Microstructural stability and mechanisms of fatigue and creep crack growth in Ti–24Al–11Nb

Abstract

Titanium intermetallics are being developed for long term applications at elevated temperatures, particularly those alloys based on the alloys Ti₃Al and TiAl. Typical approaches include the design of appropriate microstructures for room and elevated temperature fatigue and creep resistance. However, a little explored area is the stability of these microstructures at elevated temperature and its effect on fatigue crack growth. The present investigation documented the microstructural stability, fatigue crack behaviour, and stress rupture of Ti-24Al-11Nb, a Nb modified Ti₃Al alloy. A coarse two phase α₂+β Widmanstatten microstructure was found to exhibit the best resistance to fatigue crack growth. Microstructural stability and elemental segregation were studied as a function of exposure time for up to 500 h at 800°C using transmission electron microscopy (TEM). Results indicate that the Widmanstatten microstructure is metastable and the β phase breaks up into particles. The absence of a continuous β phase surrounding the α₂ phase reduces the resistance of the microstructure to fatigue crack growth at room temperature. At elevated temperature the microstructure stability does not play a role in determining the fatigue resistance. A fine Widmanstatten microstructure has the best resistance to creep deformation. Stress rupture tests were conducted in vacuum and air at 649°C and 760°C. Two types of failure mechanisms were seen in stress rupture; these include transgranular and intergranular failure within prior β grains. When tested in air at 760°C a combination of transgranular and intergranular failure occurred. Specimens that exhibited a higher proportion of transgranular failure had longer lives. When tested in vacuum at 760°C the predominant failure mode was intergranular. At 760°C extensive microstructural changes like breakup and spherodization of the β phase occurred under stress while the rate of coarsening without any stress was much slower. At 649°C the specimens tested in vacuum consistently exhibited longer lives. The creep crack growth when tested in air at 649°C was always a brittle transgranular mode while the specimens tested in vacuum always failed by an intergranular mode.