Creep and rupture properties of an austenitic Fe-30Mn-9Al-1C alloy

The creep deformation behavior and rupture properties of as-quenched austenitic Fe-30Mn-9Al-1C alloy have been studied at 923, 948, and 973 K under applied stresses ranging from 50 to 350 MPa. The creep curves of the alloy exhibited an extended tertiary stage prior to failure. The stress and temperature dependencies of the minimum creep rate indicated two regimes of creep deformation as well as a transition from creep to power-law breakdown. These two regimes of creep deformation were identified as a low-stress creep regime having an activation energy of 140 kJ/mol and a stress exponent of about 1, and a power-law creep regime having an activation energy of 350 kJ/mol and a stress exponent of about 6. Transmission electron microscope (TEM) observations of the deformed specimens revealed that a low density of dislocations, coarse dislocation networks, and profuse slip bands were developed in the low stress, power law, and power-law breakdown regimes, respectively. Optical microscope and scanning electron microscope (SEM) observations of the ruptured specimens showed that creep cavitation shifted from round-type in the low-stress creep regime to wedge-type in the power-law breakdown regime. The observed creep and rupture characteristics of the alloy are interpreted in terms of creep mechanisms, which involve the Coble creep and dislocation climb creep.     

Creep deformation and fracture behavior of types 316 and 316L(N) stainless steels and their weld metals

The creep properties of a nuclear-grade type 316(L) stainless steel (SS) alloyed with nitrogen (316L(N) SS) and its weld metal were studied at 873 and 923 K in the range of applied stresses from 100 to 335 MPa. The results were compared with those obtained on a nuclear-grade type 316 SS, which is lean in nitrogen. The creep rupture lives of the weld metals were found to be lower than those of the respective base metals by a factor of 5 to 10. Both the base and weld metals of 316L(N) SS exhibited better resistance to creep deformation compared to their 316 SS counterparts at identical test conditions. A power-law relationship between the minimum creep rate and applied stress was found to be obeyed for both the base and weld metals. Both the weld metals generally exhibited lower rupture elongation than the respective base metals; however, at 873 K, the 316 SS base and weld metals had similar rupture elongation at identical applied stresses. Comparison of the rupture lives of the two steels to the ASME curves for the expected minimum stress to rupture for 316 SS base and weld metals showed that, for 316L(N) SS, the specifications for maximum allowable stresses based on data for 316 SS could prove overconservative. The influence of nitrogen on the creep deformation and fracture behavior, especially in terms of its modifying the precipitation kinetics, is discussed in light of the microstructural observations. In welds containing δ ferrite, the kinetics of its transformation and the nature of the transformation products control the deformation and fracture behavior. The influence of nitrogen on the δ ferrite transformation behavior and coarsening kinetics is also discussed, on the basis of extensive characterization by metallographic techniques.     

Elevated temperature creep and fracture properties of the 62Cu-35Au-3Ni braze alloy

The Cu-Au-Ni braze alloys are used for metal/ceramic brazes in electronic assemblies because of their good wetting characteristics and low vapor pressure. We have studied the tensile creep properties of annealed 62Cu-35Au-3Ni alloy over the temperature range 250 °C to 750 °C. Two power-law equations have been developed for the minimum creep rate as a function of true stress and temperature. At the highest temperatures studied (650 °C and 750 °C), the minimum creep rate is well described with a stress exponent of 3.0, which can be rationalized in the context of Class I solid solution strengthening. The inverted shape of the creep curves observed at these temperatures is also consistent with Class I alloy behavior. At lower temperatures, power-law creep is well described with a stress exponent of 7.5, and normal three-stage creep curves are observed. Intergranular creep damage, along with minimum values of strain to fracture, is most apparent at 450 °C and 550 °C. The lower stress exponent in the Class I alloy regime helps to increase the strain to fracture at higher temperatures (650 °C and 750 °C). The minimum creep rate behavior of the 62Cu-35Au-3Ni alloy is also compared with those of the 74.2Cu-25. 8Au alloy and pure Cu. This comparison indicates that the 62Cu-35Au-3Ni has considerably higher creep strength than pure Cu. This fact suggests that the 62Cu-35Au-3Ni braze alloy can be used in low mismatch metal-to-ceramic braze joints such as Mo to metallized alumina ceramic with few problems. However, careful joint design may be essential for the use of this alloy in high thermal mismatch metal-to-ceramic braze joints.     

