I
THE PROPERTIEf- Of A F^.^^
GAMMA-PHASF U-."
DISCLAIMER
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UCRL-7869 Meta ls , C e r a m i c s and
Mate r i a l s , UC-25 TID-4500 (30th Ed.)
UNIVERSITY OF CALIFORNIA
Lawrence Radiation Labora to ry
L i v e r m o r e , California
Contract No. •W-7405-eng-48
THE PROPERTIES OF A METASTABLE GAMMA-PHASE
URANIUM-BASE ALLOY: U - 7 . 5 N b - 2 . 5 Zr
C. A. W. P e t e r s o n
R. R. Vandervoort
May 13, 1964
Printed in USA. P r i c e ^ ^ ^ B t s . Available from the Office o f ^ c h n i c a l Serv ices U. S. Department of Commerce Washington 25, D.C.
„ 1 „
THE PROPERTIES OF A METASTABLE GAMMA-PHASE
URANIUM-BASE ALLOY: U-7.5 Nb-2 .5 Zr
C. A. W. P e t e r s o n and R. R. Vandervoort
Lawrence Radiation Labora tory , Universi ty of California
L i v e r m o r e , California
May 13, 1964
ABSTRACT
The uran ium-niob ium-z i rconium t e rna ry sys tem exhibits a s e r i e s of
solid solutions at high t e m p e r a t u r e . Addition of both niobium and z i rconium
stabi l izes the body-centered-cubic gamma phase to a lower t e m p e r a t u r e .
A study has been made of one pa r t i cu la r t e r n a r y alloy, the U-7.5 w/o Nb-2.5
w/o Zr composit ion. The relat ionship of t rans format ion c h a r a c t e r i s t i c s of
the alloy to its physical and mechanical p rope r t i e s has been determined.
This weldable alloy is concluded to be an excellent m a t e r i a l for engineering
appl icat ions. Tensi le yield s t rengths of 100,000 to 200,000 ps i a r e obtainable,
with elongations of 12% to 4%, respect ively , depending on heat t r ea tmen t .
INTRODUCTION
While gamma-s tab i l i zed uran ium is of in te res t because of the more
isotropic nature of the body-centered-cubic gamma phase and i ts amenabil i ty
to heat t r ea tment , the binary alloys of uranium-molybdenum, u ran ium-
niobium, and u ran ium-z i r con ium that have been developed thus far each
pos se s s some shortcoming with respec t to engineering applicat ions. The
gamma uranium-niobium al loys, for instance, have low yield s t rengths ,
causing them to c reep or rupture under high loads; the gamma u ran ium-
molybdenum alloys fail in a br i t t le manner under static or l o w - s t r a i n - r a t e
loading in environments containing oxygen; the gamma uran ium-z i rcon ium
alloys t r ans fo rm to very strong, ha rd alloys but they lack ductility at room
t e m p e r a t u r e . In the gamma phase a balance of s t rength and stabili ty at
room t e m p e r a t u r e is obtained with a composit ion of U-7.5 w/o Nb-2.5 w/o Zr .
Data available from studies of the i so the rmal gamma-phase react ions
of the binary alloys ' and from work on the t e rna ry equil ibr ium d iagram of
- 2 -
uran ium-niobium-z i rconium have st imulated a detailed examination of
alloys in the u r a n i u m - r i c h corner of the t e rna ry sys tem. A t e rna ry gamma-
phase alloy with a nominal composition of U-7.5 w/o Nb-2.5 w/o Zr has been
developed as a resul t of this investigation. This alloy is not susceptible to 4 s t r e s s cracking in a i r or oxygen and can be heat t r ea t ed to obtain a -wide
range of p rope r t i e s .
EXPERIMENTAL PROCEDURE
The selection of the U-7.5 w/o Nb-2.5 w/o Zr composition for evalua-
tion was based, in par t , on the behavior of the binary uranium-niobium and
u ran ium-z i rcon ium al loys. It was des i red to combine a sufficient amount of
niobium for stabilizing the gamma phase with a sufficient amount of z i rconium
for strengthening. It was also des i red to re ta in , as much as poss ib le , the
ductility which is cha rac te r i s t i c of the binary uranium-niobium al loys .
Small a r c - m e l t e d buttons as well as plate from 10-kg and 100-kg ingots
were used to study the phase s tabi l i t ies , i so thermal t r ans format ions , and
physical and mechanical p rope r t i e s of the alloy. The re su l t s of these studies,
along with fabrication p rocedures and other per t inent information, a r e
descr ibed below.
L Mate r ia l s
The highest puri ty m a t e r i a l s available were used. Emphas i s was
placed on low in ters t i t ia l impur i t i es . Typical analyses of m a t e r i a l s used a r e
given in Table I.
2. P r e p a r a t i o n of the Alloys
The ma te r i a l which was used in the differential t h e r m a l ana lys is (DTA),
i so thermal exper iments for the TTT curves , and h a r d n e s s - v e r s u s - h e a t -
t r ea tment studies was p r epa red in the form of 200-g buttons. Ingots of
approximately 2 X 1 X 0.375 in. were formed in a wate r -coo led copper mold
by a rc -me l t ing the charge in an argon a tmosphere . Five r e m e l t s were made
and the final ingot was homogenized in vacuum for 72 hours at SSO'C. The
analyses of the ingots were substantial ly the same as the cha rges , i. e. , 90
w/o uranium, 7.5 w/o niobiumi, and 2.5 w/o z i rconium.
Ingots in the 8- to 10-kg range were a r c - m e l t e d for physical and m e -
chanical p roper ty s amples . Sandwich-type e lec t rodes of individual naetal
- 3 -
Table I. Typical analyses of ma te r i a l s used for uranium al loys.
