
spectively, the temperature for which creep begins to be a problem and the
maximum service temperature to which engineering alloys are commonly
sub jected.? |

The values of the microscopic thermal neutron absorption cross
section for these metals are of interest for nuclear applications. With
other parameters remaining constant, the use of niobium or molybdenum as
a fuel elefient cladding material would result in better neutron economy
than would the use of tantalum or tungsten.

In considering a material for engineering application, it is nec-
essary that the requirements of the particular application be carefully
evaluated and contrasted with the properties of the material. Consider
in particular the problem of choosing the structural and fuel cladding
materials for a nuclear reactor using a liquid-metal heat-transfer medium.
The material must have sufficient strength at the operating temperature,
must be capable of fabrication into the desired shapes, and must withstand
the corrosive influences of its environment.

Although the fabricability and corrosion resistance are of the
utmost importance, it is the purpose of this discussion to deal specif-

ically with the mechanical property requirements of these materials. The

1D. Mclean, "Point Defects and the Mechanical Properties of Metals
and Alloys at High Temperatures,”" p. 179, Vacancies and Other Point Defects
in Metals and Alloys, Institute of Metals Monograph No. 23, 1957.

Table 1. Physical Property Data

Microscopic
. Thermal Neutron '
o o 1/2 Absolute 2/3 Absolute Absorption ‘Modulus of
Element Density Melting Point~  Melting Point Melting Point Cross Sectio Elasticity
(8/cm?) (°c) - (°0) - (°c) (barns/atom) (psi)
‘ | x 108
 Nickel 8.90 1453 - 590 878 4.5 30.0%
Iron  7.87 1537 632 934 2.4 28.5°
Niobium 8.57 2468 1098 ' 1554 | 1.1 17.7° IS
Molybdenum 10.22 2610 1169 1649 2.4 47
Tantalun  16.6 2996 1362 1906 21 27
a

Tungsten 19.3 - 3410 1569 2183 o 19 , 50

"Phy51cal Propertles of the Elements," Metals Handbook Vol. I, pp. 4451, American Society
for Metals, 8th ed., 1961. ‘

' Samael Glasstone, Principles of Nuclear Reactor Englneerlng, pp. 841;423 Van Nostrafid, .
Princeton, N. J., 1955. - 4 . : CoeE

°L. p. Jahnke et al., ""Columbium Alloys Today," Metal Progr., 78: 77 (1960).
specific properties that must be evaluated include: (1) engineering
design data; (2) data concerning the long-time chemical stability of
the alloy; (3) the ductility between the minimum and maximum service
temperatures; (4) effect of atmosphere on the strength and ductility;
(5) the influence.of irradiation; and (6) thermal fatigue properties.

- The components of é reactor system fequire materials having con-
siderably different properties in Lhese six areas. Fér example,
materials used for fuel element cladding or radiators must be consider-
ably more dgctile than turbine blade or nozzle materials. Likewise,
resistance to damagefiby irradiatibn is of importance for core structural
materials but not for radiator materials. |

~ Although considerfible‘information is available on the high-

temperature mechanical properties of Nb-, Mo-, Ta-, and W-base alloys,
no Single alloy has been sufficiently evaluated in these six areas to
make it ready for service in a nuclear system. In this discussion an
attempt will be made to assess the state of affairs relative to these
four refractory metals. The available data will be reviewed critically.
Recommendations as to the choice of alloys for service over specific
temperature ranges will be made. Areas in which data are lacking wiil

be poihted out. .

. STRENGTHENING MECHANISM> IN HIGH-TEMPERATURE MATERIALS

- The following discussion of strengthening mechanisms is not in-
tended to be a complete "textbook" treatment of the subject, but rather
a meane of bringing to the reader's attention the many possibilitiés
that must be considered. For example, when 1% Zr is added to niobium,
it does not necessarily follow that the strengthening observed is due
to solid~solution strengthening. The entire chemistry of the metal is
changed and it is quite likely that the major portion of the strength-
ening is due to the formation of zirconium-interstitial complexes
(clusters or compounds). '

Specific data are presented ifi this discussion only where it
illustrates a partiéuléf point. Data on the mechanical behavior of |

refractory metals will be given in the ncxt scetion.
Solid-Solution Strengthening

Solid-solution strengthening may be defined as the increase in .
resistance to deformation of a material brought about by dissolving in
it another element. The introduction of atoms having a diameter dif-
ferent from those of the parent lattice introduces strains. These
distorted regions in the lattice interfere with the motion of disloca-
tion and increase the resistance of the material to deformation. The
amount of strengthening obtained by this mechanism ié proportional to
the amount of solute up to the solubility limit. The strengthening is
likewise proportional to the size difference in the solute and solvent
atoms. However, this is not an independent factor since the degree of
solubility decreases as the atomic misfit increases. This picture of
strengthening based on a size factor, as was originally proposed by
Mott and N'a'barro,2 is somewhat an oversimplification and recént
workers37% have shown the valency or electronic effects to be important.

' Another effect of solid-solution alloying is that of lowering
the stacking fault energy. This causes the dislocations to split into
partials with a faulted region in between. For cross slip to occur,
these partials must be forced together. This efféect, however, is
confined to face-centered cubic materials and has beénrdbserved in
copper and stainless steels. | |

Some of the most interesting effécts arise from the tendency of
impurity atoms to migrate to dislocations and to grain bofindaries. This
tends to anchor the dislocations and to lock the sources./’The.segfega-
tion of impurities in the grain boundaries is largely responsible for
_the large effects that impurity atoms have on the recrystallization
temperature of metals. Vandermeer et 32.5 have shown that alloy addi-
tions to high-purity aluminum alter the rate of grain boundary migration

in proportion to the diffusion rate of the solute in the solvent.

2N. F. Mott and F. R. N. Nabarro, p. l,:Réport of Conference on
Strength of Solids, Physical Society, London, 1948.

3N. P. Allen, T. H. Schofield, and A. E. Tate, Nature, 168:
378 (1951). 4

“W. R. Hibbard, Jr., Trans. Met. Soc. AIME, 212: 1 (1958).

°P. Gordon and R. A. Vandermeer, The Mechanism of Boundary
Migration in Recrystallization, Tech. Rep. No. 3, Department of

Metallurgical Engineering, Illinois Institute of Technology, August 1961.

&
However, recrystallization in the transition metals may be more complex.
Abrahamson® has shown that the effect of alloying élements on the recrys-
tallization temperature of niobium can be correlated with the atom per-
cent solute and the free atom electron configuration of the solute
element. The elements Mn, Fe, Co, Ni, W, Re, and Os lower the recrys-
tallization temperature and Ti, V, Cr, Zr, Mo, Ru, Rh, Pd, Hf, Ta, Ir,
and Pt raise the recrystallization tefiperature.

Darken’ points out that the effect of substitutional alloy
additions per se cannot account for the strength realized in materials.
Substitutional alloying may be of more importance in conjunction with
other strengthening mechanisms. The studies by Darken’ of the oxidation
of a silver-aluminum alloy illustrate this point. At the temperatures
studied, aluminum has a high affinity for oxygen whereas silver oxide is
unstable. It was felt that at low temperatures the oxygen would diffuse
to the aluminum atoms and in the limiting case the aluminum atoms would
remain stationary. If the aluminum atoms were completely surrounded by
oxygen'atoms the oxygen-to-aluminum ratio would be 6. As the aluminum
atoms migréte, the cluster size would increase and the oxygen-to-aluminum
ratio would decrease. Observations by Wriedt® on the oxidation of a
Ag—0.1% Al and a Agr0.48% Al alloy support the proposed model. The oxygen-
to-aluminum ratio decreased with increasing temperature and increasing
aluminmm content. It was also found that when an alloy was oxidized at
one température and subsequently exposed to an oxidizing atmosphere at
a higher temperature the oxygen-to-aluminum ratio did not change. This
indicates the very high stability of the aluminum-oxygen clusters. These
are considerably more effective in strengthening the alloy than would be
the strain fields due to the aluminum atoms alone.

The effect that a substitutional alloying addition has on theil
strength of a metal would be lost if' the solute element were removed;

therefore, the allny addition must be compatible with the service

'6E. P. Abrahamson II, Trans. Met. Soc. AIME, 221: 1196 (1961).
7L. S. Darken, Am. Soc. Metals Trans. Quart. 54(4): 60042 (1961).

