Effect of microstructure on fatigue and tensile properties of the gamma TiAl alloy Ti-46.5Al-3.0Nb-2.1Cr-0.2W

The relationships between the microstructure and tensile and axial load-controlled fatigue properties of the alloy Ti-46.5Al-3.0Nb-2.1Cr-0.2W (atomic per cent) have been studied. Two different microstructures, i.e. duplex (grain size, 20 μm) and fully lamellar (grain size, 300 μm), were produced, through two-step forging and subsequent heat treatments, giving similar yield strengths at room temperature. The fracture strains at room temperature were about 1.1% and 2.9% for the materials with the fully lamellar and the duplex microstructures respectively. At 600 °C, the duplex material shows a 15% higher fatigue strength than that of the fully lamellar material. At this temperature, the gamma alloy of both microstructures reaches high ratios of the fatigue strength at 107 cycles to the ultimate tensile strength (UTS), i.e. about 0.95. At 800 °C, the fully lamellar material exhibits higher fatigue strength values above 105 cycles, and both microstructures result in a two-stage behavior, in contrast to the test at 600 °C. The second stage features the characteristic conventional fatigue behavior, with a broad amplitude stress range, while the first stage is characterized by a narrow band of fatigue stress levels near the UTS. The fracture modes for the duplex material showed a general trend from transgranular to intergranular failure with increasing temperature. For the fully lamellar material, a change from predominantly translamellar failure to a mixture of inter lamellar and translamellar failure was observed, resulting in a microscopically and macroscopically rough fracture surface. The strain rate sensitivity of the fully lamellar material was negligible in the temperature range tested.