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Effect of Annealing on Strain-Temperature Response under Constant Tensile Stress in Cold-Worked NiTi Thin Wire

DOI: 10.1155/2011/160927

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The present paper aims to understand the influence of annealing on the strain-temperature response of a cold-worked NiTi wire under constant tensile stress. It was found that transformation behavior, stress-strain relationship, and strain-temperature response of the cold-worked NiTi wire are strongly affected by the annealing temperature. Large martensitic strains can be reached even though the applied stress is below the plateau stress of the martensite phase. At all stress levels transformation strain increases with increasing annealing temperature in the range of 350°–450°C and decreases with increasing annealing temperature in the range of 450°–650°C. The martensitic strain at lower stress levels exhibits the same tendency. At higher stress levels the martensitic strain increases with increasing annealing temperature. 1. Introduction Among many shape memory alloys (SMAs), NiTi has been widely used in many technological and engineering applications due to its excellent shape memory effect, superelasticity, high damping capacity, and others [1, 2]. Its remarkable properties result from a reversible martensitic phase transformation between austenite and martensite phases, which can be either stress induced or temperature driven. The transformation is sensitive to factors such as material composition, deformation processing, and heat treatments. Therefore, a mix of cold work followed by a specific annealing process has been comprehensively considered to optimize the physical and mechanical properties of a NiTi product and achieve shape memory and/or superelasticity. Significant efforts have been made to address the effects of heat treatment on transformation behavior [3–5], microstructure [6–8], recovery stress [9, 10], damping [11, 12], plateau stress and strain [13, 14], as well as lifetime of NiTi SMAs [15, 16]. To open new applications of NiTi SMAs, including smart structures, intelligent controllers, and memory devices, the understanding of strain and phase transformation behavior under constant applied stress is essential. In the present paper the influence of annealing on martensitic transformation and residual strains of cold-worked NiTi wire developed during cooling/heating under constant tensile stress has been investigated. 2. Experimental Procedure The experiments were performed on a commercial NiTi wire provided by SAES Getters (Italy) with a diameter of 0.076?mm and a normal composition of 50.2 at % Ni. As-received wires (35% cold-worked) with a length of 100?mm were annealed in an argon atmosphere for 10?min at 350°, 450°, 550°, and


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