The effect of orientation and sense of applied uniaxial stress on the morphology of coherent gamma prime precipitates in stress annealed nickel-base superalloy crystals

The coarsening of coherent γ′[Ni₃(Al, Ti)] precipitates in single crystals of a representative nickel-base superalloy, Udimet-700, is shown to be affected by a uniaxial stress applied during annealing. Depending on the sense of the applied stress and its crystallographic orientation, stress annealing results in oriented cuboidal, plate, or parallelepiped shaped γ′ precipitates. A general thermodynamic analysis of the effect of stress annealing on precipitate morphology is presented that takes into account free energy changes due to changes in bulk precipitation strain, effective modulus, coherency strain energy, and the total interphase boundary area. The analysis correctly predicts the observed γ′ precipitate morphologies as a function of stress axis orientation, stress sense, the lattice misfit of the precipitate phase, and the elastic constants of the matrix and precipitate phases. The analysis also shows that stress induced morphological changes can be completely precluded, as may be desired to optimize mechanical behavior, only if the elastic constants of the matrix and precipitate phases are equal. Changes in morphology due to changes in bulk precipitation strain, which in the case of Udimet-700 is shown to be the dominant effect, can be eliminated by alloying for zero lattice misfit or, in single crystals, by stressing parallel to < 111> . Applications to long-term creep behavior and to the fabrication of composite structures are discussed. Formerly with Advanced Materials Research and Development Laboratory, Pratt & Whitney Aircraft     

The influence of orientation on the stress rupture properties of nickel-base superalloy single crystals

The influence of orientation on the stress rapture properties of MAR-M247 single crystals was studied. Stress rupture tests were performed at 724 MPa and 774 °C where the effect of anisotropy is prominent. The mechanical behavior of the single crystals was rationalized on the basis of the Schmid factors for the operative slip systems and the lattice rotations which the crystals underwent during deformation. The stress rupture lives at 774 °C were found to be greatly influenced by the lattice rotations required to produce intersecting slip, because second-stage creep does not begin until after the onset of intersecting slip. Crystals which required large rotations to become oriented for intersecting slip exhibited a large primary creep strain, a large effective stress level at the onset of steady-state creep, and consequently, a short stress rupture life. Those crystals having orientations within about 25° of the [001] exhibited significantly longer lives when their orientations were closer to the [001]-[011] boundary of the stereographic triangle than to the [001]-[1l 1] boundary, because they required smaller rotations to produce intersecting slip and the onset of second-stage creep. Thus, the direction off the [001], as well as the number of degrees off the [001], has a major influence on the stress rapture lives of single crystals in this temperature regime. REBECCA A. MacKAY, formerly Graduate Assistant, Case Western Reserve University, Cleveland, OH RALPH D. MAIER, formerly Assistant Professor, Case Western Reserve University     

The influence of applied stress and stress sense on grain boundary precipitate morphology in a nickel-base superalloy during creep

Creep induced instability of strengthening precipitates at grain boundaries is of general concern in the applications of many high temperature alloys. Having shown that the general validity of the existing mechanism for such an instability in nickel-base superalloys may be considered suspect, this paper reports and discusses the effects of both tensile and compressive creep on γ′ grain boundary precipitate morphology in an alloy consisting of γ′ (Ni₃Al) precipitates in a γ (nickel solid solution) matrix. We find that the uniform distribution of γ′ precipitates is altered by the application of uniaxial creep stress, with the stress-induced precipitate morphology depending strongly on stress sense. Tensile creep results in the dissolution of γ′ precipitates at grain boundaries aligned more or less transverse to the stress axis, with an accompanying increase in volume fraction of γ′ precipitates at grain boundaries oriented parallel to, or almost parallel to the stress axis. In contrast, the reverse change in morphology occurs during compressive creep. The observed morphology changes and their dependence on stress sense are shown to be consistent with the flow of chromium atoms from grain boundaries that are under normal compression towards grain boundaries that are under normal tension. The results conclusively demonstrate that Herring-Nabarro type diffusion in multiphase, polycrystalline alloys can cause chemical changes in grain boundary regions which, in the extreme, result in phase changes at grain boundaries. The results and proposed mechanism are discussed in terms of the findings of other investigations.     