Effect of matrix hardness on the creep properties of a 12CrMoVNb steel

Using a creep-ductile 12CrMoVNb steel, constant-load creep tests were conducted in air at 650 °C, and the effects of matrix hardness on the creep properties were investigated. Specimens with a matrix hardness (Rc) of 30, 25, and 20 were prepared using different tempering conditions. The creep behaviors were well described by the power-law creep equation, with the stress exponents of strain rate (n) and rupture time (χ) decreasing with matrix hardness. Rupture-time analyses showed that creep rupture occurred by the nucleation of creep cavities on second-phase particles and growth by creep flow of the surrounding matrix. A hardness decrease tends to lower the rupture time and increase the strain rate (ε), and the effect of hardness was quite distinct at high applied stresses due to the short creep times, but not so at low applied stresses due to elongated creep times. After 10⁴ hours, there were almost no effects. The hardness decrease during the creep test was more severe for the specimens with higher hardness and was also more severe in the gage section than in the head section, the latter due to the stress-assisted diffusion in the coarsening of carbides. Microstructural examinations showed that subgrain boundaries grew during creep, and equiaxed carbide particles coarsened during the creep test, the rates of coarsening being greater for specimens with a higher hardness.     

Cyclic creep and stress rupture of a mechanically alloyed oxide dispersion and precipitation strengthened nickel-base superalloy

Cyclic creep and stress rupture results of Inconel MA 6000E are reported and discussed as a function of frequency. Inconel MA 6000E is a new alloy system developed for high creep resistance at intermediate as well as at high temperatures. It is a mechanically alloyed oxide dispersion (-2.5 vol pct) and y′ precipitation (-50 vol pct) strengthened nickel-base superalloy. A decrease in the minimum strain rate and increase in the rupture life were found to accompany cyclic frequency increase. The deceleration of the creep rate is related to the anelastic strains recovered during the off-load periods. The data are also discussed relative to those obtained for an alloy containing only the oxide dispersoids.     

Creep characteristics of a diecast AM50 magnesium alloy

Tensile creep tests were conducted to examine the creep behavior of a diecast AM50 magnesium alloy in the temperature range from 423 to 498 K. A normal transient creep stage is followed by a minimum creep rate stage and finally by an accelerating stage at each creep condition. The stress exponent of the minimum creep rate, n, increases from ∼5 at lower stresses to ∼10 at higher stresses at each temperature, and the value of n changes at the yield stress of the alloy. The activation energies for the creep, Q _c , are determined to be 121±10 and 162 kJ/mol, at lower and higher stresses, respectively. It is concluded that the creep of the diecast AM50 alloy is controlled by the high-temperature climb of dislocations, whereas the instantaneous plastic strain introduced by the higher stress of the creep test is assumed to cause the increased values of n and Q _c .     

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

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

Creep of metal-type organic compounds—I. Pure polycrystals and particle-hardened systems

The creep of polycrystalline camphene and succinonitrile was studied in uniaxial compression at temperatures of 273 and 293 K. Succinonitrile exhibited classic power-law creep under all conditions with a stress exponent of 5.4±0.1. Camphene showed power-law creep at higher stresses with an exponent of 4.7±0.1. However, at lower stresses, camphene exhibited diffusion creep. The minimum applied stress in each material was not sufficiently low to identify a threshold stress for creep. The creep of particle-hardened alloys was modelled using camphene and succinonitrile containing a dispersion of alumina particles of average size 0.4 μm. The volume percentage of alumina was varied from 0 to 6.6% for camphene-alumina and from 0 to 4.8% for succinonitrile-alumina. Creep measurements were made at 273 and 293 K. In all cases, creep was found to obey a power-law. The stress exponent increased by factors of up to 3 for camphene-alumina, and 2 for succinonitrile-alumina. The data could be described with the same stress exponent as that of the unalloyed systems by subtracting a back stress from the applied stress.     

The effect of predeformation on the creep and stress rupture of an oxide dispersion strengthened mechanical alloy

The effect of higher strain rate predeformation on creep behavior and stress rupture life of the oxide dispersion strengthened nickel-base alloy MA 754 was studied. Both the predeformation and creep testing were conducted at 760 °C. It was found that the minimum creep rate decreased as the amount of prestrain increased and was a factor of two lower at 1.2 pct prestrain. Predeformation also shortened the duration of primary creep. Transmission electron microscopy revealed dislocations being emitted from particle-matrix interfaces after prestraining and an increase in dislocation density with increasing prestrain. These observations are discussed with respect to the mechanical results. Formerly a Graduate Student at Columbia University     

