Thermal stability of an intermediate strength fully lamellar Ti-45Al-2Mn-2Nb-0.8Â vol.% TiB2 alloy

A fully lamellar γ-TiAl alloy Ti-45Al-2Mn-2Nb + 0.8 vol.% TiB2 was exposed at 700 °C for 10,000 h in air. No perpendicular decomposition of α2 into α2 + β/α2 + γ sections and no recrystallisation of β equiaxed grains on decomposed α2 + γ lamellae were observed all through the prolonged exposure. The dominant change in microstructure was that the α2 lamellae thinned gradually and became increasingly discontinuous with increasing exposure time. The average thickness of α2 lamellae reduced to a half and the volume fraction reduced by 1/4 after 10,000 h exposure. Correspondingly, the tensile properties were hardly changed, while fatigue strengthening was increased by 30%. The reasons for the relatively high degree of stable microstructure and mechanical properties are discussed based on production processing and alloy composition. The “exposure-induced fatigue strengthening” observed after 10,000 h exposure is assumed to be caused by structure relaxation and stress dissipation throughout the specimen.