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.     

Study of creep crack growth in 2618 and 8009 aluminum alloys

Creep crack growth (CCG) has been investigated in an 8009 (Al-Fe-V-S) P/M alloy at 175 °, 250 °, and 316 ° and in a 2618 ingot alloy at 150 °, 175 °, and 200 °. Under sustained load, subcritical crack growth is observed at stress intensity levels lower thanK _ic ; for 2618, at 200 °, crack growth is observed at stress intensities more than 40 pct lower thanK _ic . Alloys 8009 and 2618 exhibit creep brittle behavior,i.e., very limited creep deformation, during CCG. The CCG rates do not correlate with CCG parameters C* and C but correlate with the stress intensity factor,K, and theJ integral. Generally, crack growth rates increase with increasing temperature. Micromechanisms of CCG have been studied with regard to microstructural deg-radation, environmental attack, and creep damage. Although theoretical estimation indicates that CCG resistance decreases with second-phase coarsening, such coarsening has not been observed at the crack tip. Also, no evidence is found for hydrogen- or oxygen-induced crack growth in comparing test results in moist air and in vacuum. Creep deformation and cavitation ahead of crack tip are responsible for observed time-dependent crack growth. Based on the cavitation damage in the elastic field, a micromechanical model is proposed which semiquantitatively explains the correlations between the creep crack growth rate and stress intensity factor,K.     

Effect of weld heat input and creep strain on the elevated temperature and crack growth properties of austenitic steels

A potential material class for use at 600°C and more, e.g. for steam turbines with improved thermal efficiency, are austenitic steels. Using these steels with welded joints, it is to be considered that, by superposition of weld residual stresses and service stresses, extensive creep strains – and in the worst case crack formation – can occur locally. To assess the influence of these effects on service behaviour, different material states of CrNi-steels and Incoloy 800 were investigated with respect to strength, ductility and, especially, to crack and creep crack growth in the temperature range around 600°C. It is shown that creep embrittlement, not microstructural changes as effected by weld heat input, causes heat affected zone (HAZ)-reheat cracking. Creep embrittlement can be avoided by special design and fabrication rules.     

Elevated temperature creep-fatigue crack propagation in nickel-base alloys and 1 Cr-Mo-V steel

The crack growth behavior of several high temperature nickel-base alloys, under cyclic and static loading, is studied and reviewed. In the oxide dispersion strengthened (ODS) MA 6000 and MA 754 alloys, the high temperature crack propagation exhibited orientation dependence under cyclic as well as under static loading. The creep crack growth (CCG) behavior of cast nickel-base IN-738 and IN-939* superalloys at 850 °C could be characterized by the stress intensity factor,K ₁. In the case of the alloy IN-901 at 500 °C and 600 °C,K ₁ was found to be the relevant parameter to characterize the creep crack growth behavior. The energy rate line integral,C*, may be the appropriate loading parameter to describe the creep crack growth behavior of the nickel-iron base IN-800H alloy at 800 °C. The creep crack growth data of 1 Cr-Mo-V steel, with bainitic microstructure, at 550 °C could be correlated better by C * than byK ₁. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

A study of creep crack growth in 2219-T851

Creep crack growth rates were measured in high strength 2219-T851 aluminum alloy with a computerized fully automated test procedure. Crack growth tests were performed on CT specimens with side grooves. The experimental set-up is described. During a test, the specimen is cyclically loaded on a servohydraulic testing machine under computer control, maintained at maximum load for a given hold time at each cycle, unloaded, and then reloaded. Crack lengths are obtained from compliance measurements recorded during each unloading. It is shown that the measured crack growth rates per cycle do represent creep crack growth rates per unit time for hold times longer than 10 seconds. The validity of LEFM concepts for side-grooved specimens is reviewed, and compliance and stress intensity factor calibrations for such specimens are reported. For the range of testing conditions of this study, 2219-T851 is shown to be creep brittle in terms of concepts of fracture mechanics of creeping solids. It is found that, under these testing conditions, a correlation exists between the creep crack growth rates under plane strain conditions and the stress intensity factor (da/dt =A K ^3.8 at 175 °C) for simpleK histories in a regime of steady or quasi-steady state crack growth. The micromechanisms of fracture are determined to be of complex nature. The fracture mode is observed to be mixed inter- and transgranular, the relative amount of intergranular fracture decreasing asK andda/dt increase. Formerly Graduate Student, Massachusetts Institute of Technology, is Ingenieur de l’Armement, ETCA, 94114 Arcueil Cedex, France.     

