Plate-shaped transformation products in zirconium-base alloys
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INTRODUCTION
PLATE-shaped transformation products form in solidstate transformations involving martensitic, diffusional, and diffusional-displacive mechanisms. There have been controversies over the operating mechanisms of transformations in several systems, where some of the features of both diffusional and martensitic transformations are observed.[1,2,3] This point is discussed in detail in a collection of recently published articles.[1,2] The essential issue is whether it is possible to decipher the mechanism of transformation from morphological and crystallographic observables such as the orientation relationship, the habit plane, the macroscopic shear, the nature of the interface plane, and the distribution of dislocations/ledges on the interphase inS. BANERJEE, Associate Director, Materials Group, and Head, Materials Science Division, and G.K. DEY and D. SRIVASTAVA, Scientific Officers, Materials Science Division, are with Bhabha Atomic Research Center, Mumbai-400 085, India. S. RANGANATHAN, Professor and Chairman, is with the Mechanical Sciences Division, Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted March 8, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
terface. Major controversies have arisen in the bcc-fcc transformations in copper-base alloys[4] and in fcc-bct (bcc) transformations in bainitic steels,[4–7] where a consensus is yet to be established on the roles of shear and diffusion in the mechanism of transformation. In the present work, the aforementioned issue is addressed by examining the bcc-hcp transformation products in zirconium alloys produced through martensitic, diffusional, and diffusional-displacive mechanisms. A comparison is made between Widmansta¨tten a (hcp) plates and martensitic a' plates in relation to their habit planes, orientation relations, interface structure, and internal structure. The crystallography of the formation of hydride plates in the a (hcp) or the b (bcc) matrix is also discussed to highlight the similarity between the formation of g-hydride (fct) with that of the a plates in the b matrix. ‘‘Lattice correspondences,’’ which are present in all these transformations, are quite similar, and all of them lead to orientation relations close to the Burgers relation,[8] as given subsequently: {0001}a||{110}b; ,1210.a||,111.b The bcc-hcp martensitic transformation was analyzed in VOLUME 28A, NOVEMBER 1997—2201
conium alloys have been limited to dilute Zr-Nb alloys (mostly in the Zr-2.5Nb alloy*). The accurate determination *Alloy compositions are given in weight percent throughout the text.
(a)
of the habit planes in these alloys is quite difficult because of the very small volume fraction of the b phase in the product microstructure. In the present work, Widmansta¨tten a precipitates both in the Zr-2.5Nb and in the Zr-20Nb alloys were investigated, mainly for determining the habit plane and for the characterization of the interfacial dislocation structure. Based on the comparison of the crystallographic and the
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