Microtwinning and other shearing mechanisms at intermediate temperatures in Ni-based superalloys

In Ni-based superalloys, microtwinning is observed as an important deformation mechanism at intermediate temperature and low stress and strain rate conditions. Current knowledge concerning this unusual deformation mode is comprehensively reviewed, and fundamental aspects of the process are further developed using state of the art experimental and modeling techniques. The nature of microtwins and the microtwinning dislocations at the atomic level have been determined using High Angle Annular Dark Field Scanning Transmission Electron Microscopy imaging. The results unambiguously confirm that the operative twinning dislocations are identical Shockley partials a/6〈1 1 2〉, and that they propagate through the γ′ precipitates in closely-separated pairs on consecutive {1 1 1} planes. The rate-limiting process of the microtwinning deformation mechanism is the diffusion-controlled reordering in γ′-phase. It is shown that reordering requires very simple, vacancy-mediated exchange between Al and Ni atoms. The energetic aspect of the vacancy-mediated exchanges is studied for the first time using ab initio calculations. The concept of reordering as a rate-limiting process is generalized and shown to be relevant for other, previously reported deformation mechanisms in superalloys such as a〈1 1 2〉 dislocation ribbons, and superlattice intrinsic and superlattice extrinsic stacking fault formation. Other diffusion phenomena associated with microtwinning, such as segregation of heavy elements, is also discussed and supported by experimental evidence. The influence of the γ/γ′ microstructure on microtwinning deformation mode is also discussed in light of observations and phase-field dislocation modeling results.     

Interfacial reactions of zirconium and zirconium-alloy liquid metals with beryllia at elevated temperatures

Argonne National Laboratory and Integrated Thermal Sciences, Inc. are developing crucible materials for melting reactive metals. A major part of this effort involves identifying reusable materials because they would have little or no interaction with the molten metals at elevated temperatures. Sessile drop-type experiments have been performed using pure zirconium and stainless steel-zirconium alloys (e.g., HT9-15Zr) on beryllia (BeO) substrates. The system was heated in high-purity argon to about 2000°C, held for 5 minutes, and cooled to room temperature. An external video camera monitored the interfacial interaction and wetting behavior. The zirconium melted and wetted the BeO at 1600°C, far below its melting point (1855°C). Post-test examinations show beryllium and oxygen dissolving in the zirconium metal. In addition, zirconium infiltrated the BeO substrate. No third phase reaction product was present at the zirconium-beryllia interface either at the top of the substrate or in the infiltrated region. HT9-15Zr also reacted with BeO; the alloy infiltrated partially into the BeO and formed a reaction-like layer attached to the ceramic substrate at the interface with the solidified metal. The rest of the liquid metal alloy did not wet the reaction product band. The results indicate that BeO is a poor crucible for the present application, but the observed wetting and infiltration phenomena are relevant to understanding the behavior of the liquid metal-ceramic interfaces.     

Impact strength properties of nickel-based refractory superalloys at normal and elevated temperatures

The dynamic yield and fracture stress of two refractory alloys were measured under shock-wave loading at normal and elevated (up to 680°C) temperatures. The tested materials were polycrystalline nickel-based superalloy Inconel IN738LC and PWA 1483 Ni–Co–Fe alloy in quasi-monocrystalline state. Both alloys are used as turbine blade materials for the hot first stages of gas turbines. The alloys exhibit non-monotonous dependence of the dynamic yield strength upon temperature with an abrupt increase in the temperature interval of 550–600°C. The dynamic tensile strength, in general, is less sensitive to temperature and shows less pronounced increase in the vicinity of 570°C. The observed anomalies of the dynamic strength properties correlate with anomalies in the heat capacity which are associated with the equilibrating of the short-range order in the γ-matrix of the alloys.     

The preparation and properties of some lithium zinc silicate glass-ceramics

Lithium zinc silicate glasses are of interest for the preparation of moderately high thermal expansion glass-ceramics which are suitable for sealing to a number of nickel-based superalloys. The effect of composition, in particular the variation of nucleating species, on the crystallization behaviour of a number of these glasses has been examined using differential thermal analysis, X-ray diffraction, and electron microscopy. Various crystal phases have been identified, including cristobalite, quartz, tridymite and ₀ Li₂ZnSiO₄. In addition, most of the glass-ceramics also contain an unidentified phase which may be related to the-series of lithium zinc silicates. Heat-treatment schedules have been derived on the basis of these results in order to produce a number of glass-ceramic materials. The resultant thermal expansion characteristics of the glass-ceramics have been monitored using dilatometry, and expansions in the range 12.3 to 17.1×10^–6° C^–1 (20 to 460° C), have been obtained, depending on the precise glass composition and heat-treatment schedule employed. In addition, the mechanical properties of a number of selected samples have been monitored, employing a biaxial flexure technique.     

