Fatigue and creep crack growth in oxide dispersion strengthened INCONEL MA-754

The influence of temperature, orientation, and environment on fatigue and creep crack growth behavior in oxide dispersion strengthened INCONEL MA-754 was examined. With an increase in temperature, crack growth rates increase due largely to an increasing creep contribution. Environment also may influence crack growth behavior, its effect depending on orientation. Orientation has a marked effect on crack growth because of the propensity for creep void formation along particle stringers in the microstructure, which form in the processing. The rate of crack growth can be enhanced if the aligned voids are parallel to the main crack or retarded if these voids are normal to the direction of the crack. In the transverse-longitudinal (T-L) orimation crack growth is faster on a time basis in creep than in fatigue; the reverse of this is true in the longitudinal-transverse (L-T) orientation. Predicted fatigue crack growth rates based on a cumulative damage model agree with experimentally determined growth rates.     

Effect of prior creep at 1365 K on the room temperature tensile properties of several oxide dispersion strengthened alloys

A study was undertaken to determine if oxide dispersion strengthened (ODS) Ni-base alloys in wrought bar form are subject to a loss of room temperature tensile properties after elevated temperature creep similar to that found in a thin gage ODS alloy sheet. The bar products evaluated included ODS-Ni, ODS-NiCr, and advanced ODS-NiCrAl types. Tensile type test specimens were creep exposed in air at various stress levels at 1365 K and then tensile tested at room temperature. Low residual tensile properties, change in fracture mode, the appearance of dispersoid free bands, grain boundary cavitation, and/or internal oxidation in the microstructure were interpreted as creep degradation effects. This work has shown that many ODS alloys are subject to creep damage. Degradation of tensile properties occurs after very small amounts (≲0.2 pct) of creep strain; ductility being the most sensitive property. The amount of degradation is dependent on the creep strain and is essentially independent of the alloy system. All the ODS alloys which were creep damaged possessed a large grain size (>100 μm). Creep damage appears to be due to diffusional creep which produces dispersoid free bands around boundaries acting as vacancy sources. Low angle and, possibly, twin boundaries were found to act as vacancy sources. The residual tensile properties of two alloys were not affected by prior creep parallel to the extrusion axis. One of these alloys, DS-NiCr(S), was single crystalline. The other alloy, TD-Ni, possessed a small, elongated grain structure which minimized the thickness of the dispersoid free bands produced by diffusional creep.     

Convergent beam diffraction analysis of dispersoids in oxide dispersion strengthened alloys

Convergent Beam Diffraction (CBD) analysis was conducted on two of the four yttrium-aluminum oxide particles in a commercial dispersion strengthened alloy. Diffraction analysis on yttrium-aluminum garnet, YAG (5A1₂O₃ • 3Y₂O₃), and yttrium-aluminum perovskite, YAP (A1₂O₃ • Y₂O₃), indicated that diffraction effects in the High Order Laue Zones and symmetry information from the CBD patterns are extremely useful in obtaining additional information about the crystal structure from a single pattern. It was also shown that double diffraction effects in the zero layer can explain the presence of the kinematically forbidden reflections. The results of the analysis are discussed.     

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.     

The oxidation of oxide dispersion strengthened Ni-15Cr-5AI alloys

The effects of variations in both the type and amount of oxide dispersoids (La₂O₃, Y₂O₃ and ThO₂) on the isothermal and cyclic oxidation behavior of Ni-15Cr-5Al have been investigated. In isothermal oxidation, the presence of the dispersoids had no marked effect on the total weight change. Oxidation did not appear to follow any simple rate law, although the rate of weight change decreased with time, indicating the formation of a protective oxide. A layered oxide was observed to be comprised of an inner layer of A1₂O₃ and an outer layer of nickel-chromium-aluminum spinel. It is proposed that the formation of these spinels is favored in dispersoid-containing alloys. The spinels in turn promote the formation of the protective equilibrium oxide, A1₂O₃. A mechanism, based on an increase in the number of oxide nucleation sites, is presented to explain the observed results. The dispersoids markedly improved the performance of the alloys in cyclic oxidation. It is proposed that the porosity observed in the outer spinels of dispersion-hardened alloys aids in the adjustment to thermally induced expansion and contraction. These pores lead to an improvement in protective oxide scale adhesion by providing sites for the accommodation of strain, thereby reducing the amount of stress at the oxide-metal interface. Formerly Research Scientist, Paul D. Merica Laboratory, The International Nickel Company, Inc.     

