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

M. SADE, C. DAMIANI, R. GASTIEN, F.C. LOVEY, J. MALARRÍA, A. YAWNY

SMART MATERIALS AND STRUCTURES

IOP

Lugar: Bristol; Año: 2007 vol. 16 p. 126 - 136

The effects of the repeated stress induced transformation (pseudoleastic fatigue) on the mechanical behaviour and microstructure of Cu-Zn-Al and Cu-Al-Ni single crystals is presented. Several microstructural changes occur during cycling at temperatures above the martensitic transformation temperature Ms in Cu-Zn-Al alloys. Bulk defects consisting in dislocation bands with retained martensite and intrusion-extrusion type of surface defects are observed with the exact details of defects nucleation, density, as well as other microstructural features, depending on the working temperature, alloy composition, applied stresses and number of cycles. The presence of these defects alter the shape of the stress-strain curves in the pseudoleastic 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 bulk defect density. 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, defect density 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 influencing the mechanical behaviour. The defects generated during pseudoelastic cycling lead however to the inhibition of the g´ phase when a transition between the bcc parent phase and a mixture of two martensitic structures g´ (2H) and b´ (18R) is involved at the beginning of the cycling process.