Study On Phase Transformation And Magnetic Properties Of NiCoMnIn Magnetic Refrigeration Alloys

NiCoMnIn alloys exhibit excellent magnetocaloric effects resulting from the magneto-structural transition within a specific temperature range,which make them a promising room-temperature magnetic refrigeration materials.To further improve the properties,Ni45Co5.5Mn36.5In13,Ni45.2Co5.1Mn36.7In13 and Ni44.3Co6Mn36.7In13 were prepared and then subjected to the various second heat treatment.The effects of alloy composition and the second heat treatment on the martensitic transformation behavior and magnetic properties were studied with the scanning electron microscope,X-ray diffraction analysis,differential scanning calorimeter and vibrating sample magnetometer and other equipments.It was found that the martensite and austenite phases coexist in all samples at room temperature,irrespective of the composition and the heat treatment consition.The martensite structure is determined to be body-center tetragonal.For the samples after the second heat treatment,with increasing the treatment temperature,the volume fraction of martensite increases,while the volume fraction of austenite decreases.The second heat treatment could not introduce the second phase and change the crystal structure,but slightly change the lattice constant.All three alloys,including the as-annealed and that subjected to the second heat treatment,undergo one-step thermoelastic martensitic transformation.The phase transition temperatures decrease with the increase of Co/Ni ratio.For the samples after the second heat treatment,the phase transition temperatures increases gradually with the shift of the heat treatment temperature,reaching the highest value at 650?,and then decrease.Under the external magnetic field of 1 T,the magnetic field-induced martensitc reverse transformation(MFIMRT)takes place in all Ni₄₅Co_5.5Mn_36.5In₁₃ alloy samples.After the second heat treatment,the applied field leads to the decrease of transformation temperatures suggesting that such treatments effectively promote the possibility of MFIMRT.After heat treatment at650?for 10min,the dependence of transformation temperature on the magnetic field is the maximum reaching 28 K/T which is about 7 times larger than that of the as-annealed sample.The strongest effect of the magnetic field on the magnetization is found in Ni_45.2Co_5.1Mn_36.7In₁₃alloy after treatment at 350?for 10min.As compared with the as-annealed smaple,the magnetization difference between the martenstie and the austenite is improved by 96 emu/g.For the as-annealed samples,the adiabatic temperature change of Ni_45.2Co_5.1Mn_36.7In₁₃ is the largest,which is-1.9k.After the second heat treatment,the magnetic entropy change is obviously improved and reaches the maximum value of 23 J/kg K at 350?.Meanwhile,the magnetic refrigeration capacity is up to 230 J/kg.