Creep and tensile properties of several oxide dispersion strengthened nickel base alloys

The room temperature and 1365 K tensile properties and 1365 K tensile creep properties at low strain rates were measured for several oxide dispersion strengthened (ODS) alloys. The alloys examined included ODS Ni, ODS Ni-20Cr and ODS Ni-16Cr-4J5Al. Metallography of creep tested, large grain size ODS alloys indicated that creep of these alloys is an inhomogeneous process. All alloys appear to possess a threshold stress for creep. It is believed that the threshold stress is associated with diffusional creep in the large grain size ODS alloys and normal dislocation motion in perfect single crystalline ODS alloys. Threshold stresses for large grain size ODS Ni-20Cr and Ni-16Cr-4J5A1 type alloys are dependent on the grain aspect ratio. Because of the deleterious effect of prior creep on room temperature mechanical properties of large grain size ODS alloys, it is speculated that the threshold stress may be the design-limiting creep strength property.     

Zirconium alloys dispersion strengthened with yttria

Abstract

A powder metallurgy process for the manufacture of zirconium alloys dispersion strengthened with yttria particles is described. The resulting alloys con tain a suitable dispersion of stable yttria particles in a fine-grained matrix. Tensile and stress rupture tests over a range of temperature on Zircaloy-2 containing 0, 5 and 10 vol % yttria show the strength of yttria-containing alloys to be significantly improved. The post-irradiation tensile properties of Zircaloy-2 - yttria alloys have been determined with alloys irradiated at 280°C to 9.2 × 10²⁰ n/cm² thermal and 2.3 × 10²⁰ n/cm² fast. The alloys have been subjected to accelerated corrosion tests in steam and exhibit a variable response. Some possible detrimental factors and means of improvemen t of properties are discussed.

The mechanical properties of Zircaloy-2 alloys containing yttria particles are compared to some other zirconium alloys, Zircaloy-2 and zirconium-2.5 wt % Nb and the comparisons show that the dispersion- strengthened alloys arc worthy of further development.

Résumé

La fabrication par frittage des alliages de zirconium renforcés par des particules d'yttria est décrite. Ce procédé donne une dispersion convenable des particules d'yttria dans une matrice à grains fins. Des essais de traction et d'impact, effectués à diverses températures sur du Zircaloy-2 contenant 0,5 et 10 % d'yttria, ont montré que la résistance mécanique des alliages est nettement augmentée. D.autres essais de traction ont été effectués sur des alliages Zircaloy-2-yttria aprés des irradiations de 9.2 × 10²⁰ n/cm² thermique et 2.3 × 10²⁰ n/cm² rapide à 280°C. Les alliages ont été soumis à des essais de corrosion accélérée (vapeur) avec des résultats variables. Dans la discussion on considère les facteurs qui peuvent être miscibles ainsi que déautres améliorations possibles.

La comparaison des propriétés mécaniques des alliages de Zircaloy-2 contenant de l'yttria à celles d'autres alliages de zirconium, Zircaloy-2 et zirconium-2.5,% Nb indique que les alliages à particules dispersées méritent d'être étudiés davantage.     

Recovery of work-hardening and low temperature creep in dispersion strengthened Cu alloys with rod shaped particles

Low temperature recovery of work-hardening and creep in a copper single crystal, dispersion strengthened with rod shaped oxide particles, have been studied. The climb rate of an Orowan loop surrounding a rod shaped particle has been analysed on the basis of dislocation pipe diffusion. This analysis satisfactorily explains the observed shape and size dependences of softening; the softening rate is inversely proportional to the square of the rod radius times the square of the rod length. The stationary creep rate linearly depends on the applied stress, the proportional constant having the shape and size dependences similar to that of the softening rate. It can be explained by the balance of the annihilation and accumulation of the Orowan loops.     

