Compressive response of high-strength [001]-oriented single crystals of a Co35Ni35Al30 shape memory alloy

The effects of stress-free and stress-assisted aging at 673 K on microstructures, thermoelastic stress-induced B2-L1₀ martensitic transformation and superelasticity were investigated in [001]_B2-oriented Co₃₅Ni₃₅Al₃₀ (at. %) single crystals in compression. The aging of quenched crystals results in the formation of high-strength nanostructured composites containing spherical nanosized particles and elongated ε-Co (HCP structure) particles, which do not undergo martensitic transformation. The stress-free and stress-assisted aged at 673 K for 0.5 h Co₃₅Ni₃₅Al₃₀ crystals, oriented along [001]_B2-direction, demonstrate high-yield stress levels for the B2-phase of 1360÷1630 MPa and a wide superelasticity temperature range from 193 K to 563 K with good cyclic stability and narrow stress hysteresis of Δσ = 50–60 MPa. It has been shown experimentally that a difference in the effective Young's modulus of austenite and martensite determinates a decrease in the reversible strain with a test temperature in both quenched and aged crystals. The tensile two-way shape memory effect with a reversible strain of ε_TWSME = +2.2% along the [001]_B2-direction is induced due to the oriented growth of elongated ε-Co-particles in stress-assisted aged crystals. These results provide a clear demonstration of the potential of aged CoNiAl single crystals for applications involving high stresses and temperatures.