Highlight Lecture (Symp. D01)
Precipitation hardening is a very effective way to improve mechanical properties of alloys. This study illustrates the importance of understanding the fundamental features that underlie the behavior of L12 ordered nanoscale phases with coherent interfaces embedded in a solid matrix and their role in the evolution of microstructure in materials. The fundamental principles established using model alloy systems are employed in the design and testing of new materials such as systems for energy-related technologies. For the ternary AlLiSc alloy we show a way of producing coarsening resistant monodispersed Al3(LiSc) core/shell particles in an Al matrix with unusually narrow size distribution. A monodispersed particle size distribution was generated by a complex size focusing effect that drives the system in the opposite direction of typical Ostwald ripening. This approach uses differential diffusivities and solubilities of Li and Sc in an Al matrix. Our model shows that the complex precipitation pathway can be fully understood within the framework of classical theories of nucleation and growth. The effect of Li addition on core/shell precipitate formation in the ternary AlLiSc alloy has been studied by a range of advanced microscopy and spectroscopy techniques, such as high resolution transmission electron microscopy (HRTEM) with exit wave (EW) reconstruction, atomic resolution HAADF imaging, and energy filtered electron energy loss spectrum imaging (EELS), combined with first principle calculation and continuum thermodynamic modeling to uncover the role of Li. The phase of the exit wave demonstrates that Al columns can be clearly distinguished from Li columns in the Al3Li shell, and the EW data allows a quantitative analysis and comparison between experimental and calculated contrasts of Li atom columns.