Superelastic cycling of Cu–Al–Ni shape memory alloy micropillars

Superelastic nanocompression tests are performed on different micropillars milled by focused ion beam from [0 0 1]-oriented single crystals of Cu–Al–Ni shape memory alloys. Over hundreds of cycles such micropillars exhibit reproducible superelastic behavior with complete recovery at stresses around 300 MPa and stress-induced transformation strains above 5%. Upon cycling, the critical stress to induce the transformation decreases, and the transformation strain increases, both signatures of a training effect which is analyzed here in terms of the microscopic mechanisms controlling the nucleation, growth and recovery of the martensite plates. The mechanical hysteresis is characterized through the energy dissipated during closed superelastic cycles, and its evolution during cycling is discussed. The superelastic cycling of the micropillars is also found to depend on the strain rate. The highest strain rate studied here, 10−1 s−1, is found to impinge upon the nucleation and growth kinetics of γ3′ and β3′ martensites, with the result that the mechanical hysteresis and transformation strain are reduced. Finally, specific cycling tests have been conducted at increasing maximum loads to analyze the limit for plastic deformation of the micropillar, which happens between 500 and 700 MPa.