Symmetry Breakdown Related Fracture in 42CrMo4 Steel

TL;DR

This study investigates the crystallographic symmetry changes in 42CrMo4 steel during phase transformation, revealing a transition from expected 3-fold to observed 6-fold symmetry, and links this to crack propagation behavior.

Contribution

It provides new insights into symmetry breakdown mechanisms in steel during phase transformation using advanced ODF analysis and theoretical modeling.

Findings

Crystallographic symmetry shifts from 3-fold to 6-fold after transformation.

Cracks propagate through high-symmetry regions influenced by symmetry breakdown.

High symmetry areas are preferred paths for crack propagation.

Abstract

Austenite grains that underwent the f.c.c. to b.c.c. (or b.c.t.) transformation are typically composed of 24 Kurdjumov-Sachs variants that can be categorized by three axes of Bain transformations; thus, a complete transformation generally displays 3-fold symmetry in (001) pole figures. In the present work, crystallographic symmetry in 42CrMo4 steel austempered below martensite start temperature was investigated with the help of the orientation distribution function (ODF) analysis based on the FEG-SEM/EBSD technique. It is shown that, upon phase transformations, the specimens contained 6-fold symmetry in all (001), (011), and (111) pole figures of an ODF. The ODF analysis, verified by theoretical modeling, showed that under plane-strain conditions cracks prefer to propagate through areas strongly offset by the high symmetry. The origin of high symmetry was investigated, and the mechanism of the symmetry breakdown was explained.