Effect of orientation on the tensile and creep properties of coarse-grained INCONEL alloy MA754

Elevated temperature tensile and creep-rupture tests were performed on INCONEL MA754 in longitudinal and transverse orientations at temperatures from 700°C to 1000°C. The transerve orientation was weaker and less ductile than the longitudinal orientation due to a higher grain boundary density perpendicular to the applied stress axis. This effect was especially pronounced in creep tests at 900°C and 1000°C. Threshold creep behavior was observed for the longitudinal orientation, with stress exponents ranging from 29 to 40. Stress exponents in the long transverse orientation ranged from 24 at 800°C to 5 at 1000°C, indicating a temperature-varying deformation mechanism. Creep ductility in the transverse orientation was extremely low, less than 1 pct for higher temperature, lower stress conditions. Failure in all transverse specimens was controlled by grain boundary separation. Even in the relatively weak transverse direction, the strength of MA754 compares favorably with other alloys being considered for advanced power plant applications.     

Constrained cavity growth models of longitudinal creep deformation of oxide dispersion strengthened alloys

Two models of constrained cavity growth are developed to describe the long-term longitudinal creep behavior of nickel based oxide dispersion strengthened (ODS) alloys. For both models the rupture time is taken as the time for a transverse grain boundary to cavitate fully. A diffusive cavity growth law is assumed to govern cavitation. The applicability of the respective models is determined by the particular grain morphology achieved by thermal-mechanical processing. The first model assumes that longitudinal grain boundaries are unable to slide; hence displacements due to cavitation must be matched by displacements due to dislocation creep in adjoining grains. This model predicts a low stress exponent at the transition from single crystal to cavitation creep behavior, and higher stress exponents at stresses below this transition. Good agreement is found between the model predictions and creep data for MA 754 at 1000 and 1093 °C. A second model considers a grain morphology wherein longitudinal grain boundaries are able to slide by means of deformation of pockets of fine grains. Cavitation of transverse grain boundaries is thus controlled by grain boundary sliding. This model predicts a stress exponent of 1 at low stresses, and serves as an upper bound for the creep rate when a duplex grain morphology is present. Model predictions are in good agreement with creep data for a heat of MA 754 with a duplex grain morphology. Formerly Graduate Research Assistant in the Department of Materials Science and Engineering at Stanford University     

The effect of grain morphology on longitudinal creep properties of INCONEL MA 754 at elevated temperatures

The longitudinal creep behavior of two heats of coarse grained INCONEL* MA 754 have been examined at temperatures of 1000 °C and above. Both heats exhibit a pronounced transition in deformation behavior. At high stresses, dislocation creep is observed and high stress exponents (n ∼40) are measured. Fracture in this regime is transgranular with high creep ductilities. At lower stresses, the stress exponents are low and fracture is intergranular. In this regime, the stress exponent depends strongly on the grain morphology. Heat 1, with a uniform fiber grain morphology, exhibits significantly higher stress exponents than Heat 2, which has a duplex grain morphology consisting of coarse grains along with pockets of fine, equiaxed grains. Microstructural examination of specimens deformed at the lower stresses provides evidence that cavitation of the transverse grain boundaries occurs by means of diffusive cavity growth. In the heat with the uniform fiber morphology, cavity growth is constrained by creep of adjacent grains. Cavity growth for the heat with the duplex grain morphology is apparently limited by the sliding of pockets of fine grains. The implications of these results for optimizing creep resistance of MA 754 are discussed. Formerly Graduate Research Assistant, Stanford University, Stanford, CA.     

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     

Elevated temperature strength of fine-grained INCONEL alloy MA754

Elevated temperature tensile and creep-rupture tests were performed on INCONEL alloy MA754 in an as-rolled, fine-grained condition. Tensile tests were performed at 25 °C, 800 °C, 900 °C, and 1000 °C; creep-rupture tests were performed at 800 °C, 900 °C, and 1000 °C. the elevated temperature strength in the fine-grained condition was approximately 25 pct of that the coarse-grained, annealed condition. While good ductility was observed in tensile tests at a nominal strain rate of 1 × 10⁻³s⁻¹, ductility in creep-rupture tests was very low, with failure elongations less than 5 pct and no reduction in area. Creep deformation appeared to occur primarily by cavity formation and growth.     