Uranium Molybdenum Niobium Zirconium (ppm) (ppm) (ppm) (ppm)
C 35
Fe 150
Mg 21
Ni 30
Si 20
Mn 35
Pb
_4-
The machined 90-kg ingot was heated for l ~ l / 2 hours in argon at 950*C
and p re s s - fo rged to a rec tangular slab. The slab was hot - ro l led at 840 C to
a plate about 0.875 in. thick, then heated to 800°C in a salt bath and hot-
rolled to 0,625-in. p la te . The final heat t r ea tment consis ted of a 1-hour
anneal at 950°C in argon followed by cooling in an argon-f i l led chamber to
room t e m p e r a t u r e . Analyses of the 9-kg and 90-kg pla tes a r e given in Table
II.
Table II. Analyses of 9-kg and 90-kg plate of U-7,5 w/o Nb-2,5 w/o Zr alloy.
9-kg plate 90-kg plate
7.4% 7.3%
2.5% 2,5%
55 ppm 100 ppm
40 ppm 35 ppm
14 ppm
18 ppm
TRANSFORMATIONS
1. Differential The rma l Analysis
Standard 0.375-in. X 0.250-in . -diam cyl indrical samples were machined
from homogenized (72 hours at 850°C) 200-g button ingots. They were cycled
from room t empe ra tu r e to 900°C and back to room t e m p e r a t u r e at a uniform
ra te of 85°C pe r hour. F igure 1 is a photograph of a char t record ing a
cycled run for pure uran ium showing the sharp the rma l peaks of the gamma-
to-beta and be ta- to-a lpha t r ans fo rmat ions . F igu re 2 is a photograph of a
cycle made on a sample of U-7.5 w/o Nb-2.5 w/o Zr , as wrought and gamma-
annealed.
In the t e rna ry alloy the t empe ra tu r e of the gamma-phase t r ans format ion
has been lowered substantial ly and the be ta -phase react ion has been eli ininated.
The alloy t r a n s f o r m s on cooling direct ly from gamma to alpha phase . The
original gamma-phase t rans format ion of the pure uran ium now occurs at
about 500°C in the t e r n a r y alloy. It is also very sluggish and drawn out and
is not completed until the t e m p e r a t u r e has fallen to 363°C. At room
Nb
Zr
C
^ 2
^ 2 H^
- 5 -
Differential Thermal Analysis of P u r e Uranium
and U-7.5 w/o Nb-2.5 w/o Zr
GLL- 6 3 5 - I 221
T e m p e r a t u r e °C
Fig . 1. P u r e uranium. Fig . 2. U-7.5 w/o Nb-2.5 w/o Z
- 6 -
t empe ra tu re , x - r ay analysis shows the alloy to be a mixture of alpha uranium
and Y' , a bcc n iobium-r ich solid solution. No delta phase corresponding to
UZr„ was detected at this niobium concentration.
Resul ts from the rma l cycling of var ious samples made from different
mel t s and p rocessed under different conditions ag ree closely in the manner
of the gamma phase t ransformat ion.
2. Dilatometry
The l inear expansion and contract ion of samples in the range from
room t empe ra tu r e to 1000°C were determined. With the same heating and
cooling ra tes used in the differential t he rma l ana lys i s , a typical curve (Fig. 3)
for a sample of the U-7.5 w/o Nb-2,5 w/o Zr alloy (from a 90-kg hot-worked
plate) was determined.
On heating, the alloy expands normal ly to 600° C, at-which point a very
rapid expansion occurs to 640°C. These points ag ree quite well with those
from the DTA (602 and 654°C), During cooling the cycle is normal with a
somewhat exaggerated contraction at about 375°C. This point is considerably
lower than that for the DTA (500°C) and is probably due to the conditions of
cooling and the t empe ra tu r e measur ing sensit ivi ty of the equipment used.
The average l inear coefficients of expansion calculated from the char t
a r e given in Table III.
3. I so thermal Transformat ion
The t i m e - t e m p e r a t u r e - t r a n s f o r m a t i o n or TTT curve is a famil iar
method of deternaining the gamma phase stability of the alloy under non-
equil ibrium conditions. In the p resen t study the t ime of or igin of t r a n s f o r m a -
tion was es t imated from the beginning of changes in e lec t r ica l r e s i s t ance ,
ha rdness , and m i c r o s t r u c t u r e . F igure 4 shows TTT curves a s sembled from
the react ion data at the var ious i so the rms using both the res is t iv i ty and
hardness m e a s u r e m e n t s . The agreement is quite good. In the upper regions
from 500 to 600° C the m i c r o s t r u c t u r e s ag ree quite well , but below 500°C
metal lographic resu l t s a r e difficult to in te rpre t and hence the TTT curves
a r e made without these data. The na ic ros t ruc tures were identified in some
cases by use of x - r a y diffraction and in o thers by effects of chemical etchants
• 7-
0.04 —
20 100 200 400 600
T e m p , °C
800 1000
GLL-647-1893
F i g . 3. L i n e a r e x p a n s i o n of U - 7 . 5 w / o N b - 2 . 5 w / o Z r .
m By ha rdness
jk. By e lec t r ica l r e s i s t ance
200 —
1000
Time, min GLL~647-l894 Fig . 4. T ime- t empe ra tu r e - t r a n s fo rma t ion (TTT) curves for U-7.5 w/o
Nb-2.5 w/o Zr .
-9 -
14.8 X 10"
124,7 X 10"
14 4 X 10-
16.9 X 10-
-6
-6
-6
• 6
T a b l e III. Coeff ic ient of l i nea r t h e r m a l e x p a n s i o n for U - 7 . 5 w / o N b - 2 . 5 w / o Z r a l loy .