Small Punch Creep Studies for Optimization of Nitrogen Content in 316LN SS for Enhanced Creep Resistance

Small punch creep (SPC) studies have been carried out to evaluate the creep properties of 316LN stainless steel (SS) at 923 K (650 °C) at various stress levels. The results have been compared with uniaxial creep rupture data obtained from conventional creep tests. The minimum deflection rate was found to obey Norton power law. SPC rupture life was correlated with uniaxial creep rupture life. The influence of nitrogen content on the creep rupture properties of 316LN SS was investigated in the range of 0.07 to 0.14 wt pct. SPC rupture life increased and the minimum deflection rate decreased with the increase in nitrogen content. The trends were found to be in agreement with the results obtained from uniaxial creep rupture tests. These studies have established that SPC is a fast and reliable technique to screen creep properties of different experimental heats of materials for optimizing the chemical composition for developing creep-resistant materials.     

Creep behaviors of an Ir-23Nb alloy at ultra-high temperatures

The ultra-high-temperature creep behaviors of an Ir-base, Ir-23Nb (in at. pct), two-phase refractory superalloy have been investigated. The compression creep experiments were performed at temperatures from 1650 °C to 1800 °C at initial applied stresses from 49 to 200 MPa. The results show that Ir-23Nb alloy has higher creep resistance and longer creep life in comparison to Ir-17Nb alloy under the same experimental conditions. The steady-state creep behavior can be described in terms of power-law creep with the apparent stress exponent of 4.5 and apparent activation energy of 653 kJ/mol. Basing on the investigation, the possible reasons for the great improvement on the creep resistance and creep life in Ir-23Nb alloy are discussed. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Effect of prior oxidation on the creep behavior of NiAl-hardened austenitic steel

The effect of prior oxidation at 1473 K on the creep behavior of an Fe-Ni-Cr-Al alloy, hardened by ordered NiAl precipitates, has been investigated at 873 K over a stress range of 275 to 450 MPa. The alloy in the as-electroslag remelted (ESR) as well as the ESR-plus-hot-worked conditions was considered. Prior oxidation causes creep strengthening in the Fe-Ni-Cr-Al alloy, resulting in a decrease in minimum creep rate and increase in time to rupture, in contrast to the observations reported on nickel-based superalloys. Creep strengthening is, however, accompanied by a significant reduction in creep ductility. Oxidation-induced creep strengthening in the current alloy can be attributed to the improved adherence of surface oxide caused by the presence of yttrium. An effective stress that incorporates the contributions of load transfer as well as substructural strengthening is used to account for the observed oxidation-induced creep strengthening. While creep strengthening is more pronounced in the ESR cast alloy, the loss in creep ductility is more intense in the ESR wrought alloy. Increasing the oxidation time beyond 1 hour has a minimal effect on creep strengthening of both the alloys, though it lowers significantly the creep ductility of the wrought alloy. The observed differences in creep behavior of the alloy in the two different conditions could be attributed to the differences in grain size as well as morphology and related oxidation-induced damage.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through theα →γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta−7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base FeTa-Cr alloy, the rupture and creep strengths were considerably increased. Formerly with Lawrence Berkeley Laboratory.     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through the α→γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta-7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base Fe-Ta-Cr alloy, the rupture and creep strengths were considerably increased. M. Dilip Bhandarkar, formerly with Lawrence Berkeley Laboratory.     

Creep behavior of an aligned Ag3Mg-AgMg eutectic

A eutectic alloy, Ag-32.2 at. pct Mg, has been directionally solidified at growth rates,R, ranging from 0.9 to 63.9 cm/h to produce an aligned lamellar structure. The alloy consists of AgMg, an ordered CsCl type phase, and a solid solution of approximate composition Ag-27 at. pct Mg. The investigation consisted of studying ordering effects in the Ag-27 pct Mg phase on creep behavior of the aligned eutectic at temperatures between 210 and 270°C. Ordering of Ag-27 pet Mg markedly reduced creep rate and increased rupture life of the eutectic alloy. An increase in R resulted in a finer interlamellar spacing and further reduced creep rate for both ordered and disordered material. An increase in the apparent activation energy for creep with order is consistent with an increase in the activation energy for vacancy motion in the Ag₃Mg phase. Creep tests on a single-phase Ag₃Mg alloy confirmed this conclusion. Both the ordered and disordered eutectic alloy exhibited a high sensitivity of steady-state creep rate, ε, to stress. The stress dependence was analyzed in terms of a proposed mechanism of creep in this alloy. Slip processes have been observed by optical microscopy, and fractographic techniques have been employed to explain the influence of long range order on rupture. Implications of this work to design of creep-resistant eutectic composites are discussed.     

