Structure and properties of a monocrystalline Cu-Al-Ni alloy submitted to thermal cycling under load, pp. 113-143
Authors: L.A. Matlakhova, E.C. Pereira, A.N. Matlakhov and S.N. Monteiro
Abstract: Monocrystalline Cu-Al-Ni alloys are promising alternatives to be used in shape memory effect (SME) actuators, in which the material suffers under temperature and applied load a combined action of many factors including thermal cycling as well as elastic and plastic deformations that modifies the initial structure and characteristic properties. In this chapter, a monocrystalline Cu-13.5wt.%Al-4wt.%Ni (Cu-13.5Al-4Ni) alloy displaying SME was submitted to thermal cycling treatments, load-free and under load, within the critical range of temperatures associated with reversible martensitic transformations (RMT). The investigation was conducted in terms of number of cycles, magnitude of applied load, resulting structure changes, characteristics of the RMT and mechanical behavior. The alloy was produced by the ―Memory Crystal Group‖ in Russia and its characterization was carried out using optical and electron microscopy, X-rays diffraction, differential scanning calorimetry and mechanical tests. A special device was developed to perform the hundreds of thermal cycling treatments under load. It was found that the thermal cycling treatment promotes changes in the amount of phases - a martensite 1, a high temperature stable 1 and an intermediate R phase - associated with the RMT. These changes depend on the way the treatment ends, either by heating or by cooling. Load-free thermal cycling contributes to enlarge the temperature interval of RMT occurrence, but does not change the interval of hysteresis. After 300 cycles, if the end occurs by cooling below Mf, an intermediate R phase, related to the 1 1, is obtained. The alloy submitted to thermal cycling under load reveals a tendency towards a decrease in the critical intervals for both direct and reverse martensite transformation. This tendency is accentuated for 300 cycles, when the critical intervals smaller than 10 oC, are recorded. The applied load promotes the reverse transformation at lower temperatures as well as the direct martensitic transformation at higher temperatures, reorienting the structure and stabilizing the R phase, coherent with 1 and 1. The alloy‘s mechanical behavior is characterized by a complex process involving both its RMT and phase reorientation. The results of compressive tests of the initial state revealed a ―pseudoyielding‖ plateau followed by elastic deformation up to 1350 MPa and subsequent fracture, disclosing its brittleness. Thermal cycling under load reduces the pseudo-yield stress by 20-25 MPa and decreases the ultimate stress to 950 MPa. The alloy shows good resistance to irreversible changes during all thermal cycling tests. These results have a particular relevance for the practical application of the monocrystalline Cu-13.5Al-4Ni alloy as SME actuator.
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