Influence of molybdenum on the creep properties of nickel-base superalloy single crystals

The influence of Mo on the creep properties of single crystals of a model nickel-base superalloy has been investigated. The Mo content was systematically varied from 9.8 to 14.6 wt pet in an alloy series based on Ni-6 wt pet Al-6 wt pet Ta. The optimum initial γ-γ′ microstructure for raft development and creep strength was produced in each alloy prior to testing. The creep lives at 982 °C and 234 MPa exhibited a steep peak as a function of Mo content, with the maximum in life occurring at about 14.0 wt pet Mo. Deviations of less than 1 wt pet Mo from the optimum composition resulted in an order of magnitude drop in properties. As the Mo content was increased from 9.8 to 14.0 wt pct, the magnitude of lattice mismatch significantly increased, which was believed to be beneficial because of stronger γ-γ′ interfaces. As the Mo content was increased further from 14.0 to 14.6 w/o, the mechanical properties degraded because of the precipitation of a deleterious third phase. The results suggest that small variations in refractory metal content and initial gg′ size can have profound effects on mechanical properties. Hence, composition ranges and microstructures for the attainment of optimum mechanical properties may be somewhat limited and require close process control.     

The effect of internal hydrogen on a single-crystal nickel-base superalloy

The effects of a uniform, internal concentration (1.74 at. pct) of hydrogen on the room-temperature tensile behavior of a single-crystal nickel-base superalloy, PWA 1480, have been studied. In particular, the interactive role of porosity and γ/γ′ eutectic on hydrogen embrittlement was examined. It was found that while there was no effect of porosity on tensile properties, hydrogen embrittled the γ/γ′ eutectic, leading to a lower strain to failure in specimens containing this constituent. Significant reduction in ductility also occurred in hydrogen-charged specimens where both porosity and γ/γ′ eutectic were effectively absent. These observations are correlated with the deformation structure and fractography and related to existing hydrogen embrittlement processes. Since hydrogen promoted planar slip and localized deformation, it is suggested that a hydrogen-enhanced localized plasticity mechanism is operating not only in PWA 1480 but likely in other single-crystal and polycrystal nickel-base superalloys. W.S. WALSTON, formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, I.M. BERNSTEIN, formerly Chancellor, Illinois Institute of Technology, Chicago, IL,     

The growth of gamma prime precipitates in aged Ni−Ti alloys

The kinetics of growth of the γ′ precipitate in a Ni-8.74 wt pct Ti alloy were studied by magnetic analysis and transmission electron microscopy. The variation of the titanium content of the nickel-rich matrix as a function of aging time was studied by measuring the ferromagnetic Curie temperature of alloys aged at 692°, 593°, and 525°C. The kinetics of this process accurately obeyed the predictions of the Lifshitz-Wagner theory of diffusion controlled coarsening after relatively short aging times at all aging temperatures. Dark-field transmission electron microscopy was used to measure the particle-size distributions and the average particle sizes of samples aged for various times at 692°C. The kinetics of particle growth also obeyed the time law predicted by the Lifshitz-Wagner theory within the limits of experimental error. Additional analysis of the data provided a value of approximately 21 erg per sq cm for the interfacial free energy of the γ′-matrix interface, and a value for the diffusion coefficient of titanium in nickel which is in very good agreement with an independently determined value. The distribution of γ′ particle sizes was found to be significantly broader than the theoretical distribution of the Lifshitz-Wagner theory. It is suggested that this is due to the relatively large lattice parameter mismatch between γ′ and the Ni−Ti matrix. The results and conclusions of this study are critically compared with those of other investigations. A. J. ARDELL, formerly Assistant Professor, California Institute of Technology     

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

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

The mechanism of creep in gamma prime precipitation-hardened nickel-base alloys at intermediate temperatures

The creep deformation of Mar-M200 single crystals of various orientations has been studied at a temperature of 1400°F. It was found by a combination of transmission microscopy and analysis of lattice rotations that shear of the γ−γ′ structure occurs by the glide of loosely coupled intrinsic/extrinsic fault pairs with a net Burgers vector ofa <112>. The orientation dependencies of both the rate and extent of primary creep are correlated with the Schmid factors and multiplicity of slip for {111}<112> systems. It is also shown that strain hardening due to intersecting slip is necessary to obtain the transition from primary to steady-state creep. In addition, it was found that the deformation mode is a function of strain rate at 1400°F. In contrast to the observed glide mechanism during creep, tensile deformation occurs by the shear of the γ and γ′ phases bya/2 <110> superlattice pairs.     

