Evolution of local strain bands of different orientation in single crystalline NiMnGa foils under tension

We investigate the mechanically induced local deformation effects in single crystalline Ni-Mn-Ga stripes by in situ Digital Image Correlation (DIC). Ni-Mn-Ga stripes of 100 and 200 mm thickness and lateral dimensions of 15 mm × 3 mm are fabricated by cutting and electropolishing a Ni-Mn-Ga bulk single crystal having a 10 M martensite structure. Application of tensile stress causes reorientation of martensite variants being associated with the formation and growth of local strain bands. The true strain in the bands is determined to be about 6% indicating complete local variant reorientation. The stress-strain characteristics are strongly affected by the mechanical boundary conditions. (1) Fixation or clamping the foil specimens during training in a magnetic field results in the formation of a stationary twin boundary (TB) being oriented by 45° with respect to the specimen edge. Subsequent tensile loading causes formation of a strain band with 45° orientation extending in both lateral directions through TB motion. The corresponding twinning stress is about 2 MPa. (2) Bending in out-of-plane direction causes formation of a stationary TB being oriented by about 84° with respect to the specimen edge. In this case, tensile loading causes formation of a strain band with 84° orientation. The corresponding twinning stress is reduced to about 0.7 MPa.