T e m p e r a t u r e r a n g e Coeff ic ient of e x p a n s i o n (°C) ( i n . / i n . - ° C )
20 to 300
600 to 635
640 to 1000
20 to 1000
T h e r e s i s t i v i t y m e a s u r e m e n t s w e r e m a d e a t t he t e m p e r a t u r e of t r a n s -
f o r m a t i o n a f t e r a d i r e c t t r a n s f e r of the s a m p l e f r o m the g a m m a a n n e a l i n g
ba th to the i s o t h e r m a l t r a n s f o r m a t i o n ba th . The e q u i p m e n t a l l o w s r e a d i n g s
to be t a k e n a u t o m a t i c a l l y in d e c a d e s f r o m 1 m i n u t e to 10,000 m i n u t e s . I m -
m e d i a t e r e a d o u t of r e s i s t i v i t y c h a n g e s a s s m a l l a s 0 .0004 o h m i s no t ed on
s e m i - l o g p a p e r . The th in , 2 X 0.062 X 0 .062 - in . s a m p l e s a r e e n c l o s e d in
s t a i n l e s s s t e e l c o n t a i n e r s f i l led wi th h e l i u m for good h e a t t r a n s f e r .
S m a l l c u b e s 0 .375 in. on an edge w e r e u s e d a s s a m p l e s for d e t e r m i n a -
t ion of h a r d n e s s and m i c r o s t r u c t u r a l c h a n g e s a c c o m p a n y i n g t r a n s f o r m a t i o n .
T h e s p e c i m e n s w e r e t r a n s f o r m e d in h e l i u m - f i l l e d c o n t a i n e r s by quench ing
f r o m the g a m m a - a n n e a l i n g t e m p e r a t u r e d i r e c t l y into a s a l t ba th h e l d a t a
p r e s e l e c t e d t e m p e r a t u r e . Af te r i s o t h e r m a l t r a n s f o r m a t i o n the c o n t a i n e r
w a s w a t e r - q u e n c h e d . The h a r d n e s s c h a n g e wi th t i m e a t v a r i o u s t e m p e r a t u r e s
i s shown in F i g . 5. T h e da ta u s e d for h a r d n e s s p o r t i o n of the T T T c u r v e s
(F ig . 4) a r e b a s e d on t h e t i m e to a t t a i n o n e - h a l f of t h e fully a g e d h a r d n e s s .
M i c r o s t r u c t u r e s of the a l loy t r a n s f o r m e d a t 550 and 600°C show a
p e a r l i t i c type of s t r u c t u r e . T h e a l p h a p l a t e l e t s n u c l e a t e a t g r a i n b o u n d a r i e s ,
and they a d v a n c e into t h e g r a i n i n t e r i o r a s the g a m m a p h a s e t r a n s f o r m s .
T h e p l a t e s a p p e a r c o a r s e r a t t h e h i g h e r t e m p e r a t u r e . T h e m o r p h o l o g y (i, e. ,
the p l a n e s on which the g rowth o c c u r s ) h a s not b e e n d e t e r m i n e d . M i c r o s t r u c -
t u r e s showing d e c o m p o s i t i o n of the g a m m a p h a s e to the p e a r l i t e s t r u c t u r e
a t 600°C a f t e r 1, 2 .5 , and 8 h o u r s a r e r e p r o d u c e d in F i g s . 6, 1, and 8,
r e s p e c t i v e l y .
F i g u r e s 9, 10, and 11 show the ag ing effect of 0 .25 , 1, and 8 h o u r s ,
r e s p e c t i v e l y , a t 4 5 0 ° C . B e t w e e n 450 and 500° C the t r a n s f o r m e d m a t e r i a l i s
m u c h f ine r t h a n a t 600°C and i s d i s t r i b u t e d t h r o u g h o u t the g r a i n s a s a v e i n e d
-10 .
CO CD
U
XI
U
u o
Time , hours GLL-647-1895 F ig . 5. Age hardening of U-7.5 w/o Nb-2.5 w/o Z r .
1 1 .
I s o t h e r m a l T r a n s f o r m a t i o n of G a m m a
U - 7 . 5 w / o N b - 2 . 5 w / o Z r at 600°C
'•^iml.
^ ,
GLB 647 4 2 4 0
F i g . 6. 1 hou r (400X)
•"ftSjj i «f
GLB 5 4 / 4 2 4 2
F i g . 8a. 8 h o u r s (400X).
GLB 647 4241
F i g . 7 2.5 h o u r s (400X)
F i g . 8b. 8 h o u r s f
12-
I so thermal Transformat ion of Gamma U-7.5 w/o Nb-2.5 w/o Zr at 450''C (400X, Polar ized Light)
.KMlAffff
J%t ^J^i ^ C - '
h * »SV>. , « > « * * • • •
GLB-64/-4.i44 GLB-647-4245
Fig. 9. 0.25 hour. Fig. 10. 1 hour.
GLB-647-4246
Fig. 11. 8 hours.
^13-
or sa l t -and-pepper type of nucleation product. Polar ized light is used to
br ing this out more vividly.
At the t e m p e r a t u r e s below 450°C where the hardening effect is both
rapid and intensive, the m i c r o s t r u c t u r e s show a mar t ens i t i c type of strained
s t ruc ture but no d i sc re t e phase of prec ip i ta te . F igure 12, taken of the
s t ruc ture as t rans formed for 1 hour at 350°C8 and Fig . 13, of the s t ruc ture
t rans formed for 2 - l / 2 hours at 400°C, show this type of pat tern. Extensive
line broadening in the x - r a y diffraction pat terns lends evidence to support
the existence of a highly s t r e s sed in ternal s t ruc tu re .
