Revealing essence of magnetostructural coupling of Ni-Co-Mn-Ti alloys by first-principles calculations and experimental verification

In this work, the effects of Co doping on the magnetostructural coupling transformation of Ni50-xCoxMn50-yTiy (x = 0–15, y = 12.5–15) Heusler alloys were systematically investigated through the first-principles calculations and experimental verification. The calculation result indicates that the doped Co atoms prefer to occupy the Ni sublattice. The Co atoms tend to flock together in terms of the lowest energy principle. Since the formation energy of the austenite is higher than that of the martensite, the alloys will undergo martensitic transformation for the Ni50-xCoxMn37.5Ti12.5 alloys (x = 0–12.5). The magnetostructural coupling point of Ni50-xCoxMn37.5Ti12.5 alloys is predicted in the vicinity of x = 11–12. Based on the computational composition Ni37.5Co12.5Mn37.5Ti12.5, the Ni36Co14Mn36Ti14 alloy with magnetostructural coupling near room temperature was experimentally developed by simultaneously increasing the Ti and Co contents. The largest magnetization change (ΔM) and magnetic entropy changes (ΔSm) obtained under magnetic field of 5 T for the martensitic transformation in the Ni36Co14Mn36Ti14 alloy are about 87.6 A·m2·kg−1 and 21 J·kg−1·K−1, respectively. The fracture strength and strain for non-textured polycrystalline Ni36Co14Mn36Ti14 alloy reach 953 MPa and 12.3%, respectively. The results show that the alloy not only possesses a large magnetocaloric effect but also has excellent mechanical properties. In addition, the 6 M modulated martensite is evidenced in the Ni-Co-Mn-Ti alloys via transmission electron microscopy technique.