Microstructural Analysis of the Martensite Volume Fraction
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ERSTANDING the microstructure of martensite transformation is an essential ingredient in designing and producing advanced engineering and tool steels. In a recent work,[1] the present authors advocated that the development of a martensite unit may be operationally factored into midrib propagation plus thickening (M+T). Figure 1 exemplifies evidence suggestive of the M+T mechanism in Fe-31mass pct Ni-0.02mass pct C which transforms into lenticular martensite that exhibits a low relaxation rate of the transformation strains as discussed in Reference 1. As shown in Figure 1(a), the radial growth of the martensite units, thence the size of the midrib, is limited by physical obstacles, such as grain boundaries, martensite-austenite interfaces, other midribs, and by the austenite plasticity. The accommodation of the stress field generated by the martensite transformation is instrumental to the typical arrangements of martensite units.[2–5] Two regions of the micrograph labeled ‘‘A’’ and ‘‘B’’ in Figure 1(a) are of particular interest here. In A, two units are seen to meet at a right angle. Clearly, the growth of these two units stops only at the midrib of the other plate. Such an event would be unlikely to happen if both units were growing into their final lens shape in one step, since a martensite unit cannot grow into another one. The conceived mechanism for the microstructure development is schematically depicted in Figures 1(b) and (c). Figure 1(b) shows that in the first step, a network of midribs is formed and then the plates thicken as shown in Figure 1(c), resulting in the morphology observed in A. In B, the feature of interest is that the martensite units are organized in zigzag, owing to the accommodation of the martensite shape change.[6] The conceived mechanism for the formation of the zigzag plates of region B is schematically
depicted in Figures 1(d) and (e). Impingement between units in the zigzag arrangement is indicated by arrows in Figure 1(a) and represented by dotted lines between the zigzag plates in Figure 1(e). If the units in the zigzag formed one after the other by growing to their final size and shape directly from small sizes, the first formed unit would grow without impingement because the next plate would not yet have formed. The next one would then stop growing at the martensite-austenite interface of the preceding plate and so on. Therefore, only whole plates would be observed; the impingement depicted in B would not take place. In other words, the midrib network formation would have to take place before the thickening in order to observe the morphology depicted in B. Summarizing, the contacts between midribs seen in Figure 1 suggest a functional M+T mechanism.[1] In previous papers,[7–10] we have dealt with the calculation of the probable size of a martensite unit forming in a microstructure containing NV martensite units, comprising a volume fraction transformed, VV, by admitting the partitioning of the untransformed austenite by the martensite units of fixed morphology. However, the partitioning
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