Large reduction in critical stress in Co-Ni-Al upon repeated transformation

The stress-strain response of Co-Ni-Al with a two-phase microstructure displayed an unusual decrease in critical stress (84%) with strain or thermal cycling accompanied with an improved recoverability of pseudoelastic and shape memory strains. The pronounced decrease in critical stress occurred over a wide temperature range (145 degreesC), and is linked to the interaction of the transformation front with a secondary phase and the residual martensitic domains. The stabilized material is better suited for magnetic shape memory applications because of the lower elastic strain energy manifested through the lower critical stress and the reduced apparent elastic modulus. An extensive experimental program was utilized to establish the Clausius-Clapeyron relationship and the temperature dependence of critical stress for slip at elevated temperatures. A reduction of a factor of two in the slope of the Clausius-Clapeyron curve for the repeatedly deformed material was measured commensurate with the reduction in critical stress. The repeated deformation also produced a decrease in stress hysteresis and a reduction in residual strain making this material rather attractive for shape memory applications. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.