8D. ¥. Wriedt and L. S. Darken, Research Laboratories, U. S.. Steel
Corp., unpublished data.
enviromment. Two of the processes whereby the solute may be lost are
by evaporation -at high temperatures in vacuum and by selective leaching

in a corrosive environment.

Interstitial and Dispersion Strengthening

Althbugh the interstitial atoms are,smaller‘énd diffuse more
rapidly than substitutional allpying elements, they can effectiyely:alter
the motion of disloéations. They are believed to be reéponsible for the
ductile~-to-brittle transition that is characteristic of body-centered y

cubic metals. The interstitial atoms become quite immobile at low

(L%

temperatures and prevent the dislocations from moving.

Strain aging is another phenqmenon attributed to interstitial
impurity afioms. This process is brought about by interactions between
moving dislocations and mobile interstit%al solute atoms. Strain aging
may be manifested in a "return of the yiéld point" in a tensile |
specimen after interrupting a tensile test and aging the specimen or'by
strengthening during a continuofis tefisile test with accompanying reduction
in ductility and discontinuous yielding. The empirical relationship that

has been determined for the. occurrence of discontinuous yielding is

e =10° D,
where

¢ is the strain rate, and
D is the diffusion rate of the interstitial responsible for the -
strain aging.
This describes the condition for which the velocities of ‘moving disloca-
tions and impurifiy atoms afe comparable. Strain aging is a relatively
low-temperature phenomenon. For example, in niobium at strain rates of
1072 to 1074 sec™l, strain aging due to oxygen and the combined effects
of nitrogen and carbon is observed over the temperature range of 200 to

450°C. %

9B. Longson and C. Tyzack, The Effect of Hydrogen on the Mechanical
Properties of Niobium, TRG. Memo 880 (C), p. 6, March 1962.

| The precipitation of a second phase has been used as a strengthen-
ing mechanism in metals for some time. The general concept of strength-
ening by this mechanism is that the second phase particles introduce
strain fields that interfere with the motion of dislocations. In light
of this mechanism, the concept of a critical particle size (or spacing)
was proposed by Orowan. 10 Particles of sizes greater or smaller than
this critical size are relatively ineffective. However, many complica-
tions may arise that make this picture a gross oversimplification. The
particles formed may or may not produce a strain field; they may or may
not be coherent; they may have various shapes; and they may or may not
deform plastically under stress. In fact, the critical particle size
concept predicted'by Orowan has never been observed. The closest approach
has been the observation of Meiklejohn and Skodal?l on the yield strength
of solid mercury containing iron particles. However, a particle size
effect was noted that canceled out the influence of particle spacing and
gave the net result that the yield strength was a function only of the
volume fraction of the precipitate. The silver-aluminum alloys referred
to in the previous. section likewise showed only a slight dependence of
strength upon aluminum-oxygen cluster size but exhibited a marked depend-

2

ence upon the volume fraction of the précipitate.l Recent transmission

2 on iron-gold alloys and by

electron microscope studies by Horn’bogen1
Leslie et g;.13 on iron-bismuth alloys show that the second-phase par-
ticles can act as copious sources of dislocations. The cell structure
of dislocations originating from the particle offers more strengthening
than would be predicted by the Orowan concept of strengthening. Another

interesting effect is produced by cold working. Garofalol* pretreated

10E. Orowan, Discussion in Symposium on Internal Stresses in Metals
and Alloys, p. 451, Institutc of Mctalc, London, 1948.

1ly. H. Meiklejohn and R. E. Skoda, Acta Met., 8: 773 (1960).

121,. S. Darken, Am. Soc. Metals Trans. Quart., 54(4): 600—42 (1961).

1

13W. C. Leslie et al., "Annealing of Cold Worked Iron," paper
presented at the Metallurgical Society AIME Conference on High-Purity Iron
and Its Dilute Solid Solutions, to be published.

14F. Garofalo, F. Von Gemmingen, and W. F. Domis, Am. Soc. Metals
Trans. Quart., 54: 430 (1961).

10

type 316 stainless steel specimens by .solution annealing, cold working,
and agifig. It was found that the stréngth was greatly improved by pre;
treatments which resulted in fine randomly dispersed carbide formation.
The resulting dislocation networks were studied and correlated with the
mechanical properties of the steel. The desirable dislocation structure
consisted of tangles that had apparently been trapped by the precipitate
particles and the most undesirable structure was the stabilized cross
grids of dislocations which offered little back force on dislocation
fiotion.

One particularly important factor concerning the mechanical prop-
erties of refractory metals is the. influence of substitutional alloying
element on the strength attainablé-throUgh the formation'of a dispersed
phase. Thé case of the aluminum alloy addition to silver and the forma-
tion of aluminum-oxygen clusters has already been discussed. One further
- example is the influence of carbon on the properties of niobium. McCoy1?
and Cortes and Feildl® have independently shown that carbon additions up
to 0.21% do not result in measurable strengthening nor embrittlement of
niobium. The niobium-base alloys F-44 (Nb—15% Mo—1% Zr—C) and F-48
(Nb—15% W—5% Mo—1% Zr—C) are, however,.strengthened by carbide dispersions
as illustrated by the data of Chang!? given in Table 2. The formation of
Nb,C in the latter alloy is due to the zirconium content being 0.6% instead
of the nominal 1%. This illustrates the importance of the precise control
of the zirconium-to-carbon ratio in these alioys. Besides being instru-
mental in the initial formation of a strengthening dispersion, a solid-

solution alloying element can affect the solubility of the precipitated

°H. E. McCoy, Jr. , Conference on Corrosion of Reactor Matérials,
June 4—8, 1962, Proceedings, Vol. I, pp. 263-94, International Atomic
Energy Agency, Vienna, 1962.

16F. R. Cortes and A. L. Feild, Jr., J. Less-Common Metals, 4:
169 (1962). -

17w, H. Chang, p. 105, Refractory Metals and<Alloys, InterSC1ence,
NéW‘YOTk, 1960.

11

Table 2. Effect of Carbon on Niobium-Base Alloys

100-Hr Rupture

Alloy Carbon Strength at 1093°C Carbides Identified
(wt %) (psi)
P48 0.04 | 28.0 | (Wb, Zr) C
0.13 - 37.5 (Nb,Zr) C
F-24 0.02 . 20.0 (No,2r) C
0.05 30.0 (No,zr) C
0.13 22.5 (Nb,Zr) C and NosC

interstitial in the alloy and the diffusion rate of the interstitial
element. Both of these factors increase the high-temperature stability
of the precipitate. The work of Hobson'® on the solubility of oxygen in
the Nb—1% Zr alloy illustrates the marked influence of zirconium confent
on the oxygen solubility.

Some important factors and observations relative to dispersions
have been discussed, but nothing has been said of ways of introducing
dispersions. The two major ways have acquired the names of artificial
and natural. The artificial technique involves the mixing and forming
of two powders by powder-metal1urgy'techniques. The material is then
hot worked to the desired shapes. The main disadvantage of this tech-
nique is that the dispersed phase is not fine enough to obtain maximum
strengthening. Some current effort is directed toward developing tech-
niques for producing fine powders. Du Pont has also announced the com-
mercial availability of TD-Nickel, which is a nickel-base material
‘dispersion strengthened by 2% thoria.l?. The dispersed particle is 0.1lp
in diameter, which Du Pont says is introduced by a "special chemical
process, now patented, of a colloidal nature, to achieve extremely fine,

yniform dispersion of hard particles in metals.” The natural technique

18D. 0. Hobson, Aging Phenomena in Columbium-Base Alloys, ORNL-3245
(March 1962). ~

19D spersion-Strengthened Alloys Become Commercial,”
J. Metals, 1l4: 561 (1962).

12

normally involves internal oxidation; however, nitriding and carburizing
could be utilized equally as well. Zwilsky and Grant?? have used the
internal-oxidation techniqué.to form Al,05 particles in copper-aluminum
alloys. Stress-rupture properties of the dispersion-strengthened alloy
at 850°C are superior to those of pure copper at 450°C. These examples

serve to illustrate the potential of this area of materials strengthening.

