Transformation behavior in a thermomechanically cycled TiNiCu alloy
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INTRODUCTION alloys, which have been developed from the substitution of Cu for Ni in amounts of up to about 30 at pct in near-equiatomic TiNi alloys, can exhibit a shapememory effect and pseudoelasticity of the binary TiNi alloy.[1,2] Also, Cu additions can reduce the chemical-composition dependency,[3] the transformation hysteresis,[2] and the deformation stress in the condition of martensite.[4] Previous investigations[2,5] show that the TiNi40Cu10 (at pct) alloy is most attractive, as the transformation elongation and transformation hysteresis of the B2 → B19 transformation in this alloy are between those of the B2 → R and B2 → B19⬘ transformation in a binary TiNi alloy. The TiNi40Cu10 (at pct) alloy is regarded as a candidate material for application as a thermal actuator, which demands a large-transformation elongation and a small-transformation hysteresis. Typically, shape-memory alloys used as thermal actuators will be exposed to repeated thermal cycling under an applied stress. Therefore, the stability of the transformation temperature and hysteresis, the fatigue life, the recoverable strain, and the change of hot and cold shape are of concern. However, to the authors’ knowledge, there exists no systematic study of the effects of thermomechanical cycling on these parameters of a TiNiCu alloy. Even for the TiNi binary alloy, most of the tests have been performed for a limited number of cycles and are primarily concerned with the development and stability of two-way strain or the evaluation of plastic stain.[6–9] For this reason, in the present work, we systematically studied the effect of annealing temperature on the transformation behavior of a TiNi40Cu10 (at pct) alloy cycled to failure under constant stress.
TiNiCu
II. EXPERIMENTAL PROCEDURE The experimental specimens were provided by Memry Corporation, Brookfield, Connecticut, with a nominal composition of TiNi40Cu10 (at pct). All the specimens were in LIJIAN RONG, Professor, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China. Dr. DAVID A. MILLER and DIMITRIS C. LAGOUDAS, Professor, are with the Center for Mechanics of Composite, Aerospace Engineering Department, Texas A&M University, College Station, TX 77843. Manuscript submitted May 3, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
the form of wire, with a diameter of 0.6 mm after being subjected to 30 pct cold-drawn deformation. Specimens were annealed for 15 minutes within the temperature range from 573 to 973 K, at intervals of 50 K. Thermomechanical cycling experiments were performed on a self-assembled frame under a constant stress of 150 MPa (Figure 1 is the schematic of the equipment setup). As seen in the figure, the load is applied through the force of gravity and the use of a low-friction ball-bearing pulley to direct the force into the wire. The wire was heated with a constant d.c. current passing through the wire, which should ensure the transformation of the wire from the martensite to the austenite state. This constant d.
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