Grain size effects in a Ni-based turbine disc alloy in the time and cycle dependent crack growth regimes

The fatigue crack growth (FCG) behaviour in a Ni-based turbine disc alloy with two grain sized variants, in a low solvus high refractory (LSHR) superalloy has been investigated under a range of temperatures (650–725 °C) and environments (air and vacuum) with trapezoidal waveforms of 1:1:1:1 and 1:20:1:1 durations at an R = 0.1. The results indicate that a coarse grained structure possesses better FCG resistance due to the enhanced slip reversibility promoted by planar slip as well as the reduction in grain boundary area. The fatigue performance of the LSHR superalloy is significantly degraded by the synergistic oxidation effect brought about by high temperature, oxidising environment and dwell at the peak load, associated with increasingly intergranular fracture features and secondary grain boundary cracking. Secondary cracks are observed to be blocked or deflected around primary γ′, carbides and borides, and their occurrence closely relates to the roughness of the fracture surface, FCG rate and grain boundary oxidation. The apparent activation energy technique provides a further insight into the underlying mechanism of the FCG under oxidation–creep–fatigue testing conditions, and confirms that oxidation fatigue is the dominant process contributing to the intergranular failure process. At high enough crack growth rates, at lower temperatures, cycle dependent crack growth processes can outstrip crack-tip oxidation processes.     

Influence of oxidation on fatigue crack initiation and propagation in turbine disc alloy N18

Fatigue crack initiation and propagation behaviour in subsolvus heat treated turbine disc alloy N18 has been assessed in air and vacuum at 650 and 725 °C under three-point loading. Fatigue crack initiation processes have been evaluated using single edge U-notch specimens under a 1-1-1-1 trapezoidal loading waveform along with interrupted tests at 650 °C to allow intermittent observations of the notch surface. The results show apparent grain boundary (GB) oxidation can occur under an oxygen partial pressure of 10⁻²–10⁻³ Pa. Cracks mainly initiate from grain boundaries or γ/γ′ interfaces due to the formation and subsequent cracking of Cr-rich and/or Co-rich oxides, and occasionally initiate from surface pores. Fatigue life in these tests appears to be dominated by this crack initiation process and is significantly reduced by increasing temperature and/or application of an oxidizing environment. Crack growth tests conducted under 1-1-1-1 and 1-20-1-1 loading waveforms indicate that oxidation significantly degrades the crack growth resistance of N18 and is associated with more intergranular fracture surface features. Additional oxidation effects on propagation caused by higher temperature or prolonging dwell time appear limited, whereas a prolonged dwell period seems to instead promote additional creep process, which further enhance crack growth, especially at higher temperature.     

Cure path dependency of mode I fracture toughness in thermoplastic particle interleaf toughened prepreg laminates

The effect of cure cycle on fracture behaviour of a commercial thermoplastic particle interleaved prepreg system was investigated. Laminates were manufactured at 700 kPa in an autoclave using eight different thermal cycles that included both raising the cure temperature above the standard 180 °C cure cycle and incorporating an intermediate dwell stage between 150 and 170 °C prior to reaching the 180 °C cure temperature. Double cantilever beam tests were conducted on specimens from the cured laminates. The stick–slip crack behaviour, observed in samples manufactured using the standard cure cycle, changed to stable crack growth when processing deviated by 10 °C. The mode I fracture toughness values were reduced by 11–22% when incorporating an intermediate dwell stage before the final cure temperature. Scanning electron microscopy inspection of the fracture surfaces showed differences between samples made by standard cure cycles and those made using process deviations.     

The effects of texture and grain morphology on the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

The fracture toughness and fatigue crack growth resistance of nanocrystalline materials are significantly affected by the thickness of the specimen. In this work we relate the mechanical properties of nanocrystalline platinum films to their texture and grain morphology. Tensile, creep and fatigue testing of annealed, ∼1 μm films resulted in mechanical properties similar to the as-received films (yield strength of ∼1.2 GPa, fracture toughness ∼17.8 MPa √m, and a fatigue crack growth power law exponent of ∼4.2). However, the breakdown of the initially columnar grain morphology had a marked effect on the transition point from an intergranular to transgranular fatigue cracking mode. Finite element modeling suggests that cyclic (fatigue) grain coarsening and the transition from inter- to transgranular cracking modes are a result of the relative importance of dislocation slip accommodation on in-plane and through-thickness oriented slip directions.     

