Twinning behaviour of textured polychrystalline Ni-Mn-Ga alloy after hot extrusion

Up to now most of the research was concentrated on single crystals. However, there is a great interest in polycrystals for technical applications. It was proven, that coarse grained Ni-Mn-Ga polycrystals with solidification texture show magnetic-field-induced strain (MFIS) by twin boundary motion as large as 1 % after proper treatment including two step annealing and mechanical training [1]. Recently textured polycrystalline Ni50Mn29Ga21 rods of 25 mm diameter and 400 mm length were prepared by hot extrusion. The fibre texture of Ni-Mn-Ga samples has been measured with high-energy synchrotron radiation and neutron diffraction. The presence of martensitic twins can be clearly seen within the recrystallized grains. A new experimental set-up for multi-step heat treatments was used for the optimization of the microstructure. The development of the microstructure was analysed between the steps of the heat treatment by Electron Back Scatter Diffraction (EBSD). Finally, the mechanical training of cubic Ni-Mn-Ga samples leads to a high strain of several % by mechanically-induced motion of twin boundaries. The twinning behaviour is discussed with respect to texture, grain size and phase transformations including variant selection of martensite. This work is supported by DFG within SPP 1239.

Up to now most of the research was concentrated on single crystals. However, there is a great interest in polycrystals for technical applications. It was proven, that coarse grained Ni-Mn-Ga polycrystals with solidification texture show magnetic-field-induced strain (MFIS) by twin boundary motion as large as 1 % after proper treatment including two step annealing and mechanical training [1]. Recently textured polycrystalline Ni50Mn29Ga21 rods of 25 mm diameter and 400 mm length were prepared by hot extrusion. The fibre texture of Ni-Mn-Ga samples has been measured with high-energy synchrotron radiation and neutron diffraction. The presence of martensitic twins can be clearly seen within the recrystallized grains. A new experimental set-up for multi-step heat treatments was used for the optimization of the microstructure. The development of the microstructure was analysed between the steps of the heat treatment by Electron Back Scatter Diffraction (EBSD). Finally, the mechanical training of cubic Ni-Mn-Ga samples leads to a high strain of several % by mechanically-induced motion of twin boundaries. The twinning behaviour is discussed with respect to texture, grain size and phase transformations including variant selection of martensite. This work is supported by DFG within SPP 1239.