Phase Stability,Martensitic Transformation And Magnetic Properties Of Ni-Mn-In Alloys

Ni-Mn-In magnetic shape memory alloys have drawn great attention in recent years due to its large magnetic field induced stress and strain,giant magnetocaloric,giant magnetoresistance and exchange bias effect.This alloy possesses excellent physical properties and great development potential for devices especially served as sensors,magnetic micro-electro-mechanical systems and actuators.During the past ten years,researchers have proved that only the modulated martensite phase of Ni-Mn-In alloy can exhibit excellent physical properties.However,the theoretical study of it only stays in the austenite(A)and non-modulated(NM)martensite phase due to a lack of accurate atomic positions and modulated informations.In 2015,the crystal structure of six-layered modulated(6M)martensite was determined by high-resolution powder neutron diffraction in the frame of superspace theory.The crystal structure of modulated martensite was finally refined by the Rietveld method.Based on this,the first-principles calculations were performed by the Vienna Ab initio Simulation Package(VASP).The formation energies of austenitic and martensitic phases(6M and NM)for the Ni24Mn12+xIn12-x alloys and their energy difference,magnetic moment and electronic structure have been calculated.The relationship among composition,structure and performance was analyzed by the combination of theoretical and experimental method.The main innovative work and results are listed as follows:(1)The preferred site occupation of excess Mn atoms in the Ni-Mn-In alloy was investigated by the first-principles calculation method.It was confirmed that the Mn atom tend to occupy the In atom position directly,and the distribution of excess Mn atom in Ni-Mn-In supercell was revealed.(2)The formation energies of austenite and martensite(6M and NM)phases of the Ni24Mn12+xIn12-x(x=3,4,5,6,7)alloys were calculated systematically.By calculating the energy difference ?E1 between paramagnetic austenite and ferromagnetic austenite,and the energy difference ?E2 between parent phase and martensite phase of Ni-Mn-In alloy,the internal mechanism of Curie temperature TC and martensite transformation temperature TM changing with chemical composition is expounded.The predicted critical composition of martensitic transformation is in good agreement with the experimental data.The first-principles calculation results show a magnetic-structural coupling transformation in Ni24Mn12+xIn12-x alloy when x?4.(3)The total and atomic magnetic moments of austenite and martensite phases(6M and NM)for Ni24Mn12+xIn12-x alloys have been calculated.The total magnetic moment is mainly contributed by Mn and Ni atoms.The total magnetic moment of parent austenite increases with the Mn content,while decreases in the martensitic phase.The antiferromagnetic coupling between MnMn and MnIn atoms in the martensite phase is mainly caused by the shortening atomic distance between neighboring Mn atoms due to the demagnetization effect.(4)The DSC analysis shows that TM increases with the Mn content.The comprehensive analysis of XRD,SEM and M-T curves shows that there is a magnetic-structural coupling transition in Ni24Mn12+xIn12-x alloy when x>4,which proves the rationality of the first-principles calculations.The calculated results show that the Ni24Mn12+xIn12-x(5?x?7)alloys are the optimal component interval for the magnetic-structural coupling transition,and the results are verified by experiments.The transformation temperature of Ni24Mn17In7 alloy(x=5)is close to room temperature,and the TM can be controlled by the magnetic field.The ?T will change 6 K when saturation magnetization changes ?M 74 emu/g.The entropy change of 37.86 J·kg-1·K-1 of Ni24Mn17In7 alloy can be obtained by martensitic transformation,and it increases significantly with the increasing of Mn content.The above research can not only fill in the blank of martensitic transformation mechanism including modulated martensite of the Ni-Mn-In alloys but also provide a theoretical basis for composition design and property optimization.