Dynamic recrystallization of zirconium carbide

Scripta METALLURGICA

Vol. I0, pp. 251-253, 1976 Printed in the United States

Pergamon Press, Inc.

DYNAMIC RECRYSTALLIZATION OF ZIRCONIUM CARBIDE R. D a r o l i a * Division

and T.

F. Archbold

of Metallurgical Engineering U n i v e r s i t y o f Washington S e a t t l e Washington USA

(Received February 2, 1976)

Introduction Dynamic r e c r y s t a l l i z a t i o n occurs in s e v e r a l m e t a l s and a l l o y s d u r i n g hot working and has r e c e n t l y been t r e a t e d from a t h e o r e t i c a l s t a n d p o i n t by Sandstrom and Lagneborg ( I ~ During the course o f the compression t e s t i n g o f ZrC.94 and ZrC + B a l l o y s in e t e m p e r a t u r e range 1200-1800vC. we have found t h a t these m a t e r i a l s can undergo dynamic r e c r y s t a l l i z a t i o n a t 1800°C. The p r e d o m i n a n t d e f o r m a t i o n mechanism in these m a t e r i a l s , at l e a s t f o r s t r e s s e s near t h e i r y i e l d s t r e s s e s , i s the carbon d i f f u s i o n - a s s i s t e d motion o f d i s l o c a t i o n s ( 2 ) . Experimental Polycrystalline ZrC.94 and ZrC + B a l l o y s were prepared by arc m e l t i n g and subsequent h o m o g e n i z a t i o n t e c h n i q u e s d e s c r i b e d e l s e w h e r e ( 3 ) . The a l l o y s p e c i mens c o n t a i n e d I , 1 . 5 , and 2% boron by w e i g h t , and the boron u s u a l l y appeared in the m i c r o s t r u c t u r e in the form o f ZrB 2 p l a t e l e t s . The ZrC specimens were e q u i axed w i t h n e g l i g i b l e p o r o s i t y , w h i l e the a l l o y s r e t a i n e d some d e n d r i t i c s t r u c ture following homogenization. Compression t e s t i n g (2) of 2x2x2.7mm specimens was done in a vacuum t u n g s t e n f u r n a c e a t t a c h e d to an I n s t r o n u n i t . Following d e f o r m a t i o n t e s t i n g the specimens were f r a c t u r e d a t room t e m p e r a t u r e , and the f r a c t u r e s u r f a c e s were examined by SEM. R e s u l t s and D i s c u s s i o n F i g u r e s 1 and 2 are t r a c e s o f I n s t r o n c h a r t r e c o r d i n g s of the load vs. c r o s s head d i s p l a c e m e n t f o r a ZrC.94 specimen and a ZrC + I%B specimen, r e s p e c t i v e l ~ The specimen t e m p e r a t u r e in each example was loO0°C, and the s t r a i n r a t e f o r e a l l o y was s l i g h t l y g r e a t e r (3.2 x lO-"sec -I vs. 3.1 x l O - 4 s e c - 1 ) . Slight oscill a t i R n s appeared in the curves f o r a~l c o n s t a n t s t r a i n r a t e t e s t s a t a p p r o x i m a t e l y 1700vC and became pronounced at 1800vC. The f i r s t oscillations appeared at app r o x i m a t e l y I% p l a s t i c s t r a i n f o r a l l specimens. N e i t h e r the presence nor the amount o f boron in the c a r b i d e appear to a f f e c t the n a t u r e of the o s c i l l a t i o n s . Figure 3 i l l u s t r a t e s the t y p i c a l f r a c t o g r a p h i c f e a t u r e s of specimens t e s t e d at 1800vC; F i g . 3a d e p i c t s c l e a v a g e f r a c t u r e o f two ZrC g r a i n s s e p a r a t e d by a r e g i o n of r e c r y s t a l l i z e d m a t e r i a l , and F i g . 3b shows a g r a i n s i z e o f 2-4 microns in the l a t t e r area. The l a r g e g r a i n s in F i g . 3a have the a p p r o x i m a t e s i z e o f the g r a i n s o f the s t a r t i n g m a t e r i a l , 250 m i c r o n s . Owing to the n a t u r e o f the t e s t i n g p r o c e d u r e , i t was not P o s s i b l e to c o n v e r t the e n t i r e load vs. crosshead d i s p l a c e m e n t p l o t s to t r u e s t r e s s vs. t r u e s t r a i n . E s t i m a t e c a l c u l a t i o n s i n d i c a t e t h a t w h i l e the nominal load in the o s c i l l a t i n g r e g i o n i n c r e a s e s , the f l o w s t r e s s remains e s s e n t i a l l y c o n s t a n t .

