Effects Of In-plane Stress And Crystallographic Orientation On Microstructure And Magnetic Properties Of Ni-Co-Mn-Al(Sn)Films

Ni-Co-Mn-X(X=Al,Sn)ferromagnetic shape memory alloy films have broad application prospects in micro-region refrigeration devices.However,the Ni-Co-Mn-Al films prepared at present have various phase structures such as retained austenite in the film due to the mismatch stress of the substrate,and hinder the Ni-Co-Mn-Al films from obtaining large ΔM.Also,since the(001)A-oriented MgO(100)/Ni-Co-Mn-Sn films are subjected to the mismatch stress of the substrate,it is difficult to obtain large ΔM.In this paper,the intrinsic relationship between inplane stress and microstructure of Ni-Co-Mn-Al films was studied.The crystallographic orientation of Ni-Co-Mn-Sn films was changed to improve its magnetocaloric effect.Firstly,the(001)A oriented Ni-Co-Mn-Al films with different compositions were prepared on MgO(100)substrate by magnetron sputtering.There was mismatch stress between MgO(100)and Ni-Co-Mn-Al films,resulting in more pore defects in the MgO(100)/Ni-Co-Mn-AI films.Based on the above results,Ni43.4Co8.5Mn34.6Al13.5 films were prepared on the MgO(100)/V.There were two kinds of martensite lath regions in Ni43.4Co8.5Mn34.6Al13.5 film:one type is Yshaped region which is long straight and parallel to the substrate boundary[001]MgO or[010]MgO One type is X-shaped area that is curved and at an angle of 45°to the boundary of the substrate.With increasing of the thicknesses of V,the long straight strips in the Ni43.4Co8.5Mn34.6Al13.5 films gradually increase,and the X-shaped curved strips gradually decrease.Ni-Co-Mn-Al films with different chemical compositions were prepared on MgO/V(100 nm)substrates.XRD analysis showed that the Ni-Co-Mn-Al films were austenite phases when the electron concentration was less than or equal to 7.79.When the electron concentration is greater than or equal to 7.87,the film mainly existed in the form of NM martensite phase.SEM-EBSD test showed that there were four kinds of slats with different orientations in both the long straight type(Y type)and the curved slat region(X type)in the Ni43.4Co8.5Mn34.6Al13.5 films,and each slat was composed of two orientations.Four martensite strips with different orientations in the Y-type region were composed of coarse variant(1 10)NM and thin variant(001)NM.In the X-type region,two dark slats were consist of two(110)NM variants,and two bright slats were consist of a thin variant(110)NM and a thick variant(001)NM.The microstructure crystallographic analysis results were used to analyze the macroscopic texture polar diagram of the thin films.The gradually increasing density points in the(004)NM polar diagram represent the Y-type region(001)NM crystal plane parallel to the substrate.According to WLR theory,the ratio of thick variation to thin variation of strip in MgO(100)/V(100 nm)/Ni43.4Co8.5Mn34.6Al13.5 film was calculated to be 0.6732,and the ratio of(001)NM variation in X-type region was more than that in Y-type region according to six martensite variation groups in the film.Therefore,with increasing of V thicknesses,the Y-type region gradually increases and the X-type region gradually decreases,so the(001)NM variation in the microstructure of the film gradually decreases.Variable temperature XRD detection revealed that the(004)A crystal plane diffraction angle of Ni43.4Co8.5Mn34.6Al13.5 films grown on MgO/V substrates with different thicknesses shifted to the right,and the spacing between(004)A crystal planes gradually becomes smaller,and the atomic distance of the austenite crystal structure in the plane is expanded by tensile stress,which suppresses the transformation of(001)A to(001)NM martensite,thereby reducing the X-shaped slabs and increasing the Y-shaped area slats.The(110)A oriented epitaxial growth of Ni45.19Co3.48Mn38.63Sn12.70 thin films was prepared on Al2O3(1120)substrates by magnetron sputtering.The film has large ΔM under 5 T magnetic field,the maximum magnetic entropy change under 2 T magnetic field reaches 6.6 J/kg/K,and the maximum magnetic entropy change under 5 T magnetic field reaches 13 J/kg/K.To sum up,this paper analyzed the crystal structure of NM martensite in the epitaxial NiCo-Mn-Al thin film and the internal relationship between martensite variant orientation distribution and microstructure,and explained the regulation mechanism of in-plane stress on the microstructure of Ni-Co-Mn-Al thin film.By adjusting the crystallographic orientation of Ni-Co-Mn-Sn films,a large change in magnetic entropy was obtained.