Study On Microstructure Of Directionally Solidified Ni-Mn-Ga Alloys And The Evolution Affected By Magnetic Field

Ni-Mn-Ga ferromagnetic shape memory alloys equipped with high output strain andhuge response frequency not only own the characteristic of the traditional thermal induced SMAs,but also can change shape under the effect of a magnetic field,which has already absorbed wide attentions.The Ni-rich Ni-Mn-Ga high temperature shape memory alloys?HTSMAs?have attracted much research interests due to high martensitic transformation temperature?up to 350°C?and a much better ductility?over30%strain?than demonstrated by the typically brittle Ni-lean Ni-Mn-Ga alloys.However,the hierarchically structured martensite variants show special orientation distribution during fabrication and training process due to the multiple twins structures.In the process of exploring this series of complex microscopic mysteries,Ni-Mn-Ga alloys with many characteristic morphologies and microstructures which are different from the previous studies have been obtained by using directional solidification,melt-spinning,magnetic field and other preparation methods.It created a new point of view to affect the composition segregation,martensitic transformation and detwinning behavior of Ni-Mn-Ga alloy by a magnetic field-assisted directional solidification.The mysterious veil of the colorful martensite microworld is uncovered and the orientation relationship of crystallization is verified by a large number of the scanning tracing using energy spectrum?EDS?and electron backscattering diffraction?EBSD?;The dislocation and distortion at the atomic level are understood by transmission electron microscope?TEM?based on the inversion space;The evolution mechanism behind the wonderful and interesting martensitic transformation and detwinning behavior are interpreted by the material testing machine loading the external stress.The main research contents are as follows:Ni-Mn-Ga alloys with three kinds of segregations were designed:?1?A Ni-Mn-Ga alloy rod with significant axial compositional gradient has been designed by directional solidification at a low growth speed.The axial distributions of Ni and Ga gradually decrease and that of Mn gradually increases,which agrees with Burton-Prim-Slichter relationship.The axial crystal structure changes from austenite to martensite accordingly,and the crystal orientation relationship and the microstructure evolution near the phase interface have been investigated by EBSD and TEM.Moreover,the saturation magnetization along the solidification direction decreases due to the enhancement of anti-ferromagnetic coupling between Mn ions.?2?A kind of Ni-Mn-Ga alloy consisting of orderly arrayed austenite and non-modulated?NM?martensite was designed and directionally solidified.The austenite in dendrite core and the martensite in the inter-dendritic space are shown due to the micro-segregation.?3?A substructure was observed in the?phase in the Ni-rich Ni-Mn-Ga alloy bulk.The suppression of the?phase in two compositions of Ni₅₈Mn₂₅Ga₁₇ and Ni₆₀Mn₂₅Ga₁₅alloys and the formation of the induced new phase are studied.The evolution mechanism of the martensite and the effects of the magnetic field are studied:?1?Three distinct structures comprising self-accommodated solid zone,?110?_M preferred oriented liquid zone and?001?_M oriented single variant zone were discovered in directionally solidified Ni-Mn-Ga alloy rod.The preferred?110?_Morientation in liquid zone directly causes a special single variant zone to form around quenching interface.It is attributed to a longitudinal,suddenly-shrinking lattice,produced in the liquid zone after martensitic transformation,which induces a tension force at the joint.A tensile test simulates and validates the formation of single variant induced by inner force.A mechanism of variant formation is described in the present study,too.?2?In the process of directional solidification with higher growth speed,the magnetic field induces the fragment of the dendrite and the growth of austenite in the form of equiaxed crystal.The preferred orientation of?110?_M martensite matrix is destroyed,and a large number of self-accommodated variants with different orientations are formed due to the discrete distribution of?phase,.The phase distribution with three gradations along radial direction is formed in Ni₅₈Mn₂₅Ga₁₇ alloy owing to the effect of magnetic field.The compositions of?phase and martensite are not been changed by the magnetic field,but the volume fraction of?phase in different regions did.?3?The dislocations of?phase in the directionally solidified Ni₅₈Mn₂₅Ga₁₇alloy are arranged in two directions,while strain energy is released and regular arrangement of slip bands are disrupted after applying the magnetic field.At the same time,the application of magnetic field in the process of directional solidification makes the internal stress more complex.The comprehensive effect leads to the detwinning behavior in the partial areas.The stress-induced martensitic transformation and detwinning behavior of the alloys prepared with various magnetic field conditions are studied:?1?The influences of step-wise uniaxial compression on martensitic transformation and detwinning processes of the above-mentioned sample were investigated by the EBSD tracing.Experimental results show that the stress has induced martensitic transformation at ambient temperature,which is completed after the three cycles.This should be attributed to the increase of martensitic transformation temperature under compression.Moreover,the stress has caused detwinning of the twins with high Schmid factor,resulting in the martensite only composed of favorable variants with<110>_Morientation parallel to the compression axis.The dislocation density decreases during stress-induced processes.?2?The Ni₅₈Mn₂₅Ga₁₇ alloy as a high temperature shape memory alloy has been directionally solidified,which is consisted of non-modulated?NM?martensite with preferred orientation and?precipitate with a three-dimensional net structure.It has been captured how the hierarchically structured martensite variants reorient and the effect of the surrounding?phase on detwinning process with the increase of compression strain.It can be found that the volume fraction of the original minor lamella gradually increases and then it occupies the whole plate.The detwinning of new microtwins is not observed during further compression.The results indicate that the move of the intra-plate boundaries is attributed to a higher Schmid factor of the nanotwins.The mobility of the new inter-plate boundary is mainly dependent on the preferred orientation of the lamella inside martensite variants.?3?The compressive strength of Ni-rich Ni-Mn-Ga alloy with discrete?phase prepared under the magnetic field is not lower than that of the single crystal sample with the regular net shape?phase.The presence of?phase blocks the rearrangement of the martensite variant.There is no preferred orientation related to stress due to the low anisotropy.The long axis c of the martensite tends to be arranged perpendicular to the stress direction when the compressive stress is applied,and the long axis c tends to be arranged parallel to the stress direction when the tensile stress acts.The variant with smaller value of deformation gradient tensor along compression stress is preferred and retained.The variant with larger value of deformation gradient tensor along tensile stress is preferred and retained.