Development of a low-expansion bond coating for Ni-base superalloys

Ternary NiCrAl alloys were modified by the addition of Ti and Si in order to adjust their coefficients of thermal expansion (CTE) to less than 15 × 10⁻⁶ K⁻¹ (from room temperature to 1000 °C) without sacrificing essential high temperature oxidation resistance. Vacuum induction melted cast alloy samples were investigated by dilatometry and thermogravimetry (TG). Oxidative TG was conducted isothermally at 900 and 1000 °C and cyclically between 600 and 1100 °C for 100 h respectively. The CTE is reduced mainly by Ti and secondly by Cr additions. Quinary alloys which showed optimal oxidation resistance essentially exhibit phase stability in the solid state at all temperatures. Approximately 4 at.% Si is needed to attain low oxidation rates and to prevent oxide spallation as well. The beneficial effect of Si on oxidation behavior is attributed hypothetically to its ability to initiate the formation of protective alumina and subsequently silica. High Cr contents lessen the beneficial effect of Si owing to the concurrent formation of chromia and/or titania.     

High-temperature corrosion of superalloys

The performance of gas turbines has been improved by the development of alloys with progressively increasing high-temperature capabilities. While both strength and corrosion resistance are important, the strength requirements have a higher priority, and alloy developments which led to higher strengths also had the effect of reducing the corrosion resistance, particularly with nickel-base alloys. The most important form of corrosion is the accelerated oxidation which takes place when the air or fuel is contaminated with certain impurities, of which alkali metal salts are the most important. This type of attack is generally known as ‘hot corrosion’. Two different forms of hot corrosion have been distinguished. Type I, which is present over a temperature range of about 800–950°C, and type II, which is present over the range 700–800°C. Both processes involve an incubation period, an initiation step, and a propagation stage. Most attention has been given to the propagation stage but, from a technical point of view, the initiation step is the most important process. Mechanisms suggested include the salt fluxing model, the electrochemical model, and the sulphidation–oxidation model. Both the practical and theoretical aspects of the problem will be reviewed.     

Other applications of superalloys

Abstract

It is the intention in this paper to put into context the development of high-temperature alloys to their present position in non-gas-turbine applications, and to identify new alloy systems capable of improving performance in hostile industrial environments. The rise of superalloys from ferritic steels to the current γ′-hardened nickel-base materials, the best of which experience strength limitations above l000°C, is traced. Desired increases in temperature capability are possible with oxide dispersion strengthened powder alloys, which were originally developed for gas turbine usage and are now being produced on a large tonnage basis by the mechanical alloying (MA) process. The MA technique and commercial alloys are described and examples given of the replacement of more conventional materials by fabricated MA components in a diversity of industries. MA alloys exhibit combinations of strength and corrosion resistance capable of meeting many industrial demands for economic improvements to processing capabilities and efficiencies.

MST/525     

Industrial casting of superalloys

Abstract

The current status of precision vacuum casting technology is reviewed in outline and the historical background of the subject is briefly covered. Particular reference is made to its application to the manufacture in nickel-base superalloys of aerofoil and associated components for gas turbine engines, and possible directions for further development of the process and ancillary technology are examined. The current state of progress in both alloy technology and casting process development is reviewed. In connection with the latter, both the scope of currently available product geometries, from small solid equiaxed components through complex hollow shapes to integrally cast multi-aerofoil components, and the extension of the technology to include large, integrally cast, near-net-shape structural components, are discussed. Specific brief reference is made to the main contributory changes and developments which have taken place in each sector of the process from pattern technology to post-casting processing. The more recently adopted and now increasing utilization of multi- and monocrystalline directionally solidified aerofoils in gas turbine engines is addressed with reference to its impact on alloy chemistry and to the conflicting opinions on the relative merits of the diverse process technology. Also addressed are the increasingly important favourable effects on product quality and casting design flexibility of post-casting coating processes, thermal and thermomechanical treatments, advances in non-destructive testing, and the steady trend of progressively improving product quality assurance technology. Finally, consideration is given to the degree to which the casting of superalloys has reached maturity, and what, if any, prospective further improvements may be developed from which gas turbine engines could derive benefits in performance, weight, or operating efficiency in the future.

MST/243     

Nitrogen in revert superalloys

Two different ways of combining nitrogen in virgin and revert superalloy have been identified. The source of nitrogen in revert superalloy has been ascertained. It is found that nitrogen in scraps of virgin superalloy undergoes a transition from CrN to TiN when these scraps are remelted in VIM furnace. High nitrogen content in revert superalloys causes microporosity formation, that greatly deteriorates tensile and stress rupture properties of superalloys.     

