Influence of precipitate morphology on the high temperature fatigue properties of SRR99

Fatigue crack growth (FCG) tests have been conducted in air at 650°C and 850°C on 〈001〉 oriented single crystals of SRR99 having the γ′ particles in the form of: (A) 0.3 μm cuboids; (B) 0.2 μm ogdoadical cuboids; and (C) a coarse, rafted γ′ structure. In general, reducing the frequency and increasing the temperature enhances crack-tip shielding at low ΔKs due to increasing oxide induced crack closure. In material A at 650°C the crack path changes from one of γ′ precipitate cutting on {001} to propagation within the matrix as ΔK increases. Enhanced crack branching at 850°C improves the Paris regime behaviour compared with that seen at 650°C. In material B at 650°C, greater cross slip at lower frequency reduces slip reversibility, thus enhancing the fatigue crack growth rate (FCGR). At 850°C crack tip blunting and meandering, associated with γ′ cutting, improves the high ΔK FCG response and on a strength/modulus normalized basis is comparable with that seen for material A. Material C shows a similar FCG resistance to A at 650°C, but there is an acceleration in FCGR at 850°C, which can be accounted for in terms of the lower proof stress and modulus of this microstructure.     

High-cycle fatigue-life of the cast nickel base-superalloys in 738 LC and IN 939

The high cycle fatigue (HCF) properties of two cast nickel base-superalloys, IN 738 LC and IN 939, were investigated using both fracture mechanics samples and smooth specimens. The crack propagation behavior was studied in terms of linear fracture mechanics at RT and at 850 °C. In addition to the influence of temperature, the influences of frequency, mean stress, and environment (vacuum, air, sulfidizing atmosphere) were studied. At 850 °C, the fatigue thresholds were found to be higher in air than in vacuum. This could be explained by crack branching. The high scatter of fatigue crack propagation rates could be related also to this phenomenon. The S/N curves at 850 °C can be predicted treating crack growth from casting pores as the predominant failure mechanism. At RT the same method is not as successful. The reason for this may be that crack growth laws measured on long, branched cracks are not applicable to short, unbranched cracks. At RT, no significant influence of frequency on S/N-curves and fatigue crack growth rates was observed for frequencies up to 20 kHz.     

Crack growth in a nearly fully-lamellar gamma TiAl alloy at 650 °C and 800 °C under constant load conditions

The crack growth behavior of a gamma titanium aluminide alloy, K5S, was investigated at 650 °C and 800 °C under constant load conditions in a nearly fully-lamellar microstructural form. Crack growth at both temperatures ensues at stress intensities (K) much higher than anticipated from the R curves. At 650 °C, creep crack extension occurs through the formation of microcracks (interlamellar (IL) separation) and their joining to the main crack tip through ligament fracture. This results in a mainly transgranular (TG) fracture with occasional IL separation. This process features a rapid initial crack growth but at decreasing growth rate, followed by a nearly no-growth stage. At 800 °C, crack extension is accompanied by extensive plastic deformation and consists of an initial rapid transition period and a subsequent steady state. For similar K’s, crack extension and growth rate are greater at 800 °C than at 650 °C, but even these are very slow processes for this alloy. The resistance to crack propagation at 650 °C is explained in terms of work hardening that arises during the extended primary creep deformation occurring ahead of the crack tip. Increased crack propagation at 800 °C is accredited to grain boundary and lamellar-interface weakening and extensive post primary creep damage in the plastic zone. The resulting fracture at 800 °C is mainly boundary fracture, which consists of IG fracture involving formation and coalescence of voids, and IL separation.     

Fatigue crack growth of SM-1240/TIMETAL-21S metal matrix composites at elevated temperatures

A series of high-temperature fatigue crack growth experiments was conducted on a continuous-fiberreinforced SM1240/TIMETAL-21S composite using three different temperatures, room temperature (24 °C), 500 °C, and 650 °C, and three loading frequencies, 10, 0.1, and 0.02 Hz. In all the tests, the cracking process concentrated along a single mode I crack for which the principal damage mechanism was crack bridging and fiber/matrix debonding. The matrix transgranular fracture mode was not significantly influenced by temperature or loading frequency. The fiber debonding length in the crack bridging region was estimated based on the knowledge of the fiber pullout lengths measured along the fracture surfaces of the test specimens. The average pullout length was correlated with both temperature and loading frequency. Furthermore, the increase in the temperature was found to lead to a decrease in the crack growth rate. The mechanism responsible for this behavior is discussed in relation to the interaction of a number of temperature-dependent factors acting along the bridged fiber/matrix debonded zone. These factors include the frictional stress, the radial stress, and the debonding length of the fiber/matrix interface. In addition, the crack growth speed was found to depend proportionally on the loading frequency. This relationship, particularly at low frequencies, is interpreted in terms of the development of a crack tip closure induced by the relaxation of the compressive residual stresses developed in the matrix phase in regions ahead of the crack tip during the time-dependent loading process.     

