Grain boundary engineering for enhanced mechanical properties in TWIP Ti-alloys – GRAFITI

Aiming at enhancing the work-hardening behavior of strategic light-weight and corrosion resistant Ti-alloys, the TWIP (twinning induced plasticity) Ti-alloys family was developed. Playing with the stability of the ?-phase (body centered cubic), the TWIP mechanism can be triggered. Thanks to this alternative deformation mechanism, complementing dislocation glide, a large work-hardening rate could be obtained in alloys of the ?-metastable family, as simple as single-phase binary Ti-15Mo alloy. However, TWIP Ti-alloys display a rather low yield strength. To further optimize the mechanical properties without sacrificing the concept of working on simple systems, the opportunity of microstructure optimization, and in particular tuning the structure and chemistry of the grain boundary, will be investigated. Indeed, grain boundaries are in continuous interaction with the dynamically formed mechanical twins, from their nucleation to a possible twin transmission to a neighboring grain. Although this parameter seems critical to understand and ultimately control the alloy deformation, studies considering the grain boundaries of TWIP Ti-alloys are scarce, and only focus on the mechanism of twin transmission without considering other parameters, such as a possible segregation at the grain boundary. By comparing the Ti-15Mo, Ti-15Mo-xO and Ti-15Mo-1.5Sn alloys, the influence of the grain boundary character (low- or high-angle, at- or out-of-equilibrium following forging-like processes) and its chemistry (from elemental segregation of oxygen or Sn to phase precipitation) on the mechanical twinning (nucleation and transmission) will be assessed. Based on the results, strategies aiming at emphasizing some mechanical properties, such as the yield strength, through grain boundary engineering will be proposed and implemented in a proof of concept.