Al-TiC composites In Situ-processed by ingot metallurgy and rapid solidification technology: Part I. Microstructural evolution

The present work was undertaken to highlight a novel in situ process in which traditional ingot metallurgy plus rapid solidification techniques were used to produce Al-TiC composites with refined microstructures and enhanced dispersion hardening of the reinforcing phases. Microstructures of the experimental materials were comprehensively characterized by optical microscopy, electron microscopy, and X-ray diffraction. The results show that the in situ-synthesized TiC particles possess a face-centered cubic crystal structure with an atomic composition of TiC_0.8 and a lattice parameter of 0.431 nm. The typical ingot metallurgy microstructures exhibit aggregates of TiC particles segregated generally at the α-Al subgrain or grain boundaries and consisting of fine particles of 0.2 to 1.0 μm in size. The rapidly solidified microstructures formed under certain thermal history conditions contained a uniform, fine-scale dispersion of TiC phase particles with a size range of 40 to 80 nm in an α-Al supersaturated matrix of 0.30 to 0.85 μm in grain size. These dispersed TiC particles generally have a semicoherent relationship with the α-Al matrix. Based on the experimental results, a comprehensive kinetic mechanism of in situ TiC synthesis, which includes a solid-liquid interface reaction between the carbon particles and the Al melt and multiple nucleation and growth of TiC from the Al melt, was proposed. Then, the evolution of the aggregate TiC particles in a superheated melt before rapid solidification, i.e., dissolution, nucleation, and growth of the regenerated TiC dispersed particles, was analyzed. Furthermore, the behavior of rapid solidification kinetics, the nucleation of α-Al on TiC-dispersed particles, and the interaction between TiC particles and the solidification front were documented experimentally and theoretically. These studies provided the theoretical criteria and an experimental basis for the optimum design of this kind of composite.