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 transverse 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.     

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.     

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.     

Hot deformation characteristics of INCONEL alloy MA 754 and development of a processing map

The characteristics of hot deformation of INCONEL alloy MA 754 have been studied using processing maps obtained on the basis of flow stress data generated in compression in the temperature range 700 °C to 1150 °C and strain rate range 0.001 to 100 s^-1. The map exhibited three domains. (1) A domain of dynamic recovery occurs in the temperature range 800 °C to 1075 °C and strain rate range 0.02 to 2 s^-1, with a peak efficiency of 18 pct occurring at 950 °C and 0.1 s^-1. Transmission electron microscope (TEM) micrographs revealed stable subgrain structure in this domain with the subgrain size increasing exponentially with an increase in temperature. (2) A domain exhibiting grain boundary cracking occurs at temperatures lower than 800 °C and strain rates lower than 0.01 s^-1. (3) A domain exhibiting intense grain boundary cavitation occurs at temperatures higher than 1075 °C. The material did not exhibit a dynamic recrystallization (DRX) domain, unlike other superalloys. At strain rates higher than about 1 s^-1 the material exhibits flow instabilities manifesting as kinking of the elongated grains and adiabatic shear bands. The material may be safely worked in the domain of dynamic recovery but can only be statically recrystallized.     

Influence of gravity on the dispersoids during the melting of an oxide dispersion-strengthened alloy (INCONEL MA754)

Formerly Graduate Student at Columbia University, New York, NY.     

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.     

Characterization of INCONEL alloy MA 6000 powder

Mechanically alloyed powders of INCONEL* alloy MA 6000 have been characterized in order to understand the mechanical alloying process. Various analytical techniques including transmission electron microscopy, X-ray diffraction, electron microprobe analysis, and Auger electron spectroscopy were used to reveal the microstructural changes, chemical homogeneity, degree of mechanical working, and surface chemistry of powders during the powder processing. The current investigation has brought about a better understanding of the basic mechanism of the mechanical alloying process as well as a guideline for quality control of mechanically alloyed powder.     

The effect of sheet processing on the microstructure, tensile, and creep behavior of INCONEL alloy 718

The grain size, grain boundary character distribution (GBCD), creep, and tensile behavior of INCONEL alloy 718 (IN 718) were characterized to identify processing-microstructure-property relationships. The alloy was sequentially cold rolled (CR) to 0, 10, 20, 30, 40, 60, and 80 pct followed by annealing at temperatures between 954 °C and 1050 °C and the traditional aging schedule used for this alloy. In addition, this alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and the corresponding microstructure and mechanical behavior were evaluated. The creep behavior was evaluated in the applied stress (σ _a ) range of 300 to 758 MPa and the temperature range of 638 °C to 670 °C. Constant-load tensile creep experiments were used to measure the values of the steady-state creep rate and the consecutive load reduction method was used to determine the values of backstress (σ₀). The values for the effective stress exponent and activation energy suggested that the transition between the rate-controlling creep mechanisms was dependent on effective stresses (σ _e =σ _a σ₀) and the transition occurred at σ _e ≅ 135 MPa. The 10 to 40 pct CR samples exhibited the greatest 650 °C strength, while IN 718SPF exhibited the greatest room-temperature (RT) tensile strength (>1550 MPa) and ductility (ε _f >16 pct). After the 954 °C annealing treatment, the 20 pct CR and 30 pct CR microstructures exhibited the most attractive combination of elevated-temperature tensile and creep strength, while the most severely cold-rolled materials exhibited the poorest elevated-temperature properties. After the 1050 °C annealing treatment, the IN 718SPF material exhibited the greatest backstress and best creep resistance. Electron backscattered diffraction was performed to identify the GBCD as a function of CR and annealing. The data indicated that annealing above 1010 °C increased the grain size and resulted in a greater fraction of twin boundaries, which in turn increased the fraction of coincident site lattice boundaries. This result is discussed in light of the potential to grain boundary engineer this alloy. INCONEL is a registered trademark of Special Metals Corp., Huntington, WV. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Influence of microstructure on tensile and creep properties of a new castable TiAl-based alloy

Snecma Motors has been working on the development of γ-TiAl low-pressure turbine blades, including manufacturing optimization, castability evaluation of a selected alloy called G4, and heat-treatment optimization of mechanical and physical properties. The objective of this study was to evaluate microstructure variability regarding casting conditions and aluminum content. The response of cast microstructures to hot isostatic pressing (hipping) and subsequent heat treatments was determined and quantified using tensile and creep testing. Such investigations helped define an optimized heat treatment. Tensile and creep property assessment has shown a high-temperature potential for G4 alloy with respect to other γ alloys. The G4 alloy also appears to be more creep resistant than conventional nickel-based superalloys on a specific basis. The enhanced creep properties under the optimized low-temperature treatment are mainly attributed to solid solution strengthening with Re, W, and Si elements and precipitation hardening with primary β phase decorating the primary dendrites.     

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 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     

Effect of cavitation on post-deformation tensile properties of a superplastic copper-base alloy

The effects of cavitation, introduced during superplastic tensile flow, on post-deformation tensile properties have been studied for a microduplex Cu-Zn-Ni alloy. Increasing levels of cavitation of up to 4 percent by volume caused a progressive reduction in tensile strength, while ductility was decreased to a substantially greater extent. Increasing the volume fraction of cavities also changed the appearance of the fracture surface from cup and cone to macroscopically brittle. A post deformation annealing treatment reduced the level of cavitation and caused an increase in room temperature ductility, but the proportion of cavities removed during annealing became progressively less as cavitation was increased.     

