FORMATION OF GAMMA HYDRIDE IN ALPHA AND BETA ZIRCONIUM ALLOYS G. Dey, S. Banerjee, P. Mukhopadhyay
To cite this version: G. Dey, S. Banerjee, P. Mukhopadhyay. FORMATION OF GAMMA HYDRIDE IN ALPHA AND BETA ZIRCONIUM ALLOYS. Journal de Physique Colloques, 1982, 43 (C4), pp.C4327-C4-332. .
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CoZZoque C4, supple'ment au n o 12, Tome 43, dgcembre 1982
page C4-327
FORMATION OF GAMMA HYDRIDE I N ALPHA AND B E T A Z I R C O N I U M ALLOYS G.K.
Dey, S. Banerjee and P . Mukhopadhyay
MetaZZurgy Division, Bhabha Atomic Research Centre, Bombay-400 085, India (Revised t e x t accepted 25 October 1982)
A b s t r a c t . - The form ation of the g a n m a h y d r i l e phase through a s h e a r transform ation has been stu?ie? i n a Zr-20%Nb alloy and i n iodide pure zirconium. T h e crystallography of the beta to gam m a and the alpha to g a n m a transform ations has been discussed. A possible n echanisrr for the alpha to g a n n a reaction. involving the i n t e r n ediate form ation of the beta phase, has been suggested.
1. Introduction. - It has been recognised by s e v e r a l investigators [ 1-31 that the form ation of garr n a hy-lrile in al3ha zirconium o c c u r s through a h y b r i l p r o c e s s involving a s h e a r of the alpha l a t t i c e and a sirrultaneous diffusional m igration of hydrogen atorr s. The observed rr orphology of the ganmrr a hydride plates. t h e i r internal twinning. the s u r f a c e tilt associated with the transform ation and the fact that the transform ation o c c u r s at a t e n p e r a t u r e where self diffusion of zirconiun a t o m s i s insignificant, a r e all consistent with the o c c u r r e n c e of a s h e a r t r a n s f o r mation. A distribution of second phase plates has been observed i n the retained beta phase i n polished and etched s a n ples of beta quenched Z r -Nb alloys. T h e r e has been s o n e c o n t r o v e r s y o v e r the identification of this phase [ 4-6 I . I n the p r e s e n t w o r k it h a s been found that these plates a r e of the ganfrr a hydride phase and that they originate due to hydrogen contan ination occurring during electropolishing o r c h e n ical etching of the samples. T h e s e g a n m a plates i n the beta r r a t r i x a l s o exhibit s o n e of the c h a r a c t e r i s t i c f e a t u r e s of a s h e a r transformation. The aim of the present work i s to n ake a corr p a r i s o n of the beta + gam ma and the alpha ---+ garr rr a s h e a r t r a n s f o r n ations and to find out the l a t t i c e c o r r e s p o n d e n c e s am ong t h e s e closely related s t r u c t u r e s . 2. Experimental P r o c e d u r e . - Iodide pure zirconium sam ples w e r e hydrogenate? by heating i n a hydrogen atrr osphere. In sam ples of the Zr-2O%Nb alloy. hydrogen was introduce?. by ( i ) electropolishing a t 223K. ( i i ) chemical etching a t 3COK an-l ( i i i ) heating i n hydrogen a t m o s p h e r e a t 1070K. The hydrogen content of the sarr ples was d e t e r n ined by vacuum fusion analysis. Some r e p r e s e n t a t i v e values w e r e : v i r g i n sarr ale 85 opm ; c h e n ically etched Sam ple - 520 ppn and sarr ple h e a t e i i n hydrogen a t n o s ~ h e r e- 335 p n n . F h a s e analysis of these s a n ples was c a r r i e 1 out using X - r a y liffraction and the crystallography of the transform ation w a s studied by t r a n s n i s s i o n electron n icroscony and e l e c t r o n diffraction.