THE MECHANICAL PROPERTIES OF Mo, Mo, Ta, AND W

Although the authors have reviewed in detail the known available
literature on the mechanical propérties of these metals, an attempt will
not be made to recapitulate this information in detail in this report.
The length of such a recapitulation - coupled with its lack of meaning has
led to this decision. Several factors tend to discount much of the
available data.

(1) The interstitial content of the test material is seldom
specifiéd.

\(2) Much of the work has been done on 25-g buttons that have
properties difficult to reproduce in 50-1b ingots.

(3) The condition of the material (e.g., solution annealed,

- wrought, etc.) at the time of testing is frequently not specified.
Conditions such as "annealed" are often given which are not very helpful.
(4) The test atmosphere is often not designated. Thé term

"vacuum" is often used with no further qualifying statements. Whether
the vacuum is 1073 or 1039 torr can make considerable differences in the
test reSults. The questibn of when the vacuum was measured is importaht.
The vacuum may have been 107% torr at the end of a 100-hr creep test, but
may have been 1072 during the first 24 hr of the test.

(5) A large portion of the research effort has been spent in
determining tensile data. For most applications such information is not
even useful as aVSCreening criterion, since completely differenfi deforma-
tion mechanisms can be operative at lower strain rates.  The possible pit;

falls of extrapolafiing data from 0.1 to 10,000 hr need no amplifications.

20K, M. Zwilsky and N. J. Grant, Metal Progr., 80: 108 (1961).

The melting point data in Table 1 give some idea of the potential
applications of these four refractory metals. Based on the criterion of
two thirds of the absolute melting point being the maximum service temper-
ature, Nb, Mo, Ta, and W can be used up to 1554, 1648, 1906, and 2183°C,
respectively. This criterion ignores the prospect of dispersion strength-
ening, since the SAP alloys are used at three fourths of their melting
points. It is also possible to raise the useful temperature slightly'by
alloying to raise the melting point.

The available mechanical property data on several Nb-, Mo-, Ta-,
and W-base alloys are sumarized in Table 3. As is quite evident, stress-
'rupture data are not available on many of the alloys. Values of the
tensile-rupture ductility are not given because all of the alloys listed
have sufficient ductility at elevated temperatures. It is the ductility
at temperatures approaching room temperature that is a problem. The
ductile-to-brittle transition temperature for many of these alloys is
above room temperature. This is illustrated by the data??l in Fig. 1,
in which the redfiction in area is the ductility parameter. It is empha-
sized that the temperature of this transition is not a fixed property of
the material but is raised by increasing the rate of straining or tri-
axiality of loading and is also affected by composition as governed by
alloy additions and the pfesence of impurities, as well as by the heat
treatment and fabrication history. Some assessment of the fabricability
of these alloys is also indicated. The strength-to-weight ratio is

included because of its interest for space apfilication of these metals.

Niobiun-Base Alloys

Appreciable strengthening of niobium has resulted from alloy
additions of Zr, Y, W, Hf, Ta, and Mo. It is quite difficult to say
what fraction of the strength improvement occurs as a result of solution

slrengthening and what io a recult of interactions of the afi1nying element

211,. Northcott, "Some Features of the Refractory Metals," p. 8,
Niobium, Tantalum, Molybdenum, and Tungsten, (ed. by A. G. Quarrell)
Elsevier Publishing Co., New York, 196l. ‘

14

Table 3. Physicel and Mechanical Properties of Refractory-Metal Alloys

' Recrystallization Tensile Strength (psi) 100-Hr Rupture Stress (psi) 100-Hr REI([;;I;I‘G Sztress-t_o-l;)ensity Ratio
Alloy Fabricability Temperature in. )/(1b/in.”)]
3 ©C)  980°C 1204°C 1315°C 980°C 1090°C  1204°C  1315°C  “ggpoc  1090°C  1204°C  1315°C
I x 10° x 103 x 10° x 103 x 10° x 103 x 10°
Pure Niobium Tubing avanatffe ' 1090 4-369:° 3-9e.b 3-7a:b 4-9ab ~ 480 13-29 ~13
Nb—-1% Zr Tubing availabrle 29-30 12-20 94-97 39-65
AS-30 (19 to 21% W—0.8 to 1% Zr—0.8 to 1% C) |
AS-55 (5 to 10% W—0.8 to 1.2% Zr—0.2to 1%  Good to excellent? 12609 24-30 194 139 60 41
Y-0.08% C)° |
F-48 (15% W—5% Mo—1% Zr) Pilot prcsducuofrf,” 1540° 60-74° 39-50° 26-50° 3540 178 452
FS-82 (33% Ta-1% Zr) Commercial® | 12042 35-702-2 19-252-% 15-20%-2 18%
B-77 (10% W-5% V—1% Zr) Good to excellent® 34--389 > 209 > 189 12-13¢ >63 >56 38-41
B-66 (5% V—5% Mo~1% Zr) Good to excellent?  ~1260¢ 38¢ 114 36
Cb-752 (10% W—2.5% Zr)¢ Good? g ~ 12609 26¢ 18-21 18 14 8
B-33 (4% V) Excellent? l 11754 209
D-31 (10% Mo-10% Ti) Pilot producuo}ab 1204° 50° 23-26° 11-20° 148 48
D-14 (5% Zr) fi 13709 550 28¢ 177 124 5d 39 16
D-36 (10% Ti~5% Zr) 34h 17k 140
C-103 (10% Hf-1% Ti) . 13159 187 '
SCb-291 (10% Ta~10% W) ) 1150-1315¢9 374
FS$-85 (27% Ta—10% W—1% Zr) Excellent? | 13709 29-60%-9  22-414.4 20-22°
X-110 (10% W—1% Zr—0.1 C) Excellentd 13159 359 17.59 54
Nb—40% V | 980/ 33 (Rupture life of <13 hr at 102 psi and 1090°C)
Pure molybdenum Sheet avauable’g 1425-1705% 21-24° 18-28° 10-20° 12-132 9144
Mo-0.5% Ti Sheet available;fr 1340 68 20-452 16-222 29-54¢ 12¢ 7¢ 210° 952 258
TZM (0.5% Ti—0.08% Z1) ’ 1325-17057 85 67-78° 50-552 38-80°  30-51¢  35° 208 210° 90° 552
Mo—30% W 65° 35¢
Mo—25% W—0.1% Zr—0.05% C | 758
TZC (1.25% Ti-0.15% Zr-0.15 C) 15407 602 452 ~40° 30 20-30°
Mo—50% Re K 85k 30% 20k
Pure tantalum Tubing avaxlab’I;e 1090¢ 224 9-16° 8-167 ~6.5° 3f 34 -~11 5 5
Ta—10% W . 1370¢ 50-80%f  42-67f  40-45° 448
Ta-20% W g
Ta-30% W *
Ta-10% Hf-5% W 50-78¢ 46-60° 40-467
Ta—30% Nb-7.5% V° ‘ 1204/ 80° 622 422
Ta—8% W—2% HE i 15408 85¢
P 1204°C 1315°C 1650°C 1090°C  1204°C 1315°C  1650°C  1090°C  1204°C  1315°C  1650°C
Pure tungsten [;\ 558 40-50° 20-30° 222 192 44 32 27 5.7
W—3% Mo }‘ ~188.
W—30% Mo i ~ 302
W-1% ThO, { ~ 14008 38
W~2% ThO, i > 26908 ~42! >20° ~ 20!
W-30% Re 135/ 50/

4T. E. Tietz and J. W. Wilson, Mechanical, Oxidation, and ihermal Property Data for Seven Refractory Metals and Their Alloys, Lockheed Report, Code 2-36-61-1 (Sept. 15, 1961).
bE. S. Bartlett and J. A. Houk, Physical and Mechanical Pmpert:es of Columbium and (‘qumbxum-Base Alloys, DMIC Report 125 (Fcb. 1960).
€Alloys selected for study by AEC-NASA-AF Tubing Evaluatxon Committee.

. 9AEC-AF-NASA Table on Niobium Alloys.

ll

€Creep-rupture data on the 0.5% Ti-Moé alloy at 535 to 1315°C from Climax Molybdenum Company, Sept. 1957.
fAEC-AF-NASA Table on Tantalum and Vanadium Alloys. !|
8M. Semchyschen and J. J. Harwood, Refractory Metals and A,Iloys, Interscience, New York, 1961.
BDu Pont Metal Products, Product Data Sheet No. 1, 1962. |
B. R. Rajala and J. R. Van Thine, Improved Vanadxum-Base'
‘j{B R. Rajala and R. J. Van Thine, Improved Vanadmm-Base'
kManufacturer s Literature, Chase Brass and Copper Company, Waterbury, Conn.