Ultrasonic fatigue of a single crystal Ni-base superalloy at 1000 °C

An ultrasonic fatigue testing system capable of operating at temperatures up to 1000 °C has been developed and utilized to study the fatigue behavior of a single crystal superalloy (PWA 1484) at a temperature of 1000 °C and loading frequency of approximately 20 kHz. The stress-life data generated from the ultrasonic testing system were comparable to those from conventional servo-hydraulic fatigue tests for similar single crystal alloys. Interior Ta-rich carbides were the major microstructural feature responsible for crack initiation in the alloy. Crack growth under ultrasonic loading frequency at 1000 °C for PWA 1484 occurred in a crystallographic manner on {1 1 1} octahedral slip planes, in contrast to the normal Mode-I growth mode typically observed for single crystal superalloys at high temperature (>850 °C) with conventional servo-hydraulic loading frequencies (<100 Hz).     

Effects of hot-working parameters on microstructural evolution of high nitrogen austenitic stainless steel

The microstructural evolution of high nitrogen austenitic stainless steel under various deformation conditions was characterized by isothermal compression test. Special attention was paid to the variation of microhardness and its relationship with grain size was also derived. Results indicated that two kinds of strengthening mechanism acted during the whole temperature range. When the temperature is between 950 °C and 1150 °C, grain refinement plays a dominant role. But at temperatures lower than 900 °C, no recrystallization occurs and substructure (dislocations and twins) contributes massively to the strength. Furthermore, it was found that the peak precipitation of grain boundary carbides which are seriously detrimental to toughness appeared at 850 °C. Therefore, an optimizing processing route could be recommended to achieve a good combination of high strength and good toughness. Firstly, the hot-rolling at 950–1100 °C should have large stain to gain refined grains, and accelerated cooling is applied from 950 to 750 °C in order to avoid carbide precipitation along grain boundary. Lastly, at temperatures lower than 750 °C warm-rolling with medium stain can get substructure strengthening effects.     

Role of microstructural condition on fatigue damage development of AISI 316L at 20 and 300 °C

At 300 °C, when dynamic strain ageing takes place, the fatigue life of AISI 316L for lower strain amplitudes is lower than under equivalent conditions at 20 °C. Exhaustive examination of the changes in: (1) apparent elastic modulus, (2) microstructural condition, and (3) fractographic features has been performed to reveal the reason for the life reduction. The analysis of apparent elastic modulus variations and the results of fractographic observations show that the propagation rates for fatigue cracks at 20 °C are faster than for 300 °C. Crack initiation however occurs earlier at 300 °C, in particular for lower strain amplitude tests, due to the activity of localised deformation bands as a consequence of cyclic loading. In addition to persistent slip bands, a form of ladder-free deformation bands is also present at 300 °C, in particular at low strain amplitudes. When the fatigue life is rather short, the influence of the ladder-free deformation bands on cyclic endurance is negligible. The ladder-free type of localised bands have a strong influence on crack initiation once the material endurance increases with lowering strain amplitude, leading to the relative life reduction at the elevated temperature. In addition, the incidence of secondary cyclic hardening for lower strain amplitude tests at 300 °C partly contributes to the more evident life reduction. The influence of dislocation walls on the propagation of microstructurally short fatigue cracks is also examined.     

