On the prebainitic phenomenon in some alloys
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I.
INTRODUCTION
THE prebainitic phenomenon is one of the important subjects of bainitic transformation which has been greatly debated. In the 1930s, Kurjumovu~ pointed out that carbon-depleted austenite can form during the incubation period of steels holding at medium temperature and then the austenite transforms into ferrite. In the 1940s, a similar view was suggested by Klier and Lyman 321 The existence of the carbon-depleted region in austenite was confirmed indirectly by the X-ray method by Entin t31 in the 1960s and with the method of ultrasonicst4J and high-temperature X-ray diffractiontS~ in the 1970s. However, Aaronson et al.16] thought it impossible to decompose the austenite into carbon-depleted and rich regions, i.e., spinodal decomposition, according to their thermodynamic calculation. Hsu e t a / . 171 showed indirectly by means of internal friction that it is a soft mode nucleation process of bainite in the incubation period. Garwood tSl and Takezawa and S a t o 191 proposed that the bainite in/3 brasses can form in the zinc-depleted region by shear. Feng et al. ~~ pointed out that many defects exist in austenite and that, by Monte-Carlo simulation, the carbon-depleted region appears in austenite during the incubation period and then bainite forms by shear in that region. Although the prebainitic phenomenon is an important aspect of understanding the mechanism of bainite formation, the description of solute atoms' behavior in the bainitic transformation, the direct measurement of the composition change during bainitic incubation period, and direct observation of the nucleation site are scarce at present. Thus, it is necessary to study these phenomena in detail. M.K. KANG, Professor, and X.K. MENG, Ph.D., are with the Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, 710072, People's Republic of China. Y.Q. YANG, Q.M. WEI, and Q.M. YANG, formerly Ph.D. Candidates with Northwestern Polytechnical University, are Postdoctoral Researchers, Nanjing University, Nanjing, 21008, Beijing University of Science and Technology, Beijing, 10085, and Zhejiang University, Hangzhou, 310027, respectively, 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. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENT
The tested/3 brasses were prepared by melting highpurity Cu and Zn or Cu, Zn, and AI in a graphite crucible, respectively, and casting into ingots with diameters of 80 ram, followed by homogenizing at 1073 K for 48 hours and forging into rods of 20-mm and 10mm diameter, respectively. The composition is Cu42.85Zn (weight percent) and Cu-26.84Zn-4.22AI (weight percent). The specimens for internal friction measurement with a dimension of 1 x 1 • 60 mm and for metallography were cut from the rods. These s
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