Processing, microstructure and mechanical properties of TiₓAlᵧ/Al₂O₃ and Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC composites

Abstract

The TiₓAlᵧ/Al₂O₃ metal-ceramic composites materials [TiₓAlᵧ is α-Ti(Al,O), Ti₃Al, or TiAl] and Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC ceramic-ceramic composites with different volume fractions of ceramic reinforcement phases were synthesized using a complex combination of high energy mechanical milling, thermal treatment and powder consolidation processes. The thermal treatment process included reactions between carbon and TiO₂, and between Al and TiO₂, TiO₃ or TiOₙC₁₋ₙ, which was produced by reacting TiO₂ and carbon. Heat treating C/TiO₂ composite powder produced pure TiO₃. Pure TiO could not be obtained by only increasing the carbon to TiO₂ ratio in the C/TiO₂ composite powder. Instead, TiOₙC₁₋ₙ solid solution was formed. Reaction kinetic studies showed that higher carbon to TiO₂ ratios accelerated the reaction due to refinement of the C/TiO₂ composite structure. Reactions between Al and TiOₙC₁₋ₙ formed Al₂Ti₄C₂/Al₂O₃ ceramic-ceramic composite. When the aluminum content of the starting Al/TiOₙC₁₋ₙ composite powders was increased, Al₂Ti₄C₂ still formed, but small amounts of Ti₃Al or TiAl also formed. Producing TiC from Al and TiOₙC₁₋ₙ reaction depended on the value of n in the TiOₙC₁₋ₙ phase. When n is small enough, TiC formed; otherwise TiC did not form.

This study found that fully dense matrix phase of TiₓAlᵧ/Al₂O₃ and Al₂Ti₄C2/TiₓAlᵧ/Al₂O₃/TiC formed after sintering at 1550°C. The Al₂O₃ particles became fully sintered when the composites were sintered at 1650°C but not at 1550°C. The Al₂O₃ particles in the α-Ti(Al,O) and Ti₃Al based composites had a high coarsening rate above 1550°C, indicating that the oxygen and Al diffusion rates of α-Ti(Al,O) and Ti₃Al are very high at these temperatures. The Al₂O₃ particles in TiAl based composites also had a high coarsening rate above 1550°C, caused by TiAl being liquid at these temperatures. The Al₂O₃ particles in the Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC based composites had a much lower coarsening rate because of the Al₂Ti₄C₂ matrix the lower oxygen content and high melting point decreases oxygen and aluminum mobility.

The size and volume fraction of Al₂O₃ particles in the composites strongly affected the mechanical properties of the TiₓAlᵧ and Al₂Ti₄C₂ based composites. Hardness of α-Ti(Al,O), Ti₃Al, and Al₂Ti₄C₂ based composites was 800-1600 VHN and hardness of TiAl based composites was 150-500 VHN. Bending strength of TiₓAlᵧ/Al₂O₃ composites was 105-230 MPa and bending strength of Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC based composites was 250-460 MPa. It appeared that 10 μm might be the critical size of Al₂O₃ particles. Below 10 μm bending strength dramatically improved with deceasing the Al₂O₃ particle size in the composites. The fracture toughness of these composites was 2-4.5MPa•m¹/². Composite hardness increased and bending strength decreased with increasing Al₂O₃ particle size and volume fraction.