Characterization of Copper-Based Shape Memory Alloy with Zinc and Aluminum
Abstract
Copper-based shape memory alloy with zinc and aluminum was manufactured, plastically deformed, heat treated and characterized in terms of physico-mechanical, structural and micro-structural investigation. Typical martensitic microstructure with twins is revealed by optical and electron microscopy. The presence of the martensite in the structure was further confirmed through X-ray diffraction. Toughness and hardness of the alloy are investigated too. Optimal properties are obtained for the condition of the alloy that was subjected to heat treatment according to the following scheme: annealing at 850 0C and 900 0C (10 min) + quenched in water + aging at 400 0C (1 hour) + air cooling.
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Introduction
The effect of shape memory is ability of some metals and alloys deformed in martensite state or at temperature interval of martensitic transformation to regain their original shape during the heating process due to complete or almost complete absence of deformation [1,2].
The heating process causes restoration of crystals in high-temperature phase called beta or parent phase and the removal of plastic deformation. In the same time, all physical and mechanical properties are restored.
During the shape recovering process, the alloys can produce a displacement or a force, or combination of the two, as a function of temperature. The starting force of recovering shape process is difference between free energies of parent and martensitic phases during the reverse transformation. The complete shape recovering is only notice if the martensitic transformation is crystallography reverses and if the deformation process is done without plane shearing [2,3].
Shape memory effect has been studied for many binary and ternary alloys, as well as for some pure metals. However, wide application can be found only for nitinol (Ni-Ti alloys) and copper-based alloys that show shape memory effect. Copperbased alloys, compared to nitinol alloys, possess somewhat lower mechanical properties due to their larger grain size and elastic anisotropy [4]. But, they can be improve, considerably without deterioration of shape memory effect, by small grain, method of rapid solidification, sinter metallurgy or by adding the elements such as Zr, V, B, Ti, Cr, etc. [5].
Conclusion
The copper-based shape memory alloys with zinc and aluminum was manufactured, plastically deformed, heat treated and characterized in terms of physic-mechanical, structural and micro-structural analysis.
Quenching in water, following heating at 850 0C and 900 0C, lead to the observation of a typical martensitic microstructure, with twins revealed by optical and electron microscopy.
The presence of the martensite in the structure was further confirmed through X-ray diffraction.
Severe plastic deformation lead to an increase of the the hardness compared to the undeformed samples, a more pronounced increase was observed for the quenched samples.
Establishing a correlation between the state of the material, microstructure and mechanical properties it can be concluded that the combination of thermo-mechanical processing regime, can achieve such a state of the material that provides good mechanical properties.
By reducing the particle size α-solid solution of hot processing increases the hardness and impact toughness compared to the as-cast state.
Heat treatment of hot-processed alloy increases the hardness, while the impact toughness gradually decreases as the microstructure can be explained by the appearance of brittle phases, as a result of thermal deposition.
Optimal properties are obtained for the condition of the material that was subjected to heat treatment according to the following scheme: annealing at 850 0C and 900 0C (10 min) + quenched in water + aging at 400 0C (1 hour) + air cooling.