Phase transformation behavior of gamma titanium aluminide alloys during supertransus heat treatment

Abstract

Microstructure evolution in a wrought near-gamma titanium alloy, Ti-45Al-2Cr-2Nb, was investigated by a series of heat treatments comprised of initial heating high in the alpha-plus-gamma phase field followed by short-time heating in the single-phase alpha field. The initial heating step led to a dispersion of gamma particles which pinned the alpha grain boundaries. The kinetics of the gamma grain dissolution during subsequent heating in the single-phase field were interpreted in terms of models for both interface reaction-controlled and diffusion-controlled processes. The model for diffusion-controlled dissolution yielded predictions comparable to the observed times, whereas the model for interface reaction-controlled behavior predicted dissolution kinetics over an order of magnitude slower than observed. The growth of the alpha grains, both before and after the dissolution of the gamma phase, was also modeled. Section size limitations to the ability to use supertransus heating to obtain uniform and moderately fine alpha grain sizes were examined using the transformation models and a simple heat transfer analysis approach. The results were validated through the heat treatment of subscale and full-scale forgings.

An Experimental Assessment of the Effects of Heat Treatment on the Microstructure of Ti-47Al-2Cr-2Nb Powder Compacts

“…pct, respectively. Therefore, it can be stated with some confidence that increasing the isothermal temperature above a critical value (in the range 1260°C to 1300°C) leads to sufficient c-phase dissolution by a diffusion-controlled mechanism, as confirmed by Semiatin et al, [32] and to a concomitant Al-enriched a matrix (above 44 at. pct), to promote c lamellar formation upon cooling.…”

“…As a consequence, the a Þ a 2 + c phase transformation is more favored at 1300°C. By taking into account the Ti-xAl-2Cr-2Nb pseudobinary phase diagram of Semiatin et al, [32] it can be deduced that the a transus at 1260°C and 1300°C would be associated with aluminum contents of approximately 44 and 45.5 at. pct, respectively.…”

Proceedings of the 18th Annual Conference on Fossil Energy Materials.

“…hot extrusion and forging of powders and ingots at a temperature, T, higher than the α transus temperature, Tα, or lower than Tα but above the eutectoid temperature, Te [α α 2+ γ] 25-27 ; (4) thermomechanical treatment (e.g., forging plus heat treatment). [28][29][30] The third and fourth methods were widely used in previous studies, which involve mechanical treatments. The finest grain sizes reported so far are those obtained the by hot extrusion of ingots and by hot forging plus heat treatments.…”

Determination of activation energy of phase transformation and recrystallization using a modified kissinger method

“…Phase transformations in titanium aluminides [5][6][7][8][9] and recrystallization in steels are processes extensively studied in the past and present. The processes under continuous heating have been studied by many workers, using mainly thermal analysis.…”