An assessment of the effects of microstructure on room temperature fatigue crack initiation and short crack propagation in a Ni-base superalloy is presented. The assessment was carried out on microstructural variants of U720Li, including as-received U720Li, U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20Hz sinusoidal cycling waveform on plain bend bars. Tests were conducted in 3-point bend under load control with an R-ratio of 0.1. A maximum load of 95% σ y was used in all tests. Room temperature fatigue crack initiation was noted to occur due to slip band cracking and from porosity on or just beneath the surface in all materials. Crack propagation was noted to be highly faceted (due to planar slip band cracking) immediately after crack initiation followed by a transition to a flatter Stage II type crack path as crack length increases.U720Li-LP was noted to show the longest fatigue lifetime, followed by U720Li-LG while U720Li shows the shortest life. The longer lifetime of U720Li-LP was linked to a higher resistance to both fatigue crack initiation and short crack propagation. U720Li and U720Li-LG show approximately similar crack initiation resistance although U720Li-LG showed slightly improved short crack growth resistance. The observations have been rationalised in terms of the microstructural characteristics of the materials, and it is believed that larger grain size, larger coherent γ′ precipitate size and higher volume fractions of both coherent and primary γ′ precipitates will improve overall fatigue * Now at Dept. of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK lifetimes in PM Ni-base alloys which exhibit planar slip characteristics at room temperature.

No abstract

No abstract

An analysis of the microstructure of the interdendritic regions in an as-cast third generation single-crystal nickel-base superalloy CMSX-10K has been carried out using automated serial sectioning with sequential focused ion beam (FIB) milling and image capture of the secondary electron image in a combined FIB-scanning electron microscope (SEM). Two interdendritic morphologies with vastly different c/c¢ spacing were identified; these are (1) fine c/c¢ morphology and (2) coarse cellular c¢ separated by narrow c channels. The FIB results indicate that following primary solidification (L fi c), solidification of the interdendritic region commences in areas that show the fine c/c¢ morphology, (1) on the c dendrite lobe. This is followed by a gradual transition to growth of the coarse cellular c/c¢ channel structure, (2) which occur up to the termination of the solidification process.

The microstructure and fatigue performance of three sub-solvus heat treated nickel based disc superalloys for turbine disc applications are reported. The alloy variants studied are RR1000, N18 and Udimet 720 Low Interstitial (U720Li), with the latter tested both in a standard and large grain variant (LG). Their microstructures are examined in terms of grain and gamma prime size (γ'). Fatigue crack growth (FCG) rates for all materials at 650ºC show that RR1000 provides the best performance, followed by U720Li-LG, N18 and U720Li. Some of the variations in FCG rate between the alloys are due to reduction in grain boundary oxidation processes with increased grain size, but more subtle interplays between grain boundary character, alloy composition and slip character are also important.