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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
Microstructure effects on high temperature fatigue crack initiation and short crack growth in turbine disc nickel-base superalloy Udimet 720Li
No full textAn assessment of the effects of microstructure on fatigue crack initiation and short crack growth in a turbine disc nickel-base superalloy at 650?C in air is presented. U720Li and microstructural variants of U720Li, i.e. U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent g' precipitate variant) have been assessed by uninterrupted and replicated short crack tests in polished U-notch specimens using a 1-1-1-1 trapezoidal loading cycle at nominal stress levels ranging between 700 and 850 MPa (calculated in the uncracked ligament). Crack initiation was primarily due to porosity on or near the surface but also due to grain boundary oxidation. Initial transgranular crack growth across 4-6 grains in air was noted at short crack lengths before oxidation-assisted intergranular crack growth modes were established at larger crack lengths. At a nominal applied stress of 840MPa, U720Li and U720Li-LP show similar fatigue lifetimes while U720Li-LG demonstrates a significantly improved fatigue lifetime, particularly when lifetimes are compared on a local strain range basis. A larger grain size gave the most significant performance benefits in terms of overall fatigue lifetime under these test conditions
Effects of microstructural variation on fatigue crack initiation and short crack growth in Ni-base turbine disc alloys
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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 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
Comparison of time-dependent crack growth mechanisms in a range of turbine disc alloys
No full textTurbine discs experience a range of temperature and load conditions in service, but the increasing operating temperatures expected for new, more efficient engine designs, mean that increasingly their time-dependent fatigue response is of more and more importance. A variety of alloying and heat treatment approaches have now been developed to improve the high temperature fatigue performance of turbine disc alloys (whilst maintaining an appropriate balance of their other required properties). In this study we have reviewed the microstructures achieved in a range of sub-solvus and super-solvusheat treated PM alloys: U720Li, RR1000, N18 and LSHR alloys. The microstructures are examined in terms of grain and gamma prime size (?') variations, whilst fatigue crack growth (FCG) rates for all materials have been obtained at 650ºC and 725ºC in air and in vacuum. These show that RR1000 provides the best performance at the most severe conditions, i.e. the highest temperatures and longest dwells. In general failure modes become increasingly intergranular with increasing temperature and ?K. 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 clearly important.The comparison between air and vacuum conditions is especially helpful in separating out creep and environmental contributions, and the vacuum conditions have some service relevance for sub-surface defect growth. An apparent activation energy analysis approach is also presented to further assess the micromechanistic variations between these alloys. This allows a more explicit analysis of the mechanistic dependence of F.C.G rate changes between these two temperatures, where 650oC (a current possible service temperature) is compared to 725oC (a much higher, aspirational. target service temperature)
A comparison of high temperature fatigue crack propagation in sub-solvus heat treated turbine disc alloys
No full textThis paper presents the results of high temperature fatigue tests carried out in air at 725°C on four different materials: N18, RR1000, Udimet 720Li (U720Li) and U720Li Large Grain (U720Li LG) variant. The influence of composition, sub-solvus heat treatments, together with varied cooling rates on grain size and gamma prime size and distribution on fatigue crack propagation behaviour are compared and contrasted
A comparison of high temperature fatigue crack propagation in various sub-solvus heat treated turbine disc alloys
No full textThe 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. Fatigue crack growth (FCG) rates for all materials at 650ºC show that RR1000 provides the best performance, followed by U720Li-LG, N18 and U720Li. In general, the failure modes become increasingly intergranular with increasing ?K. 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 importan
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