Qualitative model for creep of AZ91D magnesium alloy

Creep properties of specimens taken from the core of AZ91D magnesium alloy ingots (9 pct Al, 1 pct Zn) were examined in the temperature range of 120 °C to 180 °C and stress range of 40 to 115 MPa. Solution-treated and aged creep specimens were also tested. The creep rates observed were about three orders of magnitude lower than those of pure magnesium, and elongations to fracture were seen to be at least twice those of pure magnesium. A minimum creep rate was reached after approximately 2/3 of the creep life of the specimens (except for the aged specimens, in which the minimum creep rate appears at the beginning of the test). A qualitative model for the creep process in proposed on the basis of the creep tests and optical, scanning electron, and transmission electron microscopy. This model proposes that dislocation motion on additional slip systems is the primary creep mechanism and that cracking acts as a stress relief mechanism. Structural instability dictates the amount of hardening and, hence, creep resistance.     

The creep fracture of wrought nickel-base alloys by a fracture mechanics approach

Creep fracture in the 500 to 750°C temperature range was by an intergranular crack growth process involving the formation of microcracks in grain boundaries slightly ahead of the main crack. The crack growth was proportional to an exponential power of the stress intensity. Wide differences in cracking behavior were seen between different alloys, but their differences were due primarily to material processing history and not to compositionper se. Transverse sample orientation and coarser grain sizes significantly improved the resistance to cracking. Both slow crack growth and the final fast fracture toughness changed appreciably with test history. A good correlation was found between the notch properties and the creep behavior of an unnotched sample loaded to the yield strength.     

Creep crack growth behavior of several structural alloys

Creep crack growth behavior of several high temperature alloys, Inconel 600, Inconel 625, Inconel X-750, Hastelloy X, Nimonic PE-16, Incoloy 800, and Haynes 25 (HS-25) was examined at 540, 650, 760, and 870 °C. Crack growth rates were analyzed in terms of both linear elastic stress intensity factor and J*-integral parameter. Among the alloys Inconel 600 and Hastelloy X did not show any observable crack growth. Instead, they deformed at a rapid rate resulting in severe blunting of the crack tip. The other alloys, Inconel 625, Inconel X-750, Incoloy 800, HS-25, and PE-16 showed crack growth at one or two temperatures and deformed continuously at other temperatures. Crack growth rates of the above alloys in terms ofJ* parameter were compared with the growth rates of other alloys published in the literature. Alloys such as Inconel X-750, Alloy 718, and IN-100 show very high growth rates as a result of their sensitivity to an air environment. Based on detailed fracture surface analysis, it is proposed that creep crack growth occurs by the nucleation and growth of wedge-type cracks at triple point junctions due to grain boundary sliding or by the formation and growth of cavities at the boundaries. Crack growth in the above alloys occurs only in some critical range of strain rates or temperatures. Since the service conditions for these alloys usually fall within this critical range, knowledge and understanding of creep crack growth behavior of the structural alloys are important.     

The effect of low gold concentrations on the creep of eutectic tin-lead joints

The effects of low Au concentrations on the creep properties of a eutectic Sn/Pb alloy were investigated. Creep testing was performed on double-shear specimens of fine-grained, eutectic Sn/Pb joints with Au concentrations of 0, 0.2, 1.0, and 1.5 wt pct Au at 90 °C, 0, 0.2, and 1.0 wt pct Au at 65°C, and 0.2 wt pct Au at 25 °C. In the absence of Au, the creep of finegrained eutectic Sn/Pb is dominated by grain-boundary sliding at high homologous temperature and intermediate stress. The addition of 0.2 wt pct Au or more suppressed this mechanism; the high-stress, bulk-creep mechanism was dominant at all stresses tested. Higher concentrations of Au increased porosity within the joints. The porosity decreased joint strength. During failure, the crack path followed softer regions of the joint; cracks propagated through Pb-rich islands or along Sn/Sn grain boundaries.     

The creep fracture of wrought nickel-base alloys by a fracture mechanics approach

Metallurgical and Materials Transactions A - Creep fracture in the 500 to 750°C temperature range was by an intergranular crack growth process involving the formation of microcracks in grain...     