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.     

The effect of frequency on the elevated temperature fatigue of a nickel-base superalloy

In this investigation, the effect of a frequency variation between 2 cpm and 6 × 10⁴ cpm on the 1400°F fatigue properties of wrought Udimet 700 was determined at a constant stress range of 85 ksi. It was found that a peak existed in the cyclic life vs frequency curve such that a) an increase in frequency from 2 to 600 cpm increased the fatigue life 100 times and b) an increase in frequency from 600 to 6 × 10⁴ cpm reduced the fatigue life sevenfold. The peak in the cyclic life vs frequency curve is the result of two competing processes: 1) there is a reduction in the effects of creep and oxidation with increased frequency that tends to increase the life and 2) there is an increase in the heterogeneity of deformation with increased frequency that tends to reduce the life. At low frequencies, crack initiation occurred at surface-connected grain boundaries. Crack propagation was initially intergranular and then proceeded noncrystallographically normal to the stress axis (Stage II mode). Crack initiation at high frequencies occurred at subsurface brittle phases located at grain boundaries or at the intersection of coherent annealing twin boundaries. Crack propagation was entirely transgranular, proceeding initially along twin boundaries or slip bands (Stage I mode) and then changing to the Stage II mode. The statistical nature of the fracture process, the significance of subsurface crack initiation, and the relation of these results to existing high temperature fatigue models are discussed.     

The effects of orientation and thickness on the notch-tensile creep strength of single crystals of a nickel-base superalloy

The effects of crystallographic orientation and thickness of specimen on the notch-tensile creep strength of single crystals of a nickel-base superalloy UDIMET∗520 has been examined at 700°, 850°, and 900 °C. It was found that the notch-tensile creep strength of thin specimens depended on the crystallographic orientations not only in the tensile direction but also in the normal direction of the specimens, and that the creep strength was superior in the thin specimens with the [011] tensile and the [011] normal orientations or the [001] tensile and the [110] normal orientations. The thick-notched specimens exhibited great creep resistance regardless of the crystallographic orientations. Formerly Graduate Student Formerly Graduate Student, Tokyo Metropolitan University     

The influence of temperature and cyclic frequency on the fatigue fracture of cube oriented nickel-base superalloy single crystals

Carbon-free single crystals of Mar-M200 were tested in pulsating tension, stress-controlled fatigue at temperatures and frequencies ranging from 1033 to 1255°K and 0.033 to 1058 Hz, respectively. The axis of loading was parallel to [001], the natural growth direction for directionally-solidified nickel-base alloys. Except for the lowest frequency at the higher temperatures where creep damage was extensive, crack initiation occurred at subsurface microporosity. Cracks initiated and propagated in the Stage I mode (crystallographic cracking on the {111} slip planes) at the lower temperatures and higher frequencies, whereas Stage (perpendicular to the principal stress axis) crack initiation and propagation was found at the higher temperatures and lower frequencies. Often a transition from Stage II to Stage I crack propagation was observed. It was established that Stage I cracking occurred under conditions of heterogeneous, planar slip and Stage II cracking under conditions of homogeneous, wavy slip. A thermally activated recovery process with an activation energy of 368 KJ/mole (88 Kcal/mole) determined the instantaneous slip character,i.e., wavy or planar, at the crack tip. In addition, it was found that an optimum frequency existed for maximizing fatigue life. At frequencies below the optimum, creep damage was detrimental, while at frequencies greater than the optimum, intense, planar slip was detrimental. The optimum frequency increased with increasing temperature.     

