Peritectic solidification of Ti—Al—Ta alloys in the region of γ-TiAl

Abstract

An investigation was made into the solidification behavior and phase equilibria of titanium alloys with 45–70 at.% Al and 0–27 at.% Ta, in which γ-TiAl is the predominant phase. A partial liquids surface was determined from studies on arc buttons. The principal features are peritectic cascades of the type L+β→α, and L+α→γ, emerging from the Ti—Al binary into the ternary, and a line of twofold saturation between γ and η. For alloys solidifying with β, α or γ as the primary phase, segregation led to significant enrichment of tantalum and depletion of aluminum in the dendrites. This gave rise to precipitation of σ, even for bulk alloy compositions within the equilibrium single-phase γ field. Transformations involving solute partitioning were frequently suppressed at the rather modest cooling rates characteristic of arc melting, presumably because of the sluggish diffusivity of tantalum in the various phases. The ensuing supersaturation results in further decomposition of the as-cast structure in the early stages of heat treatment, producing non-equilibrium precipitates which must be redissolved in order to achieve subsequent homogenization of the alloy. Typically, primary β dendrites decompose into, α, γ and σ, depending on the composition, whilst α may transform to either α2+γ or γ+σ. The γ phase is stable at low aluminum contents, but precipitates η in the richer aluminum alloys. Long-term homogenization treatments at 1373 K produced microstructures in close agreement with currently available information on the ternary phase diagram.