Elevated temperature crack growth in nickel-base alloys in carburizing environments

Fatigue crack growth rate measurements were made in various carburizing environments at intermediate temperatures ranging from 650 to 870 °C and frequencies of 0.01 to 1.0 Hz. Carburizing atmospheres increased the crack growth rates. The general effects produced by carburizing environments appeared similar to those commonly observed with oxidizing environments, except that larger concentrations of CH₄ than oxygen in the environment were required to markedly change the properties. No signs of carburization could be detected ahead of the crack tips, and there was no evidence of any significant reaction between the environment and unstressed material. The crack growth rates did not correlate with the estimated carbon activities of the environments. As in other aggressive environments it appears that kinetic rather than thermodynamic factors dominate the crack growth process at intermediate temperatures. formerly with Inco Research & Development Center, Suffern, NY     

Low-cycle fatigue and cyclic stress-strain behavior of incoloy 800

Strain-controlled low-cycle fatigue tests of solution-annealed Incoloy 800 were performed at temperatures of 538°, 649°, 704°, and 760°C using axial strain rates of 4 × 10^-3 and 4 × 10^-4 sec^-1. A few hold-time tests were also performed to indicate a noticeable reduction in fatigue life at hold times of 10 and 60 min. A comparison of these fatigue data with similar results for AISI 304 stainless steel indicates essentially identical behavior. An extensive study is made of the cyclic stress-strain behavior of Incoloy 800 and the relationship between the cyclic strain-hardening exponent and fatigue behavior is confirmed. Exponents onN _f in the elastic and plastic strain range terms of the total strain range equation are identified and compared with those used in the Universal Slopes equation.     

The influence of gaseous environments on fatigue crack growth in a nickel-copper alloy

Cyclic crack propagation rates for a 65 pct nickel-33 pct copper alloy in low pressure, 0.013 MPa (100 torr), environments of hydrogen, oxygen, and nitrogen gas were compared to a reference crack propagation rate in a 1.3μPa vacuum. Crack propagation rates were determined over a range of temperatures for vacuum and hydrogen gas at a constant cyclic stress intensity. Crack propagation in the gaseous environment results in an increased crack propagation rate compared to growth rates in vacuum and a unique fracture mor-phology for each environment. Parallel investigations using transmission electron microscopy showed a unique dislocation structure adjacent to the fracture surface corre-sponding to each fracture morphology and environment. Fracture modes were transgran-ular in vacuum and nitrogen gas, transgranular with crystallographically-oriented features in oxygen gas, and intergranular over a range of temperature in hydrogen. A mechanism is suggested to explain gaseous environmental effects based on dislocation-gas atom inter-action.     

Effects of frequency and temperature on short fatigue crack growth in aqueous environments

The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range (ΔK = 31 MPa-m^1/2) in deaerated deionized water and 0.3 M Na₂SO₄ solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions of bare steel surfaces with the deaerated 0.3 M Na₂SO₄ solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent activation energy of about 40 kJ/M.     

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.     

Near-threshold fatigue crack growth in 8090 Al-Li alloy

Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude (CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs) with different crack closure levels under identical testing parameters, the conventional ΔK _eff (=K _max —K _op) fails to correlate the test data and the modified ΔK eff (=K _max - _χK_op, where χ is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF) in terms of a microstructure-environment synergy. The transition is related to the material’s short-transverse grain size.     

Slow crystallographic fatigue crack growth in a nickel-base alloy

Fatigue crack propagation rates at very low cyclic stress intensity levels (1 to 3 MNm^-372) have been measured in cube-oriented, planar slip nickel-base superalloy monocrystals using a high frequency (20 kHz) resonant fatigue testing technique. It is found that crack propagation is entirely along the crystallographic slip planes and the crack growth rate does not drop off into a threshold behavior but follows a power law with a power law exponent close to 4, which is similar to the functional dependency observed at higher cyclic stress intensity levels in similar superalloys. The observed behaviors are discussed with respect to a new theory on threshold and the effects of strong crystallographic constraints on crack propagation behavior.     

Corrosion fatigue crack growth of titanium alloys in aqueous environments

The rate of fatigue crack propagation for Ti-6Al-6V-2Sn and Ti-6 A1-4V in aqueous environments has been measured as a function of solution chemistry, frequency, and stress wave form. Depending on the specific encironment, three types of fatigue crack growth rate behavior have been observed as a function of frequency. Crack growth rates increase with decreasing frequency in distilled water, while addition of Na₂SO₄ produces frequency-independent behavior. In solutions containing chloride or bromide ions, a reversal in frequency-dependence takes place at ΔK_scc. Below this transition ΔK level, crack growth rates decrease with decreasing frequency due to passive film formation at the crack tip. Above ΔK_scc corrosion fatigue crack growth is due to SCC under cyclic loading. The ΔK transition in fatigue is lower than the static stress corrosion threshold because of repeated rupture of the passive film at the crack tip, approaching K_Isco only for very slow cycling frequencies. This paper is based upon a thesis submitted by D. B. Dawson in partial fulfillment of the requirements of the degree of Doctor of Science at Massachusetts Institute of Technology.     

