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FORMATION OF GAMMA HYDRIDE IN ALPHAAND 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 ANDBETA ZIRCONIUM ALLOYS. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-327-C4-332.�10.1051/jphyscol:1982446�. �jpa-00222161�

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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 BETA ZIRCONIUM ALLOYS

G.K. Dey, S. Banerjee and P. Mukhopadhyay

MetaZZurgy Division, Bhabha Atomic Research Centre, Bombay-400 085, India

(Revised text accepted 25 October 1982)

Abstract . - The form ation of the gan m a hydr i le phase through a shear transform ation has been stu?ie? i n a Zr-20%Nb alloy and in iodide pure zirconium. The crystallography of the beta to gam ma and the alpha to gan m a transform ations has been discussed. A possible n echanisrr for the alpha to gan n a reaction. involving the in t e rn ediate form ation of the beta phase, has been suggested.

1. Introduction. - It has been recognised by several investigators [ 1-31 that the form ation of garr n a hy-lrile in al3ha zirconium occurs through a hybr i l process involving a shear of the alpha lat t ice and a sirrultaneous diffusional m igration of hydrogen atorr s. The observed rr orphology of the ganmrr a hydride plates. their internal twinning. the surface tilt associated with the transform ation and the fact that the transform ation occurs at a t en perature where self diffusion of zirconiun atoms i s insignificant, a r e all consistent with the occurrence of a shear transfor- mation.

A distribution of second phase plates has been observed in the retained beta phase i n polished and etched s a n ples of beta quenched Z r -Nb alloys. There has been s o n e controversy over the identification of this phase [ 4-6 I . I n the present work it has 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 or chen ical etching of the samples. These gan ma plates in the beta rratr ix a lso exhibit s o n e of the characterist ic features of a shear transformation. The aim of the present work i s to n ake a corr parison of the beta + gam ma and the alpha ---+ garr rr a shear t ransforn ations and to find out the lat t ice correspondences am ong these closely related structures.

2. Experimental Procedure. - Iodide pure zirconium sam ples were hydrogenate? by heating in a hydrogen atrr osphere. In sam ples of the Zr-2O%Nb alloy. hydro- gen was introduce?. by ( i ) electropolishing at 223K. ( i i) chemical etching a t 3COK an-l ( i i i) heating in hydrogen atmosphere at 1070K. The hydrogen content of the sarr ples was d e t e r n ined by vacuum fusion analysis. Some representative values were: virgin sarr ale - 85 opm ; chen ically etched Sam ple - 520 ppn and sarr ple hea te i in hydrogen a t n o s ~ h e r e - 335 pnn . Fhase analysis of these s a n ples was c a r r i e 1 out using X-ray liffraction and the crystallography of the transform ation was studied by t r ansn ission electron n icroscony and electron diffraction.

3. Results. - 3. 1 B e t a j G a m m a Transformation. - X-ray diffraction c lear ly den 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 in the s a n e sarr ples (Fig. 1 ) after etching a t 300K with a HF-HN03 -Hz 0 solution or electropolishing a t 223K in

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982446

C4-328 JOURNAL DE PHYSIQUE

a n electrolyte containing perchloric acid, n-butanol and rr ethanol. It was inferred, therefore, that the formation of gamma hydride occurred due to hydrogen contan i - nation. In a set of prepolished sarr ples in which hydrogen was introduced inten- tionally during the beta solutionising t r ea tn ent, garr m a hydride plates could be observed on quenching, before any electropolishing o r cherr ical etching. These plates exhibited surface rel ief characterist ic of a transform ation involving shear. X-ray diffraction data on the g a n ma hydride phase, obtained from these sarr ples, were found to be consistent with an fct s t ructure with lat t ice parameters : a = 4.50 and c = 5.22 A' - values fairly close to those reported for gan rr a hydride observed in several alpha alloys of zirconium [ 7 , 31 .

The orientation relationship between the beta and the garr ma phases was de te rn ined from superimposed electron diffraction patterns from these crystals . An exarrple i s shown i n Fig. 2. A specific variant of the observed orientation re la- tionship, {O_lO}Bl 1(_010}~; 9 0 1 > ~ 1 ( < 1 0 1 >y could a lso be viewed as follows: [ O O ~ ls l1 [ l o l l , ;[ l lO]BII[ 1271 y , with the close packed ( 1 1 0 ) ~ plane about 10' away from being parallel to ( I l l & . The la t ter representation i s ve ry useful i n visualizing the lat t ice correspondence between the beta and the g a r rr a crys ta ls and this has been discussed i n Section 4. 1.

Fig. 1: Internally twinned gamma Fig. 2(a) SADP illustrating orientation relation plate (Zr-20%Nb alloy) between beta and gan ma phases

(b) Key to the SADP in ( a )

Almost all gamma plates exhibited an internal s t ructure consisting of an a r r a y of {Oll] twins. According to the lat t ice correspondence described in Sec-

Y tion 4. 1, these planes a r e derived from the rr f r r o r planes of the parent structure. In most of the gamma plates only one varlant of {Oll}y twinning was observed but in some cases twinning along two intersecting {Ol l) planes was a lso Y noticed. These twin boundaries were found to exhibit delta fringe contrast [ 8 1 ( symmetr ic i n dark field and asymmetr ic i n bright field) characterist ic of perpen- dicular twin boundaries (Fig. 3). Single surface t race analysis demonstrated that the habit plane of the gamma plates l ay within the stereographic triangle defined by the poles ( loo) , (301) and (311).

