High Stress Regime Research Articles

Creation and annihilation of free volume during homogeneous flow of a metallic glass

Bulk samples of Pd41Ni10Cu29P20 glass were tested at constant true stress (20–636MPa) in compression at 550, 555, and 565K to study the transition from steady-state creep at low stress to deformation-induced softening at high stress. In the high-stress regime strongly accelerating creep was observed. All deformation was homogeneous. The activation volume was 106Å3. The isoconfigurational activation energy was 9.03±0.64eV. The disordering rate is proportional to the applied power density (stress×strain rate). The disordering process is inefficient in energy and in the production of free volume.

Analysis of the creep response of an Al–17Si–4Cu–0.55Mg alloy

The creep response of a hypereutectic Al–Si alloy containing 4% Cu and 0.55% Mg was investigated between 553 and 653 K. Minimum creep rate as a function of applied stress by using the power-law equation suggested the existence of two different regimes: a low-stress regime characterised by a stress exponent close to 4–5, and a high-stress regime with a higher stress exponent. Although the magnitude of the stress exponent in the low-stress regime was equivalent to that observed in pure Al, the apparent activation energy for creep was higher ( Q = 210 kJ/mol) than the activation energy for self-diffusion in Al ( Q d = 143 kJ/mol). The microstructural analysis suggested a similarity between this alloy and Al-matrix discontinuously reinforced composites. This approach, based on threshold-stress concept, permitted to rationalise both the magnitude of the stress exponent and the apparent activation energy for creep. In addition, the substantial similarity between hypereutectic Al–Si alloys and Al-based composites with similar composition was confirmed.

Hydraulic Fracture Design Optimization for Deep Foothills Gas Wells

Abstract The Deanne Glauconitic A pool is located approximately 80 km northwest of Rocky Mountain House, in the Alberta foothills. The reservoir is a thin, tight sandstone, located at a depth of approximately 3,500 m. In situ stress conditions in this area have been strongly influenced by the orogeny of the Rocky Mountains, with significant tectonic stresses imposed on the rock matrix. This high stress regime poses a major challenge to the very costly hydraulic fracturing treatments. A review of off-setting fracture treatments in the area showed that approximately 50% of previous treatments suffered from premature screenout, with poor post-stimulation productivity. A comprehensive study was conducted to understand the cause(s) of previous failures, and to make recommendations for an optimal fracture design. Once the key fracturing design parameters were identified, a recompletion design was carried out, including a refracture treatment, which successfully placed all of the fracture sand. This paper describes recommended procedures to optimize hydraulic fracture design for a deep gas reservoir in the foothills. These procedures include:Review off-setting fracture treatments;Identify the key drivers for successful sand placement;Examine core samples and design the fracture fluid, with emphasis on minimizing rock-fluid interactions;Model the hydraulic fracture using a 3D simulator;Size the fracture to optimize economic returns;Design the recompletion program and estimate costs;Implement the fracture treatment with stringent QA/QC processes; and,Post-fracture analysis by means of net pressure and radioactive logs to validate the fracture design. Introduction Field History The Deanne Glauconitic A pool was discovered with the completion of the well NUL el al Deanne 11-03-39-11 W5M in April 1983. The reported initial pool pressure was 55,360 kPaa at an MPP depth of 3,446.5 mKB giving a reservoir pressure gradient of 16.1 kPa/m. The well was tied in and placed on production in October 1987 at an initial rate of 100 103m3/d. Subsequently, a total of 10 wells have been completed in the Glauconitic zone in the immediate area, with a second pool (Glauconitic B) identified to the west of the original pool. The cumulative production from the Glauconitic A pool to February 2002 was 713.9 106m3 from a total of five producing wells. The reservoir has been produced by depletion drive, and the most recent available static reservoir pressure measurement was 28,160 kPaa as of June 2001. The reservoir natural gas is sweet, condensate rich with high heating value. The gas specific gravity is approximately 0.68, consisting of 83% methane, 11% ethane, and 3% propane. The average condensate gas ratio is approximately 0.14 m3/103m3 (25 Bbl/MMscf). Rock Properties and Reservoir Quality The Glauconitic formation in the Deanne area has variable gross pay thickness between 4 and 8 m. The open hole log derived porosity in the pay section varies between 9 and 12% with some streaks up to 15%. The reservoir is under-saturated with average water saturation values in the 15 to 25% range. The reservoir permeability, derived by in situ methods, was in the range of 0.1 to 1 mD.

