Branching of {110) twin boundaries in five-layered Ni-Mn-Ga bent single crystals


Two bending tests around two perpendicular axes were applied to 10M Ni-Mn-Ga single crystals with five-layered modulated structure. The crystal structure and microstructure evolution were examined using synchrotron radiation and electron backscatter diffraction, respectively. The bend stress results in pseudoelasto-plastic strain due to {101) twins tapering. A close examination of the microstructure reveals an additional pattern indicating microstructural changes in the form of {110) twins. As bending proceeds the {110) twins branch undergoing a significant twin refinement. Additionally, an elastic change of lattice parameters is confirmed yielding a higher total pseudoelastic strain. Unloading restores the initial twin configuration removing a large amount of the {110) twin boundaries, however, this process is followed by incomplete recovery since the samples do not retain its original shape entirely. The paper underlines the differences in mechanism for bending around two perpendicular axes explaining the amount of strain observed under pseudoelastic deformation. Additionally, the results are discussed with respect to minimization of elastic energy due to twin refinement and branching as well as mobility of the {101) and {110) twin boundaries.
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