Effects of Co content on tensile properties and deformation behaviors of Ni-based disk superalloys at different temperatures

Tensile properties of three Ni-based disk superalloys with 5 wt.%, 15 wt.% and 23 wt.% Co contents were investigated from room temperature (RT) to 725 °C with a constant strain rate of 3 × 10^− 4 s^− 1. It is found that addition of Co enhances the yield strength and the strain hardening capacity of the alloy in the studied temperature regime. It is due to the following two reasons: i.e. the interactions between a large volume fraction of fine secondary and tertiary γ′ precipitates with the dislocations in slip bands at lower temperatures and the formation of deformation microtwins at higher temperatures. However, the formation of deformation microtwins in the high Co-containing content alloy sharply decreases the ductility at higher temperatures.     

Interaction between Re and W on the microstructural stability of Ni-based single-crystal superalloys

The influences of Re and W as well as their interaction on γ′ and topologically close-packed (TCP) phases have been investigated in seven kinds of Ni-based single-crystal superalloys. The results show that after full heat treatment, the γ′ size is reduced with increasing Re, but does not change with increasing W. After thermal exposure at 1000°C for 1000?h, the TCP phase is dramatically increased with increasing Re, but increased slightly with increasing W. The TCP phase volume fraction in higher Re alloys is much more than that in lower Re alloys which have the same total content of Re and W. This indicates that W instead of Re could effectively improve the microstructural stability of Ni-based single-crystal superalloys.     

Experimental measurement of the kinetics of gamma prime dissolution during supersolvus heat treatment of powder metallurgical Ni-based disk superalloys

During heat treatment of Ni-based superalloys, the rate of change of gamma prime phase particle sizes and volume fractions affect the final grain size and thus the material’s mechanical properties. Experimental measurement of the supersolvus dissolution kinetics is difficult due to rapidly changing volume fractions at the heat treatment temperatures. By incorporating a heat transfer analysis, a method of extracting the kinetic constants from experimental observations is presented. The results are in good agreement with phase field simulation predictions.     

Thermal expansion at low temperatures of glass-ceramics and glasses

The linear thermal expansion coefficient, α, has been measured from 2 to 32 K and from 55 to 90 K for a machineable glass-ceramic, an ‘ultra-low expansion’ titanium silicate glass (Corning ULEE), and ceramic glasses (Cer-Vit and Zerodur), and for glassy carbon. α is negative for the ultra-low expansion materials below 100 K, as for pure vitreous silica. Comparative data are reported for α-quartz, α-cristobalite, common opal, and vitreous silica.     

Interfacial fracture characteristic and crack propagation of thermal barrier coatings under tensile conditions at elevated temperatures

Thermal barrier coatings (TBCs) have been extensively used in aircraft engines for improved durability and performance for more than fifteen years. In this paper, thermal barrier coating system with plasma sprayed zirconia bonded by a MCrAlY layer to SUS304 stainless steel substrate was performed under tensile tests at 1000°C. The crack nucleation, propagation behavior of the ceramic coatings in as received and oxidized conditions were observed by high-performance camera and discussed in detail. The relationship of the transverse crack numbers in the ceramic coating and tensile strain was recorded and used to describe crack propagation mechanism of thermal barrier coatings. It was found that the fracture/spallation locations of air plasma sprayed (APS) thermal barrier coating system mainly located within the ceramic coating close to the bond coat interface by scanning electron microscope (SEM) and energy dispersive X-Ray (EDX). The energy release rate and interface fracture toughness of APS TBCs system were evaluated by the aid of Suo–Hutchinson model. The calculations revealed that the energy release rate and fracture toughness ranged, respectively, from 22.15 J m⁻² to 37.8 J m⁻² and from 0.9 MPa m^1/2 to 1.5 MPa m^1/2. The results agree well with other experimental results.     

Interaction between silicon carbide and a nickel-based superalloy at elevated temperatures

A study has been made of the reaction of hot-pressed SiC and a nickel-based superalloy at temperatures between 700 and 1150° C. Under conditions of reduced oxygen pressure at the reaction interface, obtained by applying pressure to the couple, some degree of reaction was observed in both metal and ceramic at all temperatures studied. Preliminary studies utilizing the same techniques at 1000° C with a Si-SiC ceramic composite, Si₃N₄, MgO, Al₂O₃, and SiO₂ also indicated some degree of reaction in the metal for all ceramics examined.     