The influence of temperature and cyclic frequency on the fatigue fracture of cube oriented nickel-base superalloy single crystals

Carbon-free single crystals of Mar-M200 were tested in pulsating tension, stress-controlled fatigue at temperatures and frequencies ranging from 1033 to 1255°K and 0.033 to 1058 Hz, respectively. The axis of loading was parallel to [001], the natural growth direction for directionally-solidified nickel-base alloys. Except for the lowest frequency at the higher temperatures where creep damage was extensive, crack initiation occurred at subsurface microporosity. Cracks initiated and propagated in the Stage I mode (crystallographic cracking on the {111} slip planes) at the lower temperatures and higher frequencies, whereas Stage (perpendicular to the principal stress axis) crack initiation and propagation was found at the higher temperatures and lower frequencies. Often a transition from Stage II to Stage I crack propagation was observed. It was established that Stage I cracking occurred under conditions of heterogeneous, planar slip and Stage II cracking under conditions of homogeneous, wavy slip. A thermally activated recovery process with an activation energy of 368 KJ/mole (88 Kcal/mole) determined the instantaneous slip character,i.e., wavy or planar, at the crack tip. In addition, it was found that an optimum frequency existed for maximizing fatigue life. At frequencies below the optimum, creep damage was detrimental, while at frequencies greater than the optimum, intense, planar slip was detrimental. The optimum frequency increased with increasing temperature.     

Extrinsic and intrinsic nodes in the gamma prime phase of a single crystal superalloy

Transmission Electron Microscopy has been used to examine the interaction of superdislocations to form nodes and networks in the γ′ phase of the cold deformed and annealed single crystal superalloy, SRR99. The formation of these “super nodes” is traced through various metastable configurations to form, eventually, extended “super nodes” which contain superlattice stacking faults. Several dislocations interacting are observed to form networks of such nodes having alternately intrinsic and extrinsic character. The extrinsic supernodes are found to be larger than the intrinsic supernodes with the implication that the superlattice extrinsic stacking fault energy is lower than the superlattices intrinsic fault energy.     

Influence of precipitate morphology on intermediate temperature creep properties of a nickel-base superalloy single crystal

The influence of γ’ precipitate morphology on the-creep behavior of the single crystal nickel-base superalloy NASAIR 100 at 760-°C was investigtated. As-heat treated crystals with cuboidal γ’ particles and crystals given an additional pre-rafting treatment to form a continuous lamellar structure were creep tested at stress levels which produced rupture lives ranging from 40 to 2500 hours. At high applied stresses, the crystals with cuboidal γ’ had both lower minimum creep rates and longer rupture lives than the crystals with lamellar γ. At lower stress levels, the initially cubic γ’ material maintained a lower crep rate, but exhibited a similar rupture life compared to the pre-rafted crystals. Examination of the microstructures which developed during creep indicated that dislocations could shear the semi-coherent γ’ rafts relatively easily compared to the coherent cuboidal γ’. In tests at lower applied stresses, slow directional coarsening of the initially cuboidal γ’ resulted in the development of a lamellar structure similar to that in the pre-rafted material, such that the rupture lives of the two materials were similar.     

Fracture of single crystals of the nickel-base superalloy PWA 1480E in hydrogen at 22 °C

The hydrogen-induced fracture behavior of notched single crystals of the PWA 1480E nickel-based superalloy was studied. Notched single crystals with seven different crystal orientations near [100], [110], [111], [013], [112], [123], and [223] were tensile tested at 22 °C in an hydrogen atmosphere at 34 MPa. The notch tensile strength degradation in hydrogen was orientation dependent. The specimen with the [100] orientation had the greatest strength degradation, while the crystal with the [111] orientation had the least. A stereoscopic technique combined with the use of planar γ′ morphologies was applied to identify cleavage plane orientations. All specimens failed predominately by {100}-type cleavage within about 0.5 mm of the notch and {111}-type cleavage toward the center of the crystal. Cleavage on {111}-type planes in the center of the crystals was not related to testing in hydrogen. Microcracking along the {100} γ/γ′ interfaces was observed in the area near the fractured surface. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies indicated that {100}-type cleavage within the notch region controlled the hydrogen-induced cleavage fracture in notched single crystals. Formerly graduate Student, Auburn University     

Fracture of single crystals of the nickel-base superalloy PWA 1480E in helium at 22 °C