Stress and temperature dependence of creep in Alloy 600 in primary water

The stress and temperature dependence of creep of commercial nickel-base Alloy 600 was investigated through constant load, step-load, and step-temperature creep tests in deaerated primary water containing 40 to 60 cc/kg hydrogen. To analyze creep rates for Alloy 600 in the mill-annealed (MA) condition, effective stresses were estimated using applied stresses and instantaneous strains. The apparent activation area was determined to be 7b ² by the multiple regression analysis of creep rates. The apparent activation energy for creep has a weak stress dependence and was determined to lie between 188 and 281 kJ/mole for the effective stress range of 117 to 232 MPa. Creep rates were better correlated with effective stress than applied stress and the stress exponent of Alloy 600 MA was determined to be 2.2 at 337 °C and 5.1 at 360 °C. The magnitudes of the stress exponent, activation energy, and activation area can be interpreted to support a creep mechanism controlled by dislocation-climb and nonconservative motion of jogs in commercial Alloy 600 MA. The activation area agreed with that determined from carbon in solution, implying thermally activated dislocation glide as another possible creep mechanism.     

Influence of cyclic to mean load ratio on creep/fatigue crack growth

Crack growth data under combined creep and fatigue loading conditions are presented on a nickel base superalloy and a brittle and ductile low alloy steel. The main variables that have been examined are minimum to maximum load ratioR and frequency. It is shown that at high frequencies transgranular fatigue failure dominates and at low frequencies time dependent mechanisms govern. Where fatigue processes control, it is demonstrated that crack growth/cycle can be described by the Paris law and that the influence ofR ratio can be accounted for by crack closure caused by fracture surface roughness, oxidation, and creep and plastic strain developed at the crack tip. At the low frequencies where time dependent processes dominate, it is shown that crack growth can be characterized satisfactorily in terms of the creep fracture mechanics parameterC * using a model of crack extension based on ductility exhaustion in a creep damage zone at the crack tip. This model leads to enhanced resistance to creep/fatigue crack growth with increase in material creep ductility. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

Analysis of steady-state creep and creep fracture of directionally solidified eutectic γ/γ′-α alloy

The steady-state creep behavior of directionally solidified eutectic alloy Ni-30Mo-6Al-l.6V-l.2Re (wt pct) was investigated at temperatures between 1223 and 1323 K using constant strain rate tension creep tests. The steady-state stress is found to depend strongly on creep rate and temperature. The apparent power law stress exponent for steady-state stress isn = 7.5 ± 0.3, and the apparent activation energy for creep of the eutectic γ/γ′-α composite is determined to beQ = 517 ± 11 kJ mol₋₁. When the steady-state creep is analyzed in terms of the effective stress and normalized with respect to the temperature dependence of the elastic modulus, the corrected activation energy for creepQ _c is calculated to be between 412 and 424 kJ mol₋₁ and the stress exponent between 5.7 and 6.0. The kinetics of the steady-state creep deformation within the studied temperature range involves the contribution of both the fibers and the matrix which creep during steady-state. Analysis of the fracture surfaces of the composite shows ductile fracture mode. The composite fails by growth and coalescence of microvoids in the matrix and by fiber fragmentation.     

Experimental and numerical study on the relationship between creep crack growth properties and fracture mechanisms

Evaluation of creep crack growth properties taking microscopic aspects into account is effective for developing more accurate life prediction of structural components. The present study investigated the relationship between creep crack growth properties and microscopic fracture aspects for austenitic alloy 800H and 316 stainless steel. The growth rate of wedge-type intergranular and transgranular creep crack could be characterized by creep ductility. Creep damages formed ahead of the void-type crack tip accelerated the crack growth rate. Based on these experimental results, a three-dimensional finite element method (FEM) code, which simulates creep crack growth, has been developed. The effect of creep ductility on da/dt vs C* relations could be simulated based on the critical strain criteria. The diffusion of vacancies toward crack tip would accelerate the crack growth under creep conditions. The change of vacancy concentration during creep was computed for a three-dimensional compact-type (CT) specimen model by solving the diffusive equation under multiaxial stress field. The experimental results that crack growth was accelerated by creep damages formed ahead of the crack tip could be successfully simulated.     