Effect of Low Temperature on Fatigue Crack Formation and Microstructure-Scale Growth from Corrosion Damage in Al-Zn-Mg-Cu

The strong effect of cold temperature on the fatigue resistance of 7075-T651 is established. As temperature decreases from 296 K to 183 K (23 °C to ?90 °C), the formation life for cracking about pit and EXCO corrosion perimeters increases, microstructure scale crack growth rates decrease in the range from 20 to 500 μm beyond the corrosion topography, and long crack growth rates similarly decline. Fatigue crack surface features correlate with reduced hydrogen embrittlement with decreasing temperature fed by localized H produced during precorrosion for pit and EXCO-proximate cracks, as well as by crack tip H produced by water vapor reaction during stressing for all crack sizes. The importance of the former H source increases with decreasing temperature for cracks sized below 200 μm. Decreasing temperature to 223 K (?50 °C) eliminates the contribution of environmental H through interaction of reduced water vapor pressure in equilibrium with ice and reduced H diffusion. The Knudsen flow model and exposure parameter, $ P_{{{\text{H}}_{2} {\text{O}}}}/f $ , enables improved modeling of temperature dependent crack propagation, but does not fully describe low temperature fatigue behavior due to possible rate limitation by H diffusion. Further decreases in MSC da/dN to 183 K (?90 °C) are related to reduced mobility of the corrosion-precharged H which may associate with vacancies from dissolution. Crack formation, and growth rates correlate with either elastic stress intensity range or cyclic crack tip opening displacement, and are available to predict corrosion effects on airframe fatigue for the important low temperature regime.     

Micromechanism of accelerated near-threshold fatigue crack growth in Al2O3/Al-Cu composite at elevated temperature

Fatigue crack growth behavior of a peak-aged Al₂O₃/Al-Cu composite was examined at 150 °C and compared to the behavior at room temperature (RT). At 150 °C, fatigue crack growth rates showed strong dependence on loading time. At short loading time, when stress-intensity range was decreased to approach fatigue threshold, crack growth rates at 150 °C were comparable to those measured at RT. Prolonged fatigue testing at near-threshold crack growth rates resulted in oscillations of crack growth rate until the fatigue crack growth behavior was stabilized to become similar to that in an overaged composite. Measurement of the matrix hardness at different distances from the crack plane and transmission electron microscopy examination of the fatigue specimen have shown that the matrix microstructure at the tip of the fatigue crack underwent overaging during prolonged testing in the near-threshold regime. Consequently, the fatigue fracture mechanism was modified, a lower crack closure developed, and the fatigue threshold reduced to that of the overaged composite.     

Effects of Changes in Test Temperature on Tensile Properties and Notched <Emphasis Type="Italic">Vs</Emphasis> Fatigue Precracked Toughness of a Zr-Based BMG Composite

Tensile and fracture toughness behavior of a Zr-based bulk metallic glass matrix composite (BMGMC) containing a body-centered cubic crystalline phase was examined over temperatures from 77 K to 653 K (?196 °C to 380 °C). The BMGMC exhibited tensile plasticity at all test temperatures. The sample tested in tension at 173 K (?100 °C) exhibited work hardening but the remaining samples tested at higher temperatures exhibited work softening. EBSD analysis of the crystalline phase after tensile testing provides insight into active deformation mechanisms in the crystalline phase. At 603 K (330 °C), the dendrites exhibit significant plastic strain, with the dendrites oriented {101} parallel to the loading direction exhibiting the least amount of strain. Schmid factor analysis leads to the hypothesis that {110}〈111〉 dislocation mechanisms are active at this temperature. Additionally, measurements of dendrite shape as a function of macroscopic strain state in the tension experiments provide insight into cooperative deformation mechanisms in the composite. At low temperatures, the fracture toughness of the notch toughness samples exceeded that of fatigue precracked samples; but at and above room temperature, the toughness values of notched and fatigue precracked samples converge. These observations are rationalized based on the changes to the flow and fracture behavior of the glass and the crystalline phases over this temperature range. At low temperatures, the crystalline phase is sensitive to defects and changes in stress state that reduce its energy absorbing ability. At higher temperatures, both constituents possess lower strength and are less sensitive to defects, enabling more significant crack tip blunting in the fatigue precracked samples. This produces toughness values that are similar to those obtained for the notched samples.     

