Effects of {332}(113) deformation twinning on fatigue behavior of TiMn system alloys

Abstract

The effects of {332}(113) deformation twinning, one of the unique deformation modes for metastable β-type Ti alloys, on the fatigue behavior of TiMn system alloys were investigated focusing on fatigue strength, fatigue crack initiation and propagation. Ti7Mn and Ti5Mn3Mo (mass%) alloys which are primarily deformed by dislocation slips and {332}(113) deformation twins, respectively, were subjected to fatigue tests conducted in tensile-tensile mode at room temperature, followed by fracture surface and deformation microstructure analyses. We found for the first time the Ti5Mn3Mo alloy has higher fatigue strength as compared to the Ti7Mn alloy owing to the formation of the {332}(113) deformation twins. The {332}(113) deformation twins are to some extent responsible for the plastic strain accumulation in place of the dislocations during cyclic deformation. Thus, {332}(113) deformation twinning prevents the accumulation of dislocations during cyclic deformation, thereby suppressing fatigue crack initiation. Moreover, formation of the {332}(113) deformation twins around crack tip decreases stress concentration at the crack tip and changes the crack propagation direction, as a result, crack propagation speed is decreased. These results indicate that the {332}(113) deformation twining is crucial for improving the fatigue properties of metastable β-type Ti alloys.