Anisotropic cleavage fracture caused by transformation-induced internal stress in an as-quenched martensite

Abstract

The effect of microscopic internal stresses (Type-II) generated via martensitic transformation from austenite (A) to martensite (M) on {001} M cleavage fracture was investigated in an as-quenched 0.1C–5.0Mn steel (mass%). Crystallographic orientation analysis using the electron backscatter diffraction technique revealed that cryogenic {001} M cleavage fracture was promoted predominantly in mode I fracture by Charpy impact testing, and that the Bain group, where martensite variants belong to the same Bain variants, acted as an effective unit to prevent fracture. It was also discovered that the cleavage crack preferentially propagates on (001) M rather than on (100) M in the Bain group, where (001) M was nearly parallel to (001) A in the Bain lattice correspondence ([−110] A // [100] M , [110] A // [010] M , (001) A // (001) M ). In addition, by using a combination of a micro-scale focused ion beam and high-precision digital image correlation techniques, it was found that microscopic internal stresses were anisotropically developed in each Bain group. The principal axes of the internal stresses corresponded to the <001> M coordinate axes in the Bain lattice correspondence, and the principal stress parallel to [001] M is much higher than that of the other two. After the measurement of the internal stresses, it was proved that the cleavage fracture behavior obeyed the effective normal stress in the normal direction of {001} M consisting of the resolved bending stress in the impact test and the anisotropic internal stress generated via martensitic transformation. This suggests that the transformation-induced internal stress originating from the Bain strain leads to the anisotropy of the cleavage fracture in lath martensite. • The effect of microscopic Type-II internal stress on the macroscopic cleavage fracture was investigated in lath martensite. • FIB-DIC technique revealed that martensitic transformation induces the anisotropic elastic strain in each Bain group while maintaining <001> as the principal axes. • EBSD analysis proved that mode-I cleavage crack preferentially propagates on (001) rather than on (100) in a Bain group regardless of carbon contents. • Transformation-induced internal stress originating from Bain strain leads to the anisotropy of the cleavage fracture in lath martensite.