A multielement alloying strategy to tune magnetic and mechanical properties in NiMnTi alloys via Co and B

The phase stability, martensitic transformation, and magnetic and mechanical properties of (Ni2-xCoxMn1.5Ti0.5)1-yBy (0 ≤ x ≤ 0.625; y = 0.03 and 0.06) alloys are systematically studied through the first-principles calculations method. The Co and B atoms are inclined to be aggregated distribution in the Ni2Mn1.5Ti0.5 alloy, and the phase stability of the austenite and non-modulated (NM) martensite decreases by co-doping. The ferromagnetic activation effect in the austenite occurs when x = 0.03 and y = 0.625. The magnetism of the austenite changes from an antiferromagnetic to a ferromagnetic state, which is ascribed to the elongation of the nearest neighboring distance of Mn–Mn, the nearest Mn–Mn distance increases from 2.50–2.79 to 2.90–2.94 Å, while the NM martensite always shows antiferromagnetism. Additionally, the doped B accelerates the change from antiferromagnetic to ferromagnetic for the austenite, but B-doping decreases the stability of the whole alloy system. The Co and B co-doping increases the stiffness of the NiMnTi alloy but decreases toughness and plasticity. However, the toughness and plasticity of the NiCoMnTiB alloy are better than those of the NiMnTiB alloy, indicating that the Co doping increases the d-orbital hybridization in the NiMnTiB alloy. The above results are expected to support the performance design of the NiMnTi-based alloy.