Modelling of Grain Boundary Segregation for Nanocrystalline Alloys

Abstract

The Pd-based metal membranes have been focused on many studies, because they are the most promising candidates for hydrogen separation. If Pd-based membranes are designed in nanoscale, then the introduced high volume fraction of grain boundaries can act as fast diffusion paths for hydrogen atoms that leading to an increase of hydrogen separation rate. However, due to the substantial amounts of grain boundaries, the nanocrystalline material tends to show a rapid grain growth at operating temperature (that leading to the coarse grains), which may lead to the failure of this membrane in further applications. However, grain growth could be effectively inhibited by thermodynamic stabilization method-that is solute segregation to grain boundary region. In this thesis, a thermodynamic grain boundary segregation model is developed and is applied to many types of metallic binary alloys, including the Pd-Cu, Pd-Zr, Pd-Y, Fe-Zr, Y-Fe and so on. From this model, the segregation tendency of alloying constituents is determined, and the solute concentration in grain boundary is obtained. Moreover, the grain boundary energy (GB) of investigated alloys can be calculated via this model, by comparing the calculated GB energy with that of pure Pd, we are able to determine whether these alloys can be stabilized by the segregation-introduced thermodynamic stabilization. These calculated results have been compared with those reported in the literature, verifying the predictability of this model. In addition, X-ray diffraction (XRD) analysis, including the XRD residual stress measurement and X-ray peak broadening analysis have been performed on both Pd0.7Cu0.3 and pure Pd thin films, the experimental results will help us determine whether the solute segregation occurs and whether this alloy can be stabilized by the solute segregation.