The nucleation undercooling of TiB 2 /7075 Al matrix composites, the microstructure observed after solidification at different cooling rate, and the size and distribution of TiB 2 particles were investigated. The experimental results have shown that the grain sizes of TiB 2 /7075 Al matrix composites firstly decreased, then increased, and finally decreased again with the increase of TiB 2 content. The nucleation undercooling of TiB 2 /7075 Al matrix composites first increased, then decreased, and finally increased again with the increase of TiB 2 content when the cooling rates was 5 and 10 °C/min respectively, but kept decreasing with the increase of TiB 2 content at a cooling rate of 20 °C/min. The melting and solidification process showed no significant change with the decrease of cooling rate in 9.0% TiB 2 /7075 Al matrix composites. Most small particles can act as heterogeneous nucleus, which induced grain growth and were captured into the grain by the solid/liquid interface. At the same time, most of the larger particles and a minority of the small TiB 2 particles are pushed into the grain boundary; locating in the grain boundary can hinder the Al atoms from diffusing during the solidification process and restrain α-Al phase growth. The influence of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB 2 particles increased.
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