Fatigue crack growth through partially stabilized zirconia at ambient and elevated temperatures

Fatigue crack growth through magnesia stabilized zirconia at 20, 450 and 650°C has been observed dynamically in a high temperature loading stage for the scanning electron microscope. Crack tip micromechanics parameters were measured using the stereoimaging technique. Fatigue crack growth at ambient temperature was found to be very similar to crack growth through metallic alloys. With increasing temperature, the stress intensity levels in which stable fatigue crack growth could be sustained were found to narrow significantly, until fatigue is expected to not be a valid mechanism of crack growth above about 750°C. Measured crack tip parameters were used to derive the low-cycle fatigue and the stress-cycles to failure characteristics. The latter agreed with measured SN curves. Deformation within the plastic zone was shown to account for the measured value of fracture toughness. The mechanisms of crack growth are discussed.     

用原子-有限元法研究铁中裂纹的低温脆性解理扩展

运用原子 -有限元法 (FEAt)研究了裂纹在低温加载条件下的脆性解理扩展。原子模拟结果表明 :在低温和一定外部载荷条件下 ,bcc- Fe中 { 10 0 } <0 11>边界 I型裂纹的扩展过程是一个裂尖原子键断裂与层错或孪晶扩展相伴随的过程。裂纹低温脆性解理扩展的最大速率可达到 112 5m /s,即约为 0 .6 VR。相应的有限元分析表明 ,裂尖存在较大的能量和应力集中 ,这是导致裂尖原子键断裂及弹性孪晶形成的原因。     

Time-Dependent Fatigue Crack Propagation Behavior of Two Solid-Solution-Strengthened Ni-Based Superalloys—INCONEL 617 and HAYNES 230

The fatigue crack propagation (FCP) as well as the sustained loading crack growth (SLCG) behavior of two solid-solution-strengthened Ni-based superalloys, INCONEL 617 (Special Metals Corporation Family of Companies) and HAYNES 230 (Haynes International, Inc., Kokomo, IN), were studied at increased temperatures in laboratory air under a constant stress-intensity-factor (K) condition. The crack propagation tests were conducted using a baseline cyclic triangular waveform with a frequency of \frac13 \frac{1}{3} Hz. Various hold times were imposed at the maximum load of a fatigue cycle to study the hold time effect. The results show that a linear elastic fracture mechanics (LEFM) parameter, stress intensity factor (K), is sufficient to describe the FCP and SLCG behavior at the testing temperatures ranging from 873 K to 1073 K (600 °C to 800 °C). As observed in the precipitation-strengthened superalloys, both INCONEL 617 and HAYNES 230 exhibited the time-dependent FCP, steady SLCG behavior, and existence of a damage zone ahead of crack tip. A thermodynamic equation was adapted to correlate the SLCG rates to determine thermal activation energy. The fracture modes associated with crack propagation behavior were discussed, and the mechanism of time-dependent FCP as well as SLCG was identified. Compared with INCONEL 617, the lower crack propagation rates of HAYNES 230 under the time-dependent condition were ascribed to the different fracture mode and the presence of numerous W-rich M₆C-type and Cr-rich M₂₃C₆-type carbides. Toward the end, a phenomenological model was employed to correlate the FCP rates at cycle/time-dependent FCP domain. All the results suggest that an environmental factor, the stress assisted grain boundary oxygen embrittlement (SAGBOE) mechanism, is mainly responsible for the accelerated time-dependent FCP rates of INCONEL 617 and HAYNES 230.     

An investigation of the effects of interfacial microstructure on the fatigue behavior of a four-ply [75]4 continuous silicon carbide (SCS-6) fiber-reinforced titanium matrix composite

The effects of interfacial microstructure/thickness on the strength and fatigue behavior of a model four-ply [75]₄ Ti-15V-3Al-3Cr-3Sn/SiC (SCS-6) composite are examined in this article. Interfacial microstructure was controlled by annealing at 815 °C for 10, 50, or 100 hours. The reaction layer and coating thickness were observed to increase with increasing annealing duration. Damage initiation/propagation mechanisms were examined in as-received material and composites annealed at 815 °C for 10 and 100 hours. Fatigue behavior was observed to be dependent upon the stress amplitude. At high stress amplitudes, the failure was dominated by overload phenomena. However, at all stress levels, fatigue crack initiation occurred by early debonding and matrix deformation by stress-induced precipitation. This was followed by matrix crack growth and fiber fracture prior to the onset of catastrophic failure. Matrix shear failure modes were also observed on the fracture surfaces in addition to fatigue striations in the matrix. Correlations were also established between the observed damage modes and acoustic emission signals that were detected under monotonic and cyclic loading conditions.     

