Ausforming and Marforming of a Cu-Zn26.3-Al3.9 Shape Memory Alloy

Abstract
A copper-based shape memory alloy with M_s below ambient temperature is investigated in four different conditions :
1) As betatized.
2) Ausformed at 800 °C and at 780° C, subsequently quenched.
3) Marformed by different amounts of true plastic deformation at temperatures below M_f.
4) Mechanically cycled (ausformed and as betatized state). The stress amplitudes Δσa are chosen in the range between pseudo yield stress R_pp and conventional yield stress R_p0.2 ( R_pp ≤ σa ≤ R_p0.2). Different metallographic measurements are conducted to describe changes in microstructure and transformation behaviour. Ausforming results in a high defect density (dislocations, anti-phase-domain boundaries) in the ordered austenite. Cu-rich α-precipitates at grain boundaries are observed. The M_s temperature is lowered significantly. The conventional yield stress is raised. By marforming various defects and crystallographic disorder are generated in the martensite. They are responsible for suppression of the diffusion-less reverse transformation in the case of high amounts of true plastic deformation. Above a critical number of mechanical cycles of the ausformed alloy the martensitic as well as the reverse transformation take place in a two step mode which stays stable even in the following DSC cycles. An explanation can be based on the induction of defects which nucleate an additional type of diffusionless transformation.