PHYSICAL PROPERTIES
Some of the physical p roper t i e s as de te rmined for the alloy a r e as
follows:
1. Density at room t e m p e r a t u r e : cas t , 16.4 g /cc ; wrought, 16.6 g / cc .
2. Coefficient of l inear expansion: see Table III.
3. Crysta l lography (as gamma-quenched) : gamma phase, body-
centered cubic.
4 . E lec t r i ca l res i s t iv i ty : gamma-quenched from 850°C, 70 (lohm-cm;
t ransformed 8 hours 600° C, 53 |j,ohm-cm.
5. Specific heat at 23°C: 0.04 c a l / g - ° C .
MECHANICAL PROPERTIES
The yield s trength, ul t imate tensi le s t rength, and ductil i ty in tension,
as well as the hardness and the toughness of the alloy, va ry considerably
depending upon the heat t r ea tmen t given the alloy.
Table IV is a s u m m a r y of the tensi le p rope r t i e s for many heat t r e a t -
ments of the alloy. There is very lit t le effect on these p roper t i e s due to
different s t ra in r a t e s and environment in tes t ing. The ma te r i a l with the
highest s t rength and lowest ductility is the slowly cooled, gamma-annea led
alloy and the gamma-quenched alloy aged at 350 to 450°C. In the gamma-
quenched condition the metal is the softest and most duct i le .
F igure 5 shows the response to hardening of the gammia-quenched alloy
with different heat t r e a t m e n t s .
Table V, which gives tensi le data obtained on the alloy above and be
room t e m p e r a t u r e , indicates no definite t rans i t ion t empera tu re from -?
to 150° C. Slight strengthening occurs at the lower t empera tu re but d'
r e m a i n s the same.
-14-
Isothermal Transformat ion of Gamma U-7.5 w/o Nb-2.5 w/o Zr
(400X, Po la r i zed Light)
GLB-647-4247
Fig. 12. 1 hour at 350°C.
cs'.j'^iii!-• ^'.^f-^^^- '. ""-•"•. ..• iV-̂ ,.
fx. . Î ;F?
» < * •
-. *5
^,1
i
r-'fi'r .> .
'SSj,-,
GLB-647-424B
Fig. 13. 2.5 hours at 400 C.
Table IV. R o o m - t e m p e r a t u r e tensi le p roper t i e s of U-7.5 w/o Nb-2.5 w/o Zr alloy.
Tes t ing environment Conditioning of sample
Strain ra te
( in. / in. -min)
Yield s trength, 0.2% offset
(psi)
U. T. S. % elong. % (psi) in 2 in. RA
Air,
Air,
Air,
Air,
Air,
^^ir.
Air,
Vac.
Air,
Vac.
Air,
*Air,
Air,
Air,
Air,
Air,
Air,
Air,
Air,
R
F
R
R
R
F
R
, R
R
, R
R
R
R
R
R
R
R
R
R
900°C/30 min /vac . /W,Q .
840°C/sa l t ba th/30 min /AC
800°C/ l hou r /vac . /WQ
800°C/ l h o u r / v a c . / W Q
950°C/ l h o u r / a r g o n / A C
950°C/ l hour /AC + 8 hours /600°C
800°C/ l h o u r / v a c . / W Q + 8 hours /600°C It II
900°C/ l hour /vac . /WQ-I- 1 hour/600°C
900°C/ l hou r /vac . /WQ -I- 1 hour/500°C
900°C/ l hou r /vac . /W Q + 1 hour/400°C
900°C/ l hou r /vac . /WQ + 1 hour/350°C
0.05
0.05
0.05
0.05
0.005
0.05
0.05
0.05
0,0005
0.0005
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
68,300
87,000
100,300
108,300
91,600
95,000
131,400
134,300
113,500
110,300
135,400
139,700
143,800
129,400
138,400
66,500
77,000
210,000
220,000
152,300
182,300
128,300
130,300
120,300
130,200
190,800
191,300
185,500
186,900
197,000
182,400
176,200
186,200
191,500
163,600
177,000
219,000
225,000
12.3
10.5
12.5
12.0
14.4
12.0
5.2
5.2
6.5
6.3
5.0
7.8
6.9
8.4
7.8
13.0
9.5
1.0
1.0
46
24
21
21
21
34
-
-
-
_
-
-
-
_
-
41
33
1.3
0.5-
Table IV. R o o m - t e m p e r a t u r e tens i le p rope r t i e s of U-7.5 w / o Nb-2.5 w / o Zr alloy. (Continued)
Test ing environment Conditioning of sample
S t ra in Yield s t rength, r a t e 0.2% offset U. T. S. % elong, %
(psi) (psi) in 2 in. RA ( in. / in . -min)
Air , F
Air , F
Air , F
Air , F
f 1 hour/400 °C
350°C
4 hours /350°C
0.05
0.05
0.05
0.05
244,000 230,000
209,000
260,000
251,000 251,000
230,000
270,000
0.5
1.0
6
2
-
1.3
17
2
St r e s sed over 200 hours at 90% Y. S. before tes t ing.
R = round-shoulder type tens i le b a r s .
F = f la t -gr ip type tens i le b a r s .
WQ = water-quenched.
AC = a i r -coo led .
-17-
Table V. Effect of t e m p e r a t u r e of test ing on tensi le p roper t i e s of 90 U-7.5 w/o Nb-2.5 w/o Zr alloy, vacuum-annealed at 900°C and water-quenched.
T e m p e r a - Yield s t rength, U. T. S. % elong. % tu re of tes t Condition of sample 0.1%offset (psi) in 2 in. RA
(°C) (psi)
150 900°C/vac . / - | hour /WQ 74,000
100 9 0 0 ° C / v a c . / i h o u r / W Q 68,200
25 9 0 0 ° G / v a c . / i h o u r / W Q 68,300
0 9 0 0 ° C / v a c . / i h o u r / W Q 64,000
-30 9 0 0 ° C / v a c . / i h o u r / W Q 65,000
WQ = -water-quenched.