3.2 Alpha + Gamma Transformation. - There have been a number of investiga- tions on the alpha +gam ma transformation[ 1-3 I. In a recent study, the crys ta l l - ography of the formiation of the gamma plates in the alpha matrix has been discuss- ed by Weatherly [3 ] . The gamma hydride plates observed i n the present study in quenched samples exhibited a remarkable sirr i lari ty with internally twinned rr ar ten- s i te plates (Fig. 4). Such periodically spaced twins extending right upto the in ter - face have not been reported in relat ion to gamma plates formed i n slow cooled samples [ 31 . The observed orientation relationship and the indices of the habit plane and the internal twinning plane could be l isted a s follows:

Orientation relation : ( loo} 1 1 (0001)0( ; <Ol l>y 1 l<l110 >a. Habit plane : Close to- Qo io ] . Twin plane : (Oll}y. . It should be noted tha; the 101710 habit and the corresponding orientation relation, (1 1 l )y 1 1 (OOOl),; [I 101 1 1 [1~101, a s observed by Weatherly [3 1 in relat ion to some gamma hydride plates,' were not encountered in the present work. But the number of plates investigated was not so large that the exis tence of such plates could be ruled out.

3. 3 E e t a j A l p h a P r i m e Plus Gamma Transformation. - Iodide zirconium samples i n which hydrogen was introduced by diffusion annealing a t a sufficiently high temperature (above 9 7 0 ~ ) underwent a beta+ alpha prime plus gamma t rans- formation on quenching. The product micros t ructure showed a distribution of gamma plates together with alpha prime martensite laths(Fig. 5). Such a mic ro - s t ructure would be expected if the alloy, during diffusion annealing, f i r s t t r ans - 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 pr ime and gamma phases. This phase reaction may involve rejection of hydrogen from growing martensit ic alpha prime laths to the untransformed beta regions, resulting in progressive stabilization of these regions which eventual1 y transform - ed to the gamma hydride phase. Since there was no retained beta in the final microstructure, i t was difficult to determine the habit plane for the gamma plates. However, in many instances these plates were seen to be lying parallel to the alpha prime laths which exhibit a C334 1 6 habit [ 9 1. The gamma plates a l so contained periodic a r r a y s of internal twins along ( 0 11 I.,.

Fig. 3(a) B F and (b) D F images of internally twinned Fig. 5: Internally twinned gamma plate in beta matrix, showing delta fringe gamma plate and marten- contrast a t twin boundaries. si te laths in iodide

Fig. 4 : Micrographs showing internally twinned gamma plates in alpha matrix i n iodide zirconium; (b) and ( c ) constitute B F and D F pair.

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4. Discussions. - 4.1 Crvstal lography of Beta + Gamma Transform ation. - The crystal lography of

this transform ation could be described in t e r n s of the lat t ice correspondence shown in Fig. 6 i n which the relat ive positions of the lat t ice vectors of the beta and - the gan ma s t ruc tures a r e indicated on the (110)d 1 ( 1 1 1 ) ~ plane. According to this correspondence, based on the observed orientation relation, the n agnitude of the lat t ice s t ra in require? for bringing about the beta to garr ma transit ion i s not ve ry la rge . a s indicated in Fig. 6. This lat t ice s t r a in can be conceptually broken up into two con ponents. The f i r s t i s responsible for straining the (1 plane to n atch the d i n ensions of the ( 1 1 1 ) ~ plane an6 for adjusting the interplanar spacing of the form e r so that i t equals that of the la t te r . The second con ponent of the lat t ice s t r a in i s necessary for bringing the a ton s in the right positions in the gamma lat t ice, These two con ponents of the lat t ice distort ion a r e represented by the two n a t r ices S1 and S2 :

0 0 1.1322 " ; S2 = 1 0.

0 0 1. C8564 0 Both matr ices a r e expressed with reference to the axes sys ten indicated in Fig. 6. The shear 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 in ple shea r acting uniformly on every atomic plane (Fig. 7). The shea r S1, when applied to an unit sphere , leaves a se t of vectors undistorted. These vectors gene- ra te an elliptical cone, the positions of which before and af ter the application of S1

PLANE O F PROJECTION 11101fl . ~ 0 0 0 1 l ~ . l l l l l ~

loo% . [llio], : boi l - 354 3 2 0 3 4 5 P 8 -

8101, : b i o ~ ] ~ : k2 5 2 1 1 - 5 01 5 54 5 87 5 -

B C C 1OISTORTED C3Y 5 , ) HCP 1 DISTORTED] F C T

Fig. 6: Schematic diagram show- Fig. 7 : Schematic diagrarr illustrating ing crystal lographic correspon- the shea r S2 for both beta to gamma dence of alpha, beta and gan ma and alpha to garr rr a transform ations. phases.