The recrystallization of nickel-base superalloys

The effects of recrystallization on the γ′ distribution in four nickel-base superalloys of varying γ′ volume fraction (Nimonics PE16, 80A and 115, and Udimet 720) have been studied by transmission electron microscopy. These effects are explained in terms of high solubility and diffusivity in the recrystallization interface, and it is suggested that high diffusivity assumes greater importance as the amount of solute dissolved in the boundary increases. Some attention is given to the nucleation of recrystallization. It is shown that in one of the alloys (Udimet 720), nucleation at grain boundaries involves subgrain coalescence. Subsequent growth of the nucleus occurs by strain-induced boundary migration.     

Rapidly solidified nickel-base superalloys

The structures of rapidly solidified APK1, In 100 and low-carbon In 792 are described and compared with that of Nimonic 80A. Under identical processing conditions, cellular, dendritic and homogeneous equiaxed structures can be obtained. This is not due to either the influence of cooling conditions or to any single alloying addition, but depends on the combined effects of the Ti, Cr and C contents. The spinodal-type formation of , proposed for Nimonic 80A, cannot be suppressed in these alloys by pendant drop melt extraction or melt spinning techniques. However, detailed atom-probe field-ion microscopy suggests that the formation in APK1 does develop by a similar mechanism. Although not directly attributable to a modulated microstructure or to the presence of disordered particles, the extremely high strength levels observed in this alloy after heat treatment are due to the subsequent development of small, ordered, precipitates in a fine-grained matrix, together with the absence of deleterious grain boundary carbide precipitation.     

Stress corrosion of Ni-based superalloys

Abstract

The development of gas turbines to increase fuel efficiency is resulting in progressively higher operating temperatures in the under platform regions of the blades. These regions have traditionally been considered low risk areas. However, higher metal temperatures combined with stresses and the deposition of contaminants from the cooling air system may result in complex degradation mechanisms. Static stress corrosion testing has been conducted on C-ring specimens at a range of stresses in a hot corrosion environment. Cracks were observed in C-rings after exposure times greater than 100 h. Scanning electron microscopy (SEM) systems were used to image cracks and characterise deposits to improve understanding of the mechanism. Finite element analysis (FEA) has been used to model the stress intensity under test conditions. CMSX-4 specimens subject to static stresses combined with hot corrosion demonstrated significant material degradation (crack initiation and propagation) suggesting a combined stress corrosion mechanism resulting in cracking.     

Development of tool for physical simulation of skin formation during investment casting of nickel-based superalloys

Development of investment casting process has been always a challenge for manufacturers of complex shape parts with thin elements. Particularly, misruns often occur in the as-cast complex shape parts due to the formation of solid skin by freezing of melt in contact with colder ceramic mould. This work presents a new tool for physical simulation of skin formation during investment casting. Special ceramic tubes are designed and fabricated from the material used for the manufacturing of ceramic moulds for investment casting. Melting/solidification experiments are carried out in the thermo-mechanical simulator, where the melt is contained in the ceramic tube, which is heated to the temperature of ceramic mould in investment casting. Detailed microstructural characterization of the solidified specimens is performed; the obtained results predict the thickness of skin and its microstructure. This concept is applied to investment casting of complex shape nozzle guide vanes from the Mar-M247 Ni-based superalloy. Experimental casting trials are performed, and the outcomes of physical simulation tool are validated against experimental results.     

Thermal stability of advanced Ni-base superalloys

Exposures consisting of 1 to 900 h at 1000 and 1100 °C after an ageing treatment of 16 h at 870 °C were used to study the thermal stability of selected -strengthened Ni-based superalloys representing conventional, directional solidification, and single-crystal castings. Various techniques of microscopy, spectroscopy and diffraction were used to characterize the microstructure. Primary MC carbides in the alloys studied were found to be stable toward decomposition into lower carbides. In the aged condition, the strengthening phase assumed a cuboidal morphology; however, all alloys also contained varying proportions of coarse lamellar and hyperfine cooling . On an atomic scale, the nature of the cuboidal -matrix interface was found to vary from coherent to partially coherent. However, the overall lattice mismatch varied from one alloy to another depending upon its composition and the distribution of various elements in carbide phases and lamellar phase. Directional growth of the cuboidal phase upon exposure to higher temperatures was found to be accelerated by a large initial lattice mismatch leading to a considerable loss of coherency, as indicated by the observation of dislocation networks around the particles. Although the composition of the phase remained essentially unchanged, there was a marked change in matrix composition. Sigma phase was found to precipitate in all alloys, but its thermal stability was a function of alloy composition. The initial decrease in hardness followed by a hardening effect during exposure could be explained in terms of the partial dissolution of the phase and precipitation of sigma phase.     

