The Transformation And Elastocaloric Effect Of Properties Of Ni-Mn-Ga-Co Ferromagnetic Shape Memory Alloy

Driven by external fields,lattice vibration in solid-state phase transition materials always leads to the abrupt changes in physical behaviors.The significant application of giant magnetic-,elastic-and electric effect is the energy-efficient and environmentally friendly solid-state refrigeration technologies.Among them,elastocaloric cooling stems from large entropy change in stress-induced martensitic transformation has become one important potential to replace conventional vapor-compression method.However,the relatively large thermal/mechanical hysteresis,insufficient latent heat and high driven stress seriously limit the further development of elastocaloric cooling.So the thermal-induced martensitic transformation,microstructure,mechanical property,the effect of Co addition on the transformation temperatures and the elastocaloric effect of Ni-Mn-Ga-Co ferromagnetic shape memory alloys(SMAs)have been studied by the X-ray diffraction(XRD),differential scanning calorimetery(DSC)and transmission electronic microscopy(TEM).It’s shown that,for the Ni52-xMn25Ga23Cox(x=0.5,1,2,3,5)alloys,when the x is less than 0.5 percent,the alloys have tetragonal structure 5M martensite and when x is 5 percent,the alloy has L21 ordered structure and when the x is between 0.5 and 5 percent,the alloy is coexitance of parent phase and martensite phase at the room temperature.The yield strength of the alloys show the tendency to decrease after the first increase.The yield strength of alloy reaches the maximum with 3 percent Co addition is about 450 MPa.And the compressive strain increase with the addition of Co after the content of Co is more than 0.5 percent.The fracture surfaces are characterized by the intergranular cracking and cleavage.The ability to resist the generation and expansion of cracks increase,resulting in improved toughness.The Curie temperature increased with the Co addition,but the changes are not obvious.The Curie temperature of the alloys,which is distributed in large areas,is rising slightly in the area of 370 K.When the Co content is certain,the change in the magnetic field does not significantly improve the martensite transformation temperature drops slightly.Replace Ni Co changed the lattice differences between parent phase martensite,which changed the phase transformation strain of material,the material of the phase transformation strain showed a trend of decrease then increase after the first increased,and reached the maximum when Co doping amount to 2 percent a maximum of 3.18%,at this time the adiabatic temperature change is the largest,the maximum value of 2.49 K.At the same time,the magnetic entropy of alloy increases with the applied magnetic field,and under the same magnetic field strength,the magnetic entropy of alloy increases first and then decreases,and reach the maximum near the phase transition temperature.