Gigacycle fatigue initiation mechanism in Armco iron

Microstructure irreversibility plays a major role in the gigacycle fatigue crack initiation. Surface Persistent Slip Bands (PSB) formation on Copper and its alloy was well studied by Mughrabi et al. as typical fatigue crack nucleation in the very high cycle fatigue regime. In the present paper, Armco iron sheet specimens (1 mm thickness) were tested under ultrasonic frequency fatigue loading in tension–compression (R = −1). The test on the thin sheets has required a new design of specimen and new attachment of specimen. After gigacycle fatigue testing, the surface appearance was observed by optical and Scanning Electron Microscope (SEM). Below about 88 MPa stress, there is no PSBs even after fatigue cycle up to 5 × 10⁹. With a sufficient stress (above 88 MPa), PSBs in the ferrite grain was observed by optic microscope after 10⁸ cycles loading. Investigation with the SEM shows that the PSB can appear in the body-centered cubic crystal in the gigacycle fatigue regime. Because of the grain boundary, however, the local PSB did not continually progress to the grain beside even after 10⁹ cycles when the stress remained at the low level.     

Effects of crystallographic orientations and dwell types on low cycle fatigue and life modeling of a SC superalloy

Effects of temperature (760 °C and 980 °C), crystallographic orientation ([0 0 1], [0 1 1] and [1 1 1]) and dwell types (tensile, compressive and balanced dwell type) on low cycle fatigue (LCF) of a Ni-based single crystal (SC) superalloy are experimentally investigated and modeled. Since the LCF behavior shows strong crystallographic orientation and dwell type dependences, corresponding accurate life models are needed for safe application in gas turbine components. The feasibility of stress-based, strain-based and energy based models on anisotropic fatigue behavior was evaluated. A modified Cyclic damage accumulation (CDA) method combined with critical slip plane concept is developed to correlate the influence of orientation and dwell type on LCF data.     

Experimental investigation of fatigue crack growth behavior of GH2036 under combined high and low cycle fatigue

Fatigue crack growth rates have been experimentally determined for the superalloy GH2036 (in Chinese series) at an elevated temperature of 550 °C under pure low cycle fatigue (LCF) and combined high and low cycle fatigue (CCF) loading conditions by establishing a CCF test rig and using corner-notched specimens. These studies reveal decelerated crack growth rates under CCF loading compared to pure LCF loading, and crack propagation accelerates as the dwell time prolongs. Then the mechanism of fatigue crack growth at different loadings has been discussed by using scanning electron microscope (SEM) analyses of the fracture surface.     

Serrated flow induced by twin boundary–slip band interactions in a FeNi-base austenitic alloy

Tensile tests on a FeNi-base austenitic alloy were conducted at room temperature (RT) and 400 °C, when serrated flow did not and did occur, respectively. Deformation microstructures such as topographies of slip bands (SBs), morphologies of twin boundaries (TBs) and arrangements of dislocations near TBs, as well as concentrations of strain were investigated. It is shown that TBs block SBs and induce remarkable stress accumulations at 400 °C. Effect of TB-density on the serrated flow was also investigated by comparative tensile tests on specimens with different TB densities at 400 °C. Details of tensile curves reveal that more TBs induce more pronounced serrations. Therefore, interaction between TBs and SBs is proposed to induce the serrated flow of the FeNi-base alloy at 400 °C.     

Effect of stress ratio on fatigue lifetime of high Nb containing TiAl alloy at elevated temperature

The effect of stress ratio (R) on fatigue lifetime of a cast Ti–45Al–8.0Nb–0.2W–0.2B–0.1Y (at.%) alloy was investigated at 750 °C. Fatigue tests with various stress ratios ranging from 0.1 to 1 were performed using a mini servo-hydraulic fatigue machine inside a chamber of scanning electron microscope (SEM). Fatigue crack initiation and propagation behavior was studied by in situ SEM observation and fatigue fracture mode was examined by fracture surface analysis. It is found that fatigue lifetime shows a reversed S-type curve with the increase of stress ratio. At R ranging from 0.1 to 0.4, creep–fatigue interaction dominates the fatigue lifetime and the fatigue lifetime reaches its minimum value at R = 0.3. At R ranging from 0.4 to 1, creep damage dominates the fatigue lifetime and the fatigue lifetime exhibits inverse proportional relation with R. Meanwhile, with the increase of stress ratio, the fatigue crack initiation sites transform from lamellar interface at R = 0.1, to lamellar interface and colony boundary at R = 0.3, and to lamellar colony boundary at R = 0.5. Accordingly, the fatigue fracture mode transforms from transgranular cracking, to transgranular and intergranular cracking, and to intergranular cracking.     