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3. Results. 3 . 1 B e t a j G a m m a Transformation. - X - r a y diffraction c l e a r l y d e n onstrated that sarr qles of the Zr-20%Nb a110 y, quenched from 1073K. contained only the bcc beta phase. The gam rr a hydride phase appeared i n the s a n e sarr ples (Fig. 1 ) a f t e r etching a t 300K with a HF-HN03 -Hz 0 solution o r electropolishing a t 223K i n
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982446
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a n electrolyte containing perchloric acid, n-butanol and rr ethanol. It was i n f e r r e d , therefore, that the formation of g a m m a hydride o c c u r r e d due to hydrogen contan i nation. In a s e t of prepolished sarr ples i n which hydrogen w a s introduced intentionally during the beta solutionising t r e a t n ent, garr m a hydride plates could be observed on quenching, before any electropolishing o r cherr ical etching. T h e s e plates exhibited s u r f a c e r e l i e f c h a r a c t e r i s t i c of a transform ation involving s h e a r . X - r a y diffraction data on the g a n ma hydride phase, obtained from t h e s e sarr ples, w e r e found to be consistent with a n fct s t r u c t u r e with l a t t i c e p a r a m e t e r s : a = 4 . 5 0 and c = 5.22 A' - values fairly close to those r e p o r t e d for g a n rr a hydride observed i n s e v e r a l alpha alloys of zirconium [ 7 , 31
.
The orientation relationship between the beta and the garr ma phases was d e t e r n ined from superimposed e l e c t r o n diffraction p a t t e r n s from these c r y s t a l s . An exarrple i s shown i n Fig. 2. A specific v a r i a n t of the observed orientation r e l a >y1 could a l s o be viewed a s follows: tionship, {O_lO}Bl1 ( _ 0 1 0 } ~9;0 1 > ~ 1 ( < 1 0 [ O Olsl1 ~ [ l o l l , ;[ l l O ] B I I [ 1271 y , with the close packed ( 1 1 0 ) ~plane about 10' away from being parallel to ( I l l & . The l a t t e r r e p r e s e n t a t i o n i s v e r y useful i n visualizing the l a t t i c e correspondence between the beta and the g a r rr a c r y s t a l s and this has been d i s c u s s e d i n Section 4. 1.
Fig. 1: Internally twinned gamma plate ( Z r - 2 0 % N b alloy)
Fig. 2(a) SADP illustrating orientation relation between beta and g a n ma phases ( b ) Key t o the SADP in ( a )
A l m o s t all g a m m a plates exhibited a n internal s t r u c t u r e consisting of an a r r a y of { O l l ] twins. According to the l a t t i c e correspondence described i n S e c Y tion 4. 1, these planes a r e derived from the rr f r r o r planes of the parent s t r u c t u r e . In most of the gamma plates only one varlant of {Oll}y twinning w a s observed but i n s o m e c a s e s twinning along two intersecting {Ol l) planes was a l s o Y noticed. These twin boundaries w e r e found to exhibit delta fringe c o n t r a s t [ 8 1 ( s y m m e t r i c i n d a r k field and a s y m m e t r i c i n bright field) c h a r a c t e r i s t i c of perpend i c u l a r twin boundaries (Fig. 3). Single surface t r a c e analysis demonstrated that the habit plane of the g a m m a plates l a y within the s t e r e o g r a p h i c triangle defined by the poles ( l o o ) , (301) and (311). 3.2 Alpha + Gamma Transformation. - T h e r e have been a number of investigations on the alpha +gam ma transformation[ 1 - 3 I. In a r e c e n t study, the c r y s t a l l ography of the formiation of the g a m m a plates i n the alpha m a t r i x has been d i s c u s s ed by Weatherly [3 ] . The g a m m a hydride plates observed i n the p r e s e n t study i n quenched s a m p l e s exhibited a r e m a r k a b l e sirr i l a r i t y with internally twinned rr a r t e n s i t e plates (Fig. 4). Such periodically spaced twins extending right upto t h e i n t e r face have not been r e p o r t e d i n r e l a t i o n t o g a m m a plates formed i n slow cooled The observed orientation relationship and the indices of the habit s a m p l e s [ 31 plane and the internal twinning plane could be l i s t e d a s follows:
.
