Effect of Co addition on the ordering and martensitic transformations in Cu-Al-Mn shape memory alloys

The effect of Co addition on the order-disorder transition temperature, Ms temperature, ductility, hardness and the shape memory properties of ductile Cu-Al-Mn shape memory (SM) alloys was investigated by differential scanning calorimetry (DSC), cold-rolling, and tensile test techniques. It was found that the addition of Co to the Cu-Al-Mn alloys stabilized the B2 phase and decreased the Ms temperature. In the Cu₇₁Al₁₇Mn₁₀Co₂ alloy, the B2 to L2₁ ordering transformation was suppressed by quenching from 900 °C and the retained B2 parent phase transformed to the 6 M martensite. In several other alloys of this system, the presence of the CoAl (B2 type) precipitate phase was observed. Even though the CoAl precipitate caused a decrease in the cold-workability and the degree of elongation, it nonetheless improved the SM and pseudoelastic (PE) properties of the alloy by solid solution or/and precipitation hardening.