Effect of initial plastic strain on mechanical training of non-modulated Ni-Mn-Ga martensite structure

The influence of plastic pre-straining on the mechanical training process of Ni-Mn-Ga single crystals with a non-modulated martensite structure was examined using uniaxial quasi static compression tests and electron backscatter diffraction technique. Firstly, the optimal pre-straining temperature, for which a large plastic strain can be imposed to as-grown crystals with low flow stress and low rate of strain hardening, was established. Then, the maximum value of plastic pre-straining which allows performing successful room temperature mechanical training was found to be of about 20% of total sample thickness reduction. Below this value, training process leads to single variant state, which is able to accommodate true plastic strain of about 0.14 in each step of further training. Above 20% of deformation a multiple martensite variant state of characteristic triangular arrangements is generated. The latter structure cannot practically afford any plastic accommodation during further training; thus the training process fails to operate.