11
1
Orientation relation : ( l o o } (0001)0( ;
y la. Habit plane : Close to- Qoio] Twin plane : (Oll}y. . It should be noted tha; the 101710 habit and the corresponding orientation relation, [1~101, a s observed by Weatherly [3 1 i n r e l a t i o n to (1 1 l ) y (OOOl),; [I 101 s o m e g a m m a hydride plates,' w e r e not encountered i n the p r e s e n t work. But the number of plates investigated was not s o l a r g e that the e x i s t e n c e of such plates could be ruled out.
11
.
11
3. 3 E e t a j A l p h a P r i m e P l u s Gamma Transformation. - Iodide zirconium s a m p l e s i n which hydrogen was introduced by diffusion annealing a t a sufficiently ) a beta+ alpha p r i m e plus g a m m a t r a n s high t e m p e r a t u r e (above 9 7 0 ~ underwent formation on quenching. The product m i c r o s t r u c t u r e showed a distribution of g a m m a plates together with alpha p r i m e m a r t e n s i t e laths(Fig. 5). Such a m i c r o s t r u c t u r e would be expected if the alloy, during diffusion annealing, f i r s t t r a n s formed into the beta phase due to the strong beta stabilizing tendency of hydrogen and during the subsequent quenching operation decomposed into a mixture of alpha p r i m e and g a m m a phases. This phase reaction may involve r e j e c t i o n of hydrogen from growing m a r t e n s i t i c alpha p r i m e l a t h s to the untransformed beta regions, resulting i n p r o g r e s s i v e stabilization of these regions which eventual1y t r a n s f o r m ed t o the g a m m a hydride phase. Since t h e r e w a s no retained beta i n the final m i c r o s t r u c t u r e , i t w a s difficult to d e t e r m i n e the habit plane for the g a m m a plates. However, i n many instances these plates w e r e s e e n to be lying parallel to the alpha p r i m e l a t h s which exhibit a C334 1 6 habit [ 9 1. The g a m m a plates a l s o contained periodic a r r a y s of internal twins along ( 0 11 I.,.
Fig. 3 ( a ) B F and (b) D F i m a g e s of internally twinned g a m m a plate i n beta matrix, showing delta fringe c o n t r a s t a t twin boundaries.
Fig. 5: Internally twinned g a m m a plate and m a r t e n s i t e l a t h s i n iodide
Fig. 4 : Micrographs showing i n t e r n a l l y twinned g a m m a plates i n alpha m a t r i x i n iodide zirconium; (b) and ( c ) constitute B F and D F pair.
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4. Discussions. T h e c r y s t a l l o g r a p h y of G a m m a T r a n s f o r m ation. 4 . 1 C r v s t a l l o g r a p h y of Beta this transform ation could be d e s c r i b e d i n t e r n s of the l a t t i c e c o r r e s p o n d e n c e shown i n Fig. - 6 i n which the r e l a t i v e positions of the l a t t i c e v e c t o r s of the beta and According t o the g a n m a s t r u c t u r e s a r e indicated on the ( 1 1 0 ) d ( 1 1 1 ) plane. ~ this c o r r e s p o n d e n c e , based on t h e o b s e r v e d orientation relation, the n agnitude of the l a t t i c e s t r a i n r e q u i r e ? for bringing about the beta to garr ma t r a n s i t i o n i s not v e r y l a r g e . a s indicated i n Fig. 6. T h i s l a t t i c e s t r a i n c a n be conceptually broken plane to up into two c o n ponents. The f i r s t i s responsible for straining the (1 n a t c h the d i n ensions of the ( 1 1 1 ) ~ plane an6 for adjusting the i n t e r p l a n a r spacing of the form e r s o that i t equals that of the l a t t e r . T h e second c o n ponent of the l a t t i c e s t r a i n i s n e c e s s a r y for bringing the a t o n s in the right positions i n the g a m m a l a t t i c e , T h e s e two c o n ponents of the l a t t i c e d i s t o r t i o n a r e r e p r e s e n t e d by the two n a t r i c e s S1 and S2 : 0 0 1.1322 ; S2 = 1 0. 0 0 1. C8564 0 Both m a t r i c e s a r e e x p r e s s e d with r e f e r e n c e to the a x e s s y s t e n indicated i n Fig. 6. The s h e a r S 2 , which changes the ABAB.. type of stacking of the ( 1 1 0 ) ~ planes to the A ' B ' C ' A ' B ' C ' . type of stacking corresponding to the ( 1 1 l ) y planes. i s a s i n ple s h e a r acting uniformly on e v e r y a t o m i c plane (Fig. 7). T h e s h e a r S1, when applied to a n unit s p h e r e , l e a v e s a s e t of v e c t o r s undistorted. T h e s e v e c t o r s gener a t e a n elliptical cone, the positions of which before and a f t e r the application of S1
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Fig. 6: Schematic d i a g r a m showing c r y s t a l l o g r a p h i c c o r r e s p o n dence of alpha, beta and g a n ma phases.
