I

THE PROPERTIEf- Of A F^.^^

GAMMA-PHASF U-."

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

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

B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any infor-mation, apparatus, method, or process disclosed in this report.

As used in the above, "person acting on behalf of the Commission" includes any employee or contractor of the Com-mission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor.