Development of boron-microalloyed Co–V–Al–Fe shape memory alloys

This paper reported the superelasticity and excellent cycle stability of boron-microalloyed Co–V–Al–Fe shape memory alloy. The microstructures, martensitic transformation characteristics, shape memory effect, and superelastic properties of Co_56+xV_32-xAl₁₂ (x = 0, 2, 4, 6), Co_58-xFe_xV₃₀Al₁₂ (x = 2, 4, 5), and (Co₅₃Fe₅V₃₀Al₁₂)_100-xB_x (x = 0.1, 0.5, 1) alloys were studied. The martensitic transformation temperature decreases with the decrease of Co content in Co_56+xV_32-xAl₁₂ (x = 0, 2, 4, 6) alloys. The Co₅₈V₃₀Al₁₂ obtained the entirely recovered about 1.2% residual strain after 3% pre-strain. With the increased Fe content, the martensitic transformation temperature of Co_58-xFe_xV₃₀Al₁₂ (x = 2, 4, 5) alloys decreases significantly, and a Co₅₃Fe₅V₃₀Al₁₂ alloy with M_s and A_f as 233.1K and 260.1K is obtained. The results show that without the B addition, the Co₅₃Fe₅V₃₀Al₁₂ alloy only exhibits 13 superelastic cycles when applying the pre-strain of 3% at room temperature. When the microalloying of B is 0.5 at.%, the martensitic transformation temperature is slightly increased with M_s and A_f at 241.0K and 268.5K, and the thermal hysteresis (7.3K) is almost unchanged. The grain size of the alloy is significantly refined due to the precipitation of VB compounds. As a result, the mechanical and superelastic properties are improved in (Co₅₃Fe₅V₃₀Al₁₂)_99.5B_0.5 alloy. During more than 5100 superelastic cycles with the pre-strain of 3% at room temperature, the (Co₅₃Fe₅V₃₀Al₁₂)_99.5B_0.5 alloy shows excellent cycle stability. It may provide essential guiding significance for developing Co-based shape memory alloys.