Size effects in shape memory alloys: Competition between structural and microstructural features in determining grain scale performance

摘要

Stress induced phase transformation in polycrystalline NiTi is studied as a competition between two factors: extrinsic structural factors and intrinsic microstructural factors. Structural factors arise from stress concentrations around features such as holes, pores, cavities or cracks. Microstructural factors include the effect of grain orientations and interactions between grain neighbors, which determine the critical stress for transformation and constrain the martensite microstructures that can form in grains. In this work, we systematically study the relative influence of structural and microstructural factors through a combined experimental and modeling effort, spanning lengths from the macroscale to the microscale. These findings will be of significance in determining the key factor (structure or microstructure) in determining the phase transformation performance in porous SMAs when the pore size is comparable to the grain size. This knowledge is key to choosing a reliable modeling framework for miniature SMA components such as micron-sized sensors and actuators.