The standard, r o o m - t e m p e r a t u r e Cha rpy - impac t - t e s t data given in
Table VI c o r r e l a t e well with the tens i le r e su l t s . The toughness of the gamma-
quenched alloy is good. Aging of the alloy to give high tensi le s t rength adverse ly
affects the impact s t rength and ductility. Pho tomic rographs (Figs , 14 and 15)
show the great difference in f rac ture pa t te rn between gamma-quenched and
aged m a t e r i a l s . Slowly cooled ma te r i a l has an impact s trength between these
two; the s t ruc ture at the f rac ture of gamma-annea led and a i r - coo led plate is
shown in Fig . 16. Quenching gamma-annea led alloy direct ly to the t r a n s -
formation t e m p e r a t u r e instead of f i r s t quenching to room t e m p e r a t u r e produces
be t te r toughness for the same s t rength of alloy. Also, lower t e m p e r a t u r e
gamma-annea l s give bet ter Charpy impact s t rength than high t e m p e r a t u r e
anneals .
Table VII indicates that, as in the case of tens i le data, t he re is no
change in toughness from 100°C down to -40°C for the U-7.5 w/o Nb-2.5 w/o
Zr gamma-annea led alloy.
The tens i le p rope r t i e s of the alloy at t e m p e r a t u r e s up to 1000°C a r e
quite good. Some tens i le data obtained on thin s t r ip specimens heated by
e lec t r ic cu r r en t in a Marquard t tens i le t e s t e r a r e summar i zed in Table VIII
and given in graph form in Fig . 17. The t e s t s conducted at the slower s t ra in
ra te of 0.03 i n . / i n . - m i n gave questionable yie ld- load r e s u l t s , pa r t i cu la r ly
above 650 °C. The t e s t s at the higher s t ra in r a t e of 6 i n . / i n . - m i n gave much
more consis tent data for both yield loads and elongation.
The counteract ion of the na tura l annealing effect by the t r ans format ion
aging effect as the t e m p e r a t u r e of the tes t r i s e s is apparent at t e m p e r a t u r e s
132,000
137,000
150,000
159,000
166,000
12.5
12.0
12.3
13.0
15.3
45
45
46
46
47.5
-18-
Table VI. Effect of heat t r ea tment on r o o m - t e m p e r a t u r e V-notch Charpy values of U-7.5 w/o Nb-2.5 w/o Zr alloy.
Heat t r ea tment
950°C/1
900°C/1
840°C/1
800°C/ l
800°C/ l
800°C/ l
800°C/ l
hour / a rgon -1- argon Q
hour /vac . + water Q
hou r / s a l t -1- water Q
h o u r / s a l t -1- water Q
hour /vac . + water Q
hour/vac.-f tube Q
hour /vac . + oil Q
800°C/3 h o u r s / s a l t + water Q
750°C/3 h o u r s / s a l t + water Q
950' 'C/1
950°C/ l
950°C/ l
950°C/ l
800°C/ l
800°C/ l
800°C/ l
800°C/ l
800°C/ l
800°C/ l
hour / a rgon Q -f 350°C/ l hour
hour / a rgon Q + 350°C/8 hours
hour / a rgon Q + 400 ' 'C / l hour
hour / a rgon Q 4- 600°C/8 hours
hou r /vac . Q+ 350°C/ l hour
hou r /vac . Q-F 4 0 0 ° C / i hour
hou r /vac . Q-f 4 0 0 ° C / l hour
hour /vac . Q+ 400°C/2 hours
hou r /vac . Q-1- 600°C/ l hour
hou r /vac . Q + 600°C/8 hours
Charpy (ft-lb)
5.4
16.3
20.8
15.6
20.7
9.5
15.3
16.8
9.6
4.2
3.7
3.6
3.5
9.8
9.0
6.1
3.7
9.6
4 .8
Rockwell C ha rdness
41
23
17
22
23
43
22
18
18.5
46
47
52
48.5
45
44
47.5
52.0
33
43
X-ray s t ruc tu re
Y
Y
Y
Y
Y
Y
Y
Y
Y
a +
a -f
a +
a +
a +
a +
a +
a +
a +
a +
Y"
Y'
Y'
Y'
Y'
Y'
Y'
Y'
Y'
Y'
Q = quenched.
-19-
F r a c t u r e s of V-Notch Charpy Bars of U-7.5 w/o Nb-2.5 w/o Zr (400X)
GLB-647-4249
Fig . 14. As gamma-quenched. Fig. 15. As aged 1 hour a t 4 0 0 ° C .
k'v ••"v
v--̂ ^ ^
'^^
•u
GLB-S47-42St
As gamma-annealed , a i r -coo led .
- 2 0 -
Table VII. V-notch Charpy impact values of gamma-annealed U-7.5 w/o Nb-2.5 w/o Zr alloy at var ious t e m p e r a t u r e s .
Tempera tu re (°C) Charpy (ft-lb)
100
25
0
-40
11.0
12.5
10.8
10.3
Table VIII. E l eva ted - t empera tu re tensi le t e s t s of U-7.5 w/o Nb-2.5 w/o Zr alloy.
Tempera tu re Strain ra te Yield strength (°C) ( in . / in . -min) (psi)
U. T. S. (psi)
109,400
91,400
92,700
96,300
85,500
33,300
17,500
12,300
6,700
4,300
111,000
90,300
84,800
78,300
59,600
41,000
26,000
18,850
15,000
10,300
% elong. in 1 in.
