Microstructure and phase relationships in a rapidly solidified dual phase alloy based on Ti 3(Al, Si) + Ti 5(Si, Al) 3

Abstract

The microstructures of a melt-spun TiAlSi eutectic alloy containing 29 at.% Al and 6.5 at.% Si were studied. The solidification structures depend strongly on the local cooling rate and consist of microcells/microdendrites with intercellular/ interdendritic Ti 5(Si, Al) 3-silicide segregates for the high cooling rates, and eutectic structures for the slower cooling rates. This is in contrast with the arc-melted alloy which shows predominantly coupled eutectic structure, with the coupled zone skewed towards the faceted silicide phase. In the cellular/dendritic structure, it is inferred that the high temperature β-Ti(Al, Si) phase transforms massively to the hexagonal close-packed (h.c.p.) α phase which then orders to α 2-Ti 3(Al, Si). The most striking result in this study is the observation of the metastable ordered β(B2) phase in the lamellar eutectic structure, which allows the orientational relationship between β and the silicide phase during eutectic growth to be obtained. The retention of β(B2) at room temperature is discussed in terms of the effects of the lamellar structure on the kinetics of the β phase transformation.