REMOVAL OF MARTENSITE STABILIZATION IN CANTIM SHAPE-MEMORY ALLOY BY POST-QUENCH AGING

Step quenching from the solid solution temperature is a common treatme nt to enhance the shape memory properties in many copper-based shape m emory 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, martensi te stabilisation can be removed by post-quench ageing at 100 degrees C . In rapid quenching from high temperature, a large number of defects such as vacancies are usually produced. The martensite stabilisation d ue to fast quenching is attributed to the pinning effect of these exce ssive vacancies. After post-quench ageing, the concentration of vacanc ies is lowered and thus the pinning effect is reduced. This paper repo rts 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 tech niques. In contrast to the martensite stabilisation due to ageing at h igher temperatures, the martensite stabilisation due to fast quenching rate can be removed by a carefully chosen low temperature ageing prog ramme.