Pseudoelastic fatigue of Cu-Zn-Al single crystals: the effect of concomitant diffusional processes

Experiments have been performed in Cu-Zn-Al single crystals in order to further understand the evolution of material properties after continuous pseudoelastic cycling. In a fatigue experiment of this type, permanent and recoverable effects are observed for temperatures above 273 K. Experiments in the stress-strain-time space have been designed in order to separate both contributions. The occurrence of recoverable changes can be related to the ordering of the beta-phase and to the stabilization of the martensite, both depending in turn on the atomic diffusion phenomena in these alloys above 273 K. The kinetics of these processes show a considerable increase of the associated time constant after the pseudoelastic cycling procedure. The relation of these changes with the microstructural evolution of the material is analyzed and a discussion is offered on the role that each mechanism. either diffusive or due to microstructural changes, plays on fatigue. The relevance of defining a reference state in order to determine the consequences of pseudoelastic cycling is shown. A model which considers the stabilization of martensite and the beta recovery has been used to reproduce the stress deformation behavior after cycling. An enhancement of stabilization kinetics during fatigue has allowed us to obtain a closer fit with the experimental results. (C) 2000 Elsevier Science S.A. All rights reserved.