Martensitic Transformation and Grain Size in a Cu-Al-Be alloy

In copper-based shape memory alloys, within an appropriate range of compositions and temperatures, the β phase (a long range ordered bcc phase) transforms martensitically to an 18R structure by cooling, spontaneous transformation, or by application of stress. The martensitic transformation is responsible for the shape memory properties exhibited by these alloysThe influence of the grain size on Ms and σs in a β Cu-11.41Al-0.50Be (wt.%) with a DO3→18R transformation was studied. It was determined that Ms decreases and σs increases as the grain size decreases. To analyze the functional relation of σs with d, the Hall-Petch type mechanical model and modifications by three-dimensional grain constraint were analyzed. It was found that σs can not be described by any of these relations in the studied alloy. From the Claussius-Clapeyron relationship and introducing an additional component of stress associated to the grain constraint effect, the decomposition of σs in two components is proposed: one of them a function of T and Ms, and the other one a function of T and d.