Grain boundary segregation as the process enabling thermodynamic stability in nanocrystalline metal alloys

The present thesis is part of the theoretical and experimental investigations at the cutting edge of research fields in Materials Science and Physical Metallurgy in particular. Aiming at the design and fabrication of multicomponent, multiphase thermodynamically stable NC metal alloys, current studies address the unprecedented challenge of revolutionizing one of the most intensely explored areas of investigations in Materials Science, namely the one related to nanostructured metals. To such aim, the present thesis combines theoretical modelling and experimental validation. Based on the hypothesis that solute segregation at GBs can result in a significantly enhanced thermal stability of NC alloys, we developed a thermodynamic model that describes the thermodynamics of NC metal alloys involving an arbitrary number of elemental constituents and phases. We examined the possible attainment of structural stability due to GB segregation within the framework of equilibrium thermodynamics. Binary and ternary metal alloys have been systematically investigated to identify candidate systems possibly able to exhibit thermodynamic stability under suitable thermal and microstructural conditions. In particular, we focused on W- based alloys, while extending investigations to miscible binary alloys such as W-Al ones, and checked the model validity against W-Ag, predicted to exhibit a strong tendency to phase separation. These alloys were then produced by mechanical alloying and their thermal stability was evaluated. Overall, the experimental findings emphasize the potential of the thermodynamic modelling developed in the design of NC metal alloys with enhanced coarsening resistance. It definitely appears that the intrinsic microstructural instability related to the high volume density of GBs can be significantly reduced, or suppressed, in the presence of alloying element able to segregate at GBs. While the favourable influence of GB segregation has been proven so far only for immiscible alloys, we have shown that similar effects can be expected for miscible alloys.