Growth kinetics and high-temperature tem in situ observation of bainite in a Cu-Zn alloy
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I.
INTRODUCTION
A bainitic transformation in 13 brasses has been widely studied since the 1950s. Nevertheless, its mechanism is not yet clear. Studies of crystallographyt~,2,3~ showed that surface relief, habit plane, and orientation relationship of bainite are the same as those of martensite, respectively. Bainite in its early growth stage inherits the ordered structure of the matrix. [41 Therefore, bainite forms by shear. But, thermodynamic calculations tSj indicated that bainite can only form by volume diffusion. It was found by transmission electron microscopy (TEM) at room temperature that the fresh bainite is fully coherent with the matrix and free of internal substructure. |6l So, it is inconsistent with the phenomenological theory of martensite. Growth kinetics studies by optical microscopy or by TEM at room temperature also showed that bainite grows by volume diffusion. 16,71However, it should be pointed out that the optical microscopy used in kinetics studies does not seem accurate enough to measure the growth rates of very fine bainitic plates and that it is not clear whether or not the bainite plates observed statically by TEM at room temperature can really represent their different growth stages. On the other hand, some features which may be important in understanding the mechanism of bainite formation cannot be found at room temperature after quenching because these features may appear only during the growth of bainite. Therefore, in this article, high-temperature Y.Q. YANG, formerly with Northwestern Polytechnical University, is Postdoctoral Researcher, National Laboratory of Solid States Microstructure, Nanjing University, Nanjing, People's Republic of China. D.H. LIU, formerly with Northwestern Polytechnical University, is Lecturer, Pingyuau University, Xingxiang, People's Republic of China. X.K. MENG, Ph.D. Candidate, and M.K. KANG, Professor, are with the Department of Materials Science and Engineering, Northwestern Polytechnical University, Xian, 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
TEM was used to study the growth kinetics and to observe in situ the growth process of bainite in a Cu-Zn alloy.
II.
EXPERIMENTAL PROCESSES
A Cu-42.85Zn (wt ~,ct) alloy was prepared by melting high-purity (99.99 pet) Cu and Zn in a graphite crucible. The ingot, with a dimension of 80-mm diameter, was homogenized at 800 ~ for 48 hours and a 10-mm outer layer of the ingot was machined off so that the entire layer of Zn sublimation was eliminated. The machined bar was forged into rods of 20 mm in diameter. Specimens 1 mm in thickness were cut from the rods and betanized in a molten salt bath at 790 ~ for 3 minutes before water quenching to room temperature. The thin foils made from the quenched s
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