Fatigue and martensitic transitions in Cu–Zn–Al and Cu–Al–Ni single crystals: mechanical behaviour, defects and diffusive phenomena

The effects of the repeated stress-induced transformation (pseudoelastic fatigue) on the mechanical behaviour and microstructure of Cu–Zn–Al and Cu–Al–Ni single crystals are presented. Several microstructural changes occur during cycling at temperatures above the martensitic transformation temperature Ms in Cu–Zn–Al alloys. Bulk defects consisting of dislocation bands with retained martensite and intrusion–extrusion types of surface defects are observed. The fine characteristics of the defects, such as nucleation, density, as well as other microstructural features, depend on the working temperature, alloy composition, applied stresses and number of cycles. The presence of these defects alters the shape of the stress–strain curves in the pseudoelastic range for Cu–Zn–Al. Also diffusive phenomena strongly affect the mechanical behaviour of these alloys both in the parent and martensite phases, even for tests carried out slightly above room temperature. In turn, the kinetics of these diffusional processes has been found to depend on the density of bulk defects. Details of these processes are discussed and a simple model that describes the main characteristics of the mechanical behaviour evolution is considered. The interaction between the mechanical behaviour, density of defects and diffusion is discussed. In the case of Cu–Al–Ni, bulk defects are also shown to occur but the system is less prone to diffusion effects that could affect the mechanical behaviour. The defects generated during pseudoelastic cycling lead, however, to the inhibition of the γ′ phase when a transition between the bcc parent phase and a mixture of two martensitic structures γ′ (2H) and β′ (18R) is involved at the beginning of the cycling process.