Microstructural evolution during heat treatments in Ti-45 and 47Al–2Nb–2Mn+0.8vol.%TiB2 XDTM alloys

Investment cast gamma XD TiAl alloys, Ti-45 and 47Al–2Nb–2Mn+0.8vol.%TiB2, were heat treated above and below their α transus temperatures to investigate the microstructural response. Heat treatments above Tα produce fully lamellar microstructures, whereas equiaxed γ grains exist in the materials heat treated below the α transus temperature. Heat treatment time has little influence on the microstructures except for the 24 h treatment. Cooling by oil quench yields the finest lamellae in both alloys without formation of Widmanstätten and massive transformed structures. For microstructural stabilization, samples heat-treated above Tα were subsequently subjected to a secondary aging treatment at lower temperatures. A single step aging treatment was applied to minimize coarsening of lamellae. The lamellar structure in XD45 alloy is relatively more stable than that in XD47. However, equaixed γ grains are nucleated and grown at the grain boundaries during aging. Multiple step aging treatment may be needed for developing stable lamellae without causing recrystallization. In general, lamellar spacing decreases as temperature and cooling rate increase. Hardness depends on microstructural features such as grain size, lamellar spacing, and volume fraction of equiaxed grains. Hardness increases as grain size, lamellar spacing, and the volume fraction of equiaxed grains decreases.