Effects of ageing treatments on transformation temperatures and precipitation kinetics in a Cu-Zn-Al shape-memory alloy

Abstract

The effects of ageing treatments on transformation temperatures, hardness, and precipitation kinetics in a Cu-14.2Zn-8.5Al (wt%) shape-memory alloy were investigated. Quench-ageing treatment temperatures varied from 100 to 500° C with times up to 200 h after the solution treatment. The martensitic transformation temperature, M _s, of the hot-rolled material was decreased from 55 to 51 °C by the solution treatment. The temperature hysteresis (A _f-M _f) was 50° C for the hot-rolled condition, but was reduced to 30° C after the solution treatment. The maximum hardness for material aged at 500° C was lower than that for that aged at 300 or 400° C. The apparent activation energy for hardness increase in this alloy was 110 kJ mol⁻¹, compared with 72 kJ mol⁻¹ for the similar copper-based shape-memory alloy Cu-21.2 Zn- 6.0 Al. The ordering temperatures for B₂ and DO₃ superlattices were in the neighbourhood of 480 and 260° C, respectively. The tensile ductility and yield strength of this alloy were significantly reduced by the ageing treatment at 400° C.