Pseudoelasticity and Cyclic Stability in Co 49 Ni 21 Ga 30 Shape-Memory Alloy Single Crystals at Ambient Temperature
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INTRODUCTION
SHAPE-MEMORY alloys (SMAs) can yield large forces in actuators[1] and possess high damping capacity both in the austenitic state, where the stress-induced martensite (SIM) transformation takes place, and in the martensitic state because of the SIM variant reorientation.[2] In many applications, a thermomechanical loading path using the pseudoelastic (PE) effect is repeated often involving a large number of loading and unloading cycles above the austenite finish (Af) temperature. Irrecoverable strains accumulated during cyclic loading may result in loss of the PE material response, and good cyclic stability is an essential criterion for SMAs. Therefore, it is important to comprehend the evolution of the cyclic stress-strain characteristics and the associated operant mechanisms that cause the degradation of the functional properties in SMAs. For the widely studied NiTi SMA system, it has been reported that cyclic deformation initially results in an increase in the residual strain, a decrease in the critical J. DADDA and D. NIKLASCH, Research Assistants, and H.J. MAIER, Professor, are with the University of Paderborn, Lehrstuhl fu¨r Werkstoffkunde (Materials Science), Pohlweg 47-49, 33098 Paderborn, Germany. Contact e-mail: [email protected] born.de I. KARAMAN, Associate Professor, and H.E. KARACA, Research Assistant, are with the Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843. Y.I. CHUMLYAKOV is with the Siberian Physical Technical Institute, Tomsk, 634050, Russia. Manuscript submitted September 11, 2007. Article published online May 30, 2008 2026—VOLUME 39A, SEPTEMBER 2008
stress for inducing SIM, and a decrease in the stress hysteresis. However, with an increasing number of cycles, the effects become less pronounced and finally steady-state PE characteristics may be attained.[1–9] The changes in stress-strain characteristics are attributed to the introduction and accumulation of dislocations during the repeated SIM transformations.[4] Therefore, it is necessary to raise the critical stress for slip in order to stabilize the PE characteristics during cyclic loading. Strengthening against dislocation slip can be achieved through precipitation hardening or by reducing the grain size in polycrystalline material. Thus, several studies have focused on precipitated NiTi single crystals[5–7] and ultra-fine-grained NiTi wires and bulk material[8,10] in an attempt to capture the inherent orientation-dependent cyclic PE properties and to provide the parameters needed for constitutive modeling. In the present study, experimental results on the cyclic deformation behavior of single-crystalline Co49Ni21Ga30 SMAs are presented for compression loading. Recently, CoNiGa alloys have been identified as promising high-temperature PE material that can undergo a stress-induced phase transformation at temperatures as high as 425 C with a complete shape recovery of at least 2 pct strain.[11] A large PE window of about 400 C was also demonstrated.[11] Moreover, these alloys are considered
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