Martensitic Transformation And Magnetic Properties Of Ni-Co-Mn-In-Gd Ferromagnetic Shape Memory Alloys

Polycrystalline NiCoMnInGd ferromagnetic shape memory alloys are prepared by substituting Gd for In.Microstructures,crystal structure,the martensitic transformation behavior,and properties of Ni-Co-Mn-In-Gd alloys have been investigated systematically by means of OM,SEM,XRD,DSC,PPMS,compressive tests.The influence of Gd content doping on phase constitute,phase transformation,mechanical properties and magnetic properties are illuminated.It was found that the doping of the rare earth element Gd changed the microstructure of Ni-Co-Mn-In alloy.Ni₄₅Co₅Mn₃₅In_15-xGd_x?0.8?X?1.6?alloys consists of the matrix and the Gd-rich phase.With the increase of Gd content,the Gd-rich phase grows and its volume fraction increases gradually.A part of the Gd-rich phase distribute mostly along the grain boundaries,whereas another one are inside the grains with a granular morphology.When the content of Gd is 1.6at.%,the Gd-rich phase along the grain boundaries grows and connects to each other.Local enrichment of the phase is also found.According to the EDS results of Ni₄₅Co₅Mn₃₅In_15-xGd_x alloys,it can be seen that the solubility of Gd in the matrix is very low.One-step thermoelastic martensitic transformation is observed in Ni₄₅Co₅Mn₃₅I-n_15-xGd_x alloys.When the Gd content reaches 0.8 at.%,the martensite transformation temperature decreases first,and then rises with the increase of Gd content.When the content of rare earth Gd reaches 0.8 at.%,the martensite transformation temperature reaches the minimum,and then rise with the increase of the content of Gd.When x=0,M_s=286 K;The martensitic transformation start temperature of Ni₄₅Co₅Mn₃₅In_13.4Gd_1.6alloy increases about 108 K compared with Ni₄₅Co₅Mn₃₅In₁₅ alloy and still has a tendency to increase.The change of matrix composition is the main reason for the increase of martensite transition temperature.Different martensitic phase transformation appears in different Gd content.For the Ni₄₅Co₅Mn₃₅In₁₅ and Ni₄₅Co₅Mn₃₅In_14.2Gd_0.8alloys,the X-ray diffraction patterns are all indexed to coexistence of 10M modulated martensite structure and L2₁ cubic austenite structure.The magnetic-field-induced reverse martensitic transformation is observed in Ni₄₅Co₅Mn₃₅In_14.2Gd_0.8 and Ni₄₅Co₅Mn₃₅In₁₄Gd₁ alloys.The results of compression tests show that the mechanical properties of Ni-Co-Mn-In alloys can be significantly improved by doping the appropriate amount of Gd.Gd doped with rare earth element can effectively improve the toughness of the alloy,with the increase of the content of doped with rare earth Gd,strain in Gd 0.8 at.%peak,but continue to increase the Gd doped will reduce the amount of strain.With the increasing of the content of Gd,the strain and the compression strength reaches the maximum when content of Gd is 0.8 at.%.When the Gd content exceeds 1.2 at.%,the compression strain of the alloy decreases significantly with the increase of doped Gd.The main reason for the increase of Ni-Co-Mn-In alloy is the enhancement and purification of fine crystals caused by the doping of rare earth elements Gd.It can be seen from the experiments that the excessive amount of Gd can lead to the decrease of strain,and the main reason is that Gd-rich phase leads to the excessive concentration of rare earth phase and the reduction of the mechanical properties of the alloys.The magnetic measurement illustrates that the alloy magnetic entropy constantly improve with the increase of magnetic field.When the magnetic field is up to 5 T,the magnetic entropy of Ni₄₅Co₅Mn₃₅In_15-xGd_x?x=0.8,1?alloy reach maximum.The inverse magnetic entropy change??S_M?reaches 17.78 J/kg K for Ni₄₅Co₅Mn₃₅In_14.2Gd_0.8 alloy at 277 K and its R_c is about 356 J Kg^-1 at an applied field of 5 T and?S_M reaches 26.8 J/kg K for Ni₄₅Co₅Mn₃₅In_14.2Gd_0.8 alloy at 328 K and its R_c is about 255 J Kg^-1 at an applied field of 5 T.It indicates that by substituting proper Gd for In,a low hysteresis loss and large magnetocaloric effect can be obtained in NiCoMnInGd alloys,which will be a promising working material for magnetic refrigeration at room temperature.The investigation is of great guiding significance for the practical application of Ni-Mn-In alloys.