Some trends observed in the elevated-temperature kinematic and isotropic hardening of type 304 stainless steel

In this investigation, the elevated-temperature kinematic and isotropic strength of Type 304 stainless steel are related to the subgrain size and the forest dislocation density. In earlier research by the authors, a root-mean-square (rms) equation was developed that accurately predicted the isotropic strength at 1023 K (in the power-law-breakdown regime of creep) from subgrain and dislocation density measurements. In the present study, it has been found that the rms equation also accurately predicts the isotropic strength at 1138 and 1338 K (temperatures within the power-law regime). Forest dislocation hardening appears to dominate the isotropic strength in this alloy. Kinematic stresses, or back stresses, were measured by Bauschinger effect tests at 1023 and 1123 K. The back stresses appear to be provided primarily by forest dislocations. A. A. ZIAAI-MOAYYED, formerly Research Assistant, Department of Materials Science and Engineering, Stanford University, is now     

The effect of tungsten on creep

The effect of tungsten on creep behavior and microstructural evolution was investigated for tempered martensitic 9Cr steels with various W concentrations from 0 to 4 wt pct. The creep rupture testing was carried out at 823, 873, and 923 K for up to 54 Ms (15,000 hours). The creep and creep rupture strength increased linearly with W concentration up to about 3 wt pct, where the steels consisted of the single constituent of the tempered martensite. It increased only slightly above 3 wt pct, where the matrix consisted of the tempered martensite and δ-ferrite. The minimum creep rate was described by a power law. The apparent activation energy for the minimum creep rate showed a tendency similar to the W concentration dependence of the creep-rupture strength and was larger than the activation energy for self-diffusion at high W concentrations above 1 wt pct. The martensite lath microstructure with fine carbides along lath boundaries was responsible for a high resistance to creep deformation. With increasing W con- centration, the martensite lath microstructure became stabilized, which decreased the minimum creep rate and increased the apparent activation energy for the minimum creep rate.     

Modeling creep deformation of a two-phase TiAI/Ti3Al alloy with a lamellar microstructure

A two-phase TiAl/Ti₃Al alloy with a lamellar microstructure has been previously shown to exhibit a lower minimum creep rate than the minimum creep rates of the constituent TiAl and Ti₃Al single-phase alloys. Fiducial-line experiments described in the present article demonstrate that the creep rates of the constituent phases within the two-phase TiAl/Ti₃Al lamellar alloy tested in compression are more than an order of magnitude lower than the creep rates of single-phase TiAl and Ti₃Al alloys tested in compression at the same stress and temperature. Additionally, the fiducial-line experiments show that no interfacial sliding of the phases in the TiAl/Ti₃Al lamellar alloy occurs during creep. The lower creep rate of the lamellar alloy is attributed to enhanced hardening of the constituent phases within the lamellar microstructure. A composite-strength model has been formulated to predict the creep rate of the lamellar alloy, taking into account the lower creep rates of the constituent phases within the lamellar micro-structure. Application of the model yields a very good correlation between predicted and experimentally observed minimum creep rates over moderate stress and temperature ranges. Formerly with the Department of Materials Science and Engineering, University of Virginia     

Creep and stress rupture of oxide dispersion strengthened mechanically alloyed inconel alloy MA 754

The creep and stress rupture behavior of the mechanically alloyed oxide dispersion strengthened nickel-base alloy MA 754 was studied at 760, 982 and 1093 °C. Using material with a fine, highly elongated grain structure, tensile specimens oriented parallel and perpendicular to the longitudinal grain direction were tested at various stresses in air under constant load. It was found that the apparent stress dependence was large, with power law exponents ranging from 19 to 33 over the temperature range studied. The creep activation energy, after correction for the temperature dependence of the elastic modulus, was close to but slightly larger than the activation energy for self diffusion. Rupture was intergranular and the rupture ductility as measured by percentage elongation was generally low, with values ranging from 0.5 to 16 pct. The creep properties are rationalized by describing the creep rates in terms of an effective stress which is the applied stress minus a resisting stress consistent with the alloy microstructure. Values of the resisting stress obtained through a curve fitting procedure are found to be close to the values of the particle by-pass stress for this ODS alloy, as calculated from the measured oxide particle distribution. .nt]mis|Formerly at Columbia University