Characterization of hierarchical lath martensite microstructure in low carbon steels using ultra-high voltage TEM and SEM-EBSD analysis

Abstract

Lath martensite is a heterogenous microstructure in low carbon steels and plays an important part in microstructure for the high strength steels. The complex martensitic structure has been clarified as the hierarchical block and packet structures using SEM observations and electron backscatter diffraction (EBSD) analysis. As the result of variant analysis, it is found that the block consists of sub-blocks and the block size and distribution seem to be new key factors to control the mechanical properties. Each block is further subdivided by laths, which are the finest structural units in lath martensite. The morphology of laths and their variants in low carbon steels has been further investigated using the ultra-high voltage TEM (UHVEM) with 3MeV in accelerated voltage. Based on a comparison of UHVEM and SEM observations for the same field of view, it is found that the laths have a further hierarchical organization with twinned morphology and the fragmented structure within the laths. Two plane crystal analysis of the twin in a lath has been carried out using SEM-EBSD analysis after Xe-FIB large area fabrication process, resulting in the identification of the twin morphology such as the {112} twin plane perpendicular to the {110} habit plane and along the <111> direction. Although several types of twinned morphology such as the long and short length are observed in the lath microstructure under TEM observations, the twinning structure observed in various forms is explained by a simple model in the present study.