Characterisation of structural stability of (Ti(H<inf>2</inf>) + 22Al + 23Nb) powder mixtures during mechanical alloying

Abstract

Mechanical properties of Ti2AlNb-based aluminides are very sensitive to microstructural morphology. As a pre-processing technique, mechanical alloying (MA) can offer extensive constitutional and microstructural modifications for tailoring the properties. It is, therefore, essential to understand fully the structural evolution of the constitutive powder during MA in order to control the final microstructure of the sintered aluminide. Here, a comparative study using both XRD and an analytical Rietveld technique was systematically carried out to determine the mechanisms of MA and the evolutions of crystalline phases and microstructure of two powder mixtures, an elemental (Ti + 22Al + 23Nb) and a (TiH2+ 22Al + 23Nb) (at.%). The latter mixture was chosen to explore a novel idea of using the Ti hydride to enhance the microstructural evolution during MA, for the synthesis of desirable Ti2AlNb-based aluminides. Both SEM and X-Ray mapping were employed to further clarify the powder morphology and micro-chemical distribution of the alloying elements. For the (Ti + 22Al + 23Nb) mixture, formation of a bcc (Nb, Al, Ti) solid solution was identified with the lattice parameter ranging from 3.2982 to 3.2074 Å, and full amorphisation was not achieved even after extensive MA. TiH2was shown to improve considerably powder recovery, enhance powder refinement and homogenisation, and accelerate the amorphisation of the mixed powder. © 2002 Elsevier Science B.V. All rights reserved.