Abstract
The effect of the solution annealing temperature and cooling rate on the microstructure and hardness of as-cast in-situ composite with nominal composition Ti-38Al-7.5Nb-5C-0.9Mo (at.%) was studied. The composite was prepared by vacuum induction melting in graphite crucible followed by centrifugal casting into the graphite mould. Heat treatment of the composite consisted of the solution annealing at 1200, 1300, and 1400 °C followed by cooling to the room temperature at two different cooling rates. The matrix of the as-cast composite consisting of α₂(Ti₃Al) and β/B2(Ti) phases is reinforced by uniformely distributed plate-like (Ti,Nb)₂AlC and irreguraly shaped particles composed of (Ti,Nb)₂AlC and a small amount of (Ti,Nb)C phases. The applied heat treatment has no measurable effect on the volume fraction of the primary carbide particles. The solution annealing at 1300 and 1400 °C leads to the full transforamtion of the (Ti,Nb)C phase retained in the cores of irreguraly shaped particles to the (Ti,Nb)₂AlC phase. The cooling from thermodynamically stable β + α + (Ti,Nb)₂AlC phase-field at a rate of 5 °C min^–1 results in the formation of lamellar α₂ + γ colonies with β/B2 and γ(TiAl) phases on their boundaries. The cooling from thermodynamically stable β + α + γ + (Ti,Nb)₂AlC phase-field results in the formation of multiphase matrix consisting of α₂, β/B2 and γ phases in the form of single-phase regions and small amount of coarse lamellar α₂ + γ regions. Increasing volume fraction of β/B2 phase with the increasing solution annealing temperature leads to an increase in Vickers hardness of the composite.

Key words
intermetallics, TiAl, composites, casting, microstructure, hardness

Full text (1540 KB)