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

• Applicants: Professor Dr. Blazej Grabowski; Professor Dr.-Ing. Hauke Springer, since 1/2016
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2014 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 259401811

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.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1568: Design and Generic Principles of Self-Healing Materials
• International Connection: Austria, United Kingdom
• Participating Persons: Professor Dr. Jörg Behler; Dr. Andrew Duff; Dr.-Ing. Bengt Hallstedt; Sascha Maisel; Professor Dr. Stefan Müller (†); Professor Dr. Jörg Neugebauer; Professor Dr. Erwin Povoden-Karadeniz; Professor Dr.-Ing. Dierk Raabe
• Ehemaliger Antragsteller: Dr. Cemal Cem Tasan, until 1/2016