Analysis of variant selection in friction-stir-processed high-strength low-alloy steels

Abstract

Variant selection in friction-stir-welded high-strength low-alloy steels has been studied using the electron backscatter diffraction and prior austenite (PA) reconstruction techniques described in previous papers. A hypothesis for variant selection has been proposed based on grain-boundary interfacial energy and misorientation. This study focuses on austenite 〈111〉 boundaries with a two-dimensional approach. Results indicate that variant selection is strongly dependent on misorientation. Certain PA misorientations produce combinations of variants that minimize the interfacial energies between a ferrite nucleus and a neighboring austenite grain, and between adjoining ferrite nuclei along the boundary between two PA grains. PA grains that exhibit a 60° 〈111〉 misorientation between them satisfy both these conditions for a combination of variants. These PA boundaries exhibit strong variant selection. As a result, the density of these boundary types influences the overall variant selection. Additionally, variant selection is more prevalent in small PA grains (<150 µm), which is probably a result of limited intragranular nucleation. Nearly all variants are present in larger PA grains.