Removal of martensite stabilisation in CANTiM shape memory alloy by post-quench ageing

Abstract

Step quenching from the solid solution temperature is a common treatment to enhance the shape memory properties in many copper-based shape memory alloys (SMAs). However, the martensite stabilisation problem is more serious in step quenched Cu-Al-Ni-Mn-Ti (CANTiM) SMAs. After step quenching, martensite in CANTiM cannot reverse transform to its parent phase. In contrast, fast quenching rates usually lead to better shape memory properties in CANTiM SMA. When the alloy is directly quenched into KOH, martensite stabilisation is still present. However, martensite stabilisation can be removed by post-quench ageing at 100°C. In rapid quenching from high temperature, a large number of defects such as vacancies are usually produced. The martensite stabilisation due to fast quenching is attributed to the pinning effect of these excessive vacancies. After post-quench ageing, the concentration of vacancies is lowered and thus the pinning effect is reduced. This paper reports the behaviour of vacancies at different cooling rates in different quenching medium, and the effect of post-quench ageing. The evolution of vacancies in the alloy is studied using positron annihilation techniques. In contrast to the martensite stabilisation due to ageing at higher temperatures, the martensite stabilisation due to fast quenching rate can be removed by a carefully chosen low temperature ageing programme.