High-Reliability Topological Architectures for Networks Under Stress

In this paper, we consider the design of a physical network topology that meets a high level of reliability using unreliable network elements. We are motivated by the use of networks and, in particular, all-optical networks, for high-reliability applications which involve unusual and catastrophic stresses. Our network model is one in which nodes are invulnerable and links are subject to failure - a good approximation for optical networks with passive nodes and vulnerable fiber under stress of disconnection - and we focus on statistically independent link failures with initial steps taken toward generalization to dependent link failures. Our reliability metrics are the all-terminal connectedness measure and the less commonly considered two-terminal connectedness measure. We compare in the low and high stress regimes, via analytical approximations and simulations, common commercial architectures designed for all-terminal reliability when links are very reliable with alternative architectures which are mindful of both of our reliability metrics and regimes of stress. We derive new results especially for one of these alternative architectures, Harary graphs, which have been shown to possess attractive reliability properties. Furthermore, we show that for independent link failures network design should be optimized with respect to reliability under high stress, as reliability under low stress is less sensitive to graph structure; and that under high stress, very high node degrees and small network diameters are required to achieve moderate reliability performance. Finally, in our discussion of correlated failure models, we show the danger in relying on an independent failure model and the need for the network architect to minimize component failure dependencies.

High-temperature creep of synthetic calcite aggregates: influence of Mn-content

To investigate the role of impurities on the deformation of calcite rocks, we performed creep experiments on fine-grained synthetic calcite aggregates with varying Mn-content. The tests were performed in a uniaxial creep apparatus at temperatures of 700–830 °C and stresses between 15 and 65 MPa, resulting in strain rates of 10 −7 to 10 −4 s −1. Varying the oxygen fugacity, f O 2 , between 10 −18 and 10 −4 MPa did not affect the creep strength. Applying a power law constitutive relationship on the creep data, two distinct creep regimes were observed, a low stress regime (<15 MPa) and a high stress regime (>20–25 MPa). In the low stress regime, a stress exponent n of 1.7±0.3 and an activation energy of Q=300±45 kJ/mol were found. The mechanical data and microstructural observations suggest that grain-boundary sliding accommodated by grain-boundary diffusion is the dominant creep mechanism. At high stresses, we obtained a stress exponent of n=3.1±0.6 and an activation energy of Q=490±50 kJ/mol, indicating dislocation creep. Transmission-electron microscopy (TEM) observations show a dominance of recovery-related dislocation structures suggesting that dislocation climb is important. Increasing Mn-concentration from 10 to 670 ppm leads to an enhancement in creep rate in both creep regimes, but the effect is more substantial in the diffusion creep regime (one order of magnitude). In the diffusion creep regime, the enhancement of creep rate by Mn is attributed to the substitution of Ca by Mn, which may increase grain-boundary diffusivity of calcite. In the dislocation regime, it is interpreted as a combination of dislocation climb and glide with the exchange of Ca by aliovalent Mn, which may affect the point defect concentration.

Analysis of the creep response of an Al–17Si–4Cu–0.55Mg alloy

Abstract The creep response of a hypereutectic Al–Si alloy containing 4% Cu and 0.55% Mg was investigated between 553 and 653 K. Minimum creep rate as a function of applied stress by using the power-law equation suggested the existence of two different regimes: a low-stress regime characterised by a stress exponent close to 4–5, and a high-stress regime with a higher stress exponent. Although the magnitude of the stress exponent in the low-stress regime was equivalent to that observed in pure Al, the apparent activation energy for creep was higher (Q = 210 kJ/mol) than the activation energy for self-diffusion in Al (Qd = 143 kJ/mol). The microstructural analysis suggested a similarity between this alloy and Al-matrix discontinuously reinforced composites. This approach, based on threshold-stress concept, permitted to rationalise both the magnitude of the stress exponent and the apparent activation energy for creep. In addition, the substantial similarity between hypereutectic Al–Si alloys and Al-based composites with similar composition was confirmed.

