Microstructure control and mechanical properties of directionally solidified TiAl-Nb alloys

The double directional solidification (DS) technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys. Polysynthetically twinned (PST) crystals with lamellar boundaries parallel to or inclined 45° to the growth direction were achieved due to the complete peritectic transformation during directional solidification of the alloys with the dendritic solid/liquid interface. The PST crystals with aligned lamellar boundaries only parallel to the growth direction were produced when lamellar grains with lamellar boundaries in the same orientation were seeded by themselves under appropriate growth conditions. Low boron addition is harmful to align the lamellar orientation because of the growth of non-peritectic α phase. Due to the larger yttria particles and boride ribbons in the directionally solidified TiAl-Nb alloys, the tensile plastic elongations of the alloys are only close to 2%.