Study On Crystal Structure, Martensitic Transformation And Magnetic Properties Of Ni-Fe-Ga-(Co) Ferromagnetic Shape Memory Alloys

The newly developed Ni-Fe-Ga-(Co) ferromagnetic shape memory alloys in recent ten years have been one international research focuses currently in the field of metal material and condensed matter physics for their comprehensive characteristics, such as great recovery strain, large output stress, high response frequency and precise control. In order to improve the shortcomings of big workload, long research period and high cost in the previous experiments by attempts, this text combined first-principles calculations with experiments in the study. Ni-Fe-Ga-Co alloy system was designed on the foundation that the preferential occupation of Co atom was determined by first-principles calculations. Ni50+xFe25-xGa25 (x=0,1.5,3,4.5,6) and Ni56-yFe19Ga25Coy(y=0,1.5,3,4.5,6) alloys were prepared with the method of induction smelting, and the change rule with alloy composition of crystal structure, martensitic transformation and magnetic properties of Ni-Fe-Ga-(Co) ferromagnetic shape memory alloys were analyzed by first-principles calculations together with energy spectrum analysis (EDS), scanning electron microscope (SEM), X-ray diffractometer (XRD), differential scanning calorimeter (DSC), vibrating sample magnetometer (VSM) and transmission electron microscope (TEM), and the physical essence was also revealed.First-principles calculations showed that it took the lowest formation energy when normal-Ni atom in Ni-Fe-Ga alloy was substituted by Co atom, so Ni atom was took place by Co preferentially. The atomic magnetic moment of Ni was significantly lower than that of Fe, so the total magnetic moment of austenite and martensite phase both decreased as Fe atoms were substituted by Ni atoms in Ni-Fe-Ga alloys; the total magnetic moment of austenite and martensite phase increased as Ni atoms were gradually substituted by Co atoms in Ni-Fe-Ga-Co alloys on account that atomic magnetic moment of Co was significantly larger than that of Ni.DSC measurement results indicated the Tm of Ni50+xFe25-xGa25 alloys increased with the increase of Ni content, size factor and valence electron concentration both positively promoted the trend; the Tm of Ni56-yFe19Ga25Coy alloys decreased with the increase of Ni, the two internal causes of size factor and valence electron concentration had opposite effects on the change, and the latter was stronger.The study results of DSC?XRD?TEM revealed that in Ni50+xFe25-xGa25 alloys, when the content of Ni was less than or equal to 53 at.%, the Tm was under room temperature To and the microstructures at To were austenite phase and y phase. The Tm became higher than To, and the microstructures turned to martensitic phase and y phase at To when the content of Ni was greater than 53 at.%; the martensite in Ni54.5Fe2o.5Ga25 alloy was 14M, while in Ni56Fe19Ga25 it was combination of 6M and 14M. EDS showed that ? phase was rich in Fe while poor in Ga and increased with the increase of Ni.As for Ni56-yFe19Ga25Coy alloys, the Tm was all above To and the microstructures at To were martensitic phase and y phase. The martensitic phase in Co1.5 alloy was 14M. When the content of Co rised up to 3 at.% and above, the martensitic phase was made up of monocline 6M and 14M. The y phase was rich in Fe and Co while poor in Ga. It was noticed that the Ni content in matrix and y phase decreased with the increase of Co, while the content of Fe and Ga just stayed unchanged, which verified the site of Ni was replaced by Co, and that was in good agreement with the calculation results.The VSM magnetization curves of constant and varying temperature showed that the saturation magnetization M5 of Ni50+xFe25-xGa25 alloys decreased with the increase of Ni, showing the weakened magnetic property; those of Nis6-yFe19Ga25Coy alloys increased with the addition of Co, representing the enhanced magnetic property, which agreed with the calculation results. The Curie temperature Tc of Ni50+xFe25-xGa25 alloys reduced with the increase of Ni, while that of Ni56-yFe19Ga25Coy alloys rose remarkably and even became above To with the addition of Co. When the content of Co was greater than 4 at.%, TC was higher than Tm, and both of them were above room temperature, moreover, the alloys also possessed excellent magnetic properties.