The effect of precipitates on the 18R-6R martensitic transition in a CuZnAl SMA

Abstract

Abstract Cu-Zn-Al shape-memory alloy single crystals can have two sequential, stress-induced martensitic transitions, depending on their chemical composition and crystallographic orientation: first, the β-18R transition and, as the load increases, the 18R-6R transition. At electronic concentration e/a=1.48, the 18R-6R transition is immediately followed by the plastic deformation of the 6R phase. This phenomenon is usually undesired, as it compromises the reversibility of the pseudoelastic effect. Two different approaches have been applied to counteract the plastic deformation of the 6R phase: 1. A modification of the chemical composition of the alloy, reducing e/a; 2. The controlled introduction of gamma-phase precipitates, which increases the yield stress of the 6R phase. The former approach was relatively successful but, al lower e/a, the kinetics of defect formation and propagation due to pseudoelastic cycling are significantly increased, which renders these alloys unsuitable for most applications. The latter approach, on the other hand, results in 6R without plastic deformation, and does not seem to have a negative impact in the fatigue life of the alloy. However, the authors only studied one specific distribution of precipitates which, despite being successful, may not be the optimal condition to harden the 6R phase and increase pseudoelastic fatigue life. No information is available in the literature regarding the effect of different precipitate sizes on the 18R-6R transformation.