Abstract
Gas-atomised spherical powders of Ti-45Al-5Nb and Ti-45Al-10Nb alloys were produced using the plasma melting induction guided gas atomisation (PIGA) technique. The phase composition was determined by X-ray diffraction at the synchrotron beamline HEMS at PETRA III (DESY), as well as by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), two dimensional and focused ion beam (FIB) based three dimensional electron back scatter diffraction (EBSD) measurements. Due to the high cooling rates the alloy composition of both alloys consists of hexagonal-close-packed α- and body-centred-cubic β-phase. The α-phase is dominant in the larger powder size fractions. Considerable amounts of the β phase were only found in the powder particle size fractions smaller than 32 μm for the Ti-45Al-5Nballoy and smaller than 45 μm for the Ti-45Al-10Nb. A pronounced dendritic cauliflower-like structure was observed in bigger powder particles of the Ti-45Al-10Nb alloy. This gives proof that diffusion took place during the initial β-grain formation, even though there is no orientation relation between the final grain and the dendrite structure in the powder particles. The presence of dendritic structures showed that the cooling rate during powder atomization was still too low to reach the critical growth rate for a planar solidification. The absence of preferred misorientation angles between α-grains indicates that α-grains are not formed out of already solidified β-grains by a solid state phase transformation.
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