Shape memory behavior of Ni₄₅Mn₄₀Co₅Sb_10−xB_x magnetic shape memory alloys

The transformation temperatures, magnetization behavior, shape memory behavior, and mechanical properties of polycrystalline Ni₄₅Mn₄₀Co₅Sb_10−xB_x (at.%) (x = 0, 1, 2, 3, 4, 5) alloys were systematically investigated. It was revealed that substituting Sb with B drastically increases the transformation temperatures, while it decreases the saturation magnetization due to the alteration of electron concentration of the matrix and formation of Co-rich second phases. With the substitution of Sb with 5% B, martensite start temperature and activation energy were increased from 50 to 316.8 °C, and 185 to 722.6 kJ mol⁻¹, respectively. The thermal cycling under stress, superelasticity, and failure experiments showed that shape memory properties and strength were improved by the substitution of Sb with B. The shape memory effect with maximum recoverable strain of 1.6% was observed with in Ni₄₅Mn₄₀Co₅Sb₉B₁, and perfect superelasticity was exhibited at 220 °C in Ni₄₅Mn₄₀Co₅Sb₈B₂. It was concluded that NiMnCoSb alloys can be used as high-temperature magnetic shape memory alloys as they exhibit transformation temperatures above 100 °C and show promising shape memory and superelasticity behavior, and there is a magnetization difference between their transforming phases.