In-Situ Diffusion Experiment In Sparsely Fractured Granite

AbstractThe in-situ diffusion experiment was conducted at AECL's Underground Research Laboratory (URL) to improve the understanding of diffusive solute transport in sparsely fractured or intact granitic rock (SFR). The experimental program used a comparative series of laboratory and in-situ field experiments to evaluate the ability of laboratory measurements to estimate in-situ rock properties and to explore issues surrounding the influence of stress relaxation, rock texture, porosity, pore geometry, and anisotropy on derived effective diffusion coefficients (De). In-situ experiments yielded iodide Debetween 1.4 × 10−13 and 1.1 × 10−12 m2/s. Unlike laboratory results, the in-situ De estimates did not exhibit correlation with sample depth or varied stress regime. Laboratory-derived measurements of De, porosity and permeability were found to systematically increase for samples removed from greater depths and higher stress regimes. Laboratory-derived iodide De values consistently trended higher than in-situ values by a factor of 1 to 15, except on the shallowest 240-m Level (σ1 ≍ 30 MPa) where differences were negligible. Laboratory-derived estimates of permeability were consistently higher than in-situ derived values by a factor of 2 to 100. This experimental program provides evidence that laboratory steady-state diffusion experiments are most likely to yield conservative values of De for simulation of diffusive mass transport in SFR.

In situ diffusion experiment in granite: Phase I

A program of in situ experiments, supported by laboratory studies, was initiated to study diffusion in sparsely fractured rock (SFR), with a goal of developing an understanding of diffusion processes within intact crystalline rock. Phase I of the in situ diffusion experiment was started in 1996, with the purpose of developing a methodology for estimating diffusion parameter values. Four in situ diffusion experiments, using a conservative iodide tracer, were performed in highly stressed SFR at a depth of 450 m in the Underground Research Laboratory (URL). The experiments, performed over a 2 year period, yielded rock permeability estimates of 2×10 −21 m 2 and effective diffusion coefficients varying from 2.1×10 −14 to 1.9×10 −13 m 2/s, which were estimated using the MOTIF code. The in situ diffusion profiles reveal a characteristic “dog leg” pattern, with iodide concentrations decreasing rapidly within a centimeter of the open borehole wall. It is hypothesized that this is an artifact of local stress redistribution and creation of a zone of increased constrictivity close to the borehole wall. A comparison of estimated in situ and laboratory diffusivities and permeabilities provides evidence that the physical properties of rock samples removed from high-stress regimes change. As a result of the lessons learnt during Phase I, a Phase II in situ program has been initiated to improve our general understanding of diffusion in SFR.

Creep behavior of Ti–6Al–2Sn–4Zr–2Mo: I. The effect of nickel on creep deformation and microstructure

The effect of trace levels of Ni on the intermediate temperature creep behavior of the alloy Ti–6Al–2Sn–4Zr–2Mo (wt%) has been investigated. Creep experiments were performed in tension over the temperature range 510–565 °C at stress range 138–413 MPa. Two heats of commercial grade Ti–6Al–2Sn–4Zr–2Mo with Ni levels of 0.006 and 0.035 wt% were studied. The high Ni material uniformly exhibited higher primary creep strains and minimum strain rates than the lower Ni material. Stress exponents in the range 5–7 and 4–6 were obtained for the high Ni and low Ni material respectively. At 565 °C a transition to a low stress region with a stress exponent equal ∼1 is found for both materials. At all stress levels, the apparent activation energy was lower for the high Ni material. The apparent activation energy is in excellent agreement with those reported for lattice self-diffusion in α-titanium in the presence of fast diffusing impurities. The results also suggest that creep in the higher stress regime is controlled by dislocation motion within the α-phase. We suggest that trace levels of Ni in the α-phase accelerate self-diffusion therefore increasing the rate of dislocation climb leading to the higher creep rates observed in the high Ni material. In Part II, direct evidence in support of dislocation-based creep being important in both low and high stress regimes is presented.

Effect of stress axis orientation on the creep deformation behavior of Ti–48Al polysynthetically twinned (PST) crystals

The creep deformation behavior of PST crystals Ti–48Al was investigated at 1150 K under 100–400 MPa. Two hard orientations with the lamellar plates parallel or perpendicular to the compression axis and a soft orientation with the lamellar plates oriented 35° to compression axis were compared to investigate the effect of the orientation of lamellar interface on the creep behavior. The soft PST orientation exhibited the highest creep rates and larger primary creep strain than those of the hard PST orientations. The stress exponents were evaluated as 7.0 in the hard PST orientations for the stress dependence of the minimum creep rates. In the soft PST orientations, the stress exponent increased with decreasing stress from 4 at high stress regime to 8 at low stress regime. The primary creep strain increased with decreasing stress. The rotation of lamellar played an important role to accommodate the creep deformation in the soft PST orientation. The acceleration of creep rate after the minimum strain rates were attributed to the microstructural softening at strain concentrated regions. Those are kinking and dynamic recrystallization along the shear band in the hard PST orientations and recrystallization along the region at which angle of lamellar boundaries abruptly change in soft PST orientation.