Mechanical alloying of IN-738

Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y₂O₃. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.     

Mechanical alloying of IN-738

Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y₂O₃. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.     

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

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

On the influence of grain morphology on creep deformation and damage mechanisms in directionally solidified and oxide dispersion strengthened superalloys

Directionally solidified (DS) and oxide dispersion strengthened (ODS) superalloys like CM 247 LC and MA 760 exhibit elongated macrograins. In uniaxial creep tests, the creep strength of such alloys in the direction of the longitudinal grains is higher than that of an equiaxed grain structure, because significantly less grain boundary (GB) segments are perpendicular to the axis of the applied stress. The present study investigates how creep in the longitudinal direction of these alloys is influenced (1) by deviations of individual grain orientations from the optimum growth direction during casting (CM 247 LC) and (2) by the spatial distribution of the small transverse GB segments (MA 760) for a given grain aspect ratio. In the case of creep ductile CM 247 LC, it was shown that if there is a large fraction of grains that are oriented for single slip, this results in higher creep rates and lower rupture times than if there is only a small fraction of such grains. The study of the influence of grain morphology on creep damage accumulation in the creep-brittle and notch-sensitive ODS alloy MA 760 showed that large scatter in creep rupture lives is related to (1) the stochastic nature of creep damage accumulation on transverse GB segments and (2) the spatial distribution of transverse GB segments. It is the combination of these two factors that results in increased scatter in rupture lives as compared to equiaxed fine grain structures. This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as 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.     

Elevated temperature creep-fatigue crack propagation in nickel-base alloys and 1 Cr-Mo-V steel

The crack growth behavior of several high temperature nickel-base alloys, under cyclic and static loading, is studied and reviewed. In the oxide dispersion strengthened (ODS) MA 6000 and MA 754 alloys, the high temperature crack propagation exhibited orientation dependence under cyclic as well as under static loading. The creep crack growth (CCG) behavior of cast nickel-base IN-738 and IN-939* superalloys at 850 °C could be characterized by the stress intensity factor,K ₁. In the case of the alloy IN-901 at 500 °C and 600 °C,K ₁ was found to be the relevant parameter to characterize the creep crack growth behavior. The energy rate line integral,C*, may be the appropriate loading parameter to describe the creep crack growth behavior of the nickel-iron base IN-800H alloy at 800 °C. The creep crack growth data of 1 Cr-Mo-V steel, with bainitic microstructure, at 550 °C could be correlated better by C * than byK ₁. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

On the creep deformation of a cast near gamma TiAl alloy Ti48Al1 Nb

The steady-state creep deformation behavior of a cast two phase gamma TiAl alloy having the composition Ti48Al1Nb (at.%) has been studied. Tension creep tests using the stress increment technique (θθ₂θ₃) were conducted over the temperature range of 704–850°C at constant initial applied stress level of 103.4–241.3 MPa. The activation energy for creep over the temperature and stress regime of this study varied 317.5 kJ/mol (137.8 MPa) up to 341.0 kJ/mol (206.8 MPa) with an average value of 326.4 kJ/mol. This is well within the range of values previously measured for gamma TiAl alloys where creep controlled by volume diffusion has been suggested as rate controlling. The stress exponents meaured were 5.0 at 704°C, 4.9 at 750°C, 4.7 at 800°C and 4.46 at 850°C. Using the activation energy of 326.4 kJ/mol, the temperature compensated steady-state creep rate was plotted against long stress with all temperatures collapsing onto a single line having a slope equal to 4.95. Using conventional creep analysis, this value of the stress exponent can be taken as suggestive of dislocation climb controlled power law creep as the operative deformation mechanism within the stress and temperature regime of the present study. The boundary separating the lamellar grains in two phase gamma TiAl alloys having the duplex microstructure may be a very important aspect of this microstructure with respect to creep deformation resistance. The interlocking γ/α₂ laths making up these boundaries are expected to be very resistant to grain boundary sliding which may contribute to creep deformation in the dislocation creep regime. Finally, some previous observations along with a comparison of the creep behavior of the Ti48Al1Nb alloy to that of a Tiz.sbnd;50.3Al binary have been discussed in terms of the pre-exponential constant A in the power law creep equation. TiAl alloys having similar stress and temperature dependencies but differing steady-state strain rates over comparable stress-temperature regimes may be rationalized on the basis of differing power law creep constants which may reflect differences in stacking fault energies.     

