Martensite Transformation During Continuous Cooling: Analysis of Dilatation Data

The amount of athermal martensite as a function of undercooling below the martensite start temperature was quantified by analyzing the dilatation data using a novel method, and the results are compared with existing empirical equations. The discrepancy between the two results was attributed to the difference in the concentration ranges of the alloying elements considered. The importance of including the effect of substitutional elements on the lattice parameters of martensite for accurate quantitative interpretation of dilatation data was highlighted. Equations that include the effect of substitutional alloying elements were proposed to calculate martensite lattice parameters. It is further shown that it is possible to calculate the lattice parameter coefficient of a substitutional alloying element directly from the dilatation curve. It was used to estimate, for the first time, the lattice parameter coefficient of aluminum (Al) in ferrite/martensite from the dilatation curves of the two alloy steels studied in the current work. To corroborate the value of the lattice parameter coefficient of Al estimated from the dilatation data, the Bain model was also used to calculate the lattice parameter coefficient of Al independently and a good match was obtained. The lattice parameter coefficient value of Al in ferrite/martensite calculated by both these methods follows the overall trend shown by other substitutional alloying elements. The equations proposed for the lattice parameters of martensite were validated by Rietveld analysis of the X-ray diffraction (XRD) patterns.