Microscopic strain partitioning in Lüders band of an ultrafine-grained medium Mn steel

Abstract

This paper is focused on the determination and understanding of the microscopic strain partitioning in the Lüders band of an ultrafine-grained medium Mn transformation-induced plasticity (TRIP) steel with austenite-ferrite dual phase microstructures. First, a microscopic digital image correlation (μ-DIC) method is developed that enables high-resolution strain mapping at the subgrain scale. Thanks to the local Lüders strain assessment, the strain partitioning between ferrite and retained austenite is quantified. It reveals that the average Lüders strain in ferrite is about 10.1% and that in austenite about 9.1% at a macroscopic Lüders strain 9.5%, thus it indicates that both phases are important to the plastic deformation in the Lüders band. Besides the strain partitioning between the two phases, a more striking finding is the significant heterogeneity of Lüders strain within the same phase. The microstructural origins for the strain partitioning are investigated using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in combination with the strain field analysis. The results show that the main cause for strain heterogeneity formation in Lüders band can be attributed to the heterogenous distribution of mobile dislocations in the initial microstructure before tensile deformation. And the effects of grain size and crystallographic texture are proofed to be the secondary factors that may only have minor influence on Lüders strain distribution.