The internal mechanics of the intervertebral disc under cyclic loading

The mechanics of the intervertebral disc (IVD) under cyclic loading are investigated via a one-dimensional poroelastic model and experiment. The poroelastic model, based on that of Biot (J. Appl. Phys. 12 (1941) 155; J. Appl. Mech. 23 (1956) 91), includes a power-law relation between porosity and permeability, and a linear relation between the osmotic potential and solidity. The model was fitted to experimental data of the unconfined IVD undergoing 5 cyclic loads of 20 min compression by an applied stress of 1 MPa, followed by 40 min expansion. To obtain a good agreement between experiment and theory, the initial elastic deformation of the IVD, possibly associated with the bulging of the IVD into the vertebral bodies or laterally, was removed from the experimental data. Many combinations of the permeability–porosity relationship with the initial osmotic potential ( π i) were investigated, and the best-fit parameters for the aggregate modulus ( H A) and initial permeability ( k i) were determined. The values of H A and k i were compared to literature values, and agreed well especially in the context of the adopted high-stress testing regime, and the strain related permeability in the model.

Steady-Shear Rheology of Block Copolymer Melts: Zero-Shear Viscosity and Shear Disordering in Body-Centered-Cubic Systems

The steady-shear rheology of polystyrene−polyisoprene (S/I) block copolymer melts having a body-centered-cubic (bcc) morphology has been characterized as a function of applied shear stress. At temperatures below the order−disorder transition (ODT) and at low stresses (τ < 100 Pa), both diblocks and triblocks manifest finite but extremely large (η0 = 107−108 Pa·s) zero-shear viscosities. Small-angle X-ray scattering (SAXS) measurements indicate that at these low stresses the bcc lattice remains intact. This Newtonian region ends abruptly when a critical shear stress is reached (τc ≈ 200 Pa), the steady shear viscosity decreasing by almost 4 orders of magnitude with only a factor of 3 further increase in stress. The viscosity then enters a weakly shear-thinning regime very similar to the behavior of the same block copolymer above its ODT temperature. As shown by SAXS, there is no lattice order in this high-stress regime, indicating that “shear-disordering” has occurred. If the disordered polymer is held fixed or subjected to a low stress (τ < 100 Pa), it gradually regains its original equilibrium state, following the same kinetics as after a thermal quench from above to below the ODT temperature.

Predicting rock failure around boreholes and drives adjacent to stopes in Indian mines in high stress regions

Abstract During stoping in a high rock stress regime, the failure of production drillholes and drives through breakouts poses serious problems in terms of the deployment of men and machinery, and threatens the very continuance of the particular stoping method. In order to continue extraction, it becomes imperative to study the behaviour of the breakouts in the field (as laboratory investigations cannot be directly applied to the field) so that breakout can be predicted well in advance and the necessary corrective measures can be taken as appropriate. For this purpose, the failure of production drillholes and the drives were investigated during sublevel open stoping at the Zawar Group of mines in India. Three distinct stages of the breakout were identified. Analysis of these led to formulation of the empirical models, correlating the depth of the notch of breakout with rise in the field stress. Knowing the field stress, the extent of breakout of these openings can be predicted; alternatively, by using these models, the prevailing ground stress can be estimated by back analysis. The latter, however, is more useful in calibrating numerical models. The breakout of boreholes has also been used to define the limit of application of various stoping methods suitable for steeply dipping orebodies.

