Contribution to ternary system Al–Ti–B Part 2 – Study of alloys in Al–AlB 2 –TiB 2 triangle

Abstract

In arc melted and aluminothermically prepared alloys with composition in the triangle Al–AlB₂–TiB₂ four solid phases are observed: α-Al, ρ-AlB₁₂, AlB₂, and TiB₂. The univariant reaction between L, ρ-AlB₁₂, and TiB₂ is the eutectic one L⇌ α-AlB₁₂ + TiB₂. During the reaction TiB₂ platelets are sequentially formed on facets of α-AlB₁₂, but their formation may be completely suppressed by the presence of primary TiB₂. Decomposition of α-AlB₁₂ does not occur on cooling, but only during isothermal annealing under 900°C probably according to the transition reaction L + α-AlB₁₂ → TiB₂ + AlB₂. Dissolution of α-AlB₁₂ and growth of AlB₂ take place in afaceted manner owing to their high entropies of solution. The low temperature AlB₂ phase is not formed at all on cooling of concentrated alloys with an overall composition lying on the AlB₂-TiB₂ tieline, whereas in less concentrated alloys it is usually formed on the primary TiB₂ particles in the regions distant from α-AlB₁₂