Effect of Cr precipitates and He bubbles on the strength of 〈1 1 0〉 tilt grain boundaries in BCC Fe: An atomistic study

Molecular static simulations were carried out to study the fracture process of different 〈1 1 0〉 tilt grain boundaries (Σ19{3 3 1}, Σ9{2 2 1}, Σ3{1 1 1}, Σ3{1 1 2}, Σ11{1 1 3}, Σ9{1 1 4}). The main goal of this work was to investigate variation of the deformation mechanism and fracture stress in the presence of Cr precipitates, voids and He bubbles at the core of the grain boundaries (GBs). The corresponding deformation process was characterized in terms of stress–strain relationship and deformation mechanisms were inspected by visualization tools. Based on the obtained stress–strain curves, the studied GBs can be subdivided into two types, those that exhibit extensive slip and those that do not show slip at all. The presence of Cr precipitates at the GB core increases critical shear stress necessary to initiate the slip, and nucleation of a crack was regularly seen to occur at the precipitate–matrix interface. The effect of voids and He bubbles on the fracture stress is much stronger. It was revealed that the plastic deformation was essentially suppressed. The reason for the suppression was attributed to the emission of the dislocations from voids/bubbles and their pile up.     

Molecular dynamic modelling of fatigue crack growth in aluminium using LEFM boundary conditions

A molecular dynamic (MD) model of a crack in pure aluminium has been developed with isotropic Linear Elastic Fracture Mechanics (LEFMs) boundary displacements that simulates the fatigue crack growth process. The model consists of a cylindrical region filled with atoms around a crack tip and subject to boundary displacements that change due to cyclic loading. A sinusoidal load that produced a $K_{\max }=1.0\text{MPa}\sqrt{\text{m}}$ was applied to produce fatigue crack growth using three different atomic potentials for aluminium at T = 20 K, and a range of different K_min. Each run consisted of the application of fifteen or more loading cycles. In some cases, the crack tip was seen to advance in each cycle typical of fatigue, however, growth was smooth and continuous during the entire cycle with contraction occurring during the unloading phase of the cycle. The model contained 3 × 10⁶ atoms and had a diameter and width of 20 nm. This width was just large enough for fragments of sessile dislocations to form and couple with the glissile dislocations emitted from the crack tip, resulting in work hardening about the crack tip. The model was oriented for cracking on the {1 1 0} plane in the 〈1 0 0〉 direction. Crack advance was observed to be due to a combination of dislocation emission and atomic separation.     

Nanoindentation-induced phase transformation in (1 1 0)-oriented Si single-crystals

Pressure-induced plastic deformation and phase transformations manifested as the discontinuities displayed in the loading and unloading segments of the load–displacement curves were investigated by performing the cyclic nanoindentation tests on the (1 1 0)-oriented Si single-crystal with a Berkovich diamond indenter. The resultant phases after indentation were examined by using the cross-sectional transmission electron microscopy (XTEM) technique. The behaviors of the discontinuities displayed on the loading and re-loading segments of the load–displacement curves are found to closely correlate to the formation of Si-II metallic phase, while those exhibiting on the unloading segments are relating to the formation of metastable phases of Si-III, Si-XII, and amorphous silicon as identified by TEM selected area diffraction (SAD) analyses. Results revealed that the primary indentation-induced deformation mechanism in Si is intimately depending on the detailed stress distributions, especially the reversible Si-II ? Si-XII/Si-III phase transformations might have further complicated the resultant phase distribution. In addition to the frequently observed stress-induced phase transformations and/or crack formations, evidence of dislocation slip bands was also observed in tests of Berkovich nanoindentation.     

