The mean size of plate martensite: influence of austenite grain size, partitioning, and transformation heterogeneity
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I.
INTRODUCTION
II.
THE evolution
of microstmcture comprising plate martensite is "sui-generis". A major issue regarding this process is the variation of the size of the plates with the volume fraction transformed. Qualitatively, it is accepted that plate size is determined by partitioning of the austenite grains by the martensite, by autocatalysis, and by the prevailing transformation heterogeneity related to austenite grain size.t Fisher et al. 2 were probably first to attempt a quantitative description of martensitic microstructures. Several authors 3 v have considered using Fisher's equation to describe experimental data. These results, however, were generally unfavorable to the model, and the poor agreement was usually ascribed to the heterogeneity of the transformation. More recently Winchell and co-workers 8'9 and Guimarfiesl~ attempted to approach the problem from a different point of view which takes a mid-rib formation as the first step in the transformation. Although favorable results were obtained up to 0.47 fraction transformed, ~0the model does not allow the mean martensite plate volume ~ to be calculated. This is a major shortcoming since, when one considers martensite kinetics, ~ must be known in order to obtain the volume fraction transformed, V,., from the number of martensite plates per unit volume N~., V~. = N,. 9 F
[1]
In this work we reexamine the subject of austenite partitioning by martensite plates to introduce transformationheterogeneity into Fisher's model. Subsequently we describe a method to analyze the metallographic data which allows some insight into the progress of transformation in an average austenite grain. The latter was achieved with the aid of computer simulation. Finally, a new transformation model derived from the computer analysis is introduced and put to test.
J. R.C. GUIMARAES, Lecturer, and A. SAAVEDRA, Assistant Professor, are with the Department of Materials Science and Metallurgy, lnstituto Militar de Engenharia, Pca. Gem Tibfircio 80, 22290 Rio de Janeiro, RJ. Brazil. J. R. C. Guimaraes is also with Cia. Brasileira de Metalurgia e Mineraq~o, R. Pe Joao Manoel 923, 9 'h. 01411 Sao Paulo, SR Brazil. Manuscript submitted May 22, 1984.
METALLURGICAL TRANSACTIONS A
TRANSFORMATION HETEROGENEITY AND PARTITIONING
When considering the progress of martensite transformation in a polycrystalline material, it is convenient to assume that transformation results from two sequential processes: "spread" and "fill-in". lt'~2'j3 In the former, transformation takes place, by and large, in untransformed grains; but in the latter it proceeds in austenite "pockets" between previously nucleated martensite. Spread, described by the volume fraction of material comprising partially transformed grains, Gg, is easily observed in fine-grained materials. 13 Geometry suggests that when spread controls the transformation, ~ should remain constant. On the other hand, as fill-in takes over, F should decrease with increasing volume fraction transformed, V,. However, as martensite transformati
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