. IB. S. Lement and 1. Perlmutter, ‘‘Mechanical Propertles Ath}mable by Alloying of Refractory Metals,’

b
K

1
I
I

Alloys, ARF 2210-6 (Dec 20, 1961).
Alloys, ARF 2191-6 (Dec. 27, 1960).

' p. 316, Niobium, Tantalum, Molybdenum, and Tungsten (ed. by A. G. Quarrell) Elservier, New York, 1961,
15

UNCLASSIFIED
ORNL-LR-DWG 77766

o
S

Eg l e
'&J NICKEL - ~TANTALUM

@

w /

Z 60 , 4

= / —TUNGSTEN |
w

g 40 ' /

z NIOBIUM /A—MOLYBDE‘N‘UM /

pzd

© 20 : .

— .

2 4// u////

a /

lé':J 0

-300 -200 -100 0 {00 200 300 400 500 600 700 800
’ - TEMPERATURE (°C) ’ '

Fig. 1. Effect of Temperature on Ductility. [L. Northcott, "Some
Features of the Refractory Metals," p. 8, Niobium, Tantalum, Molybdenum,
and Tungsten, (ed. by A. G. Quarrell) Elsevier Publishing Co., New York,
1961. ] :

16

with interstitial impurities. The F-48 and F-50 alloys have been studied

by Chang.??

Both alloys were found to be age hardenable and the aging was
attributed to carbide precipitation. Studies?3 of the No—1% Zr alloy have
also shown it to be age hardenable under specific circumstances.

The tensile properties of niobium are improved appreciably by the
addition of vanadium. However, the creep properties are not improyed.24’25
This illustrates the fact that a tensile test is not a valuable screening
teét for engineering materials. The range of values found in the litera-

ture for the tensile strength of pure niobium at 982°C indicates the un-

reliability of much of the mechanical property data on refractory metals.

Molybdenum-Base Alloys

Additions of titanium and zirconium appreciably improve the mechan-
ical properties of molybdenum. Although this effect is often attributed
to solution strengthening, it seems more reasonable that the strengthening
is due to clusterihg.or dispersion strengthening caused by substitutional-
interstitial atom interactions. Chang22 has studied the aging response of
the MofTZC alloy and has clearly established the precipitation-hardenable
nature of the alloy. Three dispersed phases were identified, consisting
of TiC, Mo,C, and ZrC. The formation of TiC was prifiarily responsible for
the aging, and ZrC was felt to have little inflfience on the strength.
Chang suggested that another important role of the titanium was that of
enhanéing the high-temperature solubility of carbon. Molybdenum, Mo-TZ,
and Mo—0.5% Ti were found not to be age hardenable. '

The Mo—50 wt % Re (35 at. %) alloy has some very unique properties.
Figure 2 compares the ductility of this alloy with that of pure molybde-

26

num. Note that the ductile-to-brittle transition temperature is signif-

icantly lowered by the rhenium addition. This is due to the onset of

22y, H. Chang, A Study of the Influence of Heat Treatment on
Microstructure and Properties of Refractory Alloys, Report No.
ASD-TDR-62-211 (1962).

23p. o. Hobson, A Preliminary Study of the Aging Behavior of
Wrought Columbium—1% Zirconium Alloys, ORNL-2995 (Jan. 1961).

24B. R. Rajala and R. J. Van Thine, Improved Vanadium-Base Alloys,
ARF 2191-6 (Dec. 27, 1960).

25Tpid., ARF 2210-6 (Dec. 20, 1961).

261,. Northcott, "Some Features of the Refractory Metals," p. 17,
Nicbium, Tantalum, Molybdenum, and Tungsten, (ed. by A. G. Quarrell)
Elsevier Publishing Co., New York, 196l.

17

UNCLASSIFIED
ORNL—-LR-DWG 77767

Mo —35 Re (RECRYST)
S g L How
>_'
= / / Mo (RECRYST) /
: / / /
3 |
Mo (H.C.W.)

2142
wl

16 I I H.C.W.= HOT- COLD - WORKED ’

20

-200 —100 O {00
TEMPERATURE (°C)

Figs 2. Bend-Transition Curves for Molybdenum and the Mo—35% Re
Aloy in the Recrystallized and Hot- and Cold-Worked (HCW) Conditions.
[L. Northcott, "Some Features of the Refractory Metals," p. 17,
Niobium, Tantalum, Molybdenum, and Tungsten, (ed. by A. G. Quarrell)
Eleevier Publishing Co., New York, 1961.]

18

twinning in the molybdenum-rhenium alloy at low temperatures. This alloy .
is also more resistant to oxygen embrittlement than pure molybdenum. In
pure molybdenum the oxide phase accumulates in the grain boundaries, thus
forming brittle grain boundary layers. The rhenium addition influences
the surface energy of the oxide, and the oxide occurs as globules in the
grains as well as at the boundaries rather than as a continuous grain
boundary layer. The availability and cost of rhenium make the widespread
use of the Mo—50% Re alloy doubtful..

In addition to the composition wariable that influences the proper-
‘ties of molybdenum, fabrication is also an important variable.?? This is
illustrated in Fig. 3. Note the very large differences in room tempera-
ture ductility depending on whether the final rolling temperature is 1204

or 1648°C. Significant strength differences also result.

Tantalum-Base Alloys

Pure tantalum is relatively weak at elevated temperatures. Additions
of W, Hf, Nb, and V to tantalum result in significant strengthening. The
~influence of relatively low concentrations of oxygen and nitrogen on the
elevated temperature behavior of tantalum has been investigated by
Schmidt gz'gl.zs Additions of 560 ppm O and 225 ppm N were not effective
strengtheners above 1100°C. However, carbon was an effective strengthener
up to 1200°C. No systematic study has been made of strengthening due to
interstitials when a substitutional alloying element is present.

Chang29 has worked with a complex tantalum-base alloy of the
nominal composition Ta—20% Nb—10% W5% V—1% Zr—0.08% C. This alloy was
found to have a recrystallization temperature of 1704°C. Preliminary
studies have shown that severe intergranular cracking occurred when

annealed above 1648°C. Studies are continuing on this alloy.

27M. Semachyschen, R. Q. Barr, and G. D. McArdle, Effect of Thermal-
Mechanical Variables on the Properties of Molybdenum Alloys, WADD-TR-60-
451 (Nov. 1960). ‘

28F, F. Schmidt et al., WADD Report 59-13, p. 123 (Dec. 31, 1959).

29W. H. Chang, "A Study of the Influence of Heat Treatment on
Microstructure and Properties of Refractory Alloys," Quar. Rep. No. 6,
March 1, 1961, to May 31, 1961, DM62-140, pp. 10-13.

19

UNCLASSIFIED
ORNL-LR-DWG 77768

% 1o
Q
O -
o
Q
~ 100
- ]
(O]
Z [e]
& 90
- . o
wn - ]
w - ”)(\ 2200 °F
= // \
2 80 % M
= L4 N
w b3 \\
= N
g ° S 5
o S~< 3000°F
wl o SO
% e — -
60
- a
p=
3
@ 50
70
,’5
(=N
(o]
S 60
w
)
8 50
®
=
z 40 — 2200°F ——T°
5 .._-——-—-—‘—0-
o = o .
U) 30 r \\
w \ 7 Tee 3000°F
=J ’ 4 "“——-___-4_.__-‘--
(%] Vd
z 20
w
-
{0
S
~ 100 -
< [ - .
Bz oy T S =
® R T' " S
<L o
& 80 | —
z 8,8 TESTED AT 1800°F
= 0,0 TESTED AT RT.
o ———3000°F
o 60 ___22000,,} FINAL ROLLING TEMP
i) ©.,0 PROPERTY: ELONGATION
a ®;0 PROPERTY: REDUCTION IN AREA
Qa0 — : =
= ® -y — -
= e~ — —|— —
=3 ==
2 20
g fi
wl
3 0 Sl st
g 0 10 @ 30 a0 50 €0 70 an a0 100
b= .