Steady state creep behaviour of NiAl hardened austenitic steel

Creep behaviour of a Fe–Ni–Cr–Al alloy, hardened by ordered NiAl precipitates, has been studied over a temperature range 823–923 K at stresses ranging from 150 to 500 MPa. The behaviour following a stress dip or stress increment and the primary transient indicate that creep is recovery controlled. The stress dependence of steady state or minimum creep rate, covering almost five orders of magnitude, can be represented by a power-law with two distinct regimes of creep deformation. In the high stress regime, creep data reveal apparent stress exponents of 10–12 and an apparent activation energy of 282 kJ mol −1. On the other hand, the low stress regime is characterized by lower values of stress exponents (6–7) as well as activation energy (165 kJ mol −1). Creep data in the high stress regime is rationalized by invoking the concept of threshold stress. Activation energy for creep in both the regimes thus derived is equal to that for core or pipe diffusion of γ-iron (159 kJ mol −1). The transition in stress exponents can be attributed to a change in dislocation by-pass mechanism from Orowan bowing in the high stress regime to general climb over the precipitates in the low stress regime. Creep data obtained on the alloy subjected to different ageing conditions provide indirect evidence for the suggested creep mechanisms.

Micromorphological criteria for discriminating subglacial and glacimarine sediments: evidence from a contemporary tidewater glacier, Spitsbergen

Sedimentological and micromorphological evidence is presented from four sites within St. Jonsfjorden, western Spitsbergen. Sedimentological and geomorphological evidence is used to distinguish sites comprising undisturbed glacimarine sediments from glacially over-ridden glacimarine sediments and subglacial diamicts. Impregnated thin sections are used to define associations of micromorphological characteristics that allow diamicts formed under these different conditions to be discriminated. In situ glacimarine deposits display winnowed textures, dropstone structures and distinct vertical orientations of sand grains, whilst subglacial conditions result in structures indicative of deformation under moderate to high stress regimes. These criteria may help to determine the origin of ancient and Quaternary diamicts of questionable subglacial or glacimarine origin.

Experimental evidence for diffusion creep in the superplastic 3 mol% yttria-stabilized tetragonal zirconia

Although there have been numerous studies on the high temperature deformation characteristics of the superplastic 3 mol% yttria stabilized tetragonal zirconia (3YTZ), the rate controlling deformation mechanism has not been identified unambiguously. In the present study, experiments were conducted on 3YTZ at high stresses and at coarser grain sizes than used conventionally for superplasticity. The experimental results reveal, for the first time, an intragranular dislocation motion controlled high stress regime that is independent of the grain size. With a decrease in stress, there is a transition to a Newtonian viscous deformation regime consistent with Coble grain boundary diffusion creep. At sufficiently low stresses, or in materials with finer grain sizes, there is a further transition to a grain size dependent interface controlled deformation regime. Analysis of the experimental data suggests strongly that superplastic flow in 3YTZ occurs by an interface controlled deformation mechanism.

Kinetic Monte Carlo modeling of dislocation motion in BCC metals

We present a kinetic Monte Carlo (kMC) simulation method for modeling screw dislocation motion in BCC metals on the micron–second scales, using inputs from atomistic simulations of core mechanisms on the angstrom–picosecond scale. The simulations use atomistic input such as double-kink nucleation energy and kink mobility, include linear elastic (Peach–Koehler) interactions between dislocation segments, and predict overall dislocation velocity at different temperature and stress states. In addition, an important mechanism, namely, the spontaneous superjog growth and debris loop nucleation, is identified in the kMC simulation as an important factor controlling the dislocation motion in the high stress and medium temperature regime.

Experimental and theoretical studies of the system n-decyl-β-D-maltopyranoside+water

A class of non-ionic surfactants that is useful for formulating microemulsions, and that is becoming increasingly important for industrial purposes, is the series of alkyl polyglucosides (C X G Y ). These surfactants have x carbons in the hydrophobic alkyl chain and y glucose units in the hydrophilic headgroup, with commercial products typically containing noninteger values of both x and y. Commercial C X G Y blends contain many other compounds besides alkyl-β-D-glucopyranosides, including n-alkyl-α-glucopyranosides, n-alkyl-β-D-maltopyranosides, and other isomers and materials that contain a larger number of glucose units. In this paper, we investigate the physical properties of the system n-decyl-β-D-maltopyranoside (C 10G 2)+water over a wide concentration range, using various experimental techniques (surface tension measurement, rotation rheometer, DSC, polarising microscopy) and a molecular aggregation formation model. The theory is based on calculating the size distribution of the aggregates, which in turn depends on the free energy of forming an aggregate. This free energy is modelled as the sum of several free-energy contributions and an ideal entropy of mixing. For each free-energy contribution, we have highlighted schematically only the relevant characteristics of the surfactant tails or the surfactant heads. The theoretical results are compared to those found in the literature for alkyl-β-D-glucopyranosides (C X G 1) aqueous solutions. In surfactant solutions, rheological behaviour is intimately linked to internal microstructure and micellar architecture. The diluted surfactant system demonstrates Newtonian behaviour and complex non-Newtonian behaviour within the high shear stress regime. In the middle concentration range, the surfactant solutions exhibit an unexpected rheological behaviour, where the viscosities are not dependent on temperature. At high surfactant concentration phase transition, especially liquid-crystalline to isotropic solution, could be followed using rheological experiments. In performing DSC experiments, emphasis is put on the melting behaviour for the dry surfactant and C 10G 2+water systems at high surfactant concentrations. The melting behaviour can be characterised by transitions from a crystalline phase to a liquid crystalline phase and finally to an isotropic solution. The identification of the liquid-crystalline phase was carried out from textural observation, using polarising microscopy. The lyotropic behaviour follows the classical pattern established for the surfactants. Applying polarising microscopy, textures of the hexagonal and lamellar phases could be observed for the system C 10G 2+water.

