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.     

The influence of microstructure and environment on the crack growth behavior of powder metallurgy nickel superalloy RR1000

A study of crack growth in vacuum and air at 725 °C (T/T _m=0.6) highlights the relative importance of creep and environmental crack-tip damage mechanisms in Powder Metallurgy (P/M) disc alloy RR1000. Both of these mechanisms are associated with a transition to intergranular fracture during fatigue crack growth at 0.25 Hz. Crack growth under sustained loads reveals the precise nature of these mechanisms in RR1000. The severity of creep and environmental mechanisms is controlled by the grain-boundary microstructure and the crack-tip stress. Near-tip cavitation leads to fracture in vacuum. Sigma-phase precipitation causes an increase in crack growth rate through increased crack-tip cavity nucleation. Rapid near-tip stress relaxation induced by γ′ coarsening has a beneficial effect on the severity of this type of damage. In air, increases in crack growth rates are associated with near-tip intergranular oxidation. It is proposed that the extent of this damage and subsequent growth rates are increased by sigma-phase precipitation through enhanced oxidation due to chromium depletion and subsequent decreased passivation. Again, a beneficial effect of rapid near-tip stress relaxation due to selective γ′ coarsening is apparent and environmental damage is reduced under these conditions.     

Effect of Environment on Creep Crack Growth in PM/HIP René-95

The creep crack growth rates (CCGR) of PM/HIP René-95 were measured from 10^-9 m per second to 10^-4 m per second in air and in high purity argon at 760°C and 650°C using single edge notched (SEN) specimens. The crack length was monitored by the D.C. potential difference technique. The data were reported asda/dt vs the elastic stress intensity factor,K,, since PM/HIP René-95 is a creep-brittle material. The CCGR were shown to be strongly environment sensitive. The CCGR were up to 1000 times faster in air than in argon for a given value ofK _I . The temperature andK _I dependence of CCGR in air were shown to correlate with a modified formulation of the Larson-Miller parameter. Notched stress rupture (NSR) tests were performed at 650°C in air in order to study the effect of notch root radius on the time to initiate a creep crack. A comparison of the NSR data with SEN data for the same values of initialK, shows that the crack initiation times are a strong function of the notch root radius. It was observed that ninety percent of the rupture time is spent in crack initiation when the notch root radius is finite, while no incubation time was observed for creep crack growth from fatigue precracked specimens. Formerly with Massachusetts Institute of Technology, Cambridge, MA     

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.     

Time-dependent deformation behavior of near-eutectic 60Sn-40Pb solder

The compressive creep and stress-strain behavior of the near-eutectic 60Sn-40Pb solder alloy has been investigated over the temperature range of −55 °C to 125 °C. The total primary creep strain is a strong function of stress and temperature: at lower temperatures and high applied stresses (i.e., near the power-law breakdown regime), it is quite large, while it is much smaller at higher temperatures and lower applied stresses. The compressive minimum creep rate as a function of stress and temperature is fit well by the Garofalo sinh equation. A discussion of the effective stress exponent, n _eff, in the context of the Garofalo sinh equation is presented to understand trends in the creep data. The values of n _eff, for the applied stress levels studied, are found to range from 3.09 to 5.00 at 125 °C, while they have a range of 10.75 to 15.79 at −55 °C. These trends are consistent with the interpretation of climb-dominated creep at higher temperatures and plasticity-dominated power law breakdown behavior at the lower temperatures. The microstructural observations suggest that, at elevated temperatures, deformation occurs by relative displacement of eutectic colonies in the solder microstructure accompanied by extensive grain coarsening in the colony boundaries. At lower temperatures (<0 °C), deformation occurs by cell displacement with very limited coarsening and, at high stresses, is dominated by plastic deformation. The application of the Garofalo sinh equation to other data sets for creep of eutectic Sn-Pb solder is also discussed.     

Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 °C. Specially designed solution and aging treatments were used to produce γ′ strengthened microstructures with different grain sizes but without any M₂₃C₆ grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors.     

Effect of microstructure on creep and creep crack growth behaviour of titanium alloy

Creep and creep crack growth behaviour of a near α titanium alloy has been investigated at 600°C which is affected by primary α content. The alloy was heat treated at different temperatures so as to obtain different levels of equiaxed primary α in the range from 5 to 24 %. Constant load creep tests were carried out at 600°C in the stress range 250 to 400 MPa till rupture of the specimens. Creep crack growth tests were carried out at 600°C. Creep data reveals with increase in primary α content leads to creep weakening. On similar lines maximum creep crack growth resistance is associated with the alloy with lowest primary α content. Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary β / transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary β / transformed β (matrix) interfaces in the interior of the specimen.     