* now a t M a t e r i a l s

Department,

UCLA, Los A n g e l e s , 251

California.

252

DYNAMIC R E C R Y S T A L L I Z A T I O N

OF ZIRCONIUM CARBIDE

Vol.

i0, No.

3

We a t t r i b u t e the o s c i l l a t i o n s in the load-displacement curves to dynamic rec r y s t a l l i z a t i o n . Features normally associated (4,5) with a dynamic s t r a i n ageing process, a l o g i c a l a l t e r n a t i v e explanation of the present r e a u l t s , were not observed in the present or in r e l a t e d experiments at 1700-1800 C, namely: I. 2. 3. 4. 5.

T h e r e is no increase in the rate of work hardening. A peak in a p l o t of the y i e l d stress vs. temperature does not appear. There is no negative s t r a i n rate dependence of the y i e l d s t r e s s . The flow stress does not increase followiBg stress r e l a x a t i o n , and The d u c t i l i t y does not decrease. At 1800 C, several specimens were compressed to 50% of t h e i r o r i g i n a l height without f r a c t u r e .

Specimens were not quenched f o l l o w i n g t e s t i n g , and i t is possible that some s t a t i c r e c r y s t a l l i z a t ~ o n occurred during cooling. Specimens tested to s i m i l a r t o t a l s t r a i n s at 1700vC, on the other hand, do not e x h i b i t the small grains shown in Fig. 3. The recent model (1) f o r the appearance and nature of the o s c i l l a t i o n s involves successive r e c r y s t a l l i z a t i o n steps in a given volume with the specimen e v e n t u a l l y completely r e c r y s t a l l i z e d ; a steady state flow stress and grain size can be a t tained under the appropriate hot working conditions. Based on the fractographic evidence, i t would appear that any successive r e c r y s t a l l i z a t i o n steps in the present carbides are confined to grain boundary regions with adjacent grains yet to be r e c r y s t a l l i z e d or r e l a t i v e l y unaffected under the conditions of t e s t i n g . Acknowledgement This work was t o t a l l y supported from funds made a v a i l a b l e to the U n i v e r s i t y of Washington through NASA Grant NGL48-202-O04, M u l t i d i s c i p l i n a r y Research Concerning the Nature and Properties of Ceramic M a t e r i a l s . References I.

R. Sandstrom and R. Lagneborg, Acta Met., 23, 387, 1975.

2.

R. Darolia and T.F. Archbold, J. Mater. S c i . , in press.

3.

R. Darolia and T.F. Archbold, Metallography, 6, 433, 1973.

4.

Y. Bergstrom and W. Roberts, Acta Met., 21, 741, 1973.

5.

B. J. Brindley and P. J. Worthington, M e t . Rev., 15, l O l , 1970. FIG. 1

Applied load vs. crosshead displacement for a ZrC specimen at 1800°C, Strain rate 3.1 x lO-" sec-I ; original specimen length 0.272 cm,

ZrC

8O- ~' / Ii ,0'~

,0'2

.0'3 CM

.0'4

Vol.

I0, No.

3

DYNAMIC

RECRYSTALLIZATION

OF ZIRCONIUM

CARBIDE

253

120"

FIG. 2

Applied load vs. crosshead displacement f o r a ZrC + I%B specimen at 1800°C.

ZrC *

°80

Strain rate 3.2 x lO-~sec-1; o r i g i n a l

I% B

specimen length 0.256 cm.

1800"C

4O .0'1

,63

.{}2

.o'4

CM

(a)

Magnification 243X

(b)

FIG. 3 Scanning electron fractog~aph of a ZrC specimen deformed at 1800 C and fractured at room temperature.

Magnification 2275X