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.
Highlights
IntroductionAs described by Biezeno and Grammel in 1953 [8], if the crack is free to propagate along a certain path, it must choose the path which minimizes the Lagrangian function, the stable solutions of which exhibit less symmetry than the equation itself, releasing the maximum energy from the system
42CrMo4 (AISI 4140, DIN/EN 1.7225) is the most widely used quenched and tempered steel due to its outstanding properties/cost ratio
In the inverse pole figure (IPF) maps, the lath-morphology of the microstructure remained after the recalculation of EBSD maps with the maximal misorientation angle of 15◦
Summary
As described by Biezeno and Grammel in 1953 [8], if the crack is free to propagate along a certain path, it must choose the path which minimizes the Lagrangian function, the stable solutions of which exhibit less symmetry than the equation itself, releasing the maximum energy from the system This is true for dynamic as well as quasi-static crack propagation [9]. The effect of crystallographic orientation symmetry and/or symmetry breakdown on the overall properties of steel of a particular fracture, which could be significant, is still unknown [7] To figure out this problem, both experimental and theoretical investigations are performed
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