Fig. 7 : S c h e m a t i c diagrarr illustrating t h e s h e a r S2 for both beta to g a m m a and alpha to garr rr a transform ations.
a r e r e p r e s e n t e d i n the s t e r e o g r a p h i c projection i n Fig. 8. When a c e r t a i n fraction ( 11 0 ) ~plane. a habit plane soluof the s i n ple s h e a r S2 i s applied on the ( 111) tion c a n be obtained with the habit plane pole n arked within the s t e r e o g r a p h i c t r i angle defined by the poles (Too), ( T 0 l ) and ( T l l ) . T h i s i s consistent with the e x p e r i n ental d e t e r n i n a t i ~ nof the habit plane. After the a p p l i c a t i o n of S1 and S 2 , the r e q u i r e d rigid body rotation around the [OOa 8 a x i s will shift the (1 11) plane away frorr the ( 1 10) plane by about lo0. Thus the observed orientation r e l a t i o n B can be obtained. It may be noted that a habit plane solution c a n be obtained h e r e
1/
by a combination of S1 and a fraction of S 2 . both of them being con ponents of the l a t t i c e s h e a r . It can be visualised that S2 can act in either the positive (indicated by the a r r o w s in Fig. 7 ) o r the negative direction along [ i l O I B t o introduce the n e c e s s a r y change in the stacking sequence. T h e r e f o r e , by a suitable distribution of S2 in adjacent twin related don a i n s of a garr ma hydride plate, i t i s possible for the invariant plane s t r a i n condition to be satisfied. F r o m a stereographic analys i s of the crystallography of the beta to garr rr a transform ation. i t could be s e e n that a s h e a r rr agnitude of about $ [S21 i s n e c e s s a r y for satisfying the IPS condition. T h i s c o r r e s p o n d s to a twin thickness r a t i o of about 3 : 1, a value close to that experim entally observed.
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4 . 2 CrystalLography of A l p h a I u a m n a Transforwation. The crystallography of this transform ation h a s recently been discussed by Weatherly [ 3 l who shows the form ation of garr rra plates with (10iRj habit (denoted by type I) i s s i m i l a r to that of the beta to g a n ma transition discussed i n Section 4. 1. The difference between the two c a s e s a r i s e s from the difference i n the magnitudes of the elerrlents of the s t r a i n m a t r i x S1. Both the magnitude and the direction of S2 f o r the beta t o gam ma and the type I alpha to gamma transitions a r e identical. But i n the l a t t e r c a s e the sirr ple s h e a r S2 i s manifested by a displacement of e v e r y a l t e r n a t e (0001)a (11 l ) y plane through a distance a16 [ 1171
11
Y'
2 e transform ation crystallography associated with garr m a plates exhibiting a 11010 1 habit (denoted by type 11) ,-as proposed e a r l i e r E31, involves a simple s h e a r of 30°0n a (1010) plane i n a < 1210>. It i s , however, interesting t o note that the two correspondences for type I and type I1 plates can be regarded a s c r y s t a l l o graphically equivalent if one considered the alpha to gamma transition t o occur in
Fig. 8: Stereographic analysis of c r y s t a llography of beta to g a m m a transform a tion.
