Effect of milling conditions on structural evolution and phase stability of [Ti(H2) + Al + Nb] powder mixtures

Mechanical properties of structural intermetallics are very sensitive to chemical composition and to microstructure. Mechanical alloying is here applied to (Ti + 22Al + 23Nb) and (TiH2 + 22Al + 23Nb) mixtures in order to tailor both chemical and microstructural homogeneity and to refine powder size, for the synthesis of high quality Ti2AlNb-based intermetallics. Crystal structure and phase evolution during mechanical alloying were investigated by means of X-ray diffraction and Rietveld analysis. It was identified that milling of the (Ti + 22Al + 23Nb) mixture led to the formation of a bcc (Nb, Ti, Al) solid solution, which was further confirmed by TEM analysis. Replacing pure Ti with TiH2 resulted in the formation of a bcc (Nb, Ti, Al)Hx solid solution. Prolonged milling led to the transformation of a mixture of the bcc-(Nb, Ti, Al)Hx solid solution and an fcc-TiH2 into a fully amorphous phase. Selected area electron diffraction confirmed this finding. An EDS analysis showed that the amorphous phase was a (Ti, Nb, Al)Hx solid solution.