Michael V. Nathal

The role of interfacial dislocation networks in high temperature creep of superalloys

Abstract The role of interfacial dislocation networks around the γ′ precipitates during the high temperature creep of nickel-base superalloys is unclear. The networks have been shown to continually evolve during creep at relatively low temperatures or eventually reach a more stable configuration at high temperatures. The objective of this study was to examine the role of these networks in several nickel-base superalloys during creep at temperatures where directional coarsening of the γ′ precipitate occurs. It was found that dislocations were not located at the γ′ interfaces which joined together during directional coarsening. The results of this study combined with previous findings suggest that the directional coarsening process is strongly influenced by elastic strain energy. The dislocation networks formed during primary creep were stable during all subsequent creep stages. Aspects of these dislocation networks were determined to be a product of both the applied creep stress and the coherency strains caused by γ-γ′ lattice mismatch, The influence of the applied stress was seen through the prominence of octahedral slip dislocations in the interfacial networks, and the effect of lattice mismatch was manifested through an inverse dependence between dislocation spacing and lattice mismatch.

Microstructure and tensile properties of Fe-40 at. pct Al alloys with C, Zr, Hf, and B additions

The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe6Al6Zr and Fe6Al6Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions significantly improved high-temperature ductility by suppressing cavitation.

The effect of temperature on the

The tensile stress-strain behavior and failure mechanisms of Ti-24Al-11Nb and a SiC/ Ti-24Al-11Nb composite with continuous SCS-6 fibers oriented parallel to the loading direction have been examined over a range of temperatures from 23 °C to 815°C in air. Failure in Ti- 24Al-11Nb occurred at strains of approximately 4 pct soon after crack initiation at low tem- peratures. Ductility increased with temperature up to a maximum of 20 pct elongation at 600 °C, as surface-initiated cracks did not propagate readily at intermediate temperatures. At higher temperatures, the onset of grain boundary and interfacial void nucleation limited ductility. Com- posite failure appeared to be controlled by fiber fracture at all temperatures; for practical en- gineering purposes, composite failure occurred at 0.8 pct strain at all temperatures. At temperatures of 425 °C and less, fiber fractures occurred at intervals along the lengths of the fibers and appeared to be cumulative, while at temperatures of 650 °C and greater, fiber fractures were only observed locally to the fracture surfaces. The decreased radial residual stresses, interfacial strengths, and matrix properties at 650 °C and 815 °C allowed the composite to unload at 0.8 pct strain, due to fiber fractures, followed by a reloading in which fibers pulled out and the matrix failed, resulting in composite failure. The decreasing residual stresses with increasing temper- ature determined from an elastic-plastic concentric cylinder model were shown to affect the stress-strain response of the composite and were consistent with the measured decreasing inter- facial shear stresses, the increased fiber pullout with temperature, and the circumferential de- bonding observed around the fibers at higher temperatures.

Room temperature flow and fracture of Fe-40at.%Al alloys

Abstract The room temperature tensile behavior of Fe-40at.%Al alloys was investigated. Extrusion of both prealloyed powders and castings was performed to produce a wide range of grain sizes for characterization of mechanical properties. In addition, directionally solidified single crystals were also studied. It was found that the influence of processing variables such as extrusion temperature or the form of the starting material (powder vs. cast ingot) on mechanical properties could be explained primarily by their effects on grain size. Grain refinement improved both ductility and strength, whereas rapid quenching after annealing resulted in increases in yield strength and decreases in ductility. The effects of quenching were explained by the evidence of large numbers of quenched-in vacancies. In the binary alloy, fracture was primarily intergranular irrespective of cooling rate, while alloys containing boron or Zr+B failed transgranularly and maintained their ductility in the rapidly quenched condition. Single crystals oriented in the [100] direction showed evidence of slip behavior yet did not exhibit significant tensile ductility. This lack of ductility was attributed to an early onset of cleavage failure.

Properties and Potential of Two (ni,pt)ti Alloys for Use as High-temperature Actuator Materials