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.     

Stress-assisted hydrogen attack cracking in 2.25Cr-1Mo steels at elevated temperatures

Crack growth in 2.25Cr-lMo steels exposed to 3000 psi hydrogen has been investigated in the temperature range 440 °C to 500 °C, using modified wedge-opening loaded specimens to vary stress intensity. Under conditions of temperature and hydrogen pressure, where general hydrogen attack does not occur, the crack propagated by the growth and coalescence of a high density of methane bubbles on grain boundaries, driven by the synergistic influence of internal methane pressure and applied stress. Crack growth rates were measured in base metal, and the heat-affected zones (HAZs) of welds were tempered to different strength levels. The crack growth rate increased with material strength. Above a threshold of about K_l = 20 MPa√m (at 480 °C), the crack growth rate increased rapidly with stress intensity, increasing as roughly K_l ^6.5. Because of better creep resistance, stronger materials can sustain higher levels of stress intensity to drive crack growth and nucleate the high density of voids necessary for crack growth. Stress relaxation by creep reduces the stress intensity, and thus the growth rate, especially in weaker materials. The crack growth rate in the heat-affected zone was found to be substantially faster than in the base metal of the welds. Analysis indicates that K_l rather than C* is the appropriate crack-tip loading parameter in the specimen used here and in a thick-walled pressure vessel. The DC potential drop technique met with limited success in this application due to the spatially discontinuous manner of crack growth and limited crack-tip opening displacement. Formerly Graduate Student, Materials Science and Engineering Department, The Ohio State University     

Effects of load ratio and temperature on the near-threshold fatigue crack propagation behavior in a CrMoV steel

The influence of temperature in the range of 24 to 260 °C and load ratio on the near-threshold fatigue crack growth rate behavior of a CrMoV steel was characterized. At all temperatures investigated, the threshold stress intensity range, ΔK _th, for fatigue crack growth decreased with increasing load ratio. The near-threshold crack growth rates increased significantly at 149 °C when compared with the rates at room temperature. However, the crack growth rates at 260 °C were comparable to those at 149 °C. These observations are rationalized in terms of the concepts of roughness and oxide-induced crack closure. Extensive fracture surface characterization using SEM, oxide thickness measurements by Auger spectroscopy, and roughness measurements by light-section-microscopy were conducted to substantiate the explanations.     

Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 °C. Specially designed solution and aging treatments were used to produce γ′ strengthened microstructures with different grain sizes but without any M₂₃C₆ grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors. S. XU, formerly Graduate Student, Department of Engineering Physics and Materials Engineering, Ecole Polytechnique de Montreal A. K. KOUL, formerly Senior Research Officer, NRC, Ottawa     

Investigations on the influence of internal nitridation on creep crack growth in alloy 800 H

For two batches of Alloy 800 H with Al contents of 0.02 and 0.34 pct, creep crack growth was investigated at 1000 °C in air and in an Ar-5 pct H₂ atmosphere. TheK concept, the net-section stress concept, and theC ^* concept of creep fracture mechanics were applied when plotting the experimental results. TheC ^* concept proved to give the best correlation between load parameter and crack growth rate. A good agreement was observed between the experimental results and the model calculations for crack extension by constrained diffusive cavity growth. Strong internal nitridation, which occurred in the air tests and which had been shown to increase the creep strength in creep rupture tests, did not show any significant influence on the creep crack growth rate in comparison with tests in an Ar-5 pct H₂ atmosphere, in which no internal nitridation was observed. Also, the differences in the Al contents of the two batches did not play a role. This behavior is explained by the fact that neither the nitride particles nor the particle matrix interface is a particularly weak site in the material. It also becomes obvious that theC ^* concept can be rather insensitive to precipitation strengthening effects, if these only affect the parameterB in Norton's creep law. M. WELKER, formerly with DECHEMA     

Creep and rupture properties of a squeeze-cast Mg-Al-Ca alloy

A squeeze-cast Mg-Al-Ca alloy (MRI153) was creep tested at 150, 175, and 200 °C under applied stresses in the range of 30 to 120 MPa. The creep curves were characterized by an extended tertiary stage in which the creep rate increases progressively with the creep strain. Microstructural examinations revealed the precipitation and coarsening of new particles during creep. The stress dependence of the minimum creep rate suggests a transition from power-law creep at low stresses to power-law breakdown at high stresses. Creep rupture of this alloy occurred as a result of cavitation damage at dendritic grain boundaries, with the creep rupture time and the minimum creep rate following the empirical Monkman-Grant equation. A comparison is made between the creep and rupture properties of MRI153 and those of a squeeze-cast Mg-Al alloy (AZ91).     

