Effect of grain size on the observed pseudoelastic behavior of a Cu-Zn-Al shape memory alloy
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INTRODUCTION
THE stress-strain characteristics of a pseudoelastic polycrystalline shape memory alloy (SMA) can be strongly influenced by grain size. Although the effect of grain size on copper-based SMA properties has not been widely reported, two studies of this topic have been previously published. Results from the most recent of these investigations are detailed in an article by Sure and Brown,[1] which correlates the mechanical properties of a Cu-Al-Ni alloy with sample grain size. A set of stress-strain curves in the pseudoelastic range for one alloy composition was included. The authors noted that as the grain size–to–sample thickness ratio (gs/t) increased, the number of distinct regions of the stress-strain curve increased from two to three as the larger grain size samples developed an intermediate ‘‘plateau’’ region of lower slope than the initial or final regions. However, these investigators elected to focus their study on the effect of grain size on the mechanical behavior of the alloy in the martensitic regime rather than in the pseudoelastic temperature regime. An earlier study by Dvorak and Hawbolt[2] also examined grain size effects on the pseudoelastic response of a copperbased SMA. Tensile specimens of a Cu-Zn-Sn alloy were prepared with varying gs/t. The experiments demonstrated that a decrease in gs/t led to an increase in the applied stress required to produce a given strain increment. More specifically, as gs/t decreased, the transition stress from the elastic to the pseudoelastic regime increased, and the amount of recoverable strain decreased. Based on the results of their experimental work, the authors proposed that variation of the degree of grain constraint with gs/t was responsible for the observed trends in mechanical behavior. M. SOMERDAY, Research Associate and Graduate Student, and J.A. WERT, Professor, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. R.J. COMSTOCK, Jr., formerly with the Department of Materials Science and Engineering, University of Virginia, is with Armco, Inc., Middletown, OH 45044. Manuscript submitted February 17, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
The prior investigators have noted the grain size effect on SMA stress-strain curves and have attributed these observations to variations in constraint. Progress in modeling SMAs now enables evaluation of the grain size effects in further detail and incorporation of texture effects into a model for pseudoelastic response. Models of the stressstrain characteristics of pseudoelastic SMAs in the form of unconstrained single crystals, constrained single crystals, and textured polycrystals have been previously described.[3,4,5] To apply the models, knowledge of the following SMA characteristics is needed: transformation systems, texture, and degree of grain constraint. A transformation system is one habit plane/shear direction combination associated with the martensitic transformation. All possible transformation systems have been determined fo
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