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.     

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.     

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.     

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.     

Effect of strain rate on the fracture behavior at 1366 K of the bcc iron base oxide dispersion strengthened alloy MA 956

The tensile behavior of the oxide dispersion strengthened iron-base alloy MA 956 was investigated as a function of strain-rate ranging from 3.3×10⁻² to 8.3×10⁻⁸ s⁻¹ at 1366 K. All tests were conducted in the longitudinal direction on specimens machined from bar stock. Because of the microstructure of this alloy, all specimens were either single crystals or bicrystals with the boundary parallel to the gage length. Testing revealed that the strength was rather insensitive to strain-rate, the tensile ductility decreased with decreasing strain-rate, and for strain-rates ≤8.3×10⁻⁵ s⁻¹, the alloy fractured in brittle manner. Evidence of transgranular cracking perpendicular to the applied stress was observed at all strain-rates; failure at strain-rates ≤8.3×10⁻⁵ s⁻¹ was due to cracks which grow by the joining together of cavities ahead of the running crack. This alloy appears to possess a critical stress intensity factor for rapid crack growth.     

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     

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.     

氮化物弥散强化铁基合金的显微组织和力学性能研究

本实验向雾化Fe-12.8Cr-3.4W铁基粉末中引入氮化物粉末进行球磨,然后对球磨粉末进行热挤压和热轧制制备得到合金样品。对雾化粉末和球磨粉末的形貌和成分、各组成形合金的显微组织、各组成形合金的力学性能以及断口形貌进行了研究。实验结果表明,球磨对粉末晶粒起到了细化作用,同时球磨后粉末与空气接触会引入少量的氧,YN(氮化钇)的引入起到了细化球磨粉末的作用;在氮化物弥散强化合金中检测到了尺寸小于100 nm的Ti N和TiO_2的复合弥散颗粒,少量弥散颗粒与合金中的Cr、W、Y和O元素结合长大形成大于100 nm的复杂化合物颗粒。采用氮化钇制备的合金抗拉强度与氧化钇合金基本相同,延伸率增加,同时显微硬度提高;采用氮化钛和氮化钇进行制备的合金显微硬度进一步增加。     

Elevated temperature compressive steady state deformation and failure in the oxide dispersion strengthened alloy MA 6000E

A study of the compressive flow strength-strain rate behavior of the oxide dispersion strengthened (ODS) alloy MA 6000E has been conducted between 1144 and 1366 K. Specimens taken in the longitudinal, long transverse, and short transverse bar directions were tested at strain rates ranging from 2.1 × 10⁻⁵ s⁻¹ to 2.1 × 10⁻⁷ s⁻¹. The inherent compressive strength of MA 6000E was essentially independent of orientation. Testing at the higher temperatures and slower strain rates produced large scale cracking. Such cracks formed and propagated in bands in which slip had dissolved and redistributed the γ’ precipitates. Steady state deformation could be described through use of a threshold stress model of creep where threshold stresses were calculated as functions of temperature and orientation from the relatively fast flow stress-strain rate data and the assumption that the effective stress exponent was 3.5.     

The effect of interstitial sinks on creep of a dispersion strengthened columbium-base alloy

The effects of interstitial sinks on the structure and creep behavior of the dispersion-strengthened columbium base alloy., D43 were examined. Interstitial sinks change the structure of the alloy and render it less creep resistant by reducing the carbon concentration and, therefore, the volume fraction of the dispersed phase. Where the interparticle spacing is small (<≈1 μ) the alloy is strengthened by the direct interaction of dislocations with particles. Where the interparticle spacing is large, subgrains are probably a more important structural feature strengthening the alloy. The creep rate can be described by the empirical expression \(\dot \varepsilon = A\sigma ^n e^{ - H/RT} \) , whereH (106 to 112 kcal per mole) andn (7.7 to 8.3) are independent of the amount of dispersed phase, andA is proportional to the reciprocal carbon concentrations in the range 100 to 800 ppm.     

Effect of simulated earth reentry exposure on mechanical properties of several oxide dispersion strengthened and superalloy sheet materials

The effects of simulated multiple reentry into the Earth’s atmosphere on the mechanical properties of several high temperature metallic sheet materials were evaluated. The materials included five thin gage (nominally 0.025 or 0.037 cm) oxide dispersion strengthened (ODS) alloys and two thin gage (nominally 0.037 cm) superalloys. Multiple reentry conditions were simulated through cyclic Plasma Arc Tunnel (PAT) exposure. PAT exposure generally consisted of 100 cycles of 600 s duration at 1255, 1366, or 1477 K in a Mach 4.6 airstream with an impact pressure of nominally 800 N/m². PAT exposure generally produced a uniform oxide scale, oxide pits or intergranular oxidation, Kirdendall porosity, and alloy depletion zones except for the aluminum containing ODS alloys. Only a uniform oxide scale was formed on the aluminum containing ODS alloys. In some cases, the oxide scales contained copper, apparently from the PAT electrodes, thus PAT exposure may not be truly representative of reentry. Within the limits of the PAT exposures evaluated in this study, PAT exposure did not significantly affect the mechanical properties of the thin gage alloys evaluated. Thus, it appears that the microstructural changes produced by Plasma Arc Tunnel exposure have little influence on mechanical properties. He is currently on leave of absence from the Lewis Research Center and is at Der Deutschen Forschung-und Versuchsanstalt für Luft-und Raumfahnt (DFVLR) Cologne, West Germany.     

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     

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.     

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.     

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.     

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     

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.