Martensitic Transformation And Properties Of Doped Ni-Mn-Sn Magnetic Shape Memory Alloys

Ni-Mn-Sn magnetic shape memory alloys has thermoelastic martensite shape memory effect and can be induced by magnetic field,and they are a new type of intelligent material that integrates sensing and driving.However,they suffer from the drawbacks of the low operating temperature and the poor mechanical properties.So far,none of Ni-Mn-Sn alloys can meet these requirements simultaneously.In this paper,the mechanical properties of Ni-Mn-Sn alloy were improved by doping rare earth?Tb,Dy,Y?and transition group metal Cu.The effects of rare earth and Cu doping on microstructure,martensitic transformation,mechanical properties and magnetic properties of the alloys were studied systematically.On this basis,by means of Cu and Co co-doping,the working temperature range of Ni-Mn-Sn magnetic memory alloy is widened while the brittleness of the alloy is improved.It was found that the rare earth elements Tb,Dy and Y all refined the grain structure of the alloy,and the grain size gradually decreased with the increase of rare earth doping content.The addition of rare earth elements leads to the formation of a second phase riched in rare earth in the matrix of Ni-Mn-Sn-RE alloy.With the increase of rare earth doping content,the number and volume fraction of the second phase increase gradually.When the content is 2.0 at.%,the second phase is mainly distributed along the grain boundary.When the doping content reached 5.0 at.%,the matrix and the second phase presented eutectic microstructure characteristics.The temperature of martensite phase transition of the alloy was significantly affected by rare earth doping.The influence of Y on martensitic transformation temperature is greatest,and the increase as high as522.4?.The formation of the second phase rich in rare earth results in the increase of Mn content in the matrix,which is the main reason for the increase of the martensitic transformation temperature.The compressive tests at room temperature show that the rare earth content has a great influence on the compressive fracture strength and strain of the alloy.The compressive fracture strength and strain increase first and then decrease with the increase of the rare earth content.The addition of rare earth elements?Tb,Dy,Y?can improve the mechanical properties of the alloy.Among them,the rare earth element Dy has the most significant improvement on the mechanical properties.When the doping content of Dy is 2.0 at.%,the compressive fracture strength and compressive strain of Ni-Mn-Sn-Dy reach the maximum 562.3 MPa and 13.5%,respectively.Combined with microstructure and fracture morphology,it was found that the fracture morphology before rare earth doping presented brittle intergranular fracture characteristics.With the increase of doping content,the fracture gradually turns into transgranular cleavage fracture,and the toughness tearing edge appears on the fracture.However,when rare earth is overdope d,the fracture morphology is stripped along the phase boundary,leading to brittleness increase.The results show that Cu doping increases the fracture strength and strain of Ni-Mn-Sn alloy.When the doping content of Cu reaches 6.0 at.%,the compressive fracture strength and compressive strain of the alloy sample reach 546.7MPa and14.5%respectively.The main reason for the improvement of the mechanical properties of the alloy is that the second phase distributed along the grain boundary in the alloy matrix hinders the crack propagation.In addition,Cu doping can increase the martensite phase transition temperature of the alloy.With the increase of Cu content,the martensite phase transition temperature increases gradually.When Cu content is 6.0 at.%,the martenstic transformation temperature of Ni₄₇Mn₃₈Sn₉Cu₆ is 167.9?,with an increase of 150.0?.The first-principle calculation shows that the total energy difference between the paramagnetic state and the ferromagnetic state of the parent phase of Ni-Mn-Sn-Cu decreases when Cu replaces Sn,and the total energy difference between the parent phase and the martensite phase increases,resulting in the decrease of Curie temperature and the increase of martensite phase transition temperature.When Co replaces Ni,the total energy difference between the paramagnetic state and the ferromagnetic state of parent phase in Ni-Co-Mn-Sn increases,and the total energy difference between the parent phase and the martensite phase decreases,resulting in the increase of Curie temperature and the decrease of martensite phase transition temperature.By balancing the complementary effects of Cu and Co on martensitc transforamtion temperature and Curie temperature regulation,it was found that the alloys with 6.25 at.%Cu and 6.25 at.%-12.5 at.%Co may show the excellent properties we want.Based on the calculation results,Ni_48-x8-x Co_x Mn₃₇Sn₉Cu₆?x=0,6,8,10,12?alloy series were designed and prepared.Higher martensite phase transition temperature and Curie temperature?342.7 K and 375.3 K?were obtained in Ni₄₀Co₈Mn₃₇Sn₉Cu₆ alloy series,and they met the requirements of<₍?8?.At the same time,the compressive fracture strength and compressive strain of Ni₄₀Co₈Mn₃₇Sn₉Cu₆ alloy reached higher levels?1072.0 MPa and 11.9%?.These results show that Cu and Co co-doping can simultaneously improve the brittleness and broaden the working temperature range of Ni-Mn-Sn magnetic memory alloy,providing reference and ideas for optimizing the performance of Ni-Mn-Sn magnetic alloy and developing new magnetic shape memory alloy.