Elevated-temperature fatigue crack growth

The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally occurred normal to the principal stress direction and in a direction along which ΔK _II vanished. Consequently, the effective ΔK was reduced to ΔK_I and the rate of propagation was controlled by ΔK _I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and 982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model.     

Fracture and fatigue behavior of sintered steel at elevated temperatures: Part I. Fracture toughness

This study investigates fracture resistance of a sintered steel in the temperature range from 25 °C to 300 °C. The temperature-dependent fracture resistance is experimentally determined by fracture toughness tests. The fracture toughness, K _IC , decreases from 28.8 at room temperature to 23 MPa√m at 300 °C. The finite element analysis shows an insight of the rationale of using K _IC as the parameter to characterize the fracture resistance of porous sintered steel in which the stress intensity (K) field has been severely distorted at the porous crack tip. The analysis indicates that crack onset of sintered steel is controlled by a critical stress mechanism.     

Tensile behavior of inconel alloy X-750 in air and vacuum at elevated temperatures

Investigations carried out on the hot tensile properties of Inconel alloy X-750 at 700 °C in air and vacuum at different strain rates, in the range of 1 × 10⁻⁷ to 1.2 × 10⁻⁶ s⁻¹, have shown that testing in air had a weakening effect on properties. Creep ductility in vacuum (p ₀₂ = 2.7 × 10⁻⁵ Pa) did not change appreciably with strain rate, but ductility varied markedly when tested in the air. Further, the ductility minimum occurred at 625 °C in air whereas considerable improvements in the creep ductilities were observed at 575 °C and 625 °C in the vacuum. The results indicated that the environmental interaction during testing enhanced the rate of cavitation damage causing premature failure in the material.     

Creep and fatigue crack propagation in a directionally-solidified carbide eutectic alloy

Creep and fatigue crack growth rates and threshold stress intensity amplitudes have been measured for a directionally solidified carbide-eutectic alloy, C73. Fatigue testing temperatures have been ambient, 750 and 950°C for cracking perpendicular and parallel to the solidification direction. In the former cracking direction comparative propagation rates may be understood in terms of the properties of the matrix, which shows a phase transformation from hexagonal to cubic above ~900°C. A situation where crack growth rates decrease with increasing apparent stress intensity amplitudes (ΔK) has been found to exist for propagation parallel to the solidification direction at low ΔK values and high temperatures only. This phenomenon can be related to the occurrence of crack branching and multiple cracking of the carbide fibers. Considerations of plastic zone sizes and critical defect sizes for crack propagation are consistent with the conditions necessary for such crack deceleration to occur. Transformation of the M₇C₃ fibers, present in the as-cast condition, to M₂₃C₆ at cell boundaries of the solidification structure occurs at a temperature of 950°C. Although M₂₃C₆ carbides are easily cracked and therefore probably reduce propagation rates by causing secondary cracking, their presence is known to be detrimental to creep properties. High cycle fatigue threshold stress intensity amplitudes for C73 in either loading direction at room temperature, 750 and 950°C are ~20 pct lower than for the cast nickel-base alloy, EST 738LC,i.e. critical defect sizes are ~10 pct smaller in C73. Despite the known sensitivity of cracking rates and threshold values to factors such as minor fluctuations in loading amplitude it is believed that these differences are significant.     

Effect of creep strain on microstructural stability and creep resistance of a TiAi/Ti3ai lamellar alloy

Creep of a TiAl/Ti₃Al alloy with a lamellar microstructure causes progressive spheroidization of the lamellar microstructure. Microstructural observations reveal that deformation-induced spheroidization (DIS) occurs by deformation and fragmentation of lamellae in localized shear zones at interpacket boundaries and within lamellar packets. Deformation-induced spheroidization substantially increases the interphase interfacial area per unit volume, demonstrating that DIS is not a coarsening process driven by reduction of interfacial energy per unit volume. Creep experiments reveal that DIS increases the minimum creep rate (ε_min) during creep at constant stress and temperature; the activation energy (Q _c ) and stress exponent (n) for creep are both reduced as a result of DIS. Values ofn andQ _c for the lamellar microstructure are typical of a dislocation creep mechanism, while estimated values ofn andQ _c for the completely spheroidized microstructure are characteristic of a diffusional creep mechanism. The increase in (ε_min) associated with DIS is thus attributed primarily to a change of creep mechanism resulting from microstructural refinement.