Hot isostatic pressing and heat treatment development of powder metallurgy beta gamma titanium aluminide alloys

Joining of Powder Metallurgy Beta Gamma Titanium Aluminide Through Diffusion Brazing

“…Further hot isostatic pressing (HIP'ing) this alloy produces a near lamellar microstructure with approximately 2.0 vol.% β phase present in the microstructure (Figure 2.7b) . Group A specimens produced through a HIP'd P/M route exhibit a fine triplex structure with a volume fraction of β phase of approximately 1.0% [40,41]. For group B alloys, significant increase in β phase volume fraction at room temperature has been achieved.…”

“…Under high temperature operating conditions in the 700-850 • C range, typical of gas turbine components, the presence of the β phase in significant volume fraction greater than 10 vol.% is detrimental to strength and creep resistance [9,10,33]. Efforts have been made to remove the β phase during final processing to improve on high temperature performance [40,43]. In order to achieve adequate high temperature creep performance, the purpose of the solutionizing heat treatment is to transform the high β volume fraction microstructure to a fully lamellar two phase (α 2 + γ) structure with a fine colony size, fine lamellar spacing, well interlocked colony boundaries, with β phase volume fractions reduced to less than 3 vol%, similar to conventional fully lamellar γ-TiAl.…”

“…However, the minimization of the β phase is achieved through quick air cooling through the α + γ + β phase field below the eutectoid temperature which promotes the formation of fine lamellae known to improve high temperature creep resistance [46]. Group B beta gamma TiAl microstructures taken through a SCHT heat treatment have been reported [40]. These group B solution heat treated microstructures exhibit fully lamellar colonies with lamellar spacings on the order of 0.04 µm and colony sizes on the order of 40 µm.…”

“…Also, by increasing the Nb and Mo content at the expense of the Al content, a decrease in the linear grain growth rate is achieved due to low diffusion mobility in the γ and α 2 phases [23,47]. [40].…”

“…Thus, with the proper thermomechanical heat treatment, a reduction in primary β to a near-fully lamellar microstructure, while initiating the precipitation of secondary beta phase precipitates (β 0,sec ) may be exploited to improve on the high temperature creep performance of beta gamma titanium aluminides. For P/M Ti-xAl-2Nb-2Mo (at.%) beta gamma TiAl, aging treatments have been performed from 800 • C to 950 • C with various holding times ranging from 0-96 hours followed by air cooling to room temperature [40]. A mix of quantitative and qualitative assessments of the various microstructural transformations that occur in beta gamma TiAl have been carried out.…”