The microstructure, transformation temperatures, basic tensile properties, shape memory behavior, and work output for two (Ni,Ti)Pt high-temperature shape memory alloys have been characterized. One was a Ni30Pt20Ti50 alloy (referred to as 20Pt) with transformation temperatures above 230 °C and the other was a Ni20Pt30Ti50 alloy (30Pt) with transformation temperatures above 530 °C. Both materials displayed shape memory behavior and were capable of 100% (no-load) strain recovery for strain levels up to their fracture limit (3-4%) when deformed at room temperature. For the 20Pt alloy, the tensile strength, modulus, and ductility dramatically increased when the material was tested just above the austenite finish (Af) temperature. For the 30Pt alloy, a similar change in yield behavior at temperatures above the Af was not observed. In this case the strength of the austenite phase was at best comparable and generally much weaker than the martensite phase. A ductility minimum was also observed just below the As temperature in this alloy. As a result of these differences in tensile behavior, the two alloys performed completely different when thermally cycled under constant load. The 20Pt alloy behaved similar to conventional binary NiTi alloys with work output due to the martensite-to-austenite transformation initially increasing with applied stress. The maximum work output measured in the 20Pt alloy was nearly 9 J/cm3 and was limited by the tensile ductility of the material. In contrast, the martensite-to-austenite transformation in the 30Pt alloy was not capable of performing work against any bias load. The reason for this behavior was traced back to its basic mechanical properties, where the yield strength of the austenite phase was similar to or lower than that of the martensite phase, depending on temperature. Hence, the recovery or transformation strain for the 30Pt alloy under load was essentially zero, resulting in zero work output.

Effect of fiber strength on the room temperature

To increase understanding of what controls SCS-6 SiC/Ti-24Al-11Nb (at. pct) composite strength, fibers of known strength were incorporated into composites and the effect of fiber strength variability on room temperature composite strength was investigated. Fiber was also etched out of a composite fabricated by the powder cloth technique, and the effect of the fabrication process on fiber strength was assessed. The strength of the composite was directly correlated with the strength of the as-received fiber. Fabrication by the powder cloth technique resulted in only a slight degradation of fiber strength. Examination of failed tensile specimens revealed periodic fiber cracks, and the failure mode was concluded to be cumulative.

The Coming ICME Data Tsunami and What Can be Done

With the increased emphasis on reducing the cost and time to market of new materials the need for robust automated materials information management system(s) enabling sophisticated data mining tools is increasing, as evidenced by the emphasis on Integrated Computational Materials Engineering (ICME) and the recent establishment of the Materials Genome Initiative (MGI). Further, this need is also fueled by the demands for higher efficiency in material testing; consistency, quality and traceability of data; product design; engineering analysis; as well as control of access to proprietary or sensitive information. Further, the use of increasingly sophisticated nonlinear, anisotropic and/or multi-scale models requires both the processing of large volumes of test data and complex materials data necessary to establish processing-microstructure-property-performance relationships. Fortunately material information management systems have kept pace with the growing user demands and evolved to enable the: (i) capture of both point wise data and full spectrum of raw data curves, (ii) data management functions such as access, version, and quality controls; (iii) a wide range of data import, export and analysis capabilities; (iv) data “pedigree” traceability mechanisms; (v) data searching, reporting and viewing tools; and (vi) access to the information via a wide range of interfaces. This paper discusses key principles for the development of a robust materials information management system to enable the data mining of microstructure / property/ performance relationships critical to ICME. Furthermore NASA Glenn’s attempt at establishing such a database, for a model turbine disk Ni-based alloy (i.e., ME3) is articulated.

In Situ Growth of a Yb 2 O 3 Layer for Sublimation Suppression for Yb 14 MnSb 11 Thermoelectric Material for Space Power Applications

The compound Yb14MnSb11 is a p-type thermoelectric material of interest to the National Aeronautics and Space Administration (NASA) as a candidate replacement for the state-of-the-art Si-Ge used in current radioisotope thermoelectric generators (RTGs). Ideally, the hot end of this leg would operate at 1000°C in the vacuum of space. Although Yb14MnSb11 shows the potential to double the value of the thermoelectric figure of merit (zT) over that of Si-Ge at 1000°C, it suffers from a high sublimation rate at elevated temperatures and would require a coating in order to survive the required RTG lifetime of 14xa0years. The purpose of the present work is to measure the sublimation rate of Yb14MnSb11 and to investigate sublimation suppression for this material. This paper reports on the sublimation rate of Yb14MnSb11 at 1000°C (∼3xa0×xa010−3xa0g/cm2xa0h) and efforts to reduce the sublimation rate with an inxa0situ grown Yb2O3 layer. Despite the success in forming thin, dense, continuous, and adherent oxide scales on Yb14MnSb11, the scales did not prove to be sublimation barriers.

Microstructure-Property Relationships in Directionally Solidified Single-Crystal Nickel-Base Superalloys

This paper discusses some of the microstructural features which influence the creep properties of directionally solidified and single-crystal nickel-base superalloys. Gamma prime precipitate size and morphology, gamma-gamma (prime) lattice mismatch, phase instability, alloy composition, and processing variations are among the factors considered. Recent experimental results are reviewed and related to the operative deformation mechanisms and to the corresponding mechanical properties. Special emphasis is placed on the creep behavior of single-crystal superalloys at high temperatures, where directional gamma (prime) coarsening is prominent, and at lower temperatures, where gamma (prime) coarsening rates are significantly reduced. It can be seen that very subtle changes in microstructural features can have profound effects on the subsequent properties of these materials.