32
24
15 *
7
19
25
59
60
49
20
14
10
8
12
19
29
40
60
RA (%) 51
46
41
28
14
60
47
38
36
63
42
39
38
36
44
52
59
60
74
85
R. T.
150
250
350
450
550
650
750
850
950
R. T.
150
250
350
450
550
650
750
850
950
0.03 95,700
71,400
60,200
56,800
46,700
25,700
17,500
101,000
71,200
69,400
71,400
53,800
37,600
24,100
18,800
14,100
* *
Broke outside gauge length.
Yield load is questionable.
- 2 1 -
® U. T. S. Strain Rate: ( in . / in . -min)
0 200 400 600 800 1000
Temp, °C GLL-647-1896
Fig . 17. Tensi le p roper t i e s of U-7.5 w/o Nb-2.5 w/o Zr at elevated m p e r a t u r e s .
- 2 2 -
in the range of 500-600°C. As the t empera tu re becomes elevated to the
point where nucleation p r o c e s s e s predominate , the alloy anneals and softens
rapidly.
FABRICATION PROPERTIES
F o r the best p r i m a r y or secondai'y working p rope r t i e s the alloy must
be in the gamma-phase condition above 700°C. F o r forging and breakdown
rolling a s tar t ing t empera tu re of 950° C is recommended. Fo r hot-forming
thin sections (finish-rolling), and to keep a fine grain s ize , the fabrication
range is r e s t r i c t e d between 800 and 700°C. Warm-ro l l ing below 650®C is un-
successful due to duplex s t ruc ture formation caused by t ransformat ion reac t ions .
Such ma te r i a l is "hot short" and f rac tures in the grain boundaries with l i t t le
or no reduction.
Homogeneous m a t e r i a l , gamma-annea led above 7 50°C and rapidly cooled,
may be cold-worked at room t empe ra tu r e with reductions of 80% in th ickness
between anneals . F igure 18 shows the low work-hardening c ha ra c t e r i s t i c s
of the gamma-annealed-and-quenched alloy.
The recrys ta l l i za t ion t e m p e r a t u r e for the 50% cold-ro l led alloy is
between 600 and 650°C, as indicated by the annealing curve of Fig . 19.
F igure 20 shows that the cold-worked s t ruc ture has not been removed by a
1-hour anneal at 600°C; Fig . 21 shows that after 1 hour at 650°C the s t ruc tu re
has been completely rec rys ta l l i zed .
Heavy sections such as thick p la tes , when slowly cooled from the gamma-
phase t e m p e r a t u r e , age-harden before reaching room t e m p e r a t u r e . Cold-
rolling before f rac ture is l imited to approximately 30% reduction in th ickness
with very l i t t le work-hardening. Trans format ion of the s t ruc tu re to pear l i t e
at 600°C allows about the same degree of cold-rol l ing. Alloy t r ans fo rmed
to maximum ha rdness at lower t e m p e r a t u r e s has no cold-reduct ion capability.
WELDABILITY
P a r t s made from the U-7.5 w/o Nb-2.5 w/o Zr alloy a r e as readi ly
joined by TIG or e lec t ron beam fusion vv^elding as p a r t s made of unalloyed
uranium. In the TIG p r o c e s s ei ther argon or helium forms the protect ive
a tmosphere within the welding chamber . The e lec t ron beam p r o c e s s opera tes -4 in a vacuum of at leas t 10 t o r r . The alloy becomes suddenly quite fluid and
some skill is requi red when deep penetra t ion is requ i red .
- 2 3 -
Percen t reduction by cold rolling GLL-64T-189T
Fig. 18. Hardness of U-7.5 w/o Nb-2.5 w/o Zr as a function of percent reduction by cold rol l ing.
•24-
500 600 700 800
Temp, °C GLL-647-1898
Fig. 19. Hardness of U-7.5 w/o Nb-2.5 w/o Zr as a function of annealing t empera tu re .
•25-
Recrys ta l l iza t ion of 50% Cold-Rolled Gamma
U-7.5 w/o Nb-2.5 w/o Zr (400X)
: •'•A . * - l . ,
!-s-:':v-•
• •
GLB- 647 -4253
Fig . 20. Annealed 1 hour at 600°C.
GLB- 647-4254
Fig . 21 . Annealed 1 hour at 650°C.
-26-
The cracking under res idual s t r e s s which is comLmon in the U-10 w/o
Mo binary alloy does not occur in the U-7.5 w/o Nb-2.5 w/o Zr alloy welds.
Table IX shows that the gamma phase alloy in the as-welded condition
p o s s e s s e s excellent mechanical p rope r t i e s . The strengthening of the weld
which occurs as a resu l t of some t ransformat ion on cooling to room t e m p e r a -
tu re may be enhanced by post-heat ing at a low t e m p e r a t u r e .
STRESS-CRACKING
Static loading or s t r e s s ing at low s t ra in r a t e s in environments contain-4 ing oxygen have no de t r imenta l effects upon the r o o m - t e m p e r a t u r e tensi le
p rope r t i e s of the U-7.5 w/o Nb-2.5 w/o Zr alloy. Table X shows the p r o p e r -
t ies after preloading 200 hours at 100,000 ps i compared with r e su l t s on
sanaples that were not preloaded. The samples t es ted in a i r give the same
resu l t s as those tes ted in vacuum. As would be expected, t e s t s conducted at
a s t ra in ra te of 0.0005 in . / i n . -min had a slightly lower yield s t r e s s and a
small i nc rease in ductility.
Table IX. Tensi le p rope r t i e s of TIG-welded, gamma-annea led U-7.5 w/o Nb-2.5 w/o Zr alloy plates |3 X 4 X 0,350 in. ).