REDUCTION N AREA BY ROLLING (%)

Fig. 3. Tensile Properties of Ae-Rolled Molybdemm at Room
Temperature and 1800°F (982°C) vs Amount of Reduction by Rolling at
2200°F (1204°C) and 3000°F (1649°C). [T. E. Tietz and J. W. Wilson,
Mechanical, Oxidation, and Thermal Property Data for Seven Refractory
Metals and Their Alloys, Lockheed Aircraft Corporation, Missiles and
Bpace Division, Suwunyvale, California, Topical Report, September 1961.]

20

Tungsten-Base Alloys

The data available on tungsten-base alloys are quite limited. The
.benefit of the molybdenum addition is questionable in light of the avail-
able data given in Table 3. Since the atomic radii of tungsten and molyb-
denum are 1.37 and 1.36 A, respectively, since they are in the same valence
group, and since the melting point of molybdenum is considerably less than
that of tungsten, this is not contrary to expectations. The W30% Re alloy
has attractive mechanical properties both with respect to strength and
ductility. These benefits are believed to be derived by processes similar
to those described for the molybdenum-rhenium alloy. The availability and
- cost of rhenium are factors against this alloy.

An interesting piece of work has been done by Steigerwald et gl.30
on the influence of surface conditions on the ductile-to-brittle transi-
tion of tungsten. The primary points of this study are illustrated in
Fig. 4. The transition temperature was increased as the depth of surface
imperfection was increased. . Either surface oxidafion or electropolishing
was effective in decreasing the transition temperature. It is of consid-
erable practical importance that the transition temperature of a material

can be increased 204°C by fabrication variables.

INTERNAL FRICTION STUDIES OF REFRACTORY-METAL SYSTEMS

Internal friction is a very useful tool for Studying the behavior
of interstitial atoms in body-centered cubic metals. When the metal is
unstressed, each of the three types of tetrahedral sites is equally
favorable for interstitial atoms. However, the application of a stress
distorts the lattice and causes certain sites to become more favorable
than others. This ordering of iriterstitial atoms upon the application
of a stress dissipates energy and results in the material having a high-
damping capacity. The measurement of this damping, which is commonly
called internal friction, gives a measure of the number of interstitial
atoms that are moving. A study of internal friction at various fre-

quencies can also give information about the kinetics of the process.

30E. A. Steigerwald and G. J. Guarnieri, Am. Soc. Metals Trans.
Quart., 55(2): 307 (1962).

21

UNCLASSIFIED
ORNL-LR—-DWG 77769

® GROUND
O GROUND PLUS ELECTROPOLISHED
A GROUND PLUS OXIDIZED, 1500°F FOR 4 hr
A MAGCHINED
¥ MACHINED PLUS OXIDIZED, 4500°F FOR 4 hr
v MACHINED PLUS OXIDIZED, 1800°F FOR {5 min
100
AP g ——gF — — AT~
/ ’
({ I, ] I
80 / J
. :
| I
ELEGTROPOLISHED
o OXIDIZED : I 6GROUND | MAGHINED
: [
- Y
g 60 —t ;
= vi '
w |
o " '
= : ;
2 o) ' |
(]
v | | -' ;
/] /
/ / "
20 / y ;/
/ ’
/ / /
(4
‘ / b/ '
0 /7 /|

0 200 400 600 800 {000
TEST TEMPERATURE (°F)

Fig. 4. Influence of Oxidation on the Bend Transition of Tungsten
Sheet (Material A). [E. A. Steigerwald and G. J. Guarnieri, Trans. Am,
Soc. Metals, 55, 314 (1962).]

22

Since the internal friction is dependent upon the quantity of
interstitial atoms in solution, measurements of internal friction offer
a very attractive way of following a precipitation process. The prac-
tical aspects of such a technique are illustrated by studies of DijkstréBl
on the precipitation of nitrogen in Fe-Mn, Fe-Cr, Fe-Mo, and Fe-V alloys.
It was fofind that the presence of the binary substitutional alloy addition
gfeatly influenced the behavior of nitrogen from that observed in pure
iron: 'Additionél peaks were observed that were attributed to the stress-
induced motion of nitrogen atoms in the vicinity of the alloy atoms.
Studies by Stephenson and.NbCoy32 have shown that similar behavior is
observed when zirconium is added to niobium. When either nitrogen or
oxygen is added to a niobium-zirconium alloy, peaks are observed that
are not present in pure niobium. As the interstitial content is increased,
the normal interstitial-niobium»interaction peaks are'observed. This has
been intérpreted to mean that the interstitial atoms are clustered about

the zirconium atoms.

EFFECTS. OF IRRADIATION ON REFRACTORY METAIS

The proposed use of niobium, molybdenum, tantalum, tungsten, and
alloys of these‘metals as structural components in nuclear reactors
requires that some knowledge be gained as to the effects of irradiation
upon the properties of these materials. The purpose of the following
section is to critically analyze the work which has been done to date

concérning this problem,
Niobium

A limited number of tube-burst tests have been run at 982 and
1093°C to evaluate the properties of the Nb—1% Zr alloy. It was origi-

nally reported that the rupture life was less in an irradiation field

3L, J. Dijkstra and R. J. Sladek, Trans. Am. Inst. Mining
Met. Petrol. Engr., 197: 69 (1953).

32H. E. McCoy and R. L. Stephenson, Metals and Ceramics Div.
Ann. Prog. Rep. May 31, 1962, ORNL-3313, pp. 42—%.

O

23

than out of the reactor.3? Recent hot-cell examinations have led the
authors to retract their original conclusion, since all failures occurred
in the brazed joints rather than in the specimens.34 However, these
results show that the rupture life of niobium is not drastically reduced
by irradiation, since the rupture lives of the in-pile specimens which
failed in the brazed joints were only slightly less than the out-of-pile

control specimens.

Molybdenum

Bruch, McHugh, and Hockenbury>® studied the mechanical properties
of commercially pure molybdenum irradiated in the MIR for an estimated
exposure of 1.9 to 5.9 X 10°° thermal nvt. The specimen temperature was
maintained at 90°C. The material used in this investigation was arc
melted by the Climax Molybdenum Company. Two heats of material were
used having carbon contents of 0.061 and 0.066 wt %. No other analytical
details were given. The material was hot worked to 5/8-in. diam,
annealed at 1100°C in hydrogen, and swaged to 1/2-in. diam. The impli-
cation is that this last fabrication step was carried out at room
temperature and represents a reduction in area of 36%. The test material
had an average hardpess of 264 VPN (99.2 RB) and an average of
5000 grain/mm?®.

The tensile specimens were rods having a gage section 1.00 in.
long and 0.182 in. in diameter. The strain rate used in the tensile
tests was 1.3 X 107% per second.

The results of tensile tests performed in this program are
summarized in Table 4. The unirradiated material was ductile at —20°C
but was completely brittle at'—40 and —60°C. The irradiated material
was completely brittle in tests conducted at room temperature and 60°C

but was ductile at 80°C. Hence, the ductile-to-brittle transition

33y, E. Brundage et al., Solid Slule Div. Ann. Prog. Rcp.
August 1961, ORNL-3213, pp. 124—33.

3%W. E. Brundage et al., Solid State Div. Ann. Prog. Rep.
August 1962, ORNL-3364, pp. l44—45.

35¢. A. Bruch, W. E. McHugh, and R. W. Hockenbury, "Embrittlement
ol Mulybdenum by Neutron Irradiation," Trans. ATME, 203: 281-85 (1955).

Table 4. Tensile Properties of Mblybdenum?