Investigation of the reactions of SiCl4 and O2 at elevated temperatures by infra-red spectroscopy

The modified chemical vapour deposition (MCVD) process for making fibre optic preforms uses direct oxidation of SiCl₄, together with dopant chlorides to produce core glass. This reaction has been studied by direct infra-red absorption spectrophotometry of the flowing gas system. The spectrum of the SiCl₄ reactant is recognizable, its decrease with increasing temperature was measured, and the absorption spectra of SiO₂ and of silicon oxychloride reaction products were recognized in the flowing gas stream. The oxychlorides began to appear in the spectra at 900° C and increased with temperature to 1110° C. Their concentration decreased drastically between 1110° C and 1160° C above which only the spectrum of SiO₂ was observed. The identities of the oxychloride molecular species produced are uncertain, but small amounts of Si₂OCl₆ and the cyclic Si₄O₄Cl₈ were present together with several unidentified higher molecular weight compounds. The particulate matter formed in the MCVD process shows infra-red spectra varying with conditions of formation. At temperatures below 1160° C the particulate matter shows absorption bands attributable to silicon oxychlorides. At higher temperatures, the particulate matter is free of oxychloride bands and shows only the proper SiO₂ spectrum. If moisture is present, these spectra are modified.     

Anomalous deformation behavior and twin formation of Ni-base superalloys at the intermediate temperatures

The tensile behaviors of polycrystalline Ni-base superalloys have been studied in the temperature range of 25-980 °C. Anomalous increase of yield strength was observed in precipitation hardened superalloys at intermediate temperature. The alloy with high γ′ volume fraction showed a remarkable increase of yield strength at intermediate temperature. A peak of yield strength was observed in the alloy with low γ′ volume fraction at intermediate temperature while solid solution strengthened alloys did not have such peak. Abrupt decrease of ductility in the intermediate temperature regime was observed not only in the γ′ strengthened superalloys but also in the solid solution strengthened superalloy. This result implies that γ′ precipitation is not a substantial cause for the occurrence of the ductility minimum in the superalloys. It was found that twinning was an important deformation mechanism of the superalloys at intermediate temperature where ductility was abnormally low. Deformation twins formed easily in the superalloys whose reduction of stacking fault energy was high regardless of strengthening mechanisms because alloys with low stacking fault energy was prone to extend stacking faults.     

Modelling of the thermo-physical and physical properties for solidification of Ni-based superalloys

The thermo-physical and physical properties of the liquid and solid phases are critical components in the modelling of casting simulations. Such properties include the fraction solid transformed, enthalpy release, thermal conductivity, volume and density all as a function of temperature. Due to the difficulty in experimentally determining such properties at solidification temperatures, little information exists for multi-component alloys. As part of the development of a new computer programme for modelling of materials properties (JMatPro), extensive work has been carried out on the development of sound, physically based models for these properties. Wide ranging results will presented for Ni-based alloys, which will include more detailed information concerning the phases formed during solidification and their composition and the density change of the liquid that intrinsically occurs during solidification due to its change in composition.     

Effect of hafnium and tantalum on the microstructure of PM Ni-based superalloys

Journal of Materials Science - The effect of Hf and Ta on the microstructure of powder metallurgy Ni-based superalloys after heat treatment was investigated. Hf and Ta change the distribution and...     

Modelling the coefficient of thermal expansion in Ni-based superalloys and bond coatings

The coefficient of thermal expansion (CTE) of nickel-based superalloys and bond coat layers was modelled by considering contributions from their constituent phases. The equilibrium phase composition of the examined materials was determined using thermodynamic equilibrium software with an appropriate database for Ni-based alloys, whereas the CTE and elastic properties of the principal phases were modelled using published data. The CTEs of individual phases were combined using a number of approaches to determine the CTE of the phase aggregate. As part of this work, the expansion coefficients of the superalloy IN-738LC and bond coat Amdry-995 were measured as a function of temperature and compared with the model predictions. The predicted values were also validated with the published data for the single-crystal superalloy CMSX-4 and a number of other Ni-based alloy compositions at 1000 K. A very good agreement between experiment and model output was found, especially up to 800 \(^\circ \)C. The modelling approaches discussed in this paper have the potential to be an extremely useful tool for the industry and for the designers of new coating systems.     

The effect of alloying additions on the oxide morphologies of super-heater tubes at elevated temperatures

Abstract

This paper considers the oxidation of boiler tube alloys; viz. 321H which contains titanium, and E1250 which contains niobium and vanadium, at temperatures of 650 and 750°C, under laboratory conditions, for 720 hours. The resulting oxides have been characterised using advanced techniques including scanning electron microscopy, energy dispersive X-ray spectroscopy and site specific crosssections using focused ion beam milling.

The results have shown that small concentrations of titanium and vanadium have a pronounced effect on the morphologies of the oxide layers. Where titanium is present, as in alloy 321H, titanium rich nodules form leading to the development of “whisker” growths from the nodules. On the E1250 alloy, vanadium rich, gem like, crystals form on the surface of the chromium/manganese rich scale. Cross-sectional analysis has shown a possible relationship between the surface features and underlying niobium particles.