In-situ deformation characterization of mesosegregated high-strength low-alloy steel

The inhomogeneous microstructures caused by mesosegregation deeply influence the mechanical properties and stability of materials. However, the relationship between tensile deformation and inhomogeneous microstructures has received limited attention. This paper reports an in-depth investigation of the tensile deformation behavior and strain distribution of mesosegregated high-strength low-alloy (HSLA) steel. A multi-scale digital image correlation (DIC) analysis of in-situ scanning electron microscopy images reveals that the discrepancies in the tensile deformation behavior are caused by the microstructural discrepancies between the positive segregation zones (PSZs) and negative segregation zones (NSZs). The larger number of crossed-slip bands in the NSZs indicates a more serious plastic deformation caused by the softer microstructures of this zone, i.e., tempered upper bainite and allotriomorphic ferrite. On the contrary, the harder microstructures of the PSZs, i.e., bulk bainite ferrite, tempered martensite, and lower bainite, having fewer crossed-slip bands, display a higher plastic deformation resistance. Additionally, the lower plastic deformation resistance in the NSZs leads to a lower failure threshold and causes a fracture in the NSZs. This work reveals the influence of mesosegregation on deformation behavior and provides the guideline on the post-heat treatment of heavy forgings.