Stress-induced martensitic phase transformations in polycrystalline CuZnAl shape memory alloys under different stress st
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INTRODUCTION
THE experimental study of reversible stress-induced martensitic phase transformations has evolved greatly since the discovery of the shape memory effect by Chang and Read[1] in the early 1950s. Countless researchers have conducted uniaxial monotonic and cyclic tests in the temperature range where stress-induced martensitic transformations occur (temperatures greater than the martensite start temperature (Ms) and below the martensite deformation-induced temperature (Md), Ms , T , Md). These experiments have determined the uniaxial stress-strain response of many different shape memory alloy systems. It is widely accepted that stress-induced martensitic transformations produce two different macroscopic stress-strain responses: pseudoelasticity and the shape memory effect.[2,3] Although pseudoelasticity and shape memory are sometimes a result of martensite reorientation, this work will focus on stress-induced martensitic transformations. To utilize the beneficial properties of shape memory alloys in engineering applications, it is essential to conduct experimental studies to understand the effects of temperature, strain rate, cyclic loading conditions, and stress state on the critical transformation stress level. Although the effects of temperature (Clausius–Clapeyron relationship), strain rate,[4,5,6] and cyclic deformation[7–18] on pseudoelasticity and shape memory are all well established, there is a shortage of work on stress-state effects. This gap in research efforts is most likely due to the fact that the first large-scale application of shape memory alloys was in orthodontics.[19] The small NiTi wires used in braces proKEN GALL and KURT JACOBUS, Graduate Research Assistants, HUSEYIN SEHITOGLU, Associate Head, and HANS J. MAIER, Visiting Researcher on leave from the Institut fur Werkstofftechnik, Universita¨tGH-Siegen, D57068, Siegen, Germany, are with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801. Manuscript submitted October 21, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
vided little motivation for the full-scale testing of polycrystalline shape memory alloys under compression or even three-dimensional stress states. Unfortunately, it is not an easy task to experimentally impose multidimensional stress states on specimens. However, there are many new applications which underline the need for a more thorough understanding of stress-state effects on polycrystalline shape memory alloy stress-strain behavior. In many new shape memory alloy components,[20,21] there exist three-dimensional tensile and compressive stress states during the stress-induced martensitic transformation. For example, shape memory alloys which are embedded in composite structures for active vibration control[22] and in pressure vessels for hoop stress control[23] are subjected to three-dimensional stress states. To properly control either parameter, the critical transformation stress level and stressstrain response of the alloy under a constraining hydrostatic pressur
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