Role of microstructure on initiation and propagation of fatigue cracks in precipitate strengthened Cu–Ni–Si alloy

Fatigue tests were conducted on round-bar specimens to understand the fatigue behavior of precipitate-strengthened Cu–6Ni–1.5Si alloy. Aging at 500 °C for 0.5 h produced δ-Ni₂Si precipitates in the matrix, homogeneously and heterogeneously precipitated δ-Ni₂Si particles, and a precipitate-free zone around the grain boundaries. The cracks were initiated at the grain boundaries, followed by growth along the crystallographic slip planes in the adjacent grains. Crack propagation from the crack origin along the grain boundaries was occasionally observed. The physical background of fatigue damage is discussed in light of the role of microstructure on the behavior of fatigue cracks.     

The role of oxidation damage in fatigue crack initiation of an advanced Ni-based superalloy

The effects of prior oxidation on the room temperature fatigue life of coarse-grained Ni-based superalloy, RR1000, have been investigated. High cycle fatigue tests were conducted, on both machined and pre-oxidised testpieces, at room temperature at an R ratio of 0.1. The oxidation damage was produced by pre-exposures at 700 °C for either 100 or 2000 h. Pre-oxidised testpieces tended to fail with shorter fatigue lives than those obtained from the as-machined testpieces although they were also observed to outperform the as-machined test pieces at peak stress levels around 900 MPa. The chromia scale and intergranular alumina intrusions formed during pre-oxidation are prone to crack under fatigue loading leading to early crack nucleation and an associated reduction in fatigue life. This has been confirmed to be the case both below and above a peak stress level of ∼900 MPa. The better fatigue performance of the pre-oxidised specimens around this stress level is attributed to plastic yielding of the weaker γ′ denuded zone, which effectively eases the stress concentration introduced by the cracking of the chromia scale and intergranular internal oxides. This γ′ denuded zone is also a product of pre-oxidation and develops as a result of the selective oxidation of Al and Ti. Over a limited stress range, its presence confers a beneficial effect of oxidation on fatigue life.     

Effect of strain-modified particles on the formation of fined grains and the properties of AA7050 alloy

With a new two-pass deformation, a fine-grained AA7050 alloy was obtained owing to small particles which can affect the grain refinement. The banded structures were produced in the elongated grain interiors after the 1st-pass deformation at 300 °C. And deformation bands containing dislocation arrays and small spherical particles were obtained. A few new fined grains appeared along the elongated grain boundaries. After the 2nd-pass deformation at 430 °C, isolated chains of new fine grains were developed in the elongated grain interiors. The boundary glide and the increase of grain boundary misorientation due to cumulative strain could refine the elongated grains. The pinning effect of the particles accelerated the formation of deformation bands. The increase of deformation temperature promoted the rapid evolution of grain refinement during the deformation. The strength of the fine-grained AA7050 alloy was enhanced while the ductility was decreased.     

Crack growth in discontinuous glass fibre reinforced polypropylene under dynamic and static loading conditions

Crack propagation in single edge notched tensile specimens of isotactic polypropylene reinforced with short E-glass fibres has been investigated under both fatigue and creep loading conditions. Fatigue crack propagation (FCP) experiments have been performed at three different frequencies (0.1, 1, 10 Hz) and at a mean applied tensile load of 1200 N. Isothermal creep crack propagation (CCP) tests have been conducted under a constant tensile applied load of 1200 N at various temperatures in the range from 32 to 60 °C. Analysis of FCP data allowed an estimation of the pure fatigue and pure creep components of the crack velocity under the adopted cyclic loading conditions. Crack growth at low frequencies (0.1 and 1 Hz) is mainly associated with a non-isothermal creep process. At higher frequency (10 Hz), the pure fatigue contribution appeared more pronounced. Finally, the comparison of FCP and CCP as a function of the mean applied stress intensity factor confirmed the major contribution of creep crack growth during FCP process at low frequencies.