Creep of magnesium strengthened with high volume fractions of yttria dispersoids

Creep experiments were performed on dispersion-strengthened-cast magnesium (DSC-Mg), consisting of unalloyed magnesium with 1 μm grain size containing 30 vol.% of 0.33 μm yttria particles. Strain rates were measured for temperatures between 573 and 723 K at compressive stresses between 7 and 125 MPa. DSC-Mg exhibits outstanding creep strength as compared with other magnesium materials, but is less creep resistant than comparable DSC-Al and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses ( σ<30 MPa), both the apparent stress exponent ( n app≈2) and the apparent activation energy ( Q app≈48 kJ mol −1) are low, while at high stresses ( σ>34 MPa), these parameters are much higher ( n app=9–15 and Q app=230–325 kJ mol −1) and increase, respectively, with increasing temperature and stress. The low-stress regime can be explained by an existing model of grain-boundary sliding inhibited by dispersoids at grain-boundaries. The unexpectedly low activation energy (about half the activation energy of grain boundary diffusion in pure magnesium) is interpreted as interfacial diffusion at the Mg/Y 2O 3 interface. The high-stress regime can be described by dislocation creep with dispersion-strengthening from the interaction of the submicron particles with matrix dislocations. The origin of the threshold stress is discussed in the light of existing dislocation climb, detachment and pile-up models.

Creep-rupture behavior of forged, thick section 9Cr-1Mo ferritic steel

Detailed investigations have been performed to examine the creep-rupture behavior of a 1,000-mm diameter and 300-mm-thick tube plate forging of 9Cr-1Mo ferritic steel in quenched and tempered (Q + T), simulated postweld heat treatment (SPWHT), and thermally aged (TA) conditions. Creep tests were conducted over a wide stress range (50 to 275 MPa) at 793 and 873 K. The alloy exhibited well-defined primary, steady-state, and extended tertiary creep stages at all test conditions. At 793 K, no significant difference in the creep-rupture properties was noted between Q + T, SPWHT, and TA conditions. On the other hand, SPWHT specimens exhibited lower creep-rupture strength than that of Q + T specimens at 873 K. Applied stress ({sigma}{sub a}) dependence of rupture life (t{sub r}) exhibited two-slope behavior. Both the Monkman-Grant ({dot {epsilon}}{sub s}.t{sub r} = C{sub MG}) and modified Monkman-Grant ({dot {epsilon}}.t{sub r}/{epsilon}{sub f} = C{sub MMG}) relationships were found to be valid for 9Cr-1Mo steel, where {dot {epsilon}}{sub s} is the steady-state creep rate and {epsilon}{sub f} is the strain to failure. The two-slope behavior was also reflected as two constants in the Monkman-Grant relationship (MGR) and modified Monkman-Grant relationship (MMGR) in the two stress regimes. Further, two creep damagemore » tolerance factors ({lambda} = 1/C{sub MMG}) of 5 and 10 were also observed in the high and low stress regimes, respectively. The alloy exhibited high creep ductility, which was retained for longer rupture lives at low stresses, and the creep ductility increased with increase in test temperature. The failure mode remained transgranular under all test conditions. The extensive tertiary creep in the alloy has been attributed to microstructural degradation associated with precipitates and dislocation substructure. The creep-rupture strength of the forging was found to be lower than that of thin section bars and tubes.« less