Rapid solidification and advanced manufacturing of Cu-based shape memory alloys with complex geometries

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Cu-based shape memory alloys exhibit good shape memory properties, are relatively cheap and easy to process. A major shortcoming, however, is their susceptibility to brittle intergranular fracture. If the grain size is decreased, the mechanical properties can be significantly enhanced. This can be achieved by adding certain alloying elements (e.g. Zr), which restrict grain growth during solidification. In addition, the applied cooling rates can be increased in order to get refined microstructures. When the grain size decreases, the strength and ductility generally increases and the transformation temperatures of the shape memory alloy are lowered.In this proposal we want to investigate, which mechanisms govern the transformation behaviour of two Cu-based shape memory alloys, Cu-11.85Al-3.2Ni-3Mn and Cu-11.35Al-3.2Ni-3Mn-0.5Zr. They will be prepared by means of rapid solidification (injection casting, suction casting and selective laser melting) and will be additionally processed by severe plastic deformation (equal channel angular pressing, ECAP). Subsequent thermal treatments (ageing or annealing) under various conditions shall modify (i) the grain size, (ii) the degree of ordering/disordering, (iii) the density of defects (grain boundaries, dislocations and vacancies) and (iv) the number and volume fractions of the constituent phases. A detailed analysis of the different microstructures by means of electron microscopy, X-ray diffraction, resistivity measurements and positron annihilation is expected to reveal the contribution of each factor listed above to the shift in the transformation temperatures.Via selective laser melting, a special texture shall be obtained in the samples with a varying fraction of grain boundaries. These so called oligocrystalline shape memory alloys are known to have improved shape memory properties and within this project it shall be assessed whether or not these unique microstructures can be synthesised by employing selective laser melting.