Revisit Deformation Behavior of Lath Martensite

Abstract

Two mechanisms inconsistent each other, a relaxation of type II internal stress and a presence of mobile dislocation, were previously proposed to describe the low elastic limit of as-quenched lath martensite steels. In this study, neutron diffraction experiments were performed to revisit the deformation behavior of lath martensite steel. Dislocations with very dense in the order of 10¹⁵ m⁻² were observed in the as-quenched martensite single phase condition of a Fe–18Ni alloy. The diffraction profiles had good symmetry, showing that the inhomogeneous type II internal stresses which might be introduced in individual blocks or packets during martensitic transformation were well balanced in a bulky specimen size, and the influence to the yield stress would be very small. In a 0.22C steel, dislocations with very dense in the order of 10¹⁵ m⁻² and random arrangement were also observed in the as-tempered condition. The diffraction profiles had also good symmetry. The symmetry collapsed by tensile deformation, displaying the occurrence of load sharing between the packets with the active slip systems of in- and out-of-lath-plane. The highly dense random arrangement dislocations easily moved at the beginning of deformation, then accumulated, annihilated and changed the arrangement differently depending on the orientation of the packet with respect to the deformation direction. The movement of highly dense random arrangement dislocations played an important role as a mechanism at the beginning of deformation, and can be a true feature of mobile dislocations.