Micromechanics and fatigue crack growth in an alumina-fiber-reinforced magnesium alloy composite

A study has been made of fatigue crack growth through the magnesium alloy ZE41A and a composite of this alloy reinforced with alumina fibers. Crack growth rates were measured and failure mechanisms characterized for specimens with fibers parallel to the loading axis and for two off-axis orientations. Crack opening displacements and matrix and fiber strains in the vicinity of the crack tip were measured using the stereomaging technique. Crack growth rates through the composite were retarded by the fibers. For the composite with fibers at 22.5 deg to the loading axis, fibers were found to fracture in the composite at the same stress as measured for the fibers alone. Fiber fracture was the dominant growth-controlling mechanism for fibers oriented on and 22.5 deg to the loading axis, and little fiber pullout was observed. However, for crack growth through material with fibers oriented at 45 deg to the loading axis, crack growth was found to exist principally through the interface. Driving forces for cracks in interfaces were determined to be smaller than the applied δK. It was found that approximate fatigue crack growth rates through the composites could be predicted from those through the matrix by adjusting the tensile modulus. The upper and lower bounds of fatigue crack growth rate were also computed for the composite using a micromechanics-based model that incorporated observed failure mechanisms. A. McMINN, formerly with Southwest Research Institute, is with Failure Analysis Associates, Washington, D.C.     

Relationship between magnetic properties,sensitization, and corrosion of incoloy alloy 800 and inconel alloy 600

Sensitization processes in two high Ni-Cr-Fe alloys are studied with the Huey and accelerated Strauss tests, magnetic permeability measurements and scanning electron microscopy. There is good agreement between the corrosion tests for Incoloy* Alloy 800 but not for Inconel* Alloy 600. High Huey corrosion rates are associated with large magnetic permeabilities which result from chromium depletion near the grain boundaries. The relative contribution of the chromium depleted region and of electrochemical effects to the Huey corrosion rate could not be determined because the formation and disappearance of the chromium depleted zone and of the continuous grain boundary carbides occur concurrently.     

Influence of corrosive environments on near-threshold fatigue crack growth in 403 stainless steel

The near-threshold fatigue crack growth behavior of 403 stainless steel has been investigated in low O₂ steam (<1 ppm), high O₂ steam (40 ppm), and boiling water with various concentrations of NaCl and Na₂SO₄ at a test frequency of 160 Hz. High O₂ steam tends to increase the crack propagation rates in the threshold region, relative to low O₂ steam. Values of threshold stress intensity range, ΔK_th, slightly increase with an increase in the concentration of NaCl in the solution. During threshold crack growth, the percentage of intergranularity decreases with a decrease inAK. Varying pH from 5.0 to 10.0 in a 0.1 gm NaCl plus 1.0 gm Na₂SO₄ per 100 ml H₂O solution does not affect the rates of near-threshold crack propagation. However, increasing the hydrazine level from 30 to 10⁷ ppb in the same salt solution enhances the resistance to crack growth while reducing the percentage of intergranular fracture to nearly zero.     

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.     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

High cycle fatigue and fatigue crack growth of the oxide dispersion strengthened alloy MA 754

The high cycle fatigue (HCF) behavior of the oxide dispersion strengthened (ODS) MA 754 alloy has been determined as a function of specimen orientation. The fatigue life showed anisotropic behavior with the longest and shortest lives in the longitudinal and short transverse directions, respectively. Surface porosity, due to oxidation, was found to affect fatigue life in the long transverse orientation more than in the longitudinal orientation. The fatigue crack growth behavior in MA 754 exhibited a directional dependence. In general, the crack growth rates in the longitudinal direction were lower than those in the long transverse direction. The ΔK _th was ∼11 MN ·^-3/2 and 9 MN · m^-3/2 for the longitudinal and the long transverse orientation, respectively. This behavior was explained on the basis of the unusual grain structure and the texture exhibited by this alloy as well as different crack closure effects. It was found that a consideration based on the crack growth rates results, obtained from fracture mechanics specimens, could not explain the anisotropic behavior of the HCF properties of MA 754. However, the anisotropic HCF properties could be rationalized on the basis of the differences in the modes of crack initiation.     

Mechanisms of high-temperature fatigue failure in alloy 800H

The damage mechanisms influencing the axial strain-controlled low-cycle fatigue (LCF) behavior of alloy 800H at 850 °C have been evaluated under conditions of equal tension/compression ramp rates (fast-fast (F-F): 4 × 10⁻³ s⁻¹ and slow-slow (S-S): 4 × 10⁻⁵ s⁻¹) and asymmetrical ramp rates (fast-slow (F-S): 4 × 10⁻³ s⁻¹ / 4 × 10⁻⁵ s⁻¹ and slow-fast (S-F): 4 × 10⁻⁵ / 4 × 10⁻³ s⁻¹) in tension and compression. The fatigue life, cyclic stress response, and fracture modes were significantly influenced by the waveform shape. The fatigue lives displayed by different loading conditions were in the following order: F-F > S-S > F-S > S-F. The fracture mode was dictated by the ramp rate adopted in the tensile direction. The fast ramp rate in the tensile direction led to the occurrence of transgranular crack initiation and propagation, whereas the slow ramp rate caused intergranular initiation and propagation. The time-dependent processes and their synergistic interactions, which were at the basis of observed changes in cyclic stress response and fatigue life, were identified. Oxidation, creep damage, dynamic strain aging, massive carbide precipitation, time-dependent creep deformation, and deformation ratcheting were among the several factors influencing cyclic life. Irrespective of the loading condition, the largest effect on life was exerted by oxidation processes. Deformation ratcheting had its greatest influence on life under asymmetrical loading conditions. Creep damage accumulated the greatest amount during the slow tensile ramp under S-F conditions. Formerly USA National Research Council Associate, NASA-Lewis Research Center This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as a part of the 1994 Fall meeting of T.S., October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

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     

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.