The Studying Of The Martensitic Transformation,Magnetocaloric And Magnetoresistance Effect Of Ni-Mn-Al Ferromagnetic Shape Memory Alloys

NiMn-based ferromagnetic shape memory alloys have attracted considerable attentions due to their multifunctional properties around martensite transformation,such as magnetic shape memory effect,magnetocaloric effect,magnetoresistance and elastocaloric effect.Based on these features,these alloys are attractive for some potential applications,such as sensors,information storage and magnetic refrigeration.Ni-Mn-Al alloys are more attractive because of its relatively low cost and higher ductility.In this paper,we systemically investigated the structure,martensite phase transition,magnetic properties and the mechanical behavior of these alloys.The main content is as following:?1?The polycrystalline Ni₄₀Co_xMn_44-xAl₁₆?x=5,7?alloys were prepared by the arc melting method.The structure,phase transformation and magnetic properties were investigated by X-ray diffraction?XRD?,Vibrating Sample Magnetometer?VSM?,and Physical Property Measurement System?PPMS?.These alloys are mainly consist of B2 and 10M phase at room temperature.With Co content increasing,the martensitic transformation temperature tends to higher temperature and the large magnetic change can be found through phase transformation.Since martensitic and austensite have different structure,the transport properties of the material change significantly with temperature.For x=5 and 7 samples,large values of magnetoresistance of-46.39%and-41.75%are achieved around their martensitic transformation temperature at the field of90 kOe,respectively,which show a large magnetoresistance effect.?2?The Ni_43.3Mn_31.5Fe_11.7Al_13.5 ribbon was prepared by melt-spinning technique and annealed for 10 min at 1373 K.The structure and magnetic properties have been systemically investigated.The results show that the ribbon surface exhibits large granular and some?phase is observed at the grain boundaries.By measuring the magnetic and transport properties,the magnetosturcturealtransformationfromferromagneticaustenite to weak-magnetic martensitic was confirmed in these ribbons.The large magnetoresistance of-7.5%in the vicinity of martensitic transformation temperature is achieved.The magnetoresistance could not return to its original state in the demagnetization process,which is caused by some austenite remains in a kinetically arrested state and could not transformed to martensitic phase.?3?In the Fe and Co elements co-doped Ni₄₀Co_10-xFe_xMn₃₃Al₁₇?x=4,6,8?alloys,we careful studied the influence of different Fe/Co ratio on the magnetic properties and magnetoresistance effect.We found that these alloys are mainly consist of B2 and?phase at room temperature,and the volume fraction of?phase increases gradually with the increase of Fe content.With the ratio of Fe/Co increases,the martensitic transformation temperature tends to lower temperature.The large magnetic entropy change value reaches 10.6,8.2 and 4.4J/kg K for the alloys of x=4,6 and 8,respectively,at the field change of 3 T.At the same time,around the martensite transformation temperature,the large values of magnetoresistance of-33.3%,-25.7%and-13.1%are achieved at the field of 3 T for x=4,6,8 alloys,respectively.The compressive strength and compressive strain of alloys is notably enhanced with the increased Fe content,which indicates that the presence of?phase improves the mechanical properties of these alloys.Hence,Ni-Co-Fe-Mn-Al alloys have wide operational temperature window,large magnetic entropy change and magnetoresistance effect,good mechanical properties,which make these alloys to be a potential material for multifunctional applications.