Microstructure of dense thin sheets of γ-TiAl fabricated by hot isostatic pressing of tape-cast monotapes

A powder metallurgy route based on hot isostatic pressing (HIP) of tape-cast monotapes was used for the direct fabrication of dense thin sheets (250–300 μm thick) of gamma titanium aluminide (γ-TiAl). Polarized light microscopy revealed a fine-grained microstructure (average grain size ∼3 μm) but a few isolated larger grains (∼20 μm) were also present. The primarily metastable α2 microstructure of the rapidly solidified starting powder transformed to the equilibrium near-γ microstructure during HIP at 1100 °C. Chemical analysis revealed that the dense sheet had a carbon content of 0.13 wt.%, which was only 0.04 wt.% higher than that of the starting powder, but the oxygen content was significantly higher, presumably introduced during the decanning step. The hardness measured using Vickers microindentation technique was 384 ± 9 HV. Manipulation of the as-HIPed microstructure was performed by heating for up to 1 h in flowing argon at temperatures (1170–1385 °C) below and above the alpha transus (1355 °C). Below 1250 °C, limited grain growth and no discernable change in the as-HIPed (near-γ) microstructure occurred. Sheets heated to 1320 °C and 1365 °C had a duplex microstructure of γ and α2 grains, with some lamellar grains. Except for a thin surface layer (20–30 μm thick), the microstructure of the heat-treated sheet was uniform, but a fully lamellar microstructure was not achieved even after heating for 1 h at 1385 °C.