Integrated
Thermel Upper b
Neutron Test Yield Tensile Fracture Reduction
Material Flux Temperature Point  Strength Stress Elongation in Area
Condition (nvt) - (°c) (psi) (psi) (psi) (%) | (%)
x 102C . x10®  x10° x 103
Unirradiated -- +22 102.5 100.8 214.0 45.7 72.4
Unirradiated -- +22 93.8 98.8 193.0 41,7 65.0
Unirradiated -- —20 125.5 120.0 . 243.0 32.8 63.8
Unirradiated -- —40 -- 123.0 123.0. 0 0
Unirradiated -- —60 -- 142.0 142.0 0 0
Aged® . - +24.6 9. b 97.7 182.6 40.8 67.4
Aged® - +24.6 94.0 9.3 181.6 42.5 65.3
Irradiated 5.1 +21.8 151.7 151.7 149.0 0 0.08
Irradiated 5.1 +22.4 -- 109.7 109.7 0 0
Irradiated 5.85 +60 -- 148.5 148.5 0 0
Irradiated 5.85 +80.5 143.5 143.5 185.0 14.7 60.7
Irradiated 5.8 +100 111.5.  111.5 134.0 10 59.7

?C. A. Bruch, W. E. McHugh, and R. W. Hockenbury, "Embrittlement of Molybdenum by
NeutronEIrradiation,” Trans. ATIME, 203: 281-85 (1955).

bMaximum load divided by original area.

“Unirradiated specimen heated for 30 days at 90°C.

K4
25
temperature increased from about —30 to +70°C. Unirradiated and
irradiated specimens were not tested at comparable temperatures at
which each deformed plastically so that a meaningful comparison of the
strength could be made. Bruch et al.?? do not make any comments con-
cerning the relative values of the elongation and reduction in area.
However, it seems that an important trend exists. 1In the unirradiated
specimens, the uniform elongation is over one half the value of the
reduction in area. In the irradiated specimens, the elongation is only
one fifth to one sixth the wvalue of the reduction in area. A possible
explanafion of this observation is that the irradiation-igduced detf'ects
pin the dislocations in the metal so that the stress to cause plastic
deformation is quite high. When this stress is exceeded, the dis-
locations breask away from their pinning defects with such driving force
that the normal processes of work hardening are ineffective. Hence,
failure occurs with very high local deformation and very small unifdrm
elongation.

Several metallographic specimens were included in the tests of
Bruch et §£.35 They were polished and photographed at points marked
wifh hardness impressions before insertion into the experiment. Photo-
graphs were made of the same fields after irradiation without further
polishing. It was concluded that no visible metéllographic changes
occurred. ‘

The results of hardness tests performed before and after irradi-
ation are given in Table 5. The hardness increased by approximately
35 BHN as a result of the irradiation. In the last paragraph of their
pafier, the authors ingerted some additional data concerning irradiation
hardening. Few experimental detaiis are given. Opecimens were irradi-
ated at 400°C for an estimated 3 X 10%°0 thermal nvt (3 X 10%° fast nvt)
and found to inereage® in hardness froam 169 ta 216 BHN (converted from
RB values). These results indicate that the-defects are introduced by
the irradiation at a rate greater than they can be annealed out at

400°C.
Table 5. Hardness Test Resultsa

. b \ b
e RC RA
"Material Average o Average o
Condition Value ° Minimum  Maximum  BHN Value Minimum  Maximum  BHN
Unirradiated 23.0 - 19.7 24.9 242 62.0 59.9 63.0 243
Irradiated 28.5 24,2 31.7 275 64.9 62.8 66.5 280
Change +5.5 . +3.0 +7.7 433 +2.9 +0.5 +h 4 +37

8c. 5. Bruch, W. E. McHugh, and R. W. Hockenbury, "Embrittlement of Molybdenum -
by Neutron Irradiation,"” Trans. AIME, 203: 281-85 (1955).

Each number in the table represents the average of test results for
12 specimens. At least three Rockwell hardness measurements of each kind were made
per specimen. ‘

CConverted from the Rockwell numbers.

9¢
27

Makin and Gillies?® inveétigated the effects of neutron irradi-
ation on the mechanical properties of molybdenum. The test material
was obtained from the Johnson, Matthey and Company, ILtd., in the form
of 0.040-in.-diam wire. A comélete spectrographic analysis of the
material was given, but no mention was made of interstitial impurities.
The wires were given a stress-relief anneal at 1030°C for 30 min. The
specimens were irradiated for six months in a Windscale pile at approxi-
mately 100°C. The flux was 6 X 10%% thermal nv and the integrated
thermal flux was 5 X 10'% nvt. The ratio of fast to thermal neutrons
was estimated to be unity. Tensile tests were run on a Hounsfield
Tensometer at a strain rate of 8.2 X 1077 per second. The ductile-to-
brittle transition temperature was determined by bend tests on the
0.040-in.-diam wires. Specimens were defined as ductile when they could
be bent .90° around a pin of 6-mm diam without fracture. The strain rate
in the bend tests was designated as "slow." Eight specimens were used
to determine each transition temperature to a reported accuracy of *2°C.

The results of postirradiation tensile tests over the temperature
range of 20 to 200°C are given in Table 6. Four specimens were tested
at each condition. The yield stress was increased by irradiation over
the entire range of test temperatures, the effect becoming more pro-
nounced with increasing temfierature. The ultimate strength changed in
a similar mahner. Yield points'were observéd in the irradiated and
unirradiated specimens tested at 20°C. However, the drop in stress
associated with the yield point of the irradiated material was the
greatest and, at 83°C, only the irradiated material showed a yield
point. Neither materiai exhibited a yield point at the 200°C test
temperature. - The elongation at rupture was, in general, decreased
slightly by the irradiation. The 200°C test condition was an exception
with slightlytgreater elongation occurring in the irradiated specimen.

However, the elongation of both materials was quite low.

36M. J. Makin and E. Gillies, "The Effect of Neutron Irradiation
on the Mechanical Properties of Molybdenum and Tungsten," J. Inst.
Metals, 86: 108-12 (1958).
Table 6. Tensile Tests on Stress-Relieved MOlybdenum?

Test

8¢

Temperature ~Material Yield Sbressb Ultimate Strengtfib ‘Elongationb
(°C) Condition (psi) | (psi) ' (%)
. x10° - x 102 |
20 Irradiated 95.8-102.2 (99.4) 99.5-106.8 (104.3) 20.5-24.3 (22.0)
20 Unirradiated 90.6-95.6 (93.7) = 96.0-102.3 (99.8) 20.0-26.7 (23.6)
83 Irradiated - 93.3 . 93.5 18.5
97 Unirradiated 80.3 ‘ 90.5 23.8
200 Irradiated 85.5 ' 85.9 5.8
200 E Unirradiated 68.5-72.2 (70.4) 7. 6774.6 (74.6) 2.7-2.8 (2.8)

®M. J. Mekin and E. Gillies, "The Effect of Neutron Irradiation on the
Mechanical Properties of Molybdenum and Tungsten," J. Inst. Metals, 86: 10812 (1958).

bLimits of experimental results given with average values in parentheses.
29

Bend tests showed that the irradiation dose of 5 X 10%° nvt
raised the ductile-to-brittle transition temperature from —136 + 1°C
to =73 + 1°C, a rise of about 63°C. Attempts were made to study the
recovery characteristics of the radiation effect by annealing treat-
ments, but the complexity of the process coupled with the small number
' of specimens prevented conclusive results from being obtained. Makin
and Gillies3® explained their results in terms of both impurity atoms
~and irradiation-produced defects. It was postulated that the influence
of the defects produced by irradiation was not fully observed until the
test temperature was increased to about 200°C. Since quite large yield
points could be produced by postirradiation annealing at 200°C, it was
concluded that the defects were mobile at this temperature.

Studies of recovery in cold-worked molybdenum in a radiation

7 revealed a recovery stage at 150°C.

field by Kinchin and Thompson3
From the activation energy of 1.3 ev, the process was felt to be vacancy
migration. This observation led Makin and Gillies to conclude that
vacancies were the important defects in their specimens.

Makin and Gillies discuss their results in light of those obtained
by Bruch et §£.35 The former authors observed the transition tempera-
ture to increase from —136 to —73°C (+63°C) after a dose of 5 X 101° nvt.
The latter authors reported an increase from —30 to +70°C (+100°C) after
a dose of 1.9 to 5.9 X 1029 nvt. It was concluded by Makin and Gillies
that the magnitude of the increase in the transition temperature was not
proportional to the neutron dose. This seems a rather dangerous con-
clusion in light of the different metallurgical histories of the test
material, possible chemical differences, use of 0.04U-in.-diam wires vs
0.182-in.-diam rods as specimens, and uncertainties in flux measurements.

