The formation mechanism of plate in beta Cu-Zn and Cu-Zn-Al alloys
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INTRODUCTION
As a branch of the solid state transformation, the bainitic reaction is still a heated controversial topic as to its formation mechanism. Much research on the crystallography, morphology, thermodynamics, and kinetics of bainite plates has so far been carried out to clarify which mechanism (displacive shear or diffusional ledge) operates the bainitic transformation. In Fe-C-X alloys, however, the awkward ranking of bainite between the obviously diffusionless martensitic reaction and pearlitic reaction, which is a clear example of a reconstructive transformation, has in the past led to difficulties, t~l It is well known that the precipitating process of the fl' (/31,/32,/33 matrix phase) ~ cq (bainite plate) reaction in some nonferrous alloys, such as Cu-Zn and Cu-ZnAI, is not only undisturbed by diffusion of institial atoms but also simple and hence helpful to the analyses of crystallography and kinetics. Therefore, the fl' --~ oq transformation has recently become an important model reaction. [21 The composition analytical results of bainitic at plates in a Cu-39.3 wt pct Zn alloy and 3'1 plates in a Cr-33 X.K. MENG, Ph.D., and M.K. KANG, Professor, are with the Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, 710072, People's Republic of China. Y.Q. YANG, formerly with Northwestern Polytechnical University, Ph.D. Candidate, is Postdoctoral Researcher, Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210008, People's Republic of China. D.H. LIU, formerly with Northwestern Polytechnical University, Postgraduate, is Lecturer, Pingyuan University, Xinxiang, 453000, People's Republic of China. This article is based on a presentation made at the Pacific Rim Conference on the "Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products," held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL's Phase Transformations Committee. METALLURGICALAND MATERIALS TRANSACTIONSA
wt pct Ni alloy given by Cliff et al.t3l showed that even at the early stages of plate formation, and near the tips of the growing plates, the composition of the plates is different from that of the matrix and near the equilibrium composition of the second phase. Thus, Cliff et al. denied the initial formation of these bainitic plates by a mechanism solely involving crystallographic shear. Hsu and Zhou's thermodynamics calculations t4] also showed that the bainitic transformation in a Cu-26.77 wt pct Zn4.00 wt pct A1 alloy could only proceed with a diffusional mechanism. In a Cu-24.3 wt pct Zn-4.7 wt pct AI-0.8 wt pct Mn alloy and a Cu-19.51 wt pct Zn-5.88 wt pct AI alloy, however, Liu and co-workers Is'rJ observed the midribs in initial al plates, the slabby stress field in front of the tips of the plates, and the twisting of the intersected plates. These phenomena provided evidence for the shear mechanism of bainitic transformation. Wu et al. iT] concluded that the formation and early growth of at plates formed in a
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