Microstructural evolution during annealing of a powder metallurgical TiAl–Nb composite and its effect on mechanical properties

Abstract

Ductile Nb particles have been applied to toughen TiAl-based intermetallic alloys due to the positive effects of the Nb element on their mechanical and processing properties. However, the room-temperature (RT) fracture toughness of powder metallurgical (PM) TiAl–Nb composites needs further improvement to avoid micro-cracks in complex components prepared by PM near-net shape technologies. Herein, a facile heat treatment—annealing in the (α+γ) phase region for a short duration and air-cooling—was applied to strengthen and toughen a PM TiAl–Nb composite. The phase and microstructure evolution of the PM composite during annealing, and their effects on mechanical properties and fracture behaviors, were investigated specifically. It is found that after annealing, the volume fraction of α2 phase increased markedly, while that of the B2 phase decreased significantly, and the Nb solid solution (Nbss) phase almost completely disappeared; the microstructures of the TiAl–Nb composite became significantly finer and more uniform overall, and the microstructure type of the TiAl matrix transformed from near-gamma (NG) to duplex (DP). Both the RT tensile strength and fracture toughness of the TiAl–Nb composite improved markedly after annealing. The strengthening mechanisms for the as-annealed composite were primarily based on the phase and microstructure evolution. The toughening mechanisms for the as-annealed composite included the generation of shear ligaments, delaminations, crack deflection and branching. The improvement in tensile strength and the reduction in work-hardening exponent made the composite tougher through facile annealing. The incorporation of Nb particles and further heat treatment can efficiently facilitate the improvement in fracture toughness of PM TiAl-based alloy.