Relationship between strength and hardness in single packet and single block structures of lath martensite

Abstract

Microtensile tests were performed to elucidate the mechanical properties using micro-scale specimens extracted from single packet (SP) and single block (SB) structures of lath martensite in steels with low- and medium-carbon contents. In addition, micro-Vickers hardness and nanoindentation tests were conducted to measure the hardness corresponding to single substructural units. A habit-plane-dependent-yielding occurred in the SP structures, and the anisotropic yielding behaviour was also observed in the SB structures excluding the block boundaries, independent of the carbon content. While the low-angle grain boundaries are included in the SB structure, they are difficult to hinder the dislocation motion. Therefore, this indicates that the plastic anisotropy on microstructure of lath martensite is attributed to the dislocation cell structures formed during the early stage of deformation. The tensile strength of the SP and SB structures were correlated with the micro-Vickers hardness and nanohardness, respectively. The average correlation coefficient for the SP structures coincided with the Tabor’s constant, whereas the average coefficient for the SB structures was half lower than that of the Tabor relation.