The effect of environment on the mechanism of Stage I fatigue fracture

Fatigue crack propagation in nickel-base superalloys at low and intermediate temperatures occurs predominantly in the Stage I mode, along {111} slip planes. Cracking normally starts at an external surface and the Stage I fracture surface has a cleavage appearance. Both of these factors indicate that the environment may play an important role in this mode of propagation. To assess the role of environment in Stage I fracture and to determine the mechanism of failure, fatigue tests were run in air and vacuum on single crystals of low-carbon MAR-M200. The fatigue life at room temperature is significantly greater in vacuum than in air, and the improvement in life increases as the stress range is reduced. Fatigue crack propagation in specimens tested in air and in vacuum is entirely in the Stage I mode, but only the specimens tested at low stress ranges in air have a cleavage appearance. In vacuum and at high applied stress levels in air, fracture surfaces have a matte appearance with fewer fracture steps and river lines. At high magnifications, a dimpled structure is observed on these fracture surfaces. The fatigue life in air can be attributed to a faster rate of crack growth resulting from oxygen adsorption at the crack tip. A model for Stage I fatigue crack propagation in planar slip materials is presented which is an extension of the Griffith-Orowan criterion to cases where localized cleavage occurs at a crack tip in fatigue.     

The fatigue and fracture resistance of a Nb-Cr-Ti-Al alloy

The microstructure, fatigue, and fracture behaviors of a cast and heat-treated Nb-Cr-Ti-Al alloy were investigated. The microstructure of the cast alloy was manipulated by annealing at a temperature ranging from 500 °C to 1500 °C for 1 to 24 hours. The heat treatment produced Cr₂Nb precipitates along grain boundaries in all cases except in the 500 °C heat-treated material. Fracture toughness tests indicated low fracture resistance in both the as-cast and heat-treated materials. Fatigue crack growth tests performed on the 500 °C heat-treated material also indicated a low fatigue crack growth resistance. Direct observations of the near-tip region revealed a cleavage-dominated fracture process, in accordance with fractographic evidence. The fracture behavior of the Nb-Cr-Ti-Al alloy was compared to that of other Nb-Cr-Ti alloys. In addition, theoretical calculations of both the unstable stacking energy (USE) and Peierls-Nabarro (P-N) barrier energy are used to elucidate the role of Al additions in cleavage fracture of the Nb-Cr-Ti-Al alloy. The results indicate that an Al alloying addition increases the USE, which, in turn, prevents the emission of dislocations, promotes the nucleation and propagation of cleavage cracks from the crack tip, and leads to a reduction in the fracture toughness.     

Effects of transformed ferrite growth on the tensile fracture characteristics of a dual-phase steel

The effects of transformed ferrite growth on the tensile fracture characteristics of a dual-phase steel were investigated by observing crack initiation, propagation, and fracture behaviors. Crack initiation occurred by decohesion between martensite and ferrite. However, cracks propagated along the ferrite-martensite interface in a high temperature quenched specimen, whereas in specimens quenched from lower temperature cracks propagated into the martensite particle. Tensile fracture behaviors were not strongly influenced by the cooling rate. At both cooling rates of 5.6 and 0.1 °C/sec, specimens quenched from high temperature fractured by partially brittle fracture mode, but fracture mode changed to ductile mode as the quenching temperature decreased. The effect of transformed ferrite on the fracture mode was not substantially different from that of retained ferrite. However, the crack initiation and propagation was influenced by the variation in martensite distribution caused by different growth behavior of transformed ferrite.     

