On the oxidation of the third-generation single-crystal superalloy CMSX-10

The oxidation behavior of the third-generation nickel-base single-crystal superalloy CMSX-10 is examined. Since the in-service performance of the alloy is of the greatest practical significance, a detailed study is made of the microstructural degradation of a turbine blade that had been removed prematurely from a commercial gas turbine engine. The results are augmented with isothermal oxidation tests conducted in the laboratory for 100 hours, at temperatures of 800 °C, 900 °C, and 1000 °C. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, microhardness testing, and X-ray diffraction (XRD) were employed. It was found that the oxidation of CMSX-10 at temperatures below 1000 °C does not produce either Al₂O₃ or the spinel Ni(Cr,Al)₂O₄, both of which are found in the internal oxidation zone of the earlier generations of superalloys. Surprisingly, it is demonstrated conclusively that the oxidation of CMSX-10 generates the β phase (NiAl). This reaction, which to the authors’ knowledge has not yet been reported, is termed self-aluminization. The XRD studies demonstrate that the internal oxidation of CMSX-10 produces (Ni,Co)Ta₂O₆, (Ni,Co)WO₄, CrTaO₄, and Cr(W,Mo)O₄. There is indication that the formation of the δ phase (Ni₂Al₃) slows the oxidation rate at 1000 °C.     

Dislocation structure in a single-crystal nickel-base superalloy during low cycle fatigue

An investigation of dislocation structure in a single crystal nickel-base superalloy during low cycle fatigue (LCF) at 760 °C has been conducted. Dislocation bands are found to be produced first in the matrix in some defined directions. With an increase in cycle numbers, there is an increase in dislocation density in the bands and a decrease in the spacing between the bands, leading to the formation of the dislocation walls or cells. Sometimes, three-dimensional (3-D) networks are formed also by the interaction between two sets of parallel dislocations. The Burgers vectors of the dislocations in the network are 1/2 〈110〉. Clustering of dislocations eventually occurs at γ′/γ interfaces because of the obstruction of the γ′-particles to moving dislocations. Most of the dislocations observed in the γ′-phases are in the form of superdislocations. Dislocation shearing through theγ′-phase was found occasionally. Reprecipitation of γ′-phase induced by strain was also observed in the present study.     

Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part I

The isothermal fatigue behavior of a high-activity aluminide-coated single-crystal superalloy was studied in air at test temperatures of 600 °, 800 °, and 1000 °. Tests were performed using cylindrical specimens under strain control at ≈0.25 Hz; total strain ranges from 0.5 to 1.6 pct were investigated. At 600 °, crack initiation occurred at brittle coating cracks, which led to a significant reduction in fatigue life compared to the uncoated alloy. Fatigue cracks grew from the brittle coating cracks initially in a stage II manner with a subsequent transition to crystallographic stage I fatigue. At 800 ° and 1000 °, the coating failed quickly by a fatigue process due to the drastic reduction in strength above 750 °, the ductile-brittle transition temperature. These cracks were arrested or slowed by oxidation at the coating-substrate interface and only led to a detriment in life relative to the uncoated material for total strain ranges of 1.2 pct and above 800 °. The presence of the coating was beneficial at 800 ° for total strain ranges less than 1.2 pct. No effect of the coating was observed at 1000 °. Crack growth in the substrate at 800 ° was similar to 600 °; at 1000 °, greater plasticity and oxidation were observed and cracks grew exclusively in a stage II manner. Formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge. Formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge CB2 3QZ, United Kingdom.     

Hydrogen effects on low-cycle fatigue of the single-crystal nickel-base superalloy CMSX-2

The effects of hydrogen on the low-cycle fatigue behavior of CMSX-2 [001]-oriented single crystals were examined. Fatigue tests were conducted under constant plastic strain amplitude control. Cyclic stress-strain curves and fatigue life data at different plastic strain amplitudes were determined for hydrogen-free and hydrogen-charged specimens. Two charging procedures, leading to different hydrogen concentrations, were applied. Hydrogen was found to decrease significantly the number of cycles to failure under the various experimental conditions. The increasing hydrogen concentration and ratio of the hydrogen to nonhydrogen-containing volume were found to shorten fatigue life in hydrogen-charged specimens. Based on the analysis of cyclic stress-strain curves and optical and transmission electron microscopy (TEM), it was established that hydrogen enhanced strain localization and promoted crystallographic, stage I cracking, leading to embrittlement. The overall fracture mechanism is discussed in conjunction with Duquette and Gell’s stage I fracture model.[16]     

Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part II. effects of brittle precracking

The effect of brittle coating precracking on the fatigue behavior of a high-activity aluminide-coated single-crystal nickel-base superalloy has been studied using hollow cylindrical specimens at test temperatures of 600 °, 800 °, and 1000 °. Three types of precrack were studied: narrow precracks formed at room temperature, wide precracks formed at room temperature, and narrow precracks formed at elevated temperature. The effect of precracking on fatigue life at 600 ° was found to depend strongly on the type of precrack. No failure was observed for specimens with narrow room-temperature precracks because of crack arrestvia an oxidation-induced crack closure mechanism, while the behavior of wide precracks and precracks formed at elevated temperature mirrored the non-precracked behavior. Crack retardation also occurred for narrow room-temperature precracks tested at 800 °—in this case, fatigue cracks leading to failure initiated in a layer of recrystallized grains on the inside surface of the specimen. A significant reduction in fatigue life at 800 ° relative to non-precracked specimens was observed for wide precracks and elevated temperature precracks. The presence of precracks bypassed the initiation and growth of coating fatigue cracks necessary for failure in non-precracked material. No effect of precracking was observed at 1000 °. Formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge. Formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge CB2 3QZ, United Kingdom.     

Thermomechanical fatigue behavior of the high-temperature titanium alloy IMI 834

The isothermal and thermomechanical fatigue (TMF) behavior of the titanium alloy IMI 834 was studied between 350 °C and 650 °C in air and vacuum, respectively. Transmission electron microscopy (TEM) observations revealed that the microstructure established in the TMF tests was governed by the maximum temperature within the cycle. However, if the maximum temperature does not exceed 600 °C, planar dislocation slip prevails and similar microstructures are formed regardless of the test temperature and the testing mode (TMF and isothermal, respectively). As a result, the stress-strain response in TMF tests can be assessed from the corresponding isothermal data. Wavy dislocation slip was found to determine the stress-strain behavior if the maximum test temperature exceeded 600 °C. Moreover, in TMF tests with a maximum test temperature of 650 °C, the dislocation arrangement formed in the high-temperature part of the hysteresis loop was found to be stable throughout the cycle and to affect significantly the stress-strain response at the low temperatures. Although in-phase (IP) and out-of-phase (OP) TMF tests led to an almost identical microstructure, OP loading was always found to be most detrimental. The interaction between the embrittled subsurface layer, caused by oxygen uptake, and the high tensile stresses developing in the low-temperature part of the hysteresis loop in OP tests eases crack initiation and initial crack propagation and results in reduced fatigue life.     

Creep deformation and crack growth behavior of a single-crystal nickel-base superalloy

Uniaxial creep deformation and crack growth data are presented on the single-crystal nickel-base superalloy SC16, which is a candidate material for industrial gas turbine applications. All testing was performed at 900 °C. The uniaxial experiments were conducted with the loading direction aligned approximately along the [001] crystallographic axis of the material. Under these conditions, a small primary region followed by mainly tertiary creep was obtained, and failure initiated from cracks at interdendritic pores. The crack growth experiments were performed on single-edge notch tension specimens and compact tension test pieces containing deep side grooves to examine state-of-stress effects. A selection of crystallographic orientations was also examined. Little effect of stress state and orientation was obtained. It has been found that the creep crack growth characteristics of the alloy can be predicted satisfactorily from a model of the accumulation of damage at a crack tip using the creep fracture mechanics parameter C* and assuming plane stress conditions. 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 TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Orientation and temperature dependence of some mechanical properties of the single-crystal nickel-base superalloy René N4: Part II. Low cycle fatigue behavior

The low cycle fatigue (LCF) properties of a single-crystal nickel-base superalloy, René* N4, have been examined at 760 and 980 °C in air. Specimens having crystallographic orientations near [001], [01l], [111], [023], [236], and [145] were tested in fully reversed, total-strain-controlled LCF tests at a frequency of 0.1 Hz. At 760 °C, this alloy exhibited orientation dependent tension-compression anisotropies of yielding which continued to failure. Also at 760 °C, orientations exhibiting predominately single slip exhibited serrated yielding for many cycles. At 980 °C, orientation dependencies of yielding behavior were smaller. In spite of the tension-compression anisotropies, cyclic stress range-strain range behavior was not strongly orientation dependent for either test temperature. Fatigue life on a total strain range basis was highly orientation dependent at both 760 and 980 °C and was related chiefly to elastic modulus, low modulus orientations having longer lives. Stage I crack growth on (111) planes was dominant at 760 °C, while Stage II crack growth occurred at 980 °C. Crack initiation generally occurred at near-surface micropores, but occasionally at oxidation spikes in the 980 °C tests.     