"

Another conclusion based upon this comparison was that "...it is possible
that the greatest effect is produced in materials possessing initially
the lowest transition temperature." 1In light of the uncertainties just
mentioned, this conclusion is not supported by the available data.
Although Makin and Gillies seemed to realize this, this has been passed

on through the literature as a general rule.

37G. H. Kinchin and M. W. Thompson, "Irradiation Damage and
Recovery in Molybdenum and Tungsten," J. Nucl. Energy, 6: 275-84 (1958).

30 -,

Bruch et §£.38 discuss the general problem of irradiation damage
in a later papér. In this paper, an attempt was made to assess the
relative effects of irradiation on high-purity copper, nickel, zirconium,
and iron, commercial grade 75A titanium, commerical-purity molybdenum,
and cold-worked and annealed type 347 stainless steel. The data on
molybdenum presented in this paper are the same as presented earlier.
However, some general statements were made which have bearing upon the
properties of molybdenum. Although it is not possible to calculate the
number of vacancy interstitial pairs which exist in a metal in a radia-
‘tion field at a given time due to complex annealing and other inter-
actions, reasonable models exist for calculating the total number of
sfich pairs that have been produced by a given flux. Such calculations
for the metals mentioned above showed that only minor differences
existed in the number of pairs produced in the metals. Hence, it was
concluded that the very large differences in the effect of comparable
doses on the properties of these metals were due to factors such as
the annealing of defects or variations ambng metals in the property

change produced by a given defect.
Tantalum

Franklin‘gz gl.Bg have run a limited number of postirradiation
tests on.tantalum and two tantalum-tungsten alloys to evaluate the
effect of irradiation on the mechanical properties. Besides the lattice -
defects normally produced by irradiation, tantalum is also converted to
tungsten by the thermal neutron reaction Ta8l(n,7)Ta'®2(—8)W182. The -
intrinsic effects of the two processeé were evaluated by comparing the
properties of the irradiated specimens with those of control specimens
containing comparable amounts of tungsten. The analysis of the test

material is given in Table 7. Sheet specimens were used which had a

38¢. A. Bruch, W. E. McHugh, and R. W. Hockenbury, "Variations
in Radiation Damage to Metals," Trans. AIME, 206: 1362—72 (1956).

39C. K. Franklin, D. Stahl, F. R. Shober, and R. F. Dickerson,
Effects of Irradiation on the Mechanical Properties of Tantalum,
BMI-1476 (Nov. 18, 1960).

31

Table 7. Chemical Analysis of the gantalum.and
Tantalum-Tungsten Alloys®’

Analyses for Indicated Specimen (ppm)

Unalloyed
Tmpurity Tantalum Ta~l.5 wt % W Ta—3.0 wt % W
Aluminum - <5 15 . 20
Chromium - 10 4 5
Copper 10 20 . 15
Iron. 3 6 15
Molybdenum -- 20 _ , 15
Niobium -- 300 100
Nickel ~- 6 3
Silicon -- 30 60
Zirconium -- 15 | 10
Nitrogen < 10 20 35
Carbon 10 20 35
Hydrogen ' 0.3 1 | 1
Oxygen 40 53 22

aA.verage of two analyses taken of the alloys after they
had been cold rolled to 0.030-in. strip and vacuum annealed.

bC. K. Franklin et al., Effects of Irradiation on the
Mechanical Properties of Tantalum, BMI-1476 (Nov. 18, 1960).

gage cection 1.00 in. long, 0.75 in. wide, and 0.030 in., thick. The
tantalum specimens were annealed at 1371°C and the alloy specimens were
annealed 2 hr at 1426°C prior to test. The specimens were irradiated
in the MIR at a thermal flux level of 4 to 5 X 10%% nv and a tempera-
ture of 93°C. Two capsules were irradiated having total thermal doses
of 8.6 X 1029 and 1.8 x 10?! nvt. Chemical analyses showed that the
tungsten content of the six irradiated specimens varied from 0.6 to
2.2 wt %.

The room-temperature tensile properties of the irradiated
specimens are compared with those of unirradiated specimens having

comparable chemistry in Table 8. The strain rate was 5 X 107° per minute.
Table 8. Room-Temperature Mechanical Properties of Unirradiated and Irradlated Tantalum
and Unirradiated Ta—1.5 wt % W and Ta—3.0 wt % W Alloys?
Total Integrated Average of Properties Tests
Thermal Flux 0.2%
Based on Ultimate Offset Elongation
Number of Dosimetry Tensile Yield at Maximum Elongation Hard-

Specimen Specimens .Analysis Strength  Strength Load in 1 in. . ness
Description Tested (nvt) (psi) (psi) (%) (%) KHN -
Unirradiated 4 -- 42,000 30,000 -- 40 103
tantalum .
Unirradiated 3 -- 44, ;900 31,000 -- 39 151
Ta—1.5 wt % W |
Unirradiated 3 - 52,400 38, 500 -- 35 170
Ta~3.0 wt % W |
Irradiated 4 7.8 x 10°° 69, 500 65,800 -- 16 274
tantalum
- (66-day

irradiation) ,

Irradiated 4 1.57 x 10%% 86,300 81,400 -- 7 309
tantalum

(109-day

irradiation)

8. K. Franklln, D. Stahl, F. R. Shober, and R. F. Dickerson, Effects of Irradiation on.

the Mechanical Properties of Tantalum, BMI-1476 (Nov. 18, 1960).

ce
33

The yield strength of the irradiated specimens increased nearly three
times over that of the unirradiated tantalum. The properties of the
arc-melted alloys showed that the increase in strength could not be
accounted for in terms of the tungsten content since only minor strength
changes resulted from the addition of up to 3 wt % W. Hence, the change
in strength was attributed to some mechanism of defect production by
neutrons, Table 8 also illustrates the marked reduction in ductility
which occurred as a result of irradiation. However, the data indicate
that a large portion of the ductility effect can be attributed to the
tungsten content of the specimens.

Tantalum showed a large increase in hardness after irradiation.
The hardness changed from 103 to 309 KHN after an integrated thermal
dose of 1.8 X 1021 nvt. The hardnesses of the tantalum alloys containing
1.5 and 3.0 wt % W were 151 and 170 KHN, respectively.

40 reported the results of a limited number of

Sutton and Ieeser
room—tempefature postirradiation tensile tests on tantalum. No experi-
mental details other than neutron dose were given. The results of these
tests are given in Table 9. Irradiation appears to raise the ultimate
strength and decrease the rupture ductility.

These authors also reported that the hardness of tantalum was
increased 8 points to 57 RA by a neutron dose of 1 X 101° nvt thermal

and 5 X 10'° nvt fast.
Tungsten

The effects of bombardment by 13.7-Mev deuterons on the internal
friction and Young's modulus of polycrystalline tungsten were studied

d.4' TIrradiations were carried out at 300°%K using

by Muss and Townsen
the University of Pittsburgh cyclotron, and the effects of integrated

doses of 5 X 1014 to 8 x 101° deuterons/cm2 were studied. Because of

400, R. Sutton and D. O. Leeser, "Radiation Effects on Keactor
Materials," Nucleonics, 12: 8-16 (1954).

4lp, R. Muss and J. R. Townsend, "Internal Friction and Young's
Modulus in Irradiated Tungsten,"” J. Appl. Phys., 33: 180407 (1962).

34 -

Table 9. Postirradiation Tensile Properties
of Tantalum at Room Temperature

Ultimate
Integrated Neutron Dose  Tensile
(nvt) ‘ Strength Elongation
Thermal Fast (1b/in.?) (%)
x 1019 X 10%° x 103
O 0 72.0 19
0 0 65.0 23 i
1 5 88.0 17 )
1 5 g5.0 17

the nature of the bombarding particles, the number of defects produced
by deuterons is approximately 1000 times greater than the number pro-
duced by a comparable dose of neutrons. The test material was
0.0015-in.-diam tungsten wire supplied by the Sylvania Company. The
wire was designated as type NS-50, but no composition was givén. The
modulus and internal friction measurements were made using a mechanical
resonance system consisting of the tungsten wire mounted as a cantilever.
Preirradiation internal friction meaéurements as.a function of
temperature showed that a reproducible peak occurred at 140°%K. This
peak was found to occur at higher temperatures as the frequency of
vibration was increased. This 1s a characteristic of a relaxation
phenomenon and it was'fiBStUlatédfthat this peak was due to the thermal -
activation of dislocations. The data of this study and those of
Chambers and Schultz*? were combined to obtain an activation-enefgy for
the process of 0.21 + 0.05 ev.
The effects of integrated dose rate-op tungsten were'evaluated
at an irradiation temperature-of 295°%K. The internal friction decreased

and Young's modulus increased. Both of these effects were found to fit

42R. H. Chambers and J. Schultz, "Dislocations Relaxation Spectra
of Cold-Worked Body-Centered Cubic Transition Metals,'" Phys. Rev. Letters,
6: 273 (1961).