Fatigue crack growth behavior of an oxide dispersion strengthened MA 956 alloy

Fatigue crack growth behavior of oxide dispersion strengthened ferritic MA 956 alloy was studied at 25 °C and 1000 °C in air at 0.17 Hz. The growth rates were analyzed using the linear elastic parameter ΔK and the elastic-plastic parameter ΔJ. Crack growth, although transgranular at both temperatures, increased by nearly three orders of magnitude with increase in temperature from 25 to 1000 °C. The growth rates were essentially the same in terms of either ΔK or ΔJ parameters indicating that plasticity effects are small even at 1000 °C. Detailed fractographic analysis revealed the presence of ductile striations in the ΔK range of 25 to 40 MPa√m at 25 °C and in a much narrower range at 1000 °C. Presence of voids could be detected at 1000 °C. Using the measured load-displacement hysteresis energies for a unit increment in crack length, crack growth rates were calculated using cumulative damage models and were compared with the experimental data. At 1000 °C the predicted and the experimental values agree within a factor of two and it is concluded that the growth occurs essentially by a damage accumulation process except in a narrow range of ΔK where the plastic blunting process is superimposed, resulting in ductile striations that were observed. At 25 °C the predicted and the experimental value reasonably agree for ΔK values greater than 40 MPa√m, and below this value the two diverge with predicted values being much lower. This divergence is related to occurrence of the plastic blunting process in this ΔK range as confirmed by fractographic evidence. The cumulative damage process at 1000 °C was related to the environmentally assisted void formation at dispersoid-matrix interfaces. At 25 °C the damage is related to the formation of microcracks ahead of the crack tip. These results and interrelation between alloy microstructure and fatigue fracture path are discussed in detail.     

On high creep activation energies for dispersion strengthened metals

Creep studies on dispersion strengthened metals have often resulted in inexplicably high creep activation energies and high stress exponents. An analysis is presented which shows that the contribution of the temperature dependent elastic modulus to the apparent activation energy can be very large. Two cases are examined in which the difference between the apparent activation energy and the activation energy of self diffusion is almost entirely accounted for by the contribution of the temperature dependent elastic modulus. The modulus correction is particularly important at high temperatures and for materials with high stress exponents.     

Deformation of rapidly solidified dispersion strengthened titanium alloys

The influence of erbium on the behavior of titanium and titanium-aluminum based alloys has been investigated over a range of strain rates and temperatures (25 to 775°C). Data from hotextruded bulk specimens indicate that, although oxide dispersion strengthening can be large under certain conditions, the strengthening is minimal in fine-grain material subjected to low strain rate deformation at high temperatures. Both microstructural observations and an analysis of the flow data indicate profuse grain boundary sliding under these conditions. Grain coarsening anneals, which also coarsen the dispersoids and increase their population along grain boundaries, result in significantly improved high temperature flow strengths, especially in the high temperature/low strain rate regime. S. L. Kampe of Michigan Technological University, Houghton, MI     

On high creep activation energies for dispersion strengthened metals

Creep studies on dispersion strengthened metals have often resulted in inexplicably high creep activation energies and high stress exponents. An analysis is presented which shows that the contribution of the temperature dependent elastic modulus to the apparent activation energy can be very large. Two cases are examined in which the difference between the apparent activation energy and the activation energy of self diffusion is almost entirely accounted for by the contribution of the temperature dependent elastic modulus. The modulus correction is particularly important at high temperatures and for materials with high stress exponents.     

