Abstract
The effect of microstructure on mechanical properties of directionally solidified (DS) intermetallic Ti-46Al-8Nb (at.%) alloy was studied. After directional solidification at constant growth rates V ranging from 5.56x10^-6to 1.18x10^-4 ms^-1 and constant temperature gradients in liquid at the solid liquid interface G_L from 3.5x10³ to 8x10³ Km^-1, the DS samples contain several columnar grains aligned in a direction parallel or nearly parallel to the growth direction. The microstructure within the columnar grains is fully lamellar consisting of α₂(Ti₃Al)+γ(TiAl) lamellae. Mean α₂-α₂ interlamellar spacing λ decreases with increasing growth rate V and increasing cooling rate. Room-temperature Vickers microhardness HV_m and compressive yield stress σ_y at 700 °C increase with decreasing interlamellar spacing λ according to Hall-Petch relationship. High-temperature compressive yield stress depends on an angle Θ between lamellar boundaries and loading axis. Maximum values of the yield stress are measured at Θ = 90° and minimum values at Θ ranging from 30° to 60°. Columnar grain structure leading to a high anisotropy of mechanical properties can be transformed to fine equiaxed grains with convoluted type of α₂+γ microstructure by appropriate heat treatments. Compressive yield stress of the specimens with equiaxed grain structure continuously decreases with increasing test temperature. A simple relationship is proposed for a prediction of the compressive yield stress at temperatures ranging from 20 to 900 °C.

Key words
titanium aluminides, TiAl, crystal growth, heat treatments, phase transformations, microstructure, mechanical properties

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