Research On Microstructure And Anisotropic Properties Of Polycrystalline Ni-Mn-Ga Alloys

Ni-Mn-Ga shape memory alloys show various excellent properties,such as magnetic-field-induced strain(MFIS)and magnetocaloric effect(MCE),however,the polycrystalline alloys exhibit significantly lower performance compared to single crystals.In the present thesis,the high-temperature deformation and functional properties of the textured polycrystalline Ni-Mn-Ga alloys were studied to analyze the effect of growth and recrystallization textures.The vacuum induction casting was employed to fabricate Ni-Mn-Ga alloys with columnar grains along crystal orientation,while the alloy rods with recrystallization texture were successfully obtained via hot extrusion technique.The anisotropy under high-temperature deformation,superelasticity,and elastocaloric effect(e CE)of the Ni-Mn-Ga alloys were investigated with different textures.Moreover,the MFIS and rotating magnetocaloric effect(RMCE)were also explored in the textured Ni-Mn-Ga alloys,which laid the foundation for the application of the low-cost and high-performance multifunctional Ni-Mn-Ga alloys.The Ni-Mn-Ga alloys with coarse columnar grains structure were prepared by casting method,in which the orientation of the grains was parallel to the <001> crystal direction.A fine equiaxed grain structure was achieved after hot extrusion at 1273K-1323 K with maximum extrusion ratio 16:1,where the extrusion direction was parallel to <111> crystal direction.The columnar grains with <001> texture were remained during the martensite in the Ni-Mn-Ga alloy,while the equiaxed grains not only kept the <111> texture but also exhibited a new type of texture where the extrusion direction was parallel to the twinning plane of martensite.It was found that the L21 structure of the Ni-Mn-Ga alloy with equiaxed grains occurred the ductile-to-brittle transition(DBTT)approaching the order-disorder transition point,and the B2 structure displayed superplasticity between 1073 K and 1223 K.The maximum elongation of 232.9% was obtained at 1073 K with a strain rate of 0.001s-1.The mechanism of the superplasticity was explained by the dislocation slip and dynamic recrystallization.The DBTT of the Ni-Mn-Ga alloy with columnar grains around the order-disorder transition point exhibited anisotropic characteristics.The L21 structure displayed brittle fracture and B2 structure showed ductile fracture when passing the order-disorder transition point in the Ni-Mn-Ga alloy with columnar grains perpendicular to <001> direction.However,both the L21 and B2 structures parallel to <001> direction exhibited the excellent plasticity,especially superplasticity for B2 structure.The mechanism of the superplasticity was attributed to the dislocation slip and dynamic recovery with the maximum elongation of 168.6% at 1073 K.The anisotropy in high-temperature mechanical properties of the Ni-Mn-Ga alloys was attributed to the constraints on dislocation slip caused by the columnar grain boundaries.It was demonstrated that the magnetocrystalline anisotropy energy of the Ni-Mn-Ga alloy with columnar grains can be enhanced after mechanical training,superplastic training,and thermal-magnetic training.The “c” axis of martensite was parallel to the external field,where the twinning variants decreased during superplastic training and thermal-magnetic training.The mechanical training decreased the twinning variants and the main twins were distributed in the 2-dimensional training plane.The MFIS was increased after training.It was found that the MFIS in the present alloy consists of both reversible and irreversible parts.The irreversible MFIS could be recovered after applying the magnetic field perpendicular to the initial direction after mechanical training.The Ni-Mn-Ga alloy with equiaxed grains displayed very small MFIS due to the low magnetocrystalline anisotropy energy and the large critical twining stress.By contrast,the Ni-Mn-Ga alloy with columnar grains parallel to <001> direction displayed better superelasticity,higher superelastic strain,lower critical stress,and less stress hysteresis than those perpendicular to <001> direction.However,the stress-induced martensite transformation is difficult along <111> direction,thus the Ni-Mn-Ga alloy with equiaxed grains displayed the linear superelastic behavior both parallel and perpendicular to the <111> direction.The maximum adiabatic temperature change for e CE was 7.4 K in the Ni-Mn-Ga alloy with columnar grains along <001> direction under 150 MPa,which was very close to the theoretical value.Furthermore,it also displayed higher specific caloric effects when compared with other candidate material for elastocaloric refrigeration.However,it exhibited low fatigue resistance because of the crack initiation at the grain boundary and thus the e CE was failed at 50 cycles.While good cyclic stability of e CE up to 100 cycles was attained when applying the stress perpendicular to the <001> direction due to the small superelastic strain,low energy dissipation,and higher crack initiation/propagation resistances.On the other hand,the Ni-Mn-Ga alloy with equiaxed grains exhibited enhanced cyclic stability of e CE for more than 250 cycles under 300 MPa mainly due to grain refinement.The Ni-Mn-Ga alloy with columnar grains after superelastic training showed conventional(-9.2 J/kg·K under 30 k Oe)and inverse(2.5 J/kg·K under 5 k Oe)MCE when the magnetic field was applied parallel and perpendicular to <001>,respectively.Taking advantage of the strong magnetocrystalline anisotropy after superelastic training,the rotating MCE could first obtain in the Ni-Mn-Ga alloy with columnar grains.The maximum rotating magnetic-field-induced entropy change reached 7.3 and 4.2 J/kg·K under 30 and 20 k Oe,respectively.These facts indicate that Ni-Mn-Ga alloy may be a promising magnetic refrigerant around room temperature.