A new model-based creep equation for dispersion strengthened materials

The strongly stress-sensitive and temperature-dependent creep behaviour of dispersion strengthened materials cannot be described satisfactorily by current creep laws. In this paper a new creep equation is developed which considers as the rate-controlling event the thermally activated detachment of dislocations from dispersoid particles exerting an attractive force. The approach is motivated by recent TEM observations and theoretical calculations which strongly suggest that the “classical” view, according to which particles merely force dislocations to climb around them, is inadequate. The creep equation is applied to a dispersion-strengthened superalloy, two aluminium alloys and bubble-strengthened tungsten. Practical conclusions, regarding the optimum dispersoid size and alloy development, are drawn.     

氧化物弥散强化钢的强化机理

采用粉末冶金法制备出成分为Fe-12.5Cr-2.5W-0.4Ti-0.02V-0.4Y₂O₃(12Cr-ODS,质量分数,%)的铁素体钢.通过电镜观察及力学性能测试等手段研究了12Cr-ODS铁素体钢的组织与性能,并定量计算了不同强化机制对合金屈服强度的贡献.电镜观察发现12Cr-ODS钢为等轴的铁素体组织,平均晶粒尺寸为1.5μm,不同尺寸氧化物在基体中均匀分布.力学性能测试结果表明12Cr-ODS钢具有优异的室温拉伸性能,屈服强度达到738 MPa.合金主要强化机制为氧化物弥散强化、氧化物弥散强化钢加工强化、热错配位错强化和晶界强化机制,各种强化机制计算得到的理论屈服强度为750 MPa,与实测值吻合较好.     

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.     

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.     

Mechanisms of creep deformation in Mg-Sc-based alloys

Binary Mg-Sc alloys show only a very weak age-hardening response due to the low diffusivity of Sc in Mg and exhibit inferior creep resistance compared to WE alloys. The addition of a small amount of Mn (<1.5 wt pct) improves their creep behavior markedly, decreasing the minimum creep rates by up to about two orders of magnitude at temperatures above 300 °C compared to WE alloys. This is due to the precipitation of fine Mn₂Sc phase basal discs, which are very effective obstacles in controlling creep at temperatures at which cross-slip of basal dislocations and nonbasal slip are the rate controlling mechanisms. The addition of Ce improves the creep resistance even more due to the effect of the grain boundary eutectic. The effect of Mn₂Sc discs can still be seen in alloys with a low Sc content (∼1 wt pct) and with the addition of rare earth (RE) elements (Gd, Y, Ce ∼4 wt pct). Very thin hexagonal plates containing RE and Mn, which lie parallel to the basal plane of the Mg matrix, augment the effect of the Mn₂Sc precipitates at elevated temperatures (∼250 °C). The triangular arrangement of prismatic plates of metastable or stable phases of Mg-RE systems controls effectively the motion of basal dislocations during the creep of these alloys at elevated or high temperatures. The combined control of basal slip, cross-slip of basal dislocations, and of nonbasal slip in low Sc content alloys ensures minimum creep rates of about one order of magnitude lower than those observed in WE alloys, both at elevated and high temperatures. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee.     

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     

Structure and properties of rapidly solidified dispersion strengthened titanium alloys: Part II. tensile and creep properties

The effects of incoherent dispersoids on tensile and creep properties were determined in rapidly solidified Ti-Er and Ti-Nd alloys. Uniform distributions of. fine incoherent dispersoids in Ti matrix were produced by rapid solidification at cooling rates > 10³ °C per second and subsequent annealing at 700 to 800°C of Ti-1.0Er, Ti-2.0Er, Ti-1.5Nd, and Ti-3.0Nd alloys. The rapidly solidified particulates consolidated by vacuum hot pressing were isothermally forged, rolled, and annealed to produce fully recrystallized microstructures. The incoherent dispersoids in Ti-Er and Ti-Nd alloys increase by 40 to 110 pct the yield strength and ultimate tensile strength of Ti with no significant loss in ductility. The strength increments were analyzed in terms of the superposition of dispersion-, solid solution-, and fine grain-strengthening. Dispersion strengthening is offset to some extent by the reduction in interstitial oxygen solid solution strengthening caused by the scavenging of oxygen by Er and Nd. The dispersoids decrease the creep rates and increase the stress rupture lifetimes of Ti at 482 to 700 °C.