Fig. 9: Stereographic projection ill ustrating the equivalence of type I and type I1 correspondences.
two steps : the f i r s t s t e p being the alpha to beta t r a n s i t i o n i n accordance with the B u r g e r s correspondence followed by the second s t e p comprising the beta t o gamma transition a s discussed e a r l i e r . The stereographic projection showing the approx i m a t e orientations of the different v a r i a n t s of g a m m a c r y s t a l s a r i s i n g from a single alpha orientation through a n i n t e r n ediate beta orientation i s shown in Fig. 9. It c a n be s e e n that i n the type I correspondence the ( 1 1 0 ) ~plane, which i s derived
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from the (0001) plane. b e c o n e s a (11 1 4 w h e r e a s i n t h e type I1 c o r r e s p o n d e n c e C1 o t h e r {01 I} p l a n e s a r e c o n v e r t e d t o (11 planes.
B
4. 3 P o s s i b l e T r a n s f o r m a t i o n Sequence. - In v i e w of t h e c l o s e c o r r e s p o n d e n c e a m o n g t h e a l p h a , the b e t a a n d t h e g a m m a s t r u c t u r e s , t h e f a c t that h y d r o g e n is a s t r o n g b e t a s t a b i l i z e r a n d t h e findings of t h i s study, i t i s a t t r a c t i v e t o e n v i s a g e t h a t g a n m a h y d r i d e f o r m a t i o n i n a l p h a z i r c o n i u m involves a n i n t e r m e d i a t e s t e p of b e t a formation. It i s p o s s i b l e that a s h y d r o g e n s e g r e g a t i o n o c c u r s i n a c e r t a i n r e g i o n of a n a l p h a c r y s t a l , t h e r e g i o n f i r s t t r a n s f o r m s i n t o t h e b e t a s t r u c t u r e which finally g o e s o v e r t o t h e g a m m a s t r u c t u r e t o a c c o m m o d a t e t h e l a r g e c o n c e n t r a t i o n of h y d r o g e n a t o m s . A l t e r n a t i v e l y , the b e t a s t a t e n a y a c t a s a n "activated c o m plex s t a t e " b e t w e e n t h e a l p h a and t h e g a m m a s t a t e s b y providing a n e a s y p a t h i n the s h e a r i n g p r o c e s s involved i n t h e a l p h a t o g a m m a t r a n s i t i o n . ACKNOWLEDGEMENT T h e a u t h o r s would l i k e t o t h a n k D r . S. J. V i j a y a k a r f o r h i s h e l p i n the e x p e r i m e n t a l work. D i s c u s s i o n s with D r . R. K r i s h n a n and with D r . M. K. A s u n d i w e r e v e r y useful. REFERENCES BRADBROOK J. S . , LORIMER G. W . , and RIDLEY N . , J. of Nucl. M a t e r . 42 (1972) 142. C A R P E N T E R G. J. C. , A c t a Metall. ( 1 978) 1225. WEATHERLY G. C . , A c t a Metall. 29 (1981) 501. F L E W I T T P. E. J . , ASH F. J. a n d CROCKER A. G . , A c t a Metall. 24 (1976)669. HATT B. H. a n d ROBERTS J. A . , A c t a Metall. 8 (1960) 575. C O M E T T O D. J . , HOUZE ( J r . ) G. L . a n d HEHEMANN R. F., T r a n s . of Met. Soc. of AIME. 223 ( 1 965) 30. SIDHU S . S . , HEATON L e ROY., CAMPOS F. P. and ZAUBERIS D . D . , A d v a n c e s i n C h e m i s t r y S e r i e s , A m e r . Chem. Soc. 39 (1963). D e RIDDER R . , VAN LANDUYT J . , GEVERS R. and AMELINCKX S . , P h y s . Stat. Sol. 40 (1970) 271. BANERJEE S. and KRJSHNAN R . , A c t a Metall. 2 (1971) 1317.
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