The effects of environment on the elevated temperature fatigue behavior of nickel-base superalloy single crystals

The effects of air and vacuum on the fatigue behavior of a nickel-base superalloy, Mar-M200, in single crystal form were investigated. Between 800° and 1400°F fracture is entirely in the Stage I mode in air and vacuum, and fatigue life is unaffected by environment. At 1700° F in both environments, fracture is predominantly in the Stage II mode and fatigue life in air is greater than that in vacuum. At both temperatures, fatigue cracking in air is internally initiated, whereas in vacuum cracking is generally initiated at the specimen surface. Identical fatigue lives in air and in vacuum between 800° and 1400° F are attributed to the fact that internally initiated cracks in air are actually propagating in a high vacuum, surface cracking being inhibited by dynamic oxidation of emerging surface slip offsets. The subsurface portion of the Stage I fracture surface produced in air tests and all of the Stage I fracture produced in vacuum tests shows a dimpled structure, whereas the Stage I fracture surface produced while the crack propagation is in air shows a cleavage appearance. At 1700° F, bulk oxidation of surface initiated cracks interferes with the plastic blunting mechanism of Stage II crack growth normally observed at this temperature, internally initiated cracks causing ultimate failure. Shorter lives in vacuum are thought to result from enhanced Stage II surface crack propagation. Formerly with Materials Engineering and Research Laboratory, Pratt and Whitney Aircraft, Middletown, Conn.     

High-temperature fracture and fatigue-crack growth behavior of an XD gamma-based titanium aluminide intermetallic alloy

A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB₂ particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed, above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature. The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature.     

Environmentally induced crack propagation behavior of 2219 aluminum: microstructural effects

Alloy 2219 has been evaluated under corrosion fatigue conditions. The effect of the micro-structures present in the T851 and T6 conditions on crack propagation rates has been determined. Tests were performed on compact tension specimens in air and in NaCI solutions at 23 and 70°C. The corrosion fatigue behavior of the material under these conditions was evaluated by studying the crack propagation kinetics and also crack fractography.The results of this study show that 2219 aluminum is resistant to corrosion fatigue in aqueous solutions containing up to 10% NaCI. However, the material is more resistant in the T6 than in the T851 condition. In the absence of stress corrosion cracking, the initial stages of the crack growth were characterised by ductile fracture while a mixed fracture mode dominated the final stages of the growth. At low loading frequencies (0. 1 Hz), crack branching and blunting were observed when tests were performed at 70°C.     

Analysis of crystallographic high temperature fatigue crack growth in a nickel base alloy

Crack growth behavior of a nickel-base alloy, Udimet 700, was studied at room temperature and 850 °C in air and vacuum. Crack growth rates were higher in air than in vacuum but this increase in growth rates was nearly the same at both temperatures. In contrast to the effect of environment, an increase of temperature from 25 to 850 °C has a much larger effect on growth rates although the mode of crack growth did not change with temperature or with environment. A detailed analysis of the fracture surfaces indicated that the growth rates under all of the above experimental conditions occurs by a crystallographic faceted mode with the plane of the facet identified to be the {100} cleavage plane rather than a slip plane. Also the increase in growth rates with temperature appears not to be directly related to an environmental effect, creep effect or variation of elastic modulus with temperature.     

Effect of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base superalloy

The influence of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base precipitation hardened superalloy was studied in air from 427° to 649 °C. The tests were conducted at 100 Hz and at load ratios of 0.1 and 0.5. The threshold ΔK values were found to increase with temperature. This behavior is attributed to oxide deposits that form on the freshly created fracture surfaces which enhance crack closure. As determined from secondary ion mass spectrometry, the oxide thickness was uniform over the crack length and was of the order of the maximum crack tip opening displacement at threshold. Oxidation kinetics were important in thickening the oxide on the fracture surfaces at elevated temperatures, whereas at room temperature, the oxide deposits at near threshold fatigue crack growth rates and at low load ratios were thickened by an oxide fretting mechanism. The effect of fracture surface roughness-induced crack closure on the near threshold fatigue crack growth behavior is also discussed. Formerly with General Electric Company, Advanced Nuclear Technology Operation, Sunnyvale, CA 94086.     

High temperature-high strain rate fracture of inconel 600

The hot fracture of Inconel 600 has been studied over the temperature range from 800° to 2000°F using a hot torsion tester that is capable of superimposing either axial tensile or compressive stresses on the torsional shearing stresses. Microscopic studies of fracture initiation have been made over the entire temperature region. From 800° to 1200°F fracture initiates at inclusions and propagates by transgranular shear. In the temperature region of minimum ductility, 1300° to 1500°F, fracture initiates at grain boundaries and propagates readily in an intergranular manner. At 1600°F and above, fracture initiates easily at grain boundaries, but because recrys-tallization intervenes crack propagation is difficult and strain to fracture is high. Microcracks initiate at the peak in the torque-twist curve. The higher the temperature the smaller is the strain at which fracture initiates. Correlations have been found between the stress state and the shearing strain at crack initiation and total fracture strain. These correlations show the strong influence of a compressive normal stress on retarding crack initiation and resisting crack propagation.