A new insight into heterogeneous nucleation mechanism of Al by non-stoichiometric TiCx

Abstract

Al–Ti–C master alloy has been known as an efficient grain refiner for a series of Al alloys since 1950s. However, the grain refinement mechanism is still not fully elucidated up to now. Herein, we systematically investigate this mechanism by combining Cs-corrected transmission electron microscopy and theoretical calculations. First, the Al/TiCx nucleation interface is determined to be a “Al ∼ Interfacial transition layer ∼ TiCx” sandwich-like structure, with the orientation relationship of [011]Al//[011]TiCx and (11¯1)[011]Al plane inclined 21° to (11¯1)[011]TiCx plane that is beneficial to eliminate the lattice constant misfit between Al and TiCx. The interfacial transition layer, consisting of Al and Ti atoms, play a crucial role in reducing atomic arrangement mismatch between (13¯3)[011]Al and (11¯1)[011]TiCx, thus promoting Al nucleation. Second, rather than directly reacting with Al melt as conventionally reckoned, TiCx with x < 0.92 actually releases Ti atoms to Al melt first after inoculation (720 °C), contributing to the formation of the Ti-containing interfacial transition layer. On the other hand, after Ti releasing, C/Ti ratio in TiCx is increased, which inversely decreases the Ti-releasing capability and leads to grain refinement fading gradually. This study sheds new light on the grain refinement mechanism in Al–Ti–C-refined Al and should build up the foundation for developing high-efficiency TiCx-containing grain refiners.