Study On Evolution Of Microstructure Of Ni-Mn-Ga Alloys Directionally Solidified Under Static Magnetic Field

Ni-Mn-Ga alloys,as typical ferromagnetic shape memory alloys(FSMAs),have potential for sensor and actuator applications due to its large output and quick response under magnetic field.So far,a lot of work has been done to study the microstructure and performance in Ni-Mn-Ga alloys.However,these researches almost focused on the final microstructure and corresponding performance.Very little attention has been paid to the growth control by introducing external fields(e.g.magnetic field,or electric field)during solidification process,aiming to provide a novel method to modulate the Ni-Mn-Ga alloy microstructure.In this work,we devote ourselves into pursuing the evolution of microstructure,segregation during directional solidification under a magnetic field and the corresponding orientation and martensitic variant distribution after cooling to room temperature,and investigating the magnetic field induced melt convection during directional solidification numerically.By means of electron backscatter diffraction(EBSD),the detwinning process of a non-modulated(NM)martensite during the consequent uniaxial compression is shown and analyzed in detail.Some valuable results are summarized as follows:The evolutions of microsegregation and variant distribution in directionally solidified Ni-Mn-Ga alloys under an axial magnetic field have been investigated experimentally.The results show that upon the application of a magnetic field,the cell/dendrite is refined and microsegregation is reduced.Below 2 T,the results are enhanced by an increase in the magnetic field strength.Numerical simulation reveals that the applied magnetic field induced the thermoelectric magnetic convection(TEMC)at the cell/dendrite scale,and this convection is responsible for cell/dendrite refinement and amalgamation.Moreover,under a magnetic field(?2 T),the preferred orientation(i.e.,the <001> crystal direction oriented along the solidification direction)of an austenitic grain is maintained when the growth speed increases up to 20 ?m/s.For NM martensite variant distribution,three possible variant groups emerge in the directionally solidified sample under a magnetic field,unlike one variant group observed in the absence of a magnetic field.EBSD analysis revealed that the variant groups are composed of differently orientated martensite variants derived from a single crystal of austenite.Theoretical calculation revealed that the preferential martensitic configuration without a magnetic field is caused by the so-called martensitic-transformation-induced stress due to the pronounced microsegregation.It has been found that the application of transverse magnetic field has a great influence on the microstructure,macrosegregation and the distribution of crystal phase in directionally solidified Ni-Mn-Ga alloys.It has been observed that the application of magnetic field refined the dendrites and changed the symmetrical morphology of liquid/solid interface,resulting in a concave on one side of the specimen.The energy dispersive spectroscopy(EDS)results shows that the above-mentioned areas and “freckcle” areas near liquid/solid interface are rich in Mn element.Furthermore,numerical simulation shows during the directional solidification the magnetic field induces TEMC on the specimen scales and the dendritic scales,which are contributed to the macrosegregation and refinement of dendrites.Meanwhile,the TEMCs in both scales increase with the increase of magnetic field intensity(up to 0.8T),which are agreed with our experimental results.Moreover,it can be found that NM martensitic clusters exist in the austenite matrix due to the segregation induced by magnetic field.Further EBSD analysis suggests that each martensitic cluster has four martensitic plates,which are composed of alternatingly distributed thick and thin martensitic variants.This research is beneficial to the studying on the nucleation and evolution of NM martensite.An experimental observation of the detwinning of hierarchically structured martensitic variants in a directionally solidified non-modulated Ni-Mn-Ga alloy under uniaxial loading parallel to the parent [001]A orientation has been presented.There exist two typical martensite variant groups,composed of parallel plates with straight edges(Group I)and non-parallel plates with bent edges(Group II).The NM martensite plates consist of alternately distributed major and minor variants.After each compression(i.e.,cumulative strain up to 3%,7.3% and 10%),the crystallographic orientations are obtained by manually indexing the Kikuchi patterns and the corresponding twinning relationships between the intra-plate major-minor variants and the inter-plate major-major variants are also calculated.The results indicate that the detwinning occurs on the twins with high Schmid factor,resulting in a martensite only composed of favorable variants with ?110?NM orientation parallel to the compression direction.The findings bring about not only a comprehensive understanding to the martensitic variant detwinning in hierarchical structure but also a guideline for martensite training.