Microstructure characterization and mechanical properties of TiAl-based alloys prepared by mechanical milling and spark plasma sintering

High Nb containing TiAl alloy powders with or without mechanical milling were consolidated by spark plasma sintering (SPS) technique. The effects of SPS temperature and mechanical-milling treatment on phase constitution and microstructure were studied, and the mechanical properties at room temperature were tested. The phases in the as-atomized powder are composed of major α2 phases, a few γ phases and a trace of β phases. After milling, the diffraction peaks for α2 phase are obviously broadened, and the intensities of diffraction peaks for both γ phase and β phase are decreased. Similar phase constitution including a large quantity of γ phases and a few α2 phases are exhibited in the alloys sintered by either as-atomized powder or as-milled powder. At a sintering temperature of 1200 °C, the microstructure of the alloy sintered by using as-atomized powder consists of inhomogeneous γ and α2 phases as well as a few α2/γ lamellar colonies. By contrast, the densities increase and the microstructures are apparently refined for the alloys sintered by using as-milled powder. Fully dense alloys with uniformly distributed γ and α2 grains can be obtained at extending milling time or increasing rotating speed. The nucleation of γ phase during SPS of the powders is dependent on recrystallization. The heterogeneity of deformation should be responsible for the formation of heterogeneous γ grains in the alloys sintered by using as-atomized powder. The heterogeneous grain size for the alloys sintered by using as-milled powders is mainly derived from the inhomogeneous nucleation of γ phase and the uneven distribution of α2 grains. Good mechanical property accompanied by a fine grain size and a dense microstructure can be achieved by optimizing the process parameters.