Towards self-healing metals by employing optimally-dispersed Ti-Ni shape memorynano-particles

虽然自修复的概念对所有类别的材料和应用都很有趣，但到目前为止，材料设计的成功案例主要限于聚合系统，少数情况下限于陶瓷。金属材料的自愈--尽管金属在结构应用中起着主导作用--是研究最少的，因为将传统的(聚合物)修复概念应用于金属显然是困难的。金属块体内部缓慢的化学反应和室温下缓慢的扩散速度要求专门为金属材料设计新的自愈机制。NanoTiNi在自修复金属材料领域提出了一个新的概念，可能会开启材料设计的新纪元。其主要思想是将形状记忆效应封装在纳米分散的相干颗粒中，作为自愈剂，提供金属的自主自愈。将共格形状记忆纳米颗粒引入标准固溶体金属基质中并由其稳定。粘结的主体基质将表现出标准的机械性能，如强度、延展性和断裂韧性。通过优化形状记忆纳米颗粒的大小和分布，例如保证对纳米裂纹的最佳长期抵抗力，将产生特殊和新颖的自我愈合性能，而纳米裂纹在正常情况下会引发骨折的发生。目前，纳米裂纹及其应力/应变场将作为局部应力源，激活纳米颗粒的转变，从而实现自愈过程。纳米TiNi的想法是由最近的原子模拟激发的，这些模拟清楚地表明，应力驱动的晶界能够修复接近的纳米裂纹。然而，由于应力驱动的晶界是缓慢的，需要外部应力来激活，它不容易被认为是一种自我修复机制。因此，在我们的方法中，我们用马氏体相变过程中基质纳米颗粒界面的边界运动来代替晶界运动。这使得该机制具有局部性和自主性，从而成为真正的自我修复机制。考虑到建议目标的高度挑战性，NanoTiNi提供了一种综合方法，将最先进的有限温度从头计算模拟和现场多尺度实验表征技术与长期存在的合金设计技术相结合。为了在资金期限内使该项目可行，NanoTiNi专注于一种特定的材料体系，即Ti-Ni-V，其中Ti-Ni作为纳米颗粒的形状记忆合金。这一模型系统的机理有待研究，然而，所开发的方法和知识将适用于具有类似转化特性的其他材料。

Although the concept of self-healing is interesting for all classes of materials and applications, the material design success stories are so far mainly limited to polymeric systems, and for a few cases to ceramics. Self-healing in metallic materials--despite the leading role of metals in structural applications--is the least investigated, due to the obvious difficulty of the application of conventional (polymeric) healing concepts to metals. Sluggish chemical reactions inside the metallic bulk and slow diffusion rates at room temperatures necessitate novel self-healing mechanisms to be designed specifically for metallic materials. NanoTiNi proposes a novel concept within the field of self-healing metallic materials that might open the way to a new era in materials design. The main idea is to encapsulate the shape memory effect in nano-dispersive coherent particles which shall act as self-healing agents providing autonomous self-healing of metals. The coherent shape memory nano-particles will be introduced into and stabilized by a standard solid solution metallic matrix. The coherent host matrix will exhibit standard mechanical properties such as strength, ductility, and fracture toughness. The special and novel self-healing properties will arise by optimizing the size and distribution of the shape-memory nano-particles such as to guarantee an optimum long-term resistance to nano-cracks which--in normal circumstances--would trigger the onset of fracture. For the present purposes, nano-cracks and their stress/strain fields will act as local stress sources activating the transformation of the nano-particles and thereby the self-healing process. The NanoTiNi idea is motivated by recent atomistic simulations which clearly reveal that a stress-driven grain boundary was able to heal an approaching nano-crack. Since however a stress-driven grain boundary is slow and needs an external stress to be activated, it does not readily qualify as a self-healing mechanism. In our approach we therefore substitute the grain boundary motion by the boundary motion of the matrix nano-particle interface during the martensitic transformation. This renders the mechanism localized and autonomic and thus a true self-healing mechanism. Given the highly challenging nature of the proposed goals, NanoTiNi provides an integrated approach combining state of the art finite temperature ab initio simulations and in situ multiscale experimental characterization techniques in conjunction with long standing alloy design knowhow. To render the project feasible within the funding period, NanoTiNi focuses on a specific material system, Ti-Ni-V, with Ti-Ni as the shape memory alloy for the nano-particles. The mechanisms shall be investigated for this model system, however, the developed methodology and knowledge will apply to other materials given similar transformation properties.