Effect of microstructure on creep and creep crack growth behaviour of titanium alloy

Creep and creep crack growth behaviour of a near α titanium alloy has been investigated at 600°C which is affected by primary α content. The alloy was heat treated at different temperatures so as to obtain different levels of equiaxed primary α in the range from 5 to 24 %. Constant load creep tests were carried out at 600°C in the stress range 250 to 400 MPa till rupture of the specimens. Creep crack growth tests were carried out at 600°C. Creep data reveals with increase in primary α content leads to creep weakening. On similar lines maximum creep crack growth resistance is associated with the alloy with lowest primary α content. Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary β / transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary β / transformed β (matrix) interfaces in the interior of the specimen.     

Microstructure and phase stability of INCONEL alloy 617

INCONEL alloy 617 (54 Ni, 22 Cr, 12.5 Co, 9 Mo, 1 Al, 0.07 C) is a solid-solution alloy with good corrosion resistance and an exceptional combination of high-temperature strength and oxidation resistance. A laboratory study was performed to determine the effects of long-time (215 to over 10,000 h) exposure to temperatures up to 2000°F (1093°C) on the microstructure and phase stability of the alloy. To investigate the strengthening response exhibited by the alloy during high-temperature exposure, microstructures were correlated with mechanical properties. The major phase present in the alloy after exposure to all temperatures from 1200 to 2000°F (649 to 1093°C) was found to be M₂₃C₆. The phase precipitated as discrete particles and remained stable at aü temperatures. No MC or M₆C carbides were found. A small amount of gamma prime was found in samples exposed at 1200°F (649°C) and 1400°F (760°C). A PHACOMP analysis indicated 0.63 pct gamma prime could form. No topological close-packed phases such as sigma, mu, and chi were found. Strengthening of the alloy during exposure to temperature was found to result primarily from the precipitation of M₂₃C₆. The phase provides effective strengthening because it precipitates in discrete particles and remains stable at temperatures to 2000°F (1093°C). The amount of gamma prime formed is not sufficient to cause appreciable hardening, but it does provide some strengthening at 1200 to 1400°F (649 to 760°C).     

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.     

The tensile properties of a graphite-fiber-reinforced Al-Si alloy

Room temperature uniaxial tensile tests have shown that composites of an Al-13 wt pct Si alloy and Thornel 50 graphite fibers have strengths greater than a theoretical value that was calculated on the basis of the law of mixtures. At 28 vol pct fibers, the average uniaxial tensile strength was found to be 106,000 psi, and several values between 130,000 and 144,000 psi were obtained. The modes of deformation and failure in the composites have been studied by the microexamination of polished surfaces and fractures of tested specimens. The reasons for the high strengths and the unusual modes of fracture that were observed cannot be explained on the basis of the presented data. Specimens of the composite have been thermally cycled between −193° and +20°C twenty times and others between −193° and +500°C twenty times. Tensile tests and microexamination of these thermally cycled specimens show that thermal cycling does not degrade the tensile properties of the composites or change their microstructure.     

稀土元素对钛合金蠕变性能影响规律综述

钛合金因其所具有的高比强度和耐腐蚀等优异性能而在航空航天、舰船及石油化工等领域得到广泛应用.蠕变性能是其在高温条件下使用的一项重要指标,添加稀土元素被认为是改善钛合金高温蠕变性能行之有效的方法之一.首先介绍了稀土元素在钛合金中的应用概况,总结了稀土对钛合金综合力学性能的影响.重点分析了Nd、Y、Gd、Er、La等稀土元素的含量、添加形式及制备方法等对钛合金蠕变性能的影响规律,总结了稀土元素影响钛合金蠕变性能的微观机理.提出通过研究稀土Sc对钛合金中硅化物析出过程及其分布的影响规律,进而阐明其抗蠕变机理,提高合金蠕变性能,为设计具有更加优异性能的钛合金材料提供基础理论支撑,是稀土改性钛合金的研究方向之一.      

Effect of low-frequency electromagnetic casting on the castability,microstructure, and tensile properties of direct-chill cast Al-Zn-Mg-Cu alloy

Microstructure and grain boundary segregation of the direct-chill (DC) cast Al-Zn-Mg-Cu (7A60) alloy, with and without low-frequency electromagnetic field, were investigated. The surface quality, hot-tearing tendency, and tensile properties of the ingots manufactured by DC and low-frequency electromagnetic field casting (LFEC) were compared. The results show that LFEC significantly improves the surface quality and reduces the hot-tearing tendency of DC ingots. It is possible to generate a fine, uniform, and equiaxed microstructure with LFEC. Under optimum conditions, the average grain size varies from 30 μm near the surface to 45 μm at the center of the LFEC ingots. Decreasing electromagnetic frequency or increasing intensity significantly refines microstructure, suppresses grain boundary segregation, and increases as-cast fracture strength and elongation. In the range of frequency and ampere turns employed in the experiments, the optimum frequency is found to be 15 to 25 Hz and the number of ampere turns to be 18,000 to 20,000 A_t.     

The Microstructure and tensile properties of a splat-quenched Al-Cu-Li-Mg-Zr alloy

The microstructure and tensile behavior of an Al-3Cu-l.6Li-0.8Mg-0.2Zr alloy, produced by splatquenched powder metallurgy processing, were studied. The alloy exhibited homogeneous deformation, both in bulk samples and duringin situ TEM studies. This is in contrast to the strain localization that is frequently observed in Mg-free Al-Cu-Li-X alloys. The difference in deformation mode is attributed to a fine distribution of Ś (Al₂CuMg) which precipitates up to the grain boundaries. A processing treatment involving 2 pct stretch prior to aging resulted in a yield strength of 555 MPa, a reduction in area of 29 pct, and a strain to fracture of 8.8 pct. This represents an attractive improvement in specific properties compared with 7075-T76 having a similar texture.