Rebecca A. MacKay

The influence of orientation on the stress rupture properties of nickel-base superalloy single crystals

The influence of orientation on the stress rapture properties of MAR-M247 single crystals was studied. Stress rupture tests were performed at 724 MPa and 774 °C where the effect of anisotropy is prominent. The mechanical behavior of the single crystals was rationalized on the basis of the Schmid factors for the operative slip systems and the lattice rotations which the crystals underwent during deformation. The stress rupture lives at 774 °C were found to be greatly influenced by the lattice rotations required to produce intersecting slip, because second-stage creep does not begin until after the onset of intersecting slip. Crystals which required large rotations to become oriented for intersecting slip exhibited a large primary creep strain, a large effective stress level at the onset of steady-state creep, and consequently, a short stress rupture life. Those crystals having orientations within about 25° of the [001] exhibited significantly longer lives when their orientations were closer to the [001]-[011] boundary of the stereographic triangle than to the [001]-[1l 1] boundary, because they required smaller rotations to produce intersecting slip and the onset of second-stage creep. Thus, the direction off the [001], as well as the number of degrees off the [001], has a major influence on the stress rapture lives of single crystals in this temperature regime.

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.

The development of gamma-gamma-prime lamellar structures in a nickel-base superalloy during elevated temperature mechanical testing

Changes in the morphology of the γ′ precipitate were examined during creep and tensile testing at temperatures between 927 and 1038 °C in [001]-oriented single crystals of a model Ni-Al-Mo-Ta superalloy. In this alloy, the γ′ particles link together to form lamellae, or rafts, which are aligned with their broad faces perpendicular to the applied tensile axis. The dimensions of the γ and γ′ phases were measured as the lamellar structure developed and were related to time and strain in an attempt to trace the changing γ-γ′ morphology. The results showed that directional coarsening of γ′ began during primary creep, and the attainment of a fully developed lamellar structure did not appear to be directly related to the onset of steady-state creep. The rate of directional coarsening during creep increased as the temperature was raised and also increased as the stress level was raised at a given testing temperature. The raft thickness remained equal to the initial γ′ size from the start of the creep test up through the onset of tertiary creep for all testing conditions. It was found that extensive rafts did not develop during the shorter testing times of the tensile tests, and that tensile testing of pre-rafted structures did not alter the morphology of the rafts. The overall behavior of the alloy was a clear indication of the stability of the finely-spaced γ-γ′ lamellar structure.

A comparison of the mechanical properties and microstructures of intermetallic matrix composites fabricated by two different methods

The tensile properties of SCS-6 SiC fiber-reinforced Ti-24Al-11Nb (at. pct) have been measured over the past several years by a number of investigators. These composites have been fabricated by different techniques and tend to exhibit a large amount of scatter in the longitudinal tensile properties. To date, it is not known if one optimized fabrication method provides composites with improved mechanical properties over those produced by other optimized methods, since carefully controlled experiments have not been performed to determine this. Thus, the purpose of the present study was to compare the longitudinal tensile strengths of SCS-6 SiC/ Ti-24Al-11Nb composites that had been fabricated by the powder-cloth method and the lowpressure plasma spray (LPPS) method. In this study, the same lots of matrix powder and reinforcing fiber were used for fabricating the composites. It was determined that the powder-cloth and plasma spray methods produced composites having very similar tensile properties. Both fabrication methods induced damage in a small percentage of fibers, which manifested itself in the form of bimodal Weibull distributions of extracted fiber strengths. It appeared that the particular lot of SCS-6 fiber used in fabricating both types of composites was more susceptible to fabrication damage than those used in previous studies. This article also shows the dramatic effect that different handling and testing techniques can have on measured fiber strengths.

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.

Effects of Long Term Exposures on Fatigue of PM Disk Superalloys

Turbine disks in some advanced engine applications may be exposed to temperatures above 700°C for extended periods of time, approaching 1000 h. These exposures could affect near-surface composition and microstructure through formation of damaged and often embrittled layers. The creation of such damaged layers could significantly affect local mechanical properties. Powder metal disk superalloys LSHR and ME3 were exposed at temperatures of 704, 760, and 815°C for times up to 2020 h, and the types and depths of environmental attacked were measured. Fatigue tests were performed for selected cases at 704 and 760°C, to determine the impacts of these exposures on properties. Fatigue resistance was reduced up to 98 % in both superalloys for some exposure conditions. The changes in surface composition and phases, depths of these changed layers, failure responses, and failure initiation modes were compared.