The effect of carbon content on the c/a ratio of as-quenched martensite in Fe-C alloys

The as-quenched martensite crystal structure is widely accepted as body-centered tetragonal. The classical Honda & Nishiyama model, c/a = 1 + 0.045 wt%C, was obtained based only on the experimental results for steels containing more than 0.6 wt% of carbon. This model was used to predict that steels containing less than 0.6 wt% of carbon would follow the same relation. However, it was reported by Sherby that the steel with less than 0.6 wt% of carbon is body-centered cubic, and c/a ratio equals to 1. This debate is caused by highly overlapped martensite (002) and (200) peaks when the carbon content of steel is less than 0.6 wt%. In this paper, the martensite lattice parameters of the as-quenched steels with different carbon contents were measured using high-resolution X-ray diffractometer. Rietveld refinement was used to deconvolute overlapped martensite peaks. For the steel with less than 0.6 wt% of carbon, the structure is proved to be body-centered tetragonal. It was found that the relationship between c/a ratio and carbon content follows as c/a = 1 + 0.031 wt%C.