Formation mechanism of lath martensite in steels

Abstract

Lath Martensite is formed in low carbon steels and plays an important role in the mechanical properties of heat resistant steels containing about 0.1 mass%C. The lath martensite shows the hierarchic microstructure being composed of packet, block and lath. Martensitic transformation is the phase transformation accompanying orderly shear deformation without atomic diffusion. Relaxation of the strain energy caused by the deformation is the origin of the hierarchic microstructure but the formation mechanism of this microstructure has heretofore not been understood. In this paper, we survey first both the experimental results and phenomenological formation mechanism of the lath martensite reported so far and introduce a new mechanism (Two Types of Slip Deformation model: TTSD model), which is constructed by considering, independently, two kinds of slip deformations using the slip deformation model proposed by Khachaturyan. It is also introduced that TTSD model enables us to simulate the martensite phase formation by the phase-field method. Furthermore, it is found that TTSD model allow us to predict the features of lath martensite, such as the existence of sub-blocks and high dislocation density in lath. In particular, for the first time, the reason why laths in a block structure exist, can be clearly explained by TTSD model.