Effects of temperature and environment on the tensile and fatigue crack growth behavior of a Ni3AI-base alloy

The effects of temperature, frequency, and environment on the tensile and cyclic deformation behavior of a nickel aluminide alloy, Ni-9.0 wt pct Al-7.97 pct Cr-1.77 pct Zr (IC-221), have been determined. The tensile properties were obtained in vacuum at elevated temperatures and in air at room temperature. The alloy was not notch sensitive at room temperature or at 600 °C, unlike Cr-free Ni₃Al + B alloys. In general, crack growth rates of IC-221 increased with increasing temperature, decreasing frequency, exposure to air, or testing at higherR ratios. At 25 °C, crack growth rates were slightly higher than for a previously investigated Cr-free Ni₃Al alloy. However, at 600 °C, the crack growth rates for IC-221 were lower than for the Cr-free alloy. Substantial frequency effects were noted on crack growth of IC-221 at both 600 °C and 800 °C in both air and vacuum, especially at highK. The relative contributions of creep and environmental interactions to fatigue crack growth are discussed.     

The effect of combined low-cycle fatigue and creep on the life of austenitic stainless steels

Experiments regarding the effect of simultaneous fatigue and creep on life have been performed for a 20 pct Cr-35 pct Ni stainless steel at 700°C. It was found that a life fraction rule, comprising an interaction term besides the terms of pure fatigue and creep damage, adequately accounts for the observed lives. The interaction term contains the product of the fatigue and creep damage, and may physically be interpreted as the damage generation due to the interaction of the fatigue and creep processes. There is evidence that the surface crack density represents a measure of the total damage. The cracks created during the combined fatigue and creep test were both transgranular and intergranular. They were probably nucleated intergranularly and later, during their growth, were transformed to transgranular cracks. As the creep damage of the test was increased gradually, more cracks remained intergranular throughout the test.     

The influence of heat treatment on the structure and properties of a near-α titanium alloy

The microstructure and tensile properties of a near-α titanium alloy, IMI-829 (Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-0.5 wt pct Mo-1 wt pct Nb-0.32 wt pct Si) have been studied after solutionizing (and no subsequent aging) at two different temperatures separately, one above the β transus (1050 °C) and another below the β transus (975 °C) followed by various cooling rates (furnace, air, oil, or water). While 1050 °C treatment resulted in coarse Widmanstätten structures on furnace or air cooling, fine Widmanstätten structure on oil quenching and martensitic structure on water quenching, 975 °C treatment produced duplex microstructures consisting of equiaxed alpha and partially transformed beta phases. Transmission electron microscopy studies revealed the morphology, size, and distribution of the α, β, and martensite phases and also the presence of small ellipsoidal suicide particles and an interface phase with fcc structure at almost all α-β interfaces. The oil quenched structure from 1050 °C has been found to be a mixture of fine Widmanstätten α coexisting with martensite laths and retained beta at the lath boundaries. Silicides with hcp structure of about 0.4 μm size were observed in specimens solution treated at 975 °C. The interface phase is seen in all slowly-cooled specimens. The YS and UTS are superior for 975 °C treatment compared to 1050 °C treatment after water quenching or oil quenching. The tensile ductility values are superior for any cooling rate after 975 °C solution treatment as compared to 1050 °C solution treatment. The specimens failed in tension diagonally by shear after 1050 °C treatment and by cup and cone fracture after 975 °C treatment. In all cases fracture has taken place by microvoid coalescence and in most cases, along the α-β boundaries.     

Fatigue and creep damage resistance of a thermal barrier coated superalloy for combustor liners in aero turbines

Fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy was conducted at 800°C in air. Fatigue results reveal that the endurance limits for the TBC and bond coated substrate was substantially higher than that of the base alloy, while the opposite was found for high stress, low cyclic life times. It appears that the increase in endurance limit for the TBC and bond coated superalloy is due to load shifting to the bond coat, and the premature failure for these two materials is possibly due to high stress crack imitation/growth in the TBC/bond coat layers. In addition to fatigue, accelerated creep properties of thermal barrier coated (TBC) Superni C263 were evaluated. Creep results reveal that the life of the TBC composite under accelerated creep condition is substantially high compared to that of the bare substrate. The mode of fracture in the substrate at very high stresses was transgranular whereas that at low stresses was intergranular. Delamination of bond coat, oxidation of the substrate and spallation of the ceramic layer were evident at very high stress. It was evident that the substrate has negligible estimated rupture strength after 10,000 hours of service exposure.