Martensitic Transformation And Magnetic Functional Properties Of NiCoMnIn(Sn) Alloys Irradiated By Electrons

NiCoMnIn（Sn）alloys with large magnetostrain and magnetic entropy change have shown giant applying potential in the field of intelligent driving and cold storaging.Hence,exploring effective methods to improve magnetostrain and magnetic entropy change of these materials has become the frontier and hotspot of current research.In this work,we take the electron irradiation as a new method to improve the austenite phases’saturation magnetizationt by forming dense lattice defects in the NiCoMnIn（Sn）alloys,leading to improvement of the magnetostrain and magnetic entropy change.（Lorentz-）TEM,（HE）XRD,XAFS,M（?）ssbauer spectrum,DSC and PPMS are employed to systematically study the structures,transformation features and magnetic functional properties of high energy electron irradiated NiCoMnIn（Sn）alloy.Then,taken with the first principles calculation,the micro-mechanism of electron irradiation affecting martensitic transformation and functional properties of NiCoMnIn（Sn）alloy is also revealed.Under electron irradiation with low fluence,high-density vacancies are formed in the irradiation layer of NiCoMnIn alloys,accompanied by the appearance of some nano-scale amorphous phases.With the increase of irradiation fluence,the vacancy concentration increases,and a large number of vacancies migrate near the twin bonudaries and gather into vacancy clusters.Meanwhile,the micro-stress field appears around the irradiation defects,which leads to the martensitic lath fragmentation.According to the EPMA results,the elements segregates along the direction of electron incident.In the region of 0.15-0.45 mm,the content of Co/In element remains unchanged.However,the content of Mn increases while the content of Ni decreases due to the inverse Kirkendal effect induced by the irradiation defects.Electron irradiation does not change the martensitic transformation products of NiCoMnIn alloys.Both the unirradiated and irradiated alloys show one-step transformation characteristics and the same transformation products（monoclinic7M martensites）.However,the electron irradiation changes the phase transformation temperature which first decreases and then increases with the irradiation fluence increaseing.For the NiCoMnSn alloys,double absorption/exothermic peaks are observed in the DSC curve,exhibitting the two-step phase transformation feature.The in situ XRD testing results shows that both DSC peaks represent the 7M（?）L2₁ phase transition.The appearance of the two DSC peaks should be ascribed to the irradiation defect induced micro-stress field which results in martensitic transformation separation near the irradiation defects.Then,in the second thermal cycle,the phase transformation temperature of NiCoMnIn（Sn）alloy recovers a little and remains stable in the subsequent cycles.Electron irradiation significantly improves the magnetic and magnetic functional properties of the NiCoMnIn（Sn）alloys.Under low irradiation fluence,the sensitivity of phase transformation temperature and the austenite phases’saturation magnetizationt of NiCoMnIn alloy increases with the irradiation fluence,and reach the maximum values at the fluence of 2×10¹⁷e/cm².Then,both of them shows a slight decline with the irradiation further increasing.The saturation magnetization difference（ΔM）under 5 T magnetic field in the transformation processs of Ni₄₅Co₅Mn_36.5In_13.5 is increased from 64.58 A·m²/kg to 127.14 A·m²/kg with the irradiation fluence increasing from 0 to 2×10¹⁷e/cm²,contributing to an record-high magnetic entropy change of 48.15 J/（kg K）.The Ni₄₁Co₆Mn₄₃Sn₁₀ alloy irradiated by 2×10¹⁷e/cm² electrons present large magnetostrain only by employing a low magnetic field.It’s saturation magnetization difference under 2 T magnetic field increases from 76.60 A·m²/kg to 94.23 A·m²/kg,and the reversible magnetostrain reaches 0.22%,three times higher than that of unirradiated alloy.Meanwhile,the critical magnetic field driving the magnetostrain is reduced from 3.9 t to 2.3 T.The micro-mechanism of how the electron irradiation affacting the magnetic functional properties is displayed as follows.The lattice distortion induced by the crystal defects generating in the irradiation process induce lattice distortion and reduces the distance between the Mn-Mn atoms,thus enhancing the magnetic exchange interaction.Meanwhile,the energy exchange between the incident electrons and the lattice atoms’extranuclear electron leads to energy level transition and increasing unpaired electrons,thus destroying the aymmetry of crystal field and improving the hyperfine field and spin magnetic moment of Mn atoms.Taken together,the magnetization of irradiated NiCoMnIn（Sn）alloy is enhanced,as well as the magnetocaloric effect and magnetostrain.