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

铜基形状记忆合金具有良好的形状记忆性能，相对便宜且易于加工。然而，一个主要的缺点是它们对脆性沿晶断裂的敏感性。如果晶粒尺寸减小，则可以显著提高机械性能。这可以通过添加某些合金元素（例如Zr）来实现，其在凝固期间限制晶粒生长。此外，可以增加所施加的冷却速率以获得细化的显微组织。当晶粒尺寸减小时，形状记忆合金的强度和塑性普遍提高，而相变温度降低，本研究旨在探讨Cu-11.35Al-3.2Ni-3 Mn和Cu-11.35Al-3.2Ni-3 Mn-0.5Zr两种铜基形状记忆合金的相变机制。它们将通过快速凝固（注射铸造，吸铸和选择性激光熔化）制备，并将通过剧烈塑性变形（等通道角挤压，ECAP）进行额外处理。在各种条件下的后续热处理（时效或退火）应改变（i）晶粒尺寸，（ii）有序/无序程度，（iii）缺陷（晶界、位错和空位）密度和（iv）组成相的数量和体积分数。通过电子显微镜、X射线衍射、电阻率测量和正电子湮没等手段对不同的显微组织进行详细的分析，可以揭示上述各因素对相变温度变化的贡献。通过选择性激光熔化，可以在具有不同晶界比例的样品中获得特殊的织构。已知这些所谓的多晶形状记忆合金具有改善的形状记忆性能，在本项目中，应评估这些独特的微结构是否可以通过采用选择性激光熔化来合成。

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.