Vanadic and chloride attack of superalloys

Abstract

The high-temperature corrosion of superalloys is associated with contaminants. When comparing contaminant conditions the contaminant flux rate (CFR) should be considered rather than the contaminant level in the fuel or environment. At temperatures above 700° C, vanadates cause fluxing of the protective oxide scales and it is shown that corrosion is determined by the CFR and temperature rather than by material selection. The effects of sulphur level in the fuel on the efficiency of magnesium additives are also considered. Chloride contamination is shown to produce scale rupture and the influence of chloride contamination under gaseous and deposit conditions is examined. In particular the differences between marine gas turbine conditions and laboratory tests to simulate hot corrosion are evaluated. It is suggested that in marine turbines fluxing mechanisms are more appropriate to the alloy than to the protective scale. Finally, the influence of chlorides on low-temperature type II hot corrosion is considered. MST/445     

Development of microstructures of high grain aspect ratio during zone annealing of oxide dispersion strengthened superalloys

Abstract

Two extruded bars of the nickel base mechanically alloyed materials MA 6000 and MA 760 have been zone recrystallised in a calibrated gradient furnace. Selected area channelling patterns in the scanning electron microscope have been employed to study the crystallographic texture of the grains of large aspect ratio produced by zone annealing, and microbeam electron diffraction has enabled the orientations of the submicrometre sized equiaxed grains in the material behind the (secondary) recrystallisation front to be studied. In both alloys a curved secondary recrystallisation interface is observed, with the surface recrystallising at a lower temperature than the interior. This is considered to result indirectly from the strain gradients occurring during extrusion. A <110> texture is present, and reasons for this are discussed. In MA 6000 progressive grain rotation towards <110> has been measured behind the recrystallisation interface, although this is not observed in MA 760 as it transforms at a lower temperature. Quenching experiments have shown that nucleation of secondary recrystallisation occurs at temperatures higher than that at which the recrystallisation interface grows at the zoning speed employed. It is suggested that the microstructure develops via the thermally activated unpinning of interfaces which have mobility advantages.

MST/1948     

Quench crack behavior of nickel-base disk superalloys

There is a need to increase the temperature capability of superalloy turbine disks to allow higher operating temperatures in advanced aircraft engines. When modifying processing and chemistry of disk alloys to achieve this capability, it is important to preserve the ability to use rapid cooling during supersolvus heat treatments to achieve coarse grain, fineγ′ microstructures. An important step in this effort is an understanding of the key variables controlling the cracking tendencies of nickel-base disk alloys during quenching from supersolvus heat treatments. The objective of this study was to investigate the quench cracking tendencies of several advanced disk superalloys during simulated heat treatments. Miniature disk specimens were rapidly quenched after solution heat treatments. The responses and failure modes were compared and related to the quench cracking tendencies of actual disk forgings. Cracking along grain boundaries was generally observed to be operative. For the alloys examined in this study, the solution temperature, not alloy chemistry, was found to be the primary factor controlling quench cracking. Alloys with high solvus temperatures show greater tendency for quench cracking.     

Mechanisms and modelling of creep in superalloys

The characteristics of creep deformation of nickel-base superalloys are reviewed and the implications for the micromechanisms controlling the behaviour are considered. The development of a model of the creep deformation that is compatible with the physical mechanisms is traced, first in an isotropic form and later incorporating full crystallographic anisotropy. The validity of the model and its ability to be extrapolated to more complex loading conditions are evaluated against a wide range of experimental measurements. Much of the work described in this review has been funded by the Defence Research Agency and creep data and specimens for examination were supplied by Dr M R Winstone of DRA.     

Strengthening mechanisms in polycrystalline nickel-based superalloys

Nickel-based superalloys are currently the material of choice for use in high-temperature applications due to their excellent high-temperature strength. It is understood that many mechanisms contribute to this property, but debate exists regarding how to model these mechanisms and predict the overall strength. This review covers the different strengthening mechanisms occurring in polycrystalline Ni-based superalloys and how these may be modelled, with the aim of revealing the gaps in the literature. It is found that models for precipitation and coherency strengthening are particularly controversial, and a unified model for the yield strength of superalloys is missing from the literature. This is of commercial importance for the design of new alloys with superior mechanical properties to those currently available.

This review was submitted as part of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.