Anelastic relaxation controlled cyclic creep and cyclic stress rupture behavior of an oxide dispersion strengthened alloy

Cyclic creep deceleration relative to static creep was observed in oxide dispersion strengthened alloy Inconel MA 754 at 760 °C and cyclic stresses of 221 MPa-41 MPa, 231 MPa-41 MPa, and 241 MPa-41 MPa. Tests were run over the range of frequency from 0.05 cycles per hour to six cycles per hour. The maximum cyclic deceleration, which was manifested as a reduced net creep rate and increased rupture life, was observed at the highest cyclic frequency. Considerable anelastic strain, having a magnitude of ε_a/ε_e～ 1/3, is stored in MA 754 at 760 °C under these creep loads, and this strain may be recovered in the off-load half cycle of a cyclic creep test. During the higher frequency tests, the effect of an incomplete storage of anelastic strain on the accompanying nonrecoverable creep rate provides a mechanism for the frequency dependent cyclic creep deceleration. The proposed mechanism is in agreement with mixed-mode test results and with TEM examination of interrupted-test specimens.     

High temperature creep damage in steels and superalloys

The nucleation and growth of cavities was examined in steel bicrystals (Fe-3%-Si, X 8 CrNiNb 16 13) and in the ODS superalloy Inconel MA 754 (Inconel MA 754 (78% Ni; 20% Cr; 0.5% Ti; 0.3% Al; 0.6% Y₂O₃). Cavity density distributions were measured on metallographic sections and on cleaved grain boundaries as a function of time, strain, temperature and stress. Nucleation and growth laws were obtained by evaluating the distributions with appropriate models. For the fcc and bcc bicrystals, it was found that cavities nucleated continuously at sulfide and carbide particles during creep. They grew by grain boundary diffusion. But the growth rate was delayed with increasing creep strain due to cavities which nucleated in the surroundings of existing cavities. For the ODS alloy, however, many round cavities preexisted on quasi-boundaries consisting of the aggregate of coarse oxide and carbide particles. They grew initially by diffusion, but with increasing creep time (cavity size), the growth mechanism switched from growth controlled by grain boundary diffusion to growth controlled by power law creep. Implications for life predictions are discussed.     

Elevated temperature mechanical properties of the iron base oxide dispersion strengthened alloy ma 956 bar

1144 to 1477 K elevated temperature tensile, stress rupture, and creep tests and residual room temperature tensile tests following creep exposures were conducted on the iron-base oxide dispersion strengthened alloy MA 956, nominally Fe-20Cr-4.5Al-0.5Ti-0.5Y₂0₃. While the majority of the testing was in the longitudinal bar direction, a few tests in the long transverse bar direction were also conducted. Under slow strain rate conditions in the longitudinal direction, MA 956 deforms via a crack nucleation and growth mechanism eventually leading to sudden fracture. The longitudinal direction is stronger than the long transverse direction. Small amounts (∼0.1 pct) of prior creep strain do not degrade subsequent room temperature tensile properties.     

