Measuring the microscale origins of functional fatigue in materials with martensitic transformations with varying mechanical hysteresis

The aim of this experiment is to measure the interfacial stress fields that lead to functional fatigue in materials that undergo martensitic phase transformations (MPTs) such as shape memory alloys (SMAs). MPT is a diffusionless, often reversible phase transformation between a high-symmetry and a low-symmetry phase. It is the enabling mechanism behind the novel properties of SMAs and TRIP steels, but functional fatigue continues to be a major barrier. Functional fatigue is caused by stress fields that form at the phase interface and lead to dislocation generation. We plan to use dark-field X-ray microscopy (DFXM) to measure these interfacial stress fields during stress-induced MPT, using a mechanical load frame. A strong factor in the fatigue properties is the kinematic compatibility. NiTiPd SMAs have tunable kinematic compatibility and several compositions will be mechanically cycled to observe how dislocations develop and influence the functional fatigue behavior.