35

the model of a dislocation pinning mechanism that was proposed by

43 The pinning in this case was concluded to be

Dieckamp and Sosin.
due to interstitials. This conclusion was based on the work of Kinchin
and Thompson** which showed that radiation-induced interstitials begin
to anneal out at 80°K whereas vacancies become mobile at 650°%K. The
internal friction effect began to saturate at an integrated flux of
about 2 X 1016 deuterons/cmz. The elastic modulus went through an
inflection at approximately the safie dose and decreased linearly with
incrcasing dose. This decrease was explained in terms of the bulk
effect of vacancies being frozen into the lattice. This effect amounted
quantitatively to a 0.44% decrease in Young's modulus per atomic percent
vacancies. The amplitude of the internal friction peak at 140°%K and
the background internal‘frictjon were found to. decrease as a result of
irradiation. This was attributed to dislocation pinning. However,
after irradiation several small peaks appeared in the internal friction
spectrum that were not satisfactorily explained.

Makin and Gillies“® studied the effects of neutrofi irradiation
on the mechanical properties of tungsten. The material used in this
investigation was obtained from the Johnson, Matthey and Company, Ltd.,
in the form of 0.040-in.-diam wire. A complete spectrographic analysis
of the material was given, but no mention was made of interstitial
impurities. The specimens were fully recrystallized by annealing 30 min
at 1600°C. The specimens were irradiated for six months in a Windscale
pile at approximately 100°C. The flux was 6 X 10'? thermal nv and the
total integrated thermal flux was 5 X 102 nvt. The ratio of fast to
thermal neutrons was not known accfirately but was estimated Lo be unity.

Control specimens were annealed at an equivalent temperature and time.

“3H. Dicckaipy and A. Sosin, "Effect of Flectran Irradlatlon on
Young's Modulus,"” J. Appl. Phys., gz 1416 (1956).

44G. H. Kinchin and M. W. Thompson, "Irradiation Damzge and
Recovery in Molybdenum and Tungsten,'" J. Nucl. Energy, 6: 275-84 (1958).

45M, J. Makin and E. Gillies, "The Effect of Neutron Irradiation
on the Mechanical Properties of Mblybdenum and Tungsten," J. Inst.
Metals, 86: 108-12 (1.958).

36

Tensile tests were run on a Hounsfield Tensometer at a strain rate of
8.2 x 1072 per second. The ductile-to-brittle transition temperature
was determined by bend tests on the 0.040-in.-diam wires. Specimens
were defined as ductile when they could be bent 90° around a pin of
60-mm diam without fracture. The strain rate was given as "slow."
Eight specimens were used to determine each transition temperature
and the reported accuracy is *2°C. | _

The results of' tensile postirradiation tests at 100 and 200°C
are compared with the results obtained from control specimens in Table 10.
Although no statement is made of the exact number of specimens tested,
the implication is that these are average values. Both the irradiated
and unirradiated specimens were brittle at 100°C. However, the fracture
stress was raised by the irradiation. At 200°C the yieid strength was
increased by irradiation but the ultimate strength was unaffected. The
elongation at rupture and reduction in area at 200°C were increased by
irradiation. Smooth stress-strain curves were obtained with no yield
points being observed. The ductile-to-brittle transition temperature
was increased from 118 # 2°C to 126 * 2°C by the irradiation.

\

Table 10. Tensile Tests on Recrystallized Tungstena

Ultimate
‘ ' Test Tensile .
Material Temperature Yield Stress Strength Elongation
Condition (°C) (1b/in.?3) (1b/in.3) (%)
x 103 X 103
Irradiated 100 152.0 - 0
(fracture stress)
Unirradiated 100 137.0 ' -- 0
(fracture stress)
Irradiated 200 | 131.0 ~ 173.0 4.2

Unirradiated 200 ‘ 148.0 ©173.2 2.4

M. J. Makin and E. Gillies, "The Effect of Neutron Irradiation
on the Mechanical Properties of Molybdenum and Tungsten," J. Inst.
Metals, 86: 108-12 (1958).
37

40 reported that the room-temperature ultimate

Sutton and Leeser
tensile strength of tungsten increases 20 to 25% after irradiation with
5 X 10'° fast neutrons/cm®. The reported data (given in Table 11) do
not seem to support the statement made by Sutton and ILeeser. All .
specimens were tested below the ductile-to-brittle transition tempera-
ture; therefore, all were brittle. No'additional experimental details

were given.

Table 11. Postirradiation Tensile Properties of
Tungsten at Room Temperature

Ultimate
Integrated Neutron Dose Tensile
(nvt) Strength Elongation
Thermal Fast (1b/in.?) (%)
x 10%° X 10%° x 103
0 O ' 145.5 O
0 0 160.0 0]
1 5 132.0% 0
1 5 102.0% 0

®Decrease can be attributed partly to
difficulty of alloying brittle specimens by
remote control.

SUMMARY

The available data on the mechanical behavior of niobium, molyb-
denum, tantalum, and tungsten have been reviewed critically. Several
important conclusions have been reached as a result of this study — the
most important one being that insufficient engineering data are available
for the design of complex systems using refractory metals and structural
materials. It was also found that each of the four metals reviewed has
certain unique properties that make it desirable for specific application.

Molybdenum and tungsten have low coefficients of thermal expansion
which may more nearly match those of cermets and ceramic components.

These materials also have high moduli of elasticity which are desirable

/
38

from a design standpeoint. -However, both of these materials present
problems with respect to fabricatility. Niobium has a very low modulus
. of. elasticity — an undesirable feature. Niobium and tantalum are
relatifiely easy to fabricate and have good ductility. Niobium and
molybdenum have low neutron absorption cross sections, whereas tantalum
and tungsten are an order of magnitude higher.

Because of the complexity of high-temperature nuclear systems,
materials are needed that have a variety of properties. Hence, at this
stagé of refractory-metal technology it is important not to limit our
studies to those metals that can be fabricated into tubing or those
metals that can be welded, since there may be applications for which
such materials can be used in different parts of the reactor. Also,
service conditions such as stress, temperature, temperature cycle, and
desired nuclear properties must be known. _

In order to expedite the development of the technology necessary
for the use of refractory metals in engineering systems, it is felt that
persons involved in evaluating the mechanical behavior of these metals
should consider the following factors.

1. A lot of deception is being injected into this field by the
production of small melts of alloys and by the evaluation of these
alloys by short-time tensile tests. These small melts often are made
under nonrefiroducible conditions and are fabricated by unknown pro-
cedures. Unless the application is one requiring a short life, the
use of short-time tensile tests for screening purposes cafi be very
deceiving. Short creep tests of 10 to 100 hr duration are better
screening tests for materials to.be used in long-time applications.

2. More attention needs to be given to defofmation mechanisms
in refractory metals. Just as the strength of many superalloys far
‘exceeds that of pure iron .and nickel, so can the properties of refrac-
tory superalloys excel those of the pure refractory metals if one
learns more about the deformation mechanisms in refractory metals.
Fabrication procedure and impurity content can be used to an advantage
if understood. Dispersed phases may possibly be found important in

these alloys. It may be found that these dispersions improve the
39

strength by impeding dislocation motion as well'as serving as sources
for dislocations in normally brittle materials. Hence, the production
of ultra-pure alloys by electron-beam melting may not be the most
practical approach to ductile molybdenum and tungsten.

3. The more promising alloys need to be evaluated with respect
to their strength and metallurgical stability over long periods of time.

4. Experiments should be carried out to evaluate the behavior

of refractory metals under neutron irradiation at ambient and elevated
temperatures. These studies should be directed toward defining mecha-
nisms responeible for the different mechanical behavior under irradia-
tion and hence would make it possible to design alloys which are not

greatly altered by irradiation.
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