Effects of the Sense of Stress on Martensitic Transformations in Monocrystalline Cu–Al–Ni Shape Memory Alloys

Abstract

The effects of the sense of stress on stress-induced martensitic transformations in monocrystalline Cu–Al–Ni shape memory alloys with 13.8–15.3 mass%Al have been investigated as a function of temperature above M_s by tension-compression tests using an extensometer. In a temperature range above A_f, β₁\ ightleftarrowsβ₁′ transformations occur on the tension side regardless of the amount of strain, but on the compression side the β₁→β₁′ transformation occurs at the very beginning followed by β₁→γ₁′ transformation and their reversions do, probably because of a non-uniaxiality under compressive stress. β₁′ martensite is first stress-induced on both the tension and compression sides and undergoes a maximum resolved shear stress. Temperature dependence of the critical stress (σ) for inducing the β₁→β₁′ transformation is larger on the compression side than on the tension side. This result can not be explained in terms of Schmid’s law, but can be in terms of Clausius-Clapeyron equation by taking account for a different transformation strain associated with the formation of another variant of β₁′ martensite on the compression side. A temperature given by extrapolation of the γ vs temperature curve to zero stress is higher on the compression side than on the tension side. This can be interpreted in terms of a difference in constraint condition between the compression and tension sides.