Nanomechanical Properties of Ni-Mn-Ga
At the macroscale, the magnetization and shape of nickel-manganese-gallium (Ni-Mn-Ga) can be modified by magnetic-field-induced and stress-induced twinning. As applications predicated on small feature sizes are developed, the mobility of twin boundaries in confined volumes becomes increasingly more important for controlled shape change. The relationship between nanoscale deformation and twin boundary motion provides the motivation for evaluating the nanomechanical properties of Ni-Mn-Ga. Using instrumented nanoindentation, the nanomechanical properties of Ni-Mn-Ga were evaluated as a function of the orientation of the crystallographic c direction: in-plane versus out-of-plane. The reduced elastic modulus for c oriented out-of-plane (124 ± 5 GPa) was 33% greater than for c oriented in-plane (93 ± 3 GPa). Load-displacement curves from indentations on in-plane regions exhibit pop-in during loading and greater non-linear recovery during unloading compared to out-of-plane regions. The difference in elastic recovery during unloading may be attributed to pseudo-elastic twinning. This detwinning phenomenon may be triggered by dislocation image forces resulting in partial recovery during unloading.
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