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，价格相对低廉，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。如果减小晶粒度，可以显著提高材料的力学性能。这可以通过添加某些合金元素(例如，Zr)来实现，这些元素在凝固过程中限制了晶粒的生长。此外，还可以提高冷却速度以获得细小的显微组织。随着晶粒度的减小，形状记忆合金的强度和塑性普遍提高，相变温度降低。在本方案中，我们想要研究两种铜基形状记忆合金--Cu-11.85Al-3.2Ni-3Mn和Cu-11.35Al-3.2Ni-3Mn-0.5Zr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，并将经过严重塑性变形(等通道转角挤压，ECAP)处理。随后在不同条件下进行的热处理(时效或退火热处理)应改变(I)晶粒度，(Ii)有序/无序程度，(Iii)缺陷密度(晶界、位错和空位)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显微组织的详细分析，有望揭示上述各因素对相变温度变化的贡献。在选择性激光熔凝时，晶界比例不同的样品将获得特殊的织构。众所周知，这些所谓的寡晶形状记忆合金具有更好的形状记忆性能，在该项目中，应评估是否可以通过使用选择性激光熔化来合成这些独特的微结构。

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.