On bainite formation
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I.
INTRODUCTION
IN the
1970's debate on the mechanism of the bainitic transformation was concerned with the definition of bainite, the changes in composition at the onset of the bainite reaction, the growth mechanism of the ferritic component of bainite, and the source of the carbide precipitation, tq In recent years, the argument on the mechanism of the bainite reaction is still continuing based on the transformation kinetics in steels [2,3m and the interfacial as well as the internal structure of the bainite in Cu-Zn alloys.tS,6] This paper summarizes our recent work on the thermodynamics of the bainitic transformation, the observation of growth ledges on the bainite, the study of the relationship between the austenite strength and the B, temperature, the diffusion coefficient of C and Fe relative to the Bs temperature, and the comparison of the interfacial morphology and habit plane of the bainite and martensite in connection with the mechanism of the bainite formation. Meanwhile, this paper also presents some aspects of the prebainitic transformation from the internal friction and X-ray diffraction study within the incubation period of the bainitic transformation in Ag-Cd and CuZn-AI alloys and 18CrNiWA steel as well as its decarburized alloy. II. T H E R M O D Y N A M I C S O F THE BAINITIC TRANSFORMATION A . F e - C Alloys
In 1962, Kaufman, Radcliffe, and Cohen (KRC) tTj published an article on thermodynamics of the bainitic T.Y. HSU (XU ZUYAO), Professor, is with the Department of Materials Science, Shanghai Jiao Tong University, Shanghai 200030, People's Republic of China. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, 1L, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
transformation in Fe-C alloy in which the total driving forces were calculated for various processes, such as: (1) austenite decomposition into equilibrium phases, ferrite and cementite, i.e., y ~ a + Fe3C; (2) austenite decomposition into proeutectoid ferrite, i.e., y ~ a + "Yl (y~ is y after partitioning); and (3) austenite transformation into ferrite with the same composition, i.e., y---> a'. Recently, the statistical model of C-C interaction has been developed tS~ and the thermodynamic approach of AG T M calculation in ferrous alloys has been modified. [9,1~ The total driving force and the driving force for nucleation for the above three possible processes of bainite formation were calculated using the modified models, t',12[ and the results are shown in Figures 1 and 2, respectively. Figure 1 shows that for xr = 0.04, the total driving force for 3' ~ a + 3'~ is 500 J tool-~ larger than that for y---> a ' (with the same composition). Figure 2 shows that the driving force for nucleation for 3' ~ a + 7~ is much larger than that for y---> a'; for example, AGfv-'~ยง of xv = 0.02 at 600 K exceeds AGfv-*~
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