Young Investigator Program: Quasi-Liquid Grain Boundary Films in Refractory Metals

摘要

Nanoscale intergranular 'glassy' films often control the fabrication and mechanical properties of high-temperature structural ceramics. This AFOSR Young Investigator program unequivocally demonstrated the high-temperature formation of analogous liquid-like grain boundary (GB) films in metallic refractory alloys. The bulk computational thermodynamic (CalPhaD) methods were extended to GBs, predicting the onset of GB disordering at as low as 60-85% of the bulk solidus line. Combined experimental and modeling studies of both W and Mo based systems unambiguously demonstrated that the mysterious solid-state activated sintering is due to the increased mass transport in impurity-based liquid-like GB films that are thermodynamic stabilized below the bulk solidus line. Determining this solid-state activated sintering mechanism solved an outstanding scientific problem that has puzzled the materials community for >50 years. Subsequently, Ni-doped Mo was selected for systematical evaluation via characterizing well-quenched specimens and thermodynamic modeling. Finally, 'GB 'phase' diagrams' were developed as a new materials science tool to control microstructural evolution and forecast high-temperature properties; their correctness has been quantitatively validated by measuring GB diffusivities, direct HRTEM and Auger characterizations, and atomistic simulations. The developed high-temperature interfacial thermodynamic models can be further extended to more complex multicomponent metals and ceramics in future studies.