Large pseudoelastic strain in double β-18R–6R martensitic transformation with reduced 6R martensite stabilization in CuZAl alloys

Abstract

In Cu–Zn–Al single crystals, two sequential martensitic transformations can be induced by mechanical stress, depending on the chemical composition and the crystallographic orientation of load application. The first transition occurs between a metastable austenitic phase named β and a martensitic phase named 18R. As the applied load increases, a martensite to martensite transition occurs and the 18R transforms into the 6R phase. A large pesudoelastic tensile strain of up to 18% can be obtained. The mechanical hysteresis associated with the 18R–6R transformation is high, which makes it a potential candidate for mechanical damping applications. However, during pseudoelastic cycling the 6R phase is simultaneously submitted to plastic deformation and martensite stabilization. Mechanical behavior can be improved by introducing γ phase nanoprecipitates, which reduces plastic deformation and dynamic martensite stabilization. The effects of different mean precipitate sizes on 18R–6R mechanical cycling behavior are studied in this work. Results show that martensite stabilization kinetics decreases to a large extent as mean precipitate size increases. This leads to a rather reproducible mechanical behavior at least for 50 cycles. We propose a mechanism based on the interaction between precipitates and out of equilibrium vacancies to explain the great reduction in dynamic stabilization of 6R martensite.