Microstructure Regulation Of NiMn-based Shape Memory Alloys

The output strain of Ni-Mn-Co-In alloy is same as Ni-Mn-Ga alloy,and the output stress is about 50 times of Ni-Mn-Ga alloy.It has excellent performance.At the same time,it is found that the d-p covalent bond formed between the main group elements occupying the D crystal site and the transition group elements at the A/C site of its nearest neighbor has the same effect on the phase formation and desired stability of the Heusler alloy,and the d-d covalent bond in the full shell layer has the same effect.The Ni-Mn-Ti system all d-metal Heusler alloy without the main group elements has been developed.Ferromagnetic coupling of parent phase is effectively established by doping Co element.It breaks people's understanding that Heusler alloy must contain main group elements.Firstly,the alloy with nominal composition of Ni45Mn36.7Co5In13.3 and Mn50Ni32CO8Ti10 was melted by arc melting,and then the alloy was mechanically trained by an electronic universal testing machine.The samples were examined by SEM,X-ray diffraction,DSC and VSM.The following conclusions are drawn:(1)We found that Ni45Mn36.7Co5In13.3 and Mn50Ni32CO8Ti10 alloys were all in austenite state at room temperature after smelting.Their transformation temperatures is-2.3? and-52.0?,and their saturation magnetization is 57.96 emu/g and 21.9 emu/g,respectively.The microstructures of the alloys are all austenite,but there has a second phases in the Mn50Ni32Co8Ti10 alloy.(2)The stress-strain curves obtained from mechanical training of the alloy shows that there is a platform for each cycle of the training curve during the whole mechanical training process,which indicates that the stress-induced martensite transformation occurs in the process,and the stress threshold value of the stress-induced martensite will be reduced during the mechanical training process,which is more likely to occur.The stress value needed to reach 4%strain increases first and then decreases,which is due to the reduction of stress critical value,which makes transformation easier.At the same time,training makes a small number of defects in the alloy become the nucleation center of martensitic transformation,and then the irreversible plastic deformation in the alloy increases,which makes deformation more difficult.The stress lag of Mn50Ni32Co8Ti10 alloy is very small because of the pinning effect of internal precipitation on austenite.(3)After mechanical training,Martensite was found in Ni45Mn36.7Co5In13.3 alloy,which indicates that mechanical training can increase martensite transformation temperature,and martensite is more and more with the increase of training times.Two-way training has the same effect as one-way training.The precipitates in Mn50Ni32Co8Ti10 alloy increase and the austenite grain size decreases.(4)After mechanical training,the magnetic properties of Ni45Mn36.7Co5In13.3 and Mn50Ni32Co8Ti10 alloys have changed,their maximum magnetization has been greatly increased,and the magnetocrystalline anisotropy has also been enhanced.