Microstructure and mechanical properties of TiAl-(TiB₂+Ti₂AlN) composites produced by combustion reaction process of Ti-Al-BN powder mixtures

Abstract

To improve low room-and high-temperature strength of titanium alminides, TiAI based composites reinforced with TiB₂ and Ti₂AIN particles were made by combustion reaction process using Ti-AI-BN powder mixtures, and arc-melted for densification. The powder compositions included Ti_47.4Al_44.2(BN)_8.4, Ti_49.0Al_42.6(BN)_8.4 and Ti_50.5Al_41.1(BN)_8.4 in at%, corresponding to matrix composition of Ti₄₈A_l52, Ti₅₀Al₅₀ and Ti₅₂Al₄₈, respectively. The volume fraction of reinforced particles was about 18%. The as cast specimens had a matrix of TiAl(γ) with a lamellar structure of Ti₃Al(α₂). In the annealed specimens, such lamellar structures were completely or partially disappeared, while smaller particles (about 100-200nm) were observed in the matrix of all specimens. These particles were found to be MAIN and mainly formed in place of disappeared Ti₃Al in the lamellae. The formation of these particles was attributed to different solubility limit for nitrogen in yand α₂ and solutionizing α₂ phase by heat treatment. It was found that the composite materials had a high strength at both room and elevated(1173K) temperatures; 800-1100MPa and about 600MPa, respectively. The bending and fracture toughness tests also proved that the composite annealed is better than the as cast, and values were about 0.4% in ductility and 13-15 MPam in fracture toughness.