a r e represented in the stereographic projection in Fig. 8. When a cer ta in fraction of the s i n ple shea r S2 i s applied on the ( 11 1) 1 / (1 1 0 ) ~ plane. a habit plane solu- tion can be obtained with the habit plane pole n arked within the stereographic t r i - angle defined by the poles (Too), (T0l) and (T l l ) . T h i s i s consistent with the exper in ental de t e rn i n a t i ~ n of the habit plane. After the application of S1 and S 2 , the required rigid body rotation around the [OOa 8 axis will shift the (1 11) plane away frorr the (1 10) plane by about lo0. Thus the observed orientation relat ion

B can be obtained. It may be noted that a habit plane solution can be obtained h e r e

by a combination of S1 and a fraction of S2. both of them being con ponents of the lat t ice shear. It can be visualised that S2 can act in either the positive (indicated by the arrows in Fig. 7 ) o r the negative direction along [i lOIB to introduce the necessary change in the stacking sequence. Therefore, by a suitable distribution of S2 in adjacent twin related don ains of a garr ma hydride plate, i t i s possible for the invariant plane s t ra in condition to be satisfied. From a stereographic analy- s is of the crystallography of the beta to garr rr a transform ation. i t could be seen that a shear rr agnitude of about $ [S21 i s necessary for satisfying the IPS condition. This corresponds to a twin thickness ra t io of about 3 : 1, a value close to that expe- rim entally observed.

4.2 CrystalLography of A l p h a I u a m n a Transforwation. - The crystallography of this transform ation has recently been discussed by Weatherly [3l who shows the form ation of garr rra plates with (10iRj habit (denoted by type I) i s similar to that of the beta to gan ma transition discussed in Section 4. 1. The difference between the two cases a r i ses from the difference in the magnitudes of the elerrlents of the s t ra in matrix S1. Both the magnitude and the direction of S2 for the beta t o gam ma and the type I alpha to gamma transitions a r e identical. But i n the la t ter

case the sirr ple shear S2 i s manifested by a displacement of every alternate (0001)a 1 1 (11 l ) y plane through a distance a16 [ 1171

Y'

2 e transform ation crystallography associated with garr m a plates exhibiting a 11010 1 habit (denoted by type 11) ,-as proposed ea r l i e r E31, involves a simple shear of 30°0n a (1010) plane in a < 1210>. It i s , however, interesting to note that the two correspondences for type I and type I1 plates can be regarded a s crystallo- graphically equivalent if one considered the alpha to gamma transition to occur in

Fig. 8: Stereographic analysis of crys ta- Fig. 9: Stereographic projection llography of beta to gamma transform a - ill ustrating the equivalence of type tion. I and type I1 correspondences.

two steps : the f i rs t step being the alpha to beta transit ion in accordance with the Burgers correspondence followed by the second step comprising the beta to gamma transition a s discussed earl ier . The stereographic projection showing the appro- ximate orientations of the different variants of gamma crys ta ls arising from a single alpha orientation through an in te rn ediate beta orientation i s shown in Fig. 9. It can be seen that in the type I correspondence the ( 1 1 0 ) ~ plane, which i s derived

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from the (0001) plane. becon e s a (11 1 4 whe rea s i n the type I1 cor respondence C1

other {01 I} planes a r e converted to (11 planes. B 4. 3 Pos s ib l e Transform a t ion Sequence. - In view of the c lo se cor respondence among the alpha, the be ta and the g a m m a s t ruc tu r e s , the fact that hydrogen is a strong b e t a s tab i l ize r and the findings of this study, i t i s a t t rac t ive to envisage that g a n m a hydride format ion i n a lpha z i rconium involves a n in te rmedia te s t ep of beta formation. It i s poss ib le that a s hydrogen segrega t ion occu r s i n a ce r t a i n reg ion of a n alpha c ry s t a l , the region f i r s t t r a n s f o r m s in to the be ta s t r u c t u r e which finally goes over to t he g a m m a s t r u c t u r e to accommodate the l a r g e concent ra t ion of hydrogen a toms. Alternat ively, the be ta s t a t e n a y a c t a s a n "activated com- plex s ta te" between the alpha and the g a m m a s t a t e s b y providing a n e a s y path i n the shear ing p roce s s involved i n thea lpha to g a m m a transi t ion.

ACKNOWLEDGEMENT

The au thors would l ike to thank Dr . S. J. Vijayakar f o r h i s help i n the expe r i - mental work. D i scus s ions with D r . R. Kr i shnan and with Dr . M. K. Asundi w e r e v e r y useful.

REFERENCES

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Soc. of AIME. 223 (1 965) 30. SIDHU S .S . , HEATON L e ROY., CAMPOS F. P. and ZAUBERIS D .D . , Advances i n Chemis t ry S e r i e s , Amer . Chem. Soc. 39 (1963). D e RIDDER R . , VAN LANDUYT J . , GEVERS R. and AMELINCKX S . , Phys. Stat. Sol. 40 (1970) 271. BANERJEE S. and KRJSHNAN R . , Ac t a Metall. 2 (1971) 1317.