Abstract
The effect of two types of heat treatment, (i) cyclic heat treatment composed of annealing at temperatures ranging from 1543 to 1593 K combined with oil quenching and (ii) heat treatment consisting of annealing at a constant annealing temperature of 1623 K combined with cooling at constant rates ranging from 0.056 to 1.181 K.s^-1, on the microstructure and mechanical properties of a cast Ti-44Al-4Nb-4Zr-0.2Si-0.3B (at.%) alloy was studied. Grain size and Vickers hardness are decreased to final values after the first heat treatment cycle. Increasing number of the cycles has no statistical effect on these parameters. This heat treatment does not improve room-temperature tensile ductility. On the contrary, tensile tests at 1023 K showed clear yielding and plastic elongation before fracture ranging from 0.21 to 0.32 %. Increase of the cooling rate decreases mean interlamellar α ₂-α ₂ spacing within lamellar α ₂+γ grains and leads to formation of numerous equiaxed β(B2), γ and ω grains in the microstructure. Vickers hardness, room-temperature compressive yield stress, high-temperature tensile yield stress and high-temperature ultimate tensile stress increase with increasing cooling rate. Change of the cooling rate does not improve room-temperature ductility.

Key words
titanium aluminides, TiAl, heat treatment, microstructure, mechanical properties

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