High-temperature low-cycle fatigue behavior of K40S cobalt-base superalloy

An investigation was made on the strain-controlled low-cycle fatigue (LCF) of K40S cobalt-base superalloy at 900 °C in ambient atmosphere. The results show that K40S alloy possesses high LCF resistance in comparison with X-40 alloy. Under the testing conditions in this study, K40S alloy exhibits a cyclic stress response of initial hardening followed by softening. The cyclic stress response behavior has been attributed to dislocation-dislocation interactions and dislocation-precipitate interactions. The high response stress can lead to a large stress concentration at locations where inelastic strains of high amplitude accumulate, which account for the decreasing fatigue life with increasing strain rate. The well-distributed carbide particles are the “secondary” crack initiation sites. The secondary crack initiation relaxes the stress concentration at the crack tip, reducing the driving force of crack propagation. High-temperature LCF failure of K40S alloy results from the interaction of the mechanical fatigue and environmental oxidation.     

Creep-behavior modeling of the single-crystal superalloy CMSX-4

An investigation has been undertaken into the creep behavior of the single-crystal superalloy CMSX-4. Creep deformation in the alloy occurs largely through dislocation activity in the γ channels. Shearing of the γ′ dislocations is observed, but, at higher temperatures, this does not occur until late in life via the passage of superpartial dislocation pairs. At lower temperatures (1023 K) and high stress levels, shearing of the γ′ precipitates is observed relatively early in the creep curve through the passage of {111}〈112〉 dislocations, which leave superlattice stacking faults (SSFs) in the precipitates. The stress-rupture behavior of CMSX-4 has been modeled using a damage-mechanics technique, where the level of damage required to cause failure is defined by the effective stress reaching the material’s ultimate tensile strength (UTS). This technique ensures that short-term rupture data extrapolate back to the UTS. High-temperature steady-state and tertiary creep are modeled using modified damage-mechanics equations, where the strain and damage rates are similar functions of stress. At intermediate operating temperatures of 1023 to 1123 K, the material exhibits pronounced sigmoidal primary creep of up to 4 pct strain, which cannot be modeled using a conventional approach. This transient behavior has been explained by the effect of internal stresses acting on dislocations in the gamma matrix; such an internal stress has been included in the creep law and evolves as a function of the damage-state variable.     

Influence of test parameters on the thermal-mechanical fatigue behavior of a superalloy

The thermal-mechanical fatigue (TMF) behavior of IN-100, a cast nickel-base superalloy, was investigated with a basic mechanical strain-temperature loop applied in a temperature range from 600 °C to 1050 °C (873 to 1323 K). Peak strains were applied at intermediate temperatures, giving a faithful simulation of real component parts. Tests with or without a mean strain were used; other tests involved a longer period or a tensile hold time, and they were compared with conventional “in-phase” TMF cycles. An interrupted test procedure was used with a plastic replication technique to define a conventional TMF life to 0.3-mm crack depth, as well as a life to 50-μm, crack depth, to characterize the crack initiation period. Some stress-strain hysteresis loops were reported. Thermal-mechanical fatigue life was found to be dependent upon test parameters, while the life to crack initiation was not. Oxidation of specimens and micro-cracks was found to be important in all the tests. These results were then discussed and compared with those under low cycle fatigue at high temperature. Formerly with the Centre des Matériaux     

The fatigue crack growth behavior of electron-beam welded a286 superalloy

Fatigue crack growth curves(Δa/ΔN =f(K _max )) were measured with 2.5 mm thick sheets of electron beam welded iron base superalloy A286. Fatigue testing frequency was 21 kHz,R = −1 (mean stress zero) and the environment was noncorrosive silicone oil at 20 °C. Two series of samples with different welding conditions were tested. One series was welded perfectly, whereas the second contained microcracks within the weld and the heat affected zone. It was shown that the crack growth rate in the base metal is slower than in the weld. The threshold stress intensity factorK _th of the base metal is 14 MNm^-3/2 and that of the weld, 10 MNm ^-3/2 . However, at higherK _max values, the crack grows more rapidly in the weld than in the base metal; for example, the crack growth rate is 16 times higher at K_max = 20 MNm ^-3/2 . Microcracks introduced by an imperfect welding process do not influence the fatigue cracking behavior in the threshold regime; atK ^max = 15 MNm^-3/2, however, the crack growth rates differ by an order of magnitude. Fractographic examination shows considerable differences in the fracture appearance of weld, heat affected zone, and base material. Weld and base metal display ductile fracture surfaces and the heat affected zone is characterized by crystallographic fracture facets.     

