Acta metall, mater. Vol. 41, No. 12, pp. 3445-3454, 1993 0956-7151,'93 $6.00 + 0.00 Printed in Great Britain. All rights reserved Copyright ~" 1993 Pergamon Press Ltd SELF ACCOMMODATION MORPHOLOGY OF MARTENSITE VARIANTS IN Zr-2.5wt%Nb ALLOY D. SRIVASTAVA l, MADANGOPAL K. I, S. BANERJEE l and S. RANGANATHAN ~ tMetallurgy Division, Bhabha Atomic Research Center, Bombay 400 085 and 2Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India (Received 12 October 1992; in revised form 22 April 1993) Abstract--The role of self accommodation of the different martensite variants in controlling the morphologies of the Zr-2.5wt%Nb alloy martensite has been examined. Three distinct types of grouping of martensite variants have been found to be predominantly present. Crystallographicdescriptions of these groups have been provided and the degrees of self accommodation for these have been estimated and compared with those corresponding to other possible variant groupings around the symmetry axes of the parent phase. The frequently observed 3-variant group, which shows an "indentation mark" morphology when viewed along (111)8 directions in the transmission electron microscope, has been seen to have the highest degree of self accommodation amongst the cases considered. Based on the observations made, a growth sequence leading to the formation of the final martensitic structure has been proposed. 1. INTRODUCTION The morphologies and the structures of the marten- site in Zr and Ti base alloys have been examined in several alloy systems. A review of this aspect is available in Ref. [1]. It has been pointed out earlier that the morphology of the martensite in these alloys changes from lath to plate type with change in composition [2], similar to the case of steel marten- sites. The transition from the lath to the plate mor- phology, in Zr alloys, has been found to be correlated to the martensite start (Ms) temperature. Alloys having Ms below 650-700°C exhibit predominantly the plate type morphology. Additionally, a transition from dislocated to twinned substructure has been found to accompany the morphological change in these alloys [2]. Most of the studies made on the crystallography of the martensitic transformation and on martensite substructure [3-9] in Zr and Ti base alloys have focused attention on various crystallographic aspects of individual martensite plates, such as orientation relationship with the parent phase, habit plane, shape strain and inhomogeneous deformation. However, the nature of aggregation of martensite units (laths or plates) has not been examined in detail. The present paper deals with a crystallographic description of a hitherto unreported morphology, consisting of an aggregate of three martensite variants, which is seen to occur frequently in the Zr-2.5wt%Nb alloy martensite. The motivation for the evolution of such a morphology has also been investigated. 2. EXPERIMENTAL Sheet samples of two different thicknesses (0.5 and 4ram) of Zr-2.5wt%Nb alloy, containing about 1000 ppm oxygen, were sealed in prevacuated silica capsules backfilled with helium. Two different thick- nesses of the samples were selected to obtain different cooling rates while quenching. These samples were solution treated in the fl phase field at 1000°C for 30 min and subsequently quenched in water. The microstructures of these samples were charac- terized by light microscopy and transmission electron microscopy (TEM). Preparation of samples for light microscopy involved chemical polishing and etching in a solution containing 5% HF, 45% HNO3, and 50% H20. Jet electropolishing of TEM samples was carried out in a solution containing 30 parts perchlo- ric acid, 170 parts n-butanol and 300 parts methanol. 3. RESULTS TEM examination of the microstructure of the Zr-2.5wt%Nb alloy martensite showed that the fol- lowing three characteristic plate group morphologies occurred: (i) large internally twinned martensite plates with zig-zag habit planes, the primary plates being arranged in predominantly triangular mor- phology; (ii) secondary martensite plates arranged in an "in- dentation mark" morphology; (iii) packets of martensite laths stacked almost in parallel. Illustrative examples of these morphological fea- tures are shown in Fig. 1(a-c) respectively. It may be noted here that morphological features similar to those illustrated in Fig. l(a, c) above are, in general, observed in all martensites of Zr and Ti base alloys [8]. However, the secondary plate morphology shown in Fig. l(b) has not been reported earlier. The extent to which each of these features are present in a martensite structure will depend on the alloy compo- 3445