1,721,103 research outputs found
Microstructure variation effects on room temperature fatigue crack propagation and thresholds in Udimet 720Li Ni-base alloy
No full textAn assessment of the effects of microstructure on room temperature fatigue threshold and crack propagation behaviour has been carried out on microstructural variants of U720Li, i.e. as-received U720Li, U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent ?' variant). Fatigue tests were carried out at room temperature using a 20Hz sinusoidal cycling waveform at an R-ratio=0.1. U720Li-LG showed the highest threshold ?K (?Kth), whilst U720Li-LP showed the lowest ?Kth value. U720Li-LP also showed higher crack growth rates in the near-threshold regime and at high ?K (although at higher ?K levels the difference was less marked). Crack growth rates of U720Li and U720Li-LG were relatively similar both in the near-threshold and high ?K regime. The materials showed crystallographic stage I type crack growth in the near-threshold regime, with U720Li showing distinct crystallographic facets on the fracture surface while U720Li-LG and U720Li-LP showed mostly microfacets and a lower proportion of large facets. At high ?K, crack growth in the materials becomes flat and featureless indicative of stage II type crack growth. The observed performance of the materials is rationalised in terms of their microstructural characteristics. Enhanced room temperature fatigue threshold and long crack growth resistance are seen for larger grained materials due to increased extrinsic crack growth resistance contributions from crack closure. Differences in heterogeneity of deformation behaviour in this set of material variants appear to give approximately equivalent intrinsic crack growth resistance at room temperature due to the respective effects of each deformation behaviour on intrinsic crack growth resistance
Effects of microstructure on room temperature fatigue crack initiation and short crack propagation in Udimet 720Li Ni-base superalloy
No full textAn 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 ?? 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% ?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 ?? precipitate size and higher volume fractions of both coherent and primary ?? precipitates will improve overall fatigue lifetimes in PM Ni-base alloys which exhibit planar slip characteristics at room temperature
Fatigue crack growth mechanisms in superalloys: an overview
No full textFatigue studies on disc and blade nickel based superalloys by the author and co-workers are reviewed. Crack initiation in single crystal turbine blade alloys is dominated by interdendritic porosity with oxidation processes affecting initiation position. Lifetime trends can be modelled using a multipart Paris type lifing approach. Orientation, loading state, temperature and environment determine stage I/II crack growth mechanisms and the resulting crack path and should be considered in lifing. Mechanistic insights on how complex stress states, subsurface failures and different temperatures/environments affect fatigue processes can thus improve turbine blade lifing, and direct alloy development programmes. In polycrystalline disc alloys
cracks at high temperature may initiate at oxidised subsurface carbides or porosity. Grain size controls cycle and time dependent crack growth: the benefits of increased grain size in resisting
grain boundary attack mechanisms predominate over those of gamma' distribution variation. Optimising grain boundary character and gamma' distribution should yield the best alloy design strategy for high
temperature fatigue performance in turbine discs
Fatigue crack initiation and short crack growth in nickel-base turbine disc alloys—the effects of microstructure and operating parameters
No full textAn assessment of the effects of microstructure and operating parameters on both crack initiation and propagation of short fatigue cracks is presented. The assessment was carried out on RR1000, U720Li and microstructural variants of U720Li. Fatigue tests were carried out at room temperature (20 Hz sinusoidal cycling) and at 650 °C (1–1–1–1 trapezoidal cycling). Comparisons of the performance of the different microstructures revealed that initiation occurred predominantly at pores at both temperatures. At room temperature, stage I crack growth predominated and the presence of large primary ?? precipitates on the grain boundaries, larger grains and larger coherent ?? sizes gave improved fatigue crack growth resistance, whereas at 650 °C, larger grains gave the most significant performance benefits
Effect of environment on notch fatigue initiation resistance in CMSX4
No full textCrack initiation at high temperatures has been studied in CMSX4 in both air and vacuum environments, to elucidate the effect of oxidation on the notch fatigue initiation process. In air, crack initiation occurred at sub-surface interdendritic pores in all cases. The sub-surface crack grows initially under vacuum conditions, before breaking out to the top surface. Lifetime is then critically dependent on initiating pore size and distance from the notch root surface. In vacuum conditions, crack initiation has been observed more consistently from surface or close-to-surface pores - indicating that surface oxidation is infilling/"healing" surface pores or providing significant local stress transfer to shift initiation to sub-surface pores
Effect of grain size and environment on fatigue crack propagation in polycrystalline Udimet 720
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An investigation of the effect of load ratio on near-threshold fatigue crack propagation in a Ni-base superalloy
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