Effects of fine porosity on the fatigue behavior of a powder metallurgy superalloy

Hot-isostatically-pressed powder-metallurgy Astroloy was obtained which contained 1.4 pct, fine porosity at the grain boundaries produced by argon entering the powder container during pressing. The pores averaged about 2μ,m diam and 20 μ m spacing. This material was tested at 650 °C in fatigue, creep-fatigue, tension, and stress-rupture and the results compared with previous data on sound Astroloy. The pores influenced fatigue crack initiation and produced a more intergranular mode of propagation. However, fatigue life was not drastically reduced. A large 25 μm pore in one specimen resulting from a hollow particle did reduce life by 60 pct, however. Fatigue behavior of the porous material showed typical correlation with tensile behavior. The plastic strain range-life relation was reduced proportionately with the reduction in tensile ductility, but the elastic strain range-life relation was little changed reflecting the small reduction in strength divided by modulus for the porous material.     

Influence of microstructure on the thermal fatigue behavior of a cast cobalt-base superalloy

The influence of microstructure on the thermal fatigue (TF) behavior of MAR-M 509, a cast cobalt-base superalloy, was investigated using a burner rig and wedge-type specimens which were submitted to thermal shock from 200 °C to 1100 °C. Two microstructures were studied: a coarse microstructure using specimens machined from bulk castings and a fine microstructure using cast-to-size specimens. Metallographic observations showed that cast-to-size specimens display a gradient in microstructure since the size of secondary dendrites increases with the distance to the thin edge. As high-temperature fatigue in this superalloy is controlled by oxidation-fatigue interactions, the kinetics of interdendritic oxidation was studied at 900 °C. Interdendritic oxidation was found to be inhibited by a refinement of dendritic microstructure. A fine microstructure was shown to give a much longer TF life-to-crack initiation and a much lower crack growth rate. This behavior was mainly related to differences in interdendritic oxidation since interdendritic areas act as crack initiation sites as well as easy crack propagation paths. The influence of microstructure on crack growth rates was accounted for by using a previously proposed oxidation-fatigue crack growth model and interdendritic oxidation kinetic data. Formerly with the Centre des Matériaux, Ecole des Mines de Paris     

The influence of crystallographic orientation and strain rate on the high-temperature low-cyclic fatigue property of a nickel-base single-crystal superalloy

Fully reversed low-cyclic fatigue (LCF) tests were conducted on [001], [012], [-112], [011], and [-114] oriented single crystals of nickel-based superalloy DD3 with different cyclic strain rates at 950°C. The cyclic strain rates were chosen as 1.0×10⁻², 1.33×10⁻³ and 0.33×10⁻³ s⁻¹. The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends on the crystallographic orientation and cyclic strain rate. Except [001] orientation specimens, it is found from the scanning electron microscopy (SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flow stress and plastic behavior existed at 950°C, and an orientation and strain rate modified Lall-Chin-Pope (LCP) model was derived for the nonconformity. The influence of crystallographic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied in this article.     

The influence of crystallographic orientation and strain rate on the high-temperature low-cyclic fatigue property of a nickel-base single-crystal superalloy

Fully reversed low-cyclic fatigue (LCF) tests were conducted on [001], [012], , [011], and oriented single crystals of nickel-based superalloy DD3 with different cyclic strain rates at 950 °C. The cyclic strain rates were chosen as 1.0 × 10⁻², 1.33 × 10⁻³, and 0.33 × 10⁻³ s⁻¹. The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends on the crystallographic orientation and cyclic strain rate. Except [001] orientation specimens, it is found from the scanning electron microscopy (SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flow stress and plastic behavior existed at 950 °C, and an orientation and strain rate modified Lall-Chin-Pope (LCP) model was derived for the nonconformity. The influence of crystallographic orientation and cyclic strain rate on the LCF behavior can be predicted statisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied in this article.     

