Control of microstructure using magnetic fields and study of the mechanical behavior of Ni-rich Ni-Mn-Ga alloys. (15th October 2020) - Hou, Long
Abstract: In the present work, the influence of an axial magnetic field on the structure of directionally solidified Ni-Mn-Ga alloys is investigated experimentally. It has been found that a radial graded distribution of the gamma (γ) phase and grain fragmentation of the martensite/γ phase can occur under a magnetic field. Indeed, the structures, including a single martensite zone and martensite/γ phase cellular and eutectic mixed zones, are seen under a weak magnetic field ( B ≤ 2 T). Under a strong magnetic field ( B ≥ 4 T), the grain fragmentation of the martensite/γ phase occurs. Moreover, the application of a strong magnetic field induces a structural distortion in the martensite and dislocation rearrangement inside the γ phase. On the basis of the above investigation, a unique sample that includes columnar and equiaxed grains was successfully prepared using magnetic-field-assisted directional solidification. Subsequently, the mechanical behavior and detwinning evolution of the Ni-Mn-Ga specimen under compressive loading were studied. A general criterion is proposed to elucidate the compressive-stress-induced detwinning evolution based on the Schmid factor and deformation gradient tensor. The γ phase significantly enhanced the mechanical behavior of both the columnar and equiaxed grain samples. In particular, the random distribution of the γ phase was more prone to hindering fracture, even resulting in a larger compression strain in the polycrystalline Ni-Mn-Ga sample. This work highlights the effect of a magnetic field on the microstructure and also provides an in-depth understanding of the detwinning mechanism in Ni-Mn-Ga alloys containing the γ phase. Graphical abstract: Image, graphical abstract