Thermomechanical effects on phase transformations in single-crystal Cu-Al-Ni shape-memory alloy
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ngle-crystal rods of Cu–Al–Ni shape-memory alloy fabricated from a molten pool of 82 wt% Cu, 14 wt% Al, and 4 wt% Ni by the Czochralski method were first heated to ∼870 °C and then quenched to obtain austenitic microstructures. Various microanalysis techniques were used to determine the chemical composition, microstructure, and phase-transformation temperatures of the produced alloy. Cyclic tensile tests with in situ temperature control demonstrated the occurrence of pseudoelastic deformation at elevated and close to phase-transformation temperatures and provided insight into the temperature dependence of the phase-transformation stress, damping characteristics, and cyclic straining of single-crystal Cu–Al–Ni alloy. The stress hysteresis observed in the pseudoelastic deformation cycles decreased at elevated temperatures. The stress response at different temperatures is associated with the formation, growth, and coalescence of martensite variants. Stress-induced phase-transformation mechanisms, coalescence of twin variants, and energy dissipation by pseudoelastic deformation are discussed in the context of experimental findings. The results illustrate the potential of single-crystal Cu–Al–Ni as a structural material for dynamic microsystems and temperature sensors.
I. INTRODUCTION
Shape-memory alloys (SMAs) have gradually become candidate materials for dynamic microsystems due to the demonstrated very large reversible strains (typically ∼8%) and controllable mechanical response resulting from phase transformations that may include multiple phases, such as austenite and martensite of ordered structures. Austenite-to-martensite phase transformation in SMAs may result from a temperature decrease or an increase in applied stress. Since austenite can transform to different martensite phases, depending on the temperature and applied stress, it is often referred to as the parent phase, whereas the various types of martensite are known as derivative phases. Martensite-to-martensite phase transformation may also occur under certain conditions of stress and temperature. Stress–strain responses of various SMAs have been obtained at different temperatures.1–8 Among all SMAs, single-crystal Cu–Al–Ni exhibits the largest strain recovery (∼17%), in addition to high thermal and electrical conductivity and temperature-dependent damping ratio.1,2,9 Materials with high damping ratio are desirable
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0116 994
J. Mater. Res., Vol. 22, No. 4, Apr 2007 http://journals.cambridge.org Downloaded: 11 Mar 2015
for reducing vibrations and noise in high-speed devices involving dynamic contacts. The large strain recovery and adjustable damping ratio are due to the formation of four intriguing martensite phases (␣⬘1, ⬘1, ⬙1, and ␥⬘1) from the parent austenite 1 phase under different stress and temperature conditions. For example, excessive deformation of the parent 1 phase leads to the formation of ⬘1 martensite,5,10–12 ␥⬘1 can be obtained by
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