The effect of microstructure on mechanical properties of directionally solidified intermetallic Ti-46Al-8Nb alloy

摘要

The effect of microstructure on mechanical properties of directionally solidified (DS) intermetallic Ti-46Al-8Nb (at. percent ) alloy was studied. After directional solidification at constant growth rates V ranging from 5.56 X 10~(-6) to 1.18 X 10~(-4) m s~(-1) and constant temperature gradients in liquid at the solid liquid interface G_L from 3.5 X 10~3 to 8 X 10~3 K m~(-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 alpha_2(Ti_3Al) + gamma(TiAl) lamellae. Mean alpha_2-alpha_2 interlamellar spacing A decreases with increasing growth rate V and increasing cooling rate. Room temperature Vickers microhardness HV_m and compressive yield stress sigma_y at 700 deg C increase with decreasing interlamellar spacing A according to Hall-Petch relationship. High-temperature compressive yield stress depends on an angle & between lamellar boundaries and loading axis. Maximum values of the yield stress are measured at theta = 90 deg and minimum values at theta ranging from 30 deg to 60 deg . Columnar grain structure leading to a high anisotropy of mechanical properties can be transformed to fine equiaxed grains with convoluted type of alpha_2 + gamma 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 deg C.