Marforming and martempering of a Cu–Zn–Al shape memory alloy

A Cu–Zn–Al alloy with Ms=−25°C has been quenched in liquid nitrogen. A subsequent rolling of the thermally induced martensite (marforming (MF) at −196°C) by an amount of ϕ=20% provides oriented and high defect domains. This results in a defect stabilized structure which can not retransform diffusionless into austenite. Tempering (martempering (MT)) of this martensite is conducted in two ways: (1) Isochronal (1 h) at different temperatures 150°C<TMT≤800°C. The material is subsequently water quenched. (2) Heat-up treatments in a differential scanning calorimeter (DSC) device with different heating rates (5–50 K min−1). The microstructural changes caused by these MT treatments are investigated by optical and transmission electron microscopy. The course of different precipitation processes (γ-, β- and α-brass) is confirmed by the DSC heat-up treatments. This results in a time–temperature reaction diagram. Recovery of the diffusionless transformability of the defect martensite caused by isochronal MT and its effect on the non conventional yielding of the material is investigated by stress–strain experiments.