Martensitic Transformation And Magnetic Properties Of Ni-Mn-In-RE Magnetic Refrigeration Alloys

Ni-Mn-In alloy exhibit excellent magnetocaloric effects owing to the magneto-structural transition,which makes it a promising room-temperature magnetic refrigeration material.In present work,two rare earth elements,Dy and Er were added in order to improve the properties of Ni-Mn-In alloy.The microstructure,martensitic transformation behavior,magnetic properties and magnetocaloric effects of Ni-Mn-In-RE(RE=Dy,Er)alloys were studied with scanning electron microscope(SEM),X-ray diffractometer(XRD),differential scanning calorimetry(DSC),vibrating sample magnetometer(VSM)and other instruments.The influence of the doping of Dy and Er on the microstructure,martensitic transformation temperatures and magnetocaloric effects were revealed.It is shown that the grains of Ni-Mn-In alloys can be apparently refined by the addition of RE.The higher RE content,the smaller grains.Er is more effective for the grain refinement than Dy in case of the same content.Ni-Mn-In-RE alloys consists of the Ni-Mn-In matrix and the RE-rich second phase.With the increase of RE content,the volume fraction of the second phase increases gradually.The precipitation of the second phase leads to the change of the matrix composition,that is,the content of Ni remains almost unchanged,Mn content increases,and In content decreases.Ni-Mn-In-RE alloys undergo one-step thermoelastic martensitic transformation and its reverse one during cooling and heating.The addition of Dy and Er did not change the transformation sequence,while remarkably shifts the transformation temperatures and reduces the thermal hysteresis.The shift of transformation temperatures can be ascribed to the increase of valence electron concentration reuslting from the the increase of Mn content of the matrix.In addition,Ni-Mn-In-RE alloys possess excellent thermal cycling stability.The martensitic transformation and reverse transformation temperatures are almost kept constant even after ten thermal cycles.For Ni_49.8Mn₃₅In₁₅Dy_0.2and Ni_49.8Mn₃₅In₁₅Er_0.2 after the second heat treatment,the transformation temperatures first drops and then increases gradually with the shift of the heat treatment temperature.For Ni_49.8Mn₃₅In₁₅Dy_0.2,the transformation temperatures reach the highest value after heat treatment at 750?,while those of Ni_49.8Mn₃₅In₁₅Er_0.2 reache the highest after heat treatment at 650?.The reason can be attributed to the change of the order degree caused by the secondary heat treatment at different temperatures.The dependence of Curie temperature on the secondary heat treatment is reverse to the transformation temperatures.The magnetizations of the austenite and martensite were obviously weakened by the doping of Dy and Er due to the formation of nonmagnetic second phase and the increase of Mn content.With increasing RE content,refrigerant capacity,relative cooling power and the working temperature interval decrease.However,the maximum magnetic entropy change?S_Mrises by the addition of RE.The?S_M values of Ni_49.8Mn₃₅In_15.2-xDy_x and Ni_49.8Mn₃₅In_15.2-xEr_xalloys reach the maximum at x=0.4 at.%,which are 19.9 J/kg·K and 27.8 J/kg·K respectively under 5 T magnetic field,about 2 and 3 times higher than Ni_49.8Mn₃₅In_15.2without RE.