Condition
As welded
Pos t -hea t ed 0.5 h o u r / 2 7 5 ' 'C
Yield strength, 0.1% offset
(psi)
130,000
139,500
U. T. S. (psi)
136,500
152,300
% elong. in 1 in.
11
6
Table X. Tensi le p roper t i e s of U-7.5 w/o Nb-2.5 w/o Zr alloy.
Condition Environment Hardness , R .
Strain rate (in./in. -min)
0.05
0.05
0.0005
0.0005
0.05
0.05
0.0005
0.05
0.05
0.05
0.05
0.05
0.05
0.05
Yield strength
(psi)
131,400
134,300
113,500
110,300
135,400
100,300
91,600
108,300
153,600
157,600
163,700
159,700
190,100
192,800
U. T. S. (psi)
190,800
191,300
185,500
186,900
197,000
128,300
120,300
130,300
210,600
209,000
216,700
217,900
240,000
248,600
Elon %
5.2
5.2
6.5
6.3
5.0
12.5
14.4
12.0
4.0
3.9
4.0
3.8
3.6
3.5
Ann. 9 5 0 ^ C / A C Air
Ann. 9 5 0 ° C / A C Vac.
Ann. 9 5 0 ' ' C / A C Air
Ann. 9 5 0 * C / A C Vac.
Ann. 9 5 0 ° C / A C + 200 hours /100 ,000 ps i Air
SOO^C/vac./WQ Air
BOO^C/vac./WQ Air
8 0 0 ° C / v a c . / W Q -f 200 hours /100,000 psi Air
9 5 0 ° C / A C + 8 hours /250°C Air
9 5 0 ° C / A C + 8 hours/ZSO^C + 200 Air hours /100,000 ps i
9 5 0 ' ' C / A C + 8 hours /300°C Air
9 5 0 ° C / A C -f 8 hours /300°C + 200 Air hours /100,000 psi
9 5 0 ° C / A C + 8 hours /350 ' 'C Air
9 5 0 ° C / A C + 8 hours /350°C + 200 Air hours /100,000 ps i
42
42
42
42
42
21
21
21
45
45
48
48
54
54
- 4
AC = a i r -coo led .
WQ = water -quenched.
-28-"
STRUCTURE AND PHASE IDENTIFICATION
Metallography
Individual specimens were mounted in Bakeli te , ground through 180,
320, 600, and 4-0 grit pape r s , and then wet-pol ished on a microclo th wheel
with 1-micron diamond pas te . The final m i r r o r finish was given to the
specimens by several hours of polishing with 0.05~micron Linde B alumina
on a Syntron v ibra tory table covered with microclo th .
Cathodic etching was used to bring out cold-worked s t ruc tu res and grain
boundaries of annealed and t r ans fo rmed s t ruc tu res as well as p rec ip i ta tes and
inclusions. An electrolyt ic etch in phosphoric-ethylene glycol-ethyl alcohol
solution was useful in giving contras t to some of the t r ans fo rmed s t ruc tu re s ,
pa r t i cu la r ly pea r l i t e s . Po la r i zed light is effective in identifying and photo-
graphing alpha phase, as shown in F igs . 9, 10, and 11.
X-Ray Diffraction
This method was used mainly for phase identification. When la rge
enough in a rea , the metal lographic samples were e lectropol ished and this
surface scanned on a General E l ec t r i c XRD-5 diffractometer , using copper
K radiat ion. The intensi t ies at the var ious 20 angles were r ecorded on a a * moving chart . Diffraction pa t t e rns from a quenched specimen and from one
t rans formed at 600° C a r e shown in F i g s . 22 and 23, respect ive ly . The
diffraction l ines from specimens t r ans fo rmed at other t e m p e r a t u r e s were
very broad, and for this reason no at tempt was made to de te rmine lat t ice
p a r a m e t e r s of the t ransformat ion products .
DISCUSSION
2
According to work by Dwight and Muel ler on the t e r n a r y u ran ium-
niobium-zi rconium sys tem, a monoeutectoid valley should occur for an alloy
of the composit ion U=7,5w/o Nb-2„5 w/o Zr at about 638''C. This would
r ep re sen t the lower boundary of the gamma phase for this alloy under equilib-
r ium conditions. According to the p resen t data, the conditions of the
DTA determinat ions in this study were not near equi l ibr ium for cooling at the
ra te used (85°C per hour); thus the t ransformat ion of gamma phase was
depressed to 500°C. ThiSahowever, indicates a s luggish- to~t ransform
- 29 -
X-Ray Diffraction P a t t e r n of U-7.5 w/o Nb-2.5 w/o Zr
-'—"GLL-647- 1900
Fig. 22. Annealed at 900° C 1 hour and quenched. All gamma phase .
F ig . 23. Annealed at 900° C 1 hour and isothermal ly t r ans fo rmed 8 hours at 600°C. Alpha and gamma phases .
- 30 -
alloy that is quite gamma-s t ab le . During heating the peak of the reac t ion ' s
heat absorption is at 623''C and the mate r ia l is all gamma at 654°C. This is
in quite good agreement with the equil ibrium t e m p e r a t u r e s .
Both metal lographic and x - r a y techniques failed to detect more than
one gamma phase. This indicates that the alloy avoids any miscibi l i ty ga.p
of the gamma phases such as a re present in the b inary sys tems of u ran ium-
niobium and u ran ium-z i rcon ium.
Isothermal t ransformat ion of the gamma phase jus t below the cr i t ica l
t empera tu re at 600°C proceeds by nucleation of alpha plates at grain boundaries
followed by their inward growth towards the in ter ior of the g ra ins . The s e r i e s
of photographs in Appendix I i l lus t ra tes this p r o c e s s . This pear l i t ic eutectoid
is verified by x - r a y diffraction to be alpha plus n iob ium-r ich gamima (y').