The fracture behavior of single crystals of the PWA 1480E nickel-base superalloy was studied using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Notched single crystals with seven different crystal growth orientations near [100], [110], [111], [013], [112], [123], and [223] were tensile tested at 22 °C in a helium atmosphere at 34 MPa. Gamma prime particles were orderly and closely aligned with the cube edges along the [100], [010], and [001] directions of theγ matrix. The cuboid morphology of theγ’ precipitate was not influenced by the crystal growth orientation. The specimen with the [110] orientation was the strongest, while the crystal with the [100] orientation was the weakest. A stereoscopic technique, combined with the use of planary’ morphologies, was applied to identify the cleavage plane orientation. All specimens failed predominately by {lll}-type cleavage which originated from combined slip on various {111} planes. In most cases, deformation was found to occur inhomogeneously in intense slip bands lying on {111} planes and aligned parallel to the different slip directions. Both SEM and TEM studies indicated that {lll}-type slip was the controlling factor during cleavage fracture of single crystals of the PWA 1480E nickel-base superalloy. Formerly Graduate Student, Auburn University     

Influence of secondary precipitates and crystallographic orientation on the strength of single crystals of a Ni-based superalloy

The effect of crystallographic orientation and aging heat treatment at 850 °C on the creep rupture strength of single crystals of a nickel-based superalloy was examined at 700 °C in detail. Initial tensile orientations were selected over a wide range on the standard stereographic triangle. The {111}〈112〉-type slip systems were found to be operative during the creep deformation. The creep behavior was found to be greatly influenced by the additional aging at 850 °C for 20 hours. It was found that the effect of the aging at 850 °C was quite different between orientations favored for the slip system and those favored for the (111) slip system and that the creep deformation mechanisms of these two slip systems were different. In the orientations favored for slip systems, in the single-aged specimens, a small mean surface-to-surface spacing due to hyperfine γ′ precipitates in the matrix channel promoted the slip and the primary creep. As a result of the additional aging at 850 °C, the hyperfine γ′ precipitates were dissolved into the matrix, and the resultant large mean surface-to-surface spacing between the cuboidal precipitates inhibited extensive shearing of the γ-γ′ structure by the slip system. As a result, the creep strengths of these orientations were increased in double-aged specimens; however, the low ductility associated with the difficulty of secondary noncoplanar slip did not enlarge rupture lifetime in the double-aged [001] specimen. In the orientations favored for the (111) slip system, creep deformation occurred by twinning shear through γ and γ′ precipitates, and a distinct effect of the aging at 850 °C was not observed. In the multiple orientation of the {111} -type slip systems, i.e., the and orientations, hyperfine precipitates improved creep strength because they prevented dislocations from gliding in the matrix channel in the single-aged specimens.     

The elastic strain energy of growth ledges on coherent and partially coherent precipitates

The formation rate of growth ledges on a faceted precipitate strongly affects the growth kinetics and the shape of the precipitate. An Eshelby-type model is used to compare the strain energy associated with the nucleation of a ledge on different facet planes of a body-centered cubic (bcc) precipitate in face-centered cubic (fcc) matrix. Ledge nucleation is only likely at facet areas where the interaction energy between the ledge and the precipitate is negative. The strain energy for ledge formation is not symmetric on any of the facet planes, but it is symmetric about the center of the precipitate. For coherent precipitates comparable to those observed in the Ni-Cr system, ledges form with the lowest strain energy on the broad facet of the precipitate implying that precipitate thickening should occur faster than lengthening and widening. A procedure for modifying the Eshelby model is suggested in order to allow strain-energy calculations of partially coherent precipitates. The strain energy for ledge formation on at least one type of partially coherent lath is lowest for a ledge located on the facet perpendicular to the crystallographic invariant line (IL). This situation favors precipitate lengthening in the invariant line direction.     

Environmental effects on the creep behavior of a nickel-base superalloy

Environmental effects on the 760 °C creep behavior of a nickel-base superalloy are isolated by testing at varying stress levels, in laboratory air and vacuum, specimens of different gage diameters and grain sizes, and in a few cases, different grain boundary microstructures. For all specimens receiving a standard heat treatment that results in grain boundaries free of discrete carbides, the minimum creep rate is greater in air than in vacuum for a given specimen and grain size. In contrast, for specimens aged to precipitate carbides along the grain boundaries, the creep rate is lower in air than in vacuum. The minimum creep rate and the sensitivity of this rate to applied stress also are found to be functions of grain size, specimen size, and grain boundary microstructure to extents dependent on the test environments. Rationalizations of these environmental creep behaviors are suggested in terms of the apparently competing phenomena of the relative weakening of the alloy grain boundaries in the air environment, and the alloy creep strengthening in air due to the external surface oxide scale. Formerly Graduate Research Assistant, Henry Krumb School of Mines, Columbia University