Research On Hot Deformation Behavior And Microstructure Of Polycrystalline Ni-Mn-Ga Alloys

In this paper, polycrystalline Ni-Mn-Ga alloys were synthesized by vacuum arcmelting. The hot deformation behavior and plastic deformability of polycrystallineNi-Mn-Ga alloy were investigated by hot compression and hot tensile, respectively.According to the hot deformation results, two hot deformation methods ofdeformation induced grain growth heat treatment and hot extrusion were used toobtain texture and coarse grains of polycrystalline Ni-Mn-Ga. The structureevolution of polycrystalline Ni-Mn-Ga after hot compression was studied by opticalmicroscope (OM) and electron backscatter diffraction (EBSD), which assisted thedeformation mechanism analyzation. The structural change of polycrystallineNi-Mn-Ga after hot tensile was analyzed by scan electron microscope (SEM) andoptical microscope (OM). Texture change in polycrystalline Ni-Mn-Ga alloys afterhot deformation was investigated by electron backscatter diffraction (EBSD) andX-ray diffraction (XRD).The higher plasticity of polycrystalline Ni-Mn-Ga alloys in high tempratruewas discovered despite its high brittleness in room temperature. With the increase oftrue strain, the true stress-true strain curves shown peak flow stresses quickly in hotcompression and then attained steady-state flow period. In high strain rate tests,flow stress peaked at the true strain of0.2, then decreased rapidly because ofmacro-failure. The flow stress of polycrystalline Ni-Mn-Ga alloys rose withincreasing strain rate and decreasing temperature. The softing mechanism ofpolycrystalline Ni-Mn-Ga alloys was dynamic recovery at low temperature anddynamic recrystallization at low strain rate in high temperature range. Both dynamicrecovery and dynamic recrystallization existed at middle and high strain in hightemperature.Flow stress constitutive relationship and hot working map of polycrystallineNi-Mn-Ga alloy were established according to the results of hot compression testswhich carried out at a temperature range of600to1000oC and strain rate range of0.001/s to1/s. The deformation activation energy of polycrystalline Ni-Mn-Ga alloycalculated from the constitutive relationship was321.39kJ/mol. From the hotworking map, an ideal shaping area was obtained. The ideal temperature for shapingranged from750to800oC and950to1000oC where the stain rate was between0.03/s~0.3/s. Meanwhile, it would be hard for shaping when the strain rate washigher than0.3/s.Polycrystalline Ni-Mn-Ga alloys showed high plasticity at a high temperaturehigher than750℃during hot tensile tests. The largest elongation of polycrystalline Ni-Mn-Ga alloys was101.5%. Both intergranular and transcrystalline fracture wereexisted at750oC while it was almost plastic fracture at800oC. Grains were grownexcessively after hot tensile at800oC and formed oligocrystal. The sample onlyincluded elongated oligocrystal parallel to tensile direction. The mechanism ofpolycrystalline Ni-Mn-Ga alloys was intergranular dislocation sliding and climbing.The polycrystalline Ni-Mn-Ga alloys consisted coarse grains after deformationinduced grain growth heat treatment. The process of grain growth was promotedwhen deformation was carried out before heat preservation and could be suppressedif stress was applied during heat preservation. The polycrystalline Ni-Mn-Ga alloysinvolved <111> fiber texture parallel to compressive direction after deformationinduced grain growth heat treatment. Dynamic recrystallization existed after hotextrusion which lead to fine equiaxed grain generation. The preferred orientation ofgrains became more sharp with the increasing extrusion ratio, which form <111>fiber texture along the extrusion direction. Twin variants were prone to parallel tothe extrusion direction after martensite transformation.