The room temperature fatigue behavior of nickel-base superalloy crystals at ultrasonic frequency

Previous investigations have invariably observed strain rate related deformation effects as the fatigue frequency is raised to the ultrasonic range. Through room temperature tests on strain rate insensitive nickel-base superalloy single crystals of Mar-M200, we have shown that another effect of increasing the fatigue frequency to the ultrasonic range is in the suppression of the deleterious influence of environment. It was found that above a stress amplitude of 30,400 psi the fatigue lives of crystals ultrasonically fatiguedin air increase with decreasing stress in a manner which is functionally similar to, that of crystals conventionally fatiguedin vacuum. Similarly, the fracture surfaces of ultrasonically fatigued crystals have a dimpled appearance over most of their areas which is characteristic of locally ductile failure and identical to, the appearance of crystals failed at conventionally frequency in vacuum. These results, along with a kinetic analysis of gaseous adsorption, indicate that the major effect of increasing the fatigue frequency to the ultrasonic, range is in the suppression of the influence of oxygen in enhancing the rate of crack propagation. In addition, the short test times involved in running large numbers of cycles have allowed for the determination of the fatigue limit in a nickel-base superalloy. This is the first indication of no-fail behavior in this type of alloy.     

Deformation and recrystallization behavior during hot working of a coarse-grain,nickel-base superalloy ingot material

The deformation and dynamic recrystallization behavior of Waspaloy-ingot material with coarse, columnar grains was established using isothermal uniaxial and double-cone compression tests. Testing was conducted along different test directions relative to the columnar-grain microstructure at supersolvus temperatures (1066 °C and 1177 °C) and strain rates (0.005 and 0.1 s⁻¹), which bracket typical ingot-breakdown conditions for the material. The flow behavior of axial samples (i.e., those compressed parallel to the columnar-grain direction) showed an initial strain-hardening transient followed by steady-state flow. In contrast, the stress-strain curves of samples upset transverse to the columnar grains exhibited a peak stress at low strains, whose magnitude was greater than the steady-state flow stress of the axial samples, followed by flow softening. The two distinct flow behaviors were explained on the basis of the solidification texture associated with the starting ingot structure, differences in the kinetics of dynamic recrystallization revealed in the double-cone tests, and the evolution of deformation and recrystallization textures during hot working. Dynamic recrystallization kinetics were measurably faster for the transverse samples as well as specimens oriented at ∼45 deg to the forging direction, an effect partially rationalized based on the initial texture and its effect on the input rate of deformation work driving recrystallization. Despite these differences, the overall strains required for dynamic recrystallization were comparable to those measured previously for fine-grain (wrought) Waspaloy. However, the Avrami exponents (∼2 to 3) were somewhat higher than those for wrought material (∼1 to 2), an effect attributable to the particle-stimulated nucleation in the ingot material.     

On the microstructural instability of an experimental nickel-based single-crystal superalloy

The susceptibility of an experimental nickel-based single-crystal superalloy to the precipitation of topologically close-packed phases (TCPs) is considered. Its composition has been chosen to be enriched with regard to molybdenum with no tungsten being present, in order to promote microstructural instability and to allow this to be studied. Two conditions are examined: (1) as-cast and (2) as-cast with a solutioning and aging treatment. In the as-cast condition, it is shown that the interdendritic region is already prone to TCP formation, and that further heat treatment in the vicinity of 1000 °C increases the extent of this severely. This is attributed to the partitioning of Ta, which causes an increase in the γ′ volume fraction in the interdendritic regions and a concomitant enrichment of the γ matrix with respect to Mo and particularly Cr. In the solutioned and aged state, TCPs form after heat treatment in the range from 800 °C to 1100 °C and form preferentially at the dendrite cores; this is due to the presence of residual Re, which does not diffuse as quickly as Ta in the opposite direction. The different TCP particles exhibit very different morphologies. At 1000 °C, the P phase is prevalent; around 850 °C, the P phase is still found, but μ is predominant and is found in association with σ. The experimental data are compared with the predictions of thermodynamic software and a database of thermodynamic parameters; the predictions are reasonable, although some discrepancies are noted.     

Thermomechanical fatigue,oxidation, and creep: Part i. Damage mechanisms

Isothermal fatigue tests and both out-of-phase and in-phase thermomechanical fatigue tests were performed in air and in helium atmospheres. A wide range of temperatures from 20 ‡C to 700 ‡C was considered in these tests on 1070 steel specimens. A procedure for inert atmosphere testing using encapsulated specimens is described. Results indicate that the fatigue lives are 2 to 12 times greater in helium than in air. Interrupted tests were performed to characterize the pro-gression of damage in the material. Results indicate that oxidation-induced crack nucleation and crack growth are detrimental at high temperatures for isothermal and out-of-phase thermome-chanical fatigue tests. In these tests, transgranular cracking is observed. However, creep-induced intergranular cracking is the dominant damage mechanism during in-phase thermomechanical fatigue tests.