Abstract
The effect of Al content on the microstructure of Ti-xAl-8Nb-3.6C-0.8Mo (at.%) composites reinforced with carbide particles, where x = 38, 42, 45, and 47 at.%, was studied. The composites prepared by vacuum induction melting in graphite crucibles followed by centrifugal casting into graphite moulds were studied in the as-cast and heat-treated state. The intermetallic matrices of the as-cast composites consisting of α₂(Ti₃Al)-, γ(TiAl)-, and β/B2(Ti)-phases are reinforced with uniformly distributed carbide particles composed of (Ti,Nb) ₂AlC-phase and small amount of (Ti,Nb)C-phase preserved in the cores of some coarse irregular shaped carbides. The increasing Al content leads to a decrease in the volume fraction of β/B2-phase and an increase in the volume fraction of the lamellar α₂ + γ and single γ-phase regions in the as-cast composites. The increasing Al content increases shape factor and decreases the mean size of the primary carbide particles. In the heat-treated composites, the intermetallic matrices change from multiphase composed of lamellar α₂ + γ grains with β/B2- and γ-phase regions formed along the lamellar grain boundaries, through nearly lamellar to single γ-phase with increasing Al content. The (Ti,Nb)C regions in the core of some carbide particles transform to (Ti,Nb) ₂AlC-phase during the heat-treatment. The Vickers hardness and matrix microhardness decrease with the increasing Al content in both the as-cast and heat-treated composites.

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

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