Creep and intergranular cracking of Ni-Cr-Fe-C in 360 °C argon

The influence of carbon and chromium on the creep and intergranular (IG) cracking behavior of controlled-purity Ni-xCr-9Fe-yC alloys in 360 °C argon was investigated using constant extension rate tension (CERT) and constant load tension (CLT) testing. The CERT test results at 360 °C show that the degree of IG cracking increases with decreasing bulk chromium or carbon content. The CLT test results at 360 °C and 430 °C reveal that, as the amounts of chromium and carbon in solution decrease, the steady-state creep rate increases. The occurrence of severe IG cracking correlates with a high steady-state creep rate, suggesting that creep plays a role in the IG cracking behavior in argon at 360 °C. The failure mode of IG cracking and the deformation mode of creep are coupled through the formation of grain boundary voids that interlink to form grain boundary cavities, resulting in eventual failure by IG cavitation and ductile overload of the remaining ligaments. Grain boundary sliding may be enhancing grain boundary cavitation by redistributing the stress from inclined to more perpendicular boundaries and concentrating stress at discontinuities for the boundaries oriented 45 deg with respect to the tensile axis. Additions of carbon or chromium, which reduce the creep rate over all stress levels, also reduce the amount of IG fracture in CERT experiments. A damage accumulation model was formulated and applied to CERT tests to determine whether creep damage during a CERT test controls failure. Results show that, while creep plays a significant role in CERT experiments, failure is likely controlled by ductile overload caused by reduction in area resulting from grain boundary void formation and interlinkage. Thomas M. Angeliu, formerly Graduate Student Research Assistant, Department of Materials Science and Engineering, the University of Michigan, Ann Arbor, MI,.     

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

The creep and stress rupture behavior of a mechanically alloyed oxide dispersion strengthened (ODS) and γ′ precipitation strengthened nickel-base alloy (alloy MA 6000E) was studied at intermediate and elevated temperatures. At 760 °C, MA 6000E exhibits the high creep strength characteristic of nickel-base superalloys and at 1093 °C the creep strength is superior to other ODS nickel-base alloys. The stress dependence of the creep rate is very sharp at both test temperatures and the apparent creep activation energy measured around 760 °C is high, much larger in magnitude than the self-diffusion energy. Stress rupture in this large grain size material is transgranular and crystallographic cracking is observed. The rupture ductility is dependent on creep strain rate, but usually is low. These and accompanying microstructural results are discussed with respect to other ODS alloys and superalloys and the creep behavior is rationalized by invoking a recently-developed resisting stress model of creep in materials strengthened by second phase particles. The analysis indicates that at the intermediate temperature the creep strength is controlled by the high volume fraction of γ′ precipitates and the contribution to the creep strength from the oxide dispersion is small. At the elevated temperature, the creep strength is derived mainly from the inert oxide dispersoids. Formerly at Columbia University.     

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M₂₃C₆ type carbides, having high chromium content, and M₆C type carbides, having high molybdenum content, co-exist on the grain boundaries. M₂₃C₆ type carbides, however, are quantitatively predominant. Furthermore, M₆C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.     

Characterization of Creep-Damaged Grain Boundaries of Alloy 617

Intergranular cracking and void nucleation occur over extended periods of time in alloy 617 when subjected to stress at high temperatures. Damage occurs inhomogeneously with some boundaries suffering failure, while others are seemingly immune to creep. Crack propagation associated with grain size, and grain boundary character was investigated to determine which types of grain boundaries are susceptible to damage and which are more resistant. Electron backscatter diffraction and a stereological approach to obtain the five-parameter grain boundary distribution were used to measure the proportions of each type of boundary in the initial and damaged structures. The samples were crept at 1273.15 K (1000 °C) at 25 MPa until fracture. It was found that in addition to low-angle and coherent twin boundaries, other low index boundary plane grain boundaries with twist character are relatively resistant to creep.     

Influence of Grain Boundary Character on Creep Void Formation in Alloy 617

Alloy 617, a high-temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP), which will operate at temperatures exceeding 760 °C and a helium pressure of approximately 7 MPa. Observations of the crept microstructure using optical microscopy indicate creep stress does not significantly influence the creep void fraction at a given creep strain over the relatively narrow set of creep conditions studied. Void formation was found to occur only after significant creep in the tertiary regime (>5 pct total creep strain) had occurred. Also, orientation imaging microscopy (OIM) was used to characterize the grain boundaries in the vicinity of creep voids that develop during high-temperature creep tests (900 °C to 1000 °C at creep stresses ranging from 20 to 40 MPa) terminated at creep strains ranging from 5 to 40 pct. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular, or 45 deg to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids, while coincident site lattice (CSL)–related grain boundaries did not appear to be consistently associated with void development. Similar results were found in oxygen-free, high-conductivity (OFHC) copper, severely deformed using equal channel angular extrusion, and creep tested at 450 °C and 14 MPa.