Influence of alloying elements on the kinetics of massive transformation in gamma titanium aluminides

Abstract

The effect of alloying elements, Nb and Mn on massive transformation in Ti-45Al based alloys was studied. The alloy samples were heat treated at 1350 °C for 30 minutes and subsequently cooled to room temperature by furnace cooling, air cooling, and water quenching. The microstructural evolution in various alloys was investigated by a detailed microstructural characterization of the heattreated samples by optical, scanning, and transmission electron microscopy. It was observed that the volume fraction of the massively transformed gamma in water-quenched samples increased from nil in the Mn-free alloy to ϳ70 pct in the alloy containing 2 at. pct Mn. Nb had a minimal effect on the extent of transformation. The effects of Nb and Mn have been rationalized on the basis of the site occupancy of the alloying elements and their possible influence on the phase boundaries and grain size. An attempt has also been made to elucidate the mechanism of massive transformation in these alloys. Based on the results obtained, the influence of Mn and Nb on the kinetics of massive transformation is presented and discussed and, the CCT diagrams for different alloys used in this study are proposed.

The role of oxygen content and cooling rate on transformations in TiAl-based alloys

“…Some evidence has been published which suggests that other nucleation sites exist [12]. In addition in unpublished work on a TiAl-based alloy it has been found that contrary to this mechanism fine lamellae are observed in slowly quenched samples at both sides of grain boundaries in grains which were otherwise massively transformed.…”

A numerical model for the description of the lamellar and massive phase transformations in TiAl alloys

“…The critical cooling rate for the transition from lamellar to massive microstructures can be a crucial parameter for the heat treatment of large pieces where massive g may be difficult to obtain in the core of the component. The critical cooling rate can be lowered by the addition of heavy alloying elements such as Nb and Ta [9,10].…”

“…Although bulk nucleation has been reported [9,19], it is generally accepted that nucleation of massive g occurs predominantly at grain boundaries. Gamma nuclei generally exhibit a Blackburn orientation relationship with one of the adjacent a grains while they grow at the expense of the other grain by the movement of highly mobile incoherent interfaces [20][21][22].…”

On the massive phase transformation regime in TiAl alloys: The alloying effect on massive/lamellar competition

“…It is known [9,10] that alloys with different compositions have different responses to continuous cooling but there is at present no way of predicting the range of cooling rates required to produce massive gamma in the given TiAl-based alloys. Thus, in order to utilize the massive transformation as a method of microstructural refinement in cast TiAl alloys it is necessary to define the massive transformation regime in each alloy.…”