Transformat ion by nucleation of fine alpha within gra ins and gra in boundar ies
takes place as low as 400°C. Below 400°C the cha rac t e r i s t i c m i c r o s t r u c t u r e ,
high ha rdnes s , and d is tor ted x - r a y pa t te rns suggest t r ans format ion is by a
mar tens i t i c or shear p r o c e s s . The gamma phase decomposit ion does not
occur mar tens i t i ca l ly on quenching as it does in the u ran ium-z i r con ium binary
alloy because of the presence of niobium. This mechan i sm does operate
when a low enough t empera tu re is reached during slow cooling or i so thermal
aging and is due to the p resence of z i rconium in the alloy.
The operat ion of the aforementioned mechan i sms of t r ans format ion
offers control of the mechanical p roper t i e s by var ious heat t r e a t m e n t s which
have been d i scussed . The genera l overal l co r ros ion r e s i s t ance of this alloy
in ei ther the gamma or t rans formed condition has not been a subject of this
study. It is known that the alloy is not susceptible to s t r e s s - c r a c k i n g . It is
reasonable to believe that the gamma alloy will be more t a r n i s h - r e s i s t a n t in
air than the t rans formed alloy, based upon behavior of polished meta l lographic
samples .
CONCLUSIONS
The incorporat ion of niobium and z i rconium with u ran ium in the p r o -
port ions U-7.5 w/o Nb-2.5 w/o Zr has resul ted in an alloy which by judicious
control of heat t r e a tmen t s may have a wide range of p r o p e r t i e s . A soft,
ductile alloy, suitable for severe fabrication opera t ions , is produced by
gam ma-anneal ing and quenching. It may then be strengthened to sa t is factory
- 3 1 -
strength and toughness levels by aging at var ious t empe ra tu r e levels . The
alloy p o s s e s s e s excellent a tmospher ic cor ros ion r e s i s t ance and is not subject
to s t r e s s - c r a c k i n g in oxygen-bearing environments .
Acknowledgment s
The ass i s t ance rendered by var ious m e m b e r s of the meta l lurgy groups
at Lawrence Radiation Labora tory in L i v e r m o r e and the Bureau of Mines in
Albany, Oregon, is gratefully acknowledged. Thanks a r e given par t icu lar ly
to W. Steele for excellent exper imental work and to S. DiGiallonardo for the
metal lographic samples and photomicrographs . The electron mic rographs
were p repa red by F r a n c e s Bert ing.
REFERENCES
1. C. A. W. Peterson, 'A Study of the Iso thermal Transformat ions of Some
Binary Uran ium-Base Alloys Between 400°C and 650°C," Lawrence Radiation
Labora tory , L i v e r m o r e , Rept. UCRL-7824 (in prepara t ion) .
2. A. E. Dwight and M. H. Mueller , "Constitution of the Uranium-Rich
U-Nb and U-Nb-Zr Sys tems , " Argonne National Labora to ry Rept. ANL-5581
(1957).
3. C. A. W. P e t e r s o n and W. J. Steele, "A Study of the Effect of Alloying
on the G a m m a - P h a s e Stability of Uranium Using Vacuum Differential Thermal
Analys is , " Lawrence Radiation Labora tory , L i v e r m o r e , Rept. UCRL-7595
(1963).
4. C. A. W. P e t e r s o n and R. R. Vandervoort , "S t r e s s Cracking in the
Uranium-10 w/o Molybdenum Alloy," Lawrence Radiation Labora tory ,
L i v e r m o r e , Rept. UCRL-7767 (1964).
/ n o
- 3 2 -
APPENDIX I
Optical Photomicrographs and Elect ron Micrographs of Gamma-
Annealed and Isothermally Trans formed U-7.5 w/o
Nb-2.5 w/o Zr Alloy
TRANSFORMATION STRUCTURE OF THE ALLOY
URANIUM-7"|™ w/oNIOBIUM-Zy w/o ZIRCONIUM
SPECIMEN HISTORY
Annealed at 900°C for I hour, aged ot 6 0 0 ° C
for the indicated tinne^and water quenched.
ETCHING
Electro-etched in an orthophosphoric acid
solution.
REPLICATION
Direct positive carbon with P t -Pd shadowing.
A-MATRIX
Niobium rich b.c.c. solid solut ion.
B-TRANSFORMATION PRODUCT
a Uranium.
-33-
TRANSFORMATION STRUCTURE OF THE ALLOY
•2 URANIUM-?^ w/o NIOBIUM-2^ w/o ZIRCONIUM
One hour Photomicrograph
' ^ %
lO/x
f
- 1
"X
On® hour Electron micrograph
J «•
t 1
- 3 4 -
TRANSFORMATION STRUCTURE OF THE ALLOY
URANIUM-?-^- w/o NI0BIUM-~2-^ w/o ZIRCONIUM
Two and one half hours Photomicrograph
!_J 10^
N,
Two and Electron
•
*.
one half hours micrograph
- -- ,. -^
. • j
" - S i ;
. . . - • , > ,
V
• i.-
X ,.
7 ^X>'
K
^ *
-35-
TRANSFORMATION STRUCTURE OF THE ALLOY
URANIUM-?- w/o NI0BIUM-2Y w/o ZIRCONIUM
> 1
J.' . 51
Eight hours
Photomicrograph
LJ 10/i
. - - : . - . > •>
r- f^..
/:
:•/ /.^v>'
. ^ -,
r •• t •'•• I ;
Eight hours
Electron micrograph
This report was prepared as an account of Government sponsored work. Neither the United States, nor the Com-mission, nor any person acting on behalf of the Commission:
A. Makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, appa-ratus, method, or process disclosed in this report may not infringe privately owned rights; or
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