Study On Superelasticity,Magnetoelastic Coupling And Magnetocaloric Effect Of Magnetic Shape Memory Alloys

As one kind of novel smart materials,magnetic shape memory alloys have been extensively studied because of the excellent multi-functionality including shape memoryeffect,superelasticity,magnetic-field-inducedstrain?MFIS?and magnetocaloric effect?MCE?.Ni-Mn-Ga alloy is representative of magnetic shape memory alloys,possessing large strain output of conventional shape memory alloys and fast response of magnetostrictive materials.The MFIS up to 10%is achieved by martensitic variant reorientation,making it promising to be used as actuators,sensors and damping materials.However,Ni-Mn-Ga alloy is brittle with low deformation ability.The large MFIS is only for single crystal while the polycrystalline materials exhibit negligible value,which severely restricts its developments and application in industry.In this work,a series of glass-coated Ni-Mn-Ga microwires are prepared by Taylor-Ulitovsky method,followed by annealing to obtain the bamboo-like structure so as to reduce the constraints of grain boundary on variant reorientation.The microstructures and properties are systematically studied to discover new laws and phenomena and explore innovative ways to promote the performance.Mn-Co-Ge alloy has drawn much attention because of its magnetic field-induced martensitic transformation and magnetocaloric properties in recent years.In this work,doping Ni/Fe is employed to study the effect of alloy elements on the phase transition temperature,crystal structure and magnetic structure and performance,to optimize their functional behaviors and provide experimental and theoretical support for designing and developing novel high-performance alloys.Moreover,based on synchrotron high-energy x-ray diffraction and neutron diffraction techniques,superelasticity of Ni-Mn-Ga microwires,magnetoelastic coupling of Ni doped MnCoGe and magnetocaloric properties of Fe doped MnCoGe alloys,are in-situ studied,respectively,to trace the evolution of crystal structure,micro-structure and lattice parameters under the stress field,temperature field and magnetic field to reveal the in-depth physical mechanism behind.A series of Ni-Mn-Ga microwires are prepared with different crystal structures,covering the five-layered modulated?5M?,seven-layered modulated?7M?and non-modulated?NM?martensite.The microwires are annealed properly to make the grains grow in order to obtain the bamboo-like structure and improve the brittleness.Furthermore,the microstructures,crystal structure,shape memory effect and superelasticity are systematically studied.It is found that microwires of Ni₅₃Mn₂₅Ga₂₂and Ni_51.5Mn_26.5Ga₂₂ exhibit shape memory effect and superelasticity at ambient temperature,respectively.In both alloys,there exists intermartensitic transition from seven-layered?7M?modulated martensite into non-modulated?NM?tetragonal martensite below the martensitic transformation temperature.For the microwire of Ni_49.2Mn_29.7Ga_21.1,it crystallizes in five-layered modulated?5M?martensite.Based on the results of stress-strain curves at various temperatures,the stress-temperature diagram is established.It is found that the microwire undergoes different transformation sequences such as P-NM?P-7M-NM?P-5M-7M-NM and 5M-7M-NM during tensile test with the decrease of performing temperatures.At free stress state,5M martensite is stable till the low temperature so that intermartensitic transition cannot take place on cooling.For the microwire of Ni₅₀Mn_28.6Ga_21.4,it consists of bamboo grains with tetragonal martensite matrix and coarse?precipitates,exhibiting fully reversible superelastic behavior up to 4%tensile strain.Upon multiple tensile load-unload cycles,reproducible stress fluctuations of³ MPa are measured on the loading superelastic stress plateau of⁵0 MPa.During cycles at various temperatures spanning-70 to 55?,the plateau stress decreases from 58 to 48 MPa near linearly with increasing temperature.Based on synchrotron high-energy x-ray diffraction technique,the evolution of the crystal structure,micro-structure and lattice parameters during uniaxial tensile test are in-situ studied to reveal the mechanism for the superelasticity and fluctuations on the stress plateau.We conclude that the superelastic behavior is due to reversible martensite variant reorientation?i.e.,reversible twinning?with lattice rotation of¹3o.The reproducible stress plateau fluctuations are assigned to reversible twinning at well-defined locations along the microwire.The strain recovery during unloading is attributed to reverse twinning,driven by the internal stress generated on loading between the elastic?precipitates and the twinning martensite matrix.The temperature dependence of the twining stress on loading is related to the change in tetragonality of the martensite,as measured by x-ray diffraction.Based on neutron diffraction combining with synchrotron high-energy x-ray diffraction technique,the evolution of crystal structure and magnetic structure of Ni doped MnCoGe alloy are in-situ studied to demonstrate the existence of strong magnetoelastic coupling.It is found that MnNi_0.62Co_0.38Ge alloy exhibits paramagnetic,ferromagnetic and anti-ferromagnetic state on cooling without any structural transformation.With the increase of applied magnetic field at 20 K,50 K and 150K,anti-ferromagnetic transforms into ferromagnetic state,i.e.the spiral moments tend to align parallel,associated with the decrease of d_spacing of?004?and?020?.The magnetic-field-induced strain is at the range of 1000-1600 ppm.Substitution of Fe for Mn atoms in Mn–Co–Ge alloys is employed for tailoring concurrent structural and magnetic phase transitions.The thermal,structural and magnetic properties are systematically studies for the Mn_1-x Fe_xCoGe?x=0.05,0.09,0.1,0.11,0.15,0.2,0.25?and MnCo_1-x Fe_xGe alloys?x=0.05,0.1,0.15,0.2,0.21,0.22,0.25?.It is demonstrated that substitution of Fe atoms for Mn or Co obviously decrease the martensitic transformation temperatures,resulting in magnetostructural transition from paramagnetic hexagonal Ni₂In-type parent phase to ferromagnetic orthorhombic TiNiSi-type martensite.Giant magnetocaloric effect of|?S_M|=9.74J·kg^-1·K^-1 and|?S_M|=10.97 J·kg^-1·K^-1 for Mn_0.89Fe_0.11CoGe and MnCo_0.78Fe_0.22Ge alloy are obtained under magnetic field change of 5 T in the vicinity of phase transition temperature.In-situ synchrotron high-energy X-ray diffraction experiments were conducted to reveal the detailed change in crystallographic structure of phases and the effect of applied magnetic field on phase transformation behaviors.An anomalously huge strain of 11.89%and volume expansion of 4.35%in unit-cell were obtained between martensite and parent phase across the transformation.Furthermore,the magnetic field-induced martensitic transformation was directly evidenced at 250K,which eventually demonstrates the possibility to achieve magnetic-field-induced strain and large magnetocaloric effect simultaneously.