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 fur ther understand the evolution of material properties after continuous pseud oelastic cycling. In a fatigue experiment of this type, permanent and recov erable effects are observed for temperatures above 273 K. Experiments in th e stress-strain-time space have been designed in order to separate both con tributions. The occurrence of recoverable changes can be related to the ord ering of the beta-phase and to the stabilization of the martensite, both de pending in turn on the atomic diffusion phenomena in these alloys above 273 K. The kinetics of these processes show a considerable increase of the ass ociated time constant after the pseudoelastic cycling procedure. The relati on of these changes with the microstructural evolution of the material is a nalyzed and a discussion is offered on the role that each mechanism. either diffusive or due to microstructural changes, plays on fatigue. The relevan ce of defining a reference state in order to determine the consequences of pseudoelastic cycling is shown. A model which considers the stabilization o f martensite and the beta recovery has been used to reproduce the stress de formation 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.