Martensite Lattice Parameter Measured by Modern X-ray Diffraction in Fe-C alloy

Martensite crystal structure is widely accepted to be body-centered tetragonal (BCT). With the increase of carbon content, the c/a ratio increases, following the linear relationship. However, it is pointed out by Sherby that the steel with less than 0.6 wt% carbon has body-center cubic (BCC) structure and c/a ratio equals to 1. AISI9310, AISI4140, AISI4150, AISI1080 and AISI52100 were quenched to form fresh martensite. The lattice parameter was determined by X-ray diffraction and Rietveld Refinement. The microstructure was observed by scanning electron microscope and hardness was measured. The result shows that for the steel with less than 0.6 wt% carbon, the crystal structure should be lightly distorted BCC, not BCT.