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New Ni-base superalloys with higher temperature capability are required for future, more efficient gas turbine engines. In designing such alloys, careful consideration is required of the elemental concentrations to ensure that a suitable balance of mechanical properties and environmental resistance is obtained. In this study, the oxidation resistance of a series of new Ni-base superalloys with varying Al:Nb ratio has been assessed through long-term exposures in air at elevated temperature. The oxide scale has been characterised using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and quantitative measurements of oxide scale dimensions. The alloys were found to form continuous chromia scales at 700°C and outperformed several current commercial superalloys. However, following exposure at 800°C, significant microstructural degradation was observed due to precipitation of the δ phase.

Numerous challenges persist with the additive manufacturing of high γ′ containing Ni-based superalloys such as CM247LC. Currently, significant cracking occurs during deposition of CM247LC components using laser powder bed fusion and during post-processing. Whilst post-deposition procedures seek to eliminate or minimise cracks, current procedures do not produce a microstructure suitable for service. This study systematically investigates the microstructural evolution of CM247LC manufactured using laser powder bed fusion following multiple post processing treatments. Phase and textural changes after each processing step were consistent with previous studies, although an additional Hf-rich and Cr-depleted segregation zone was identified along intercellular boundaries in the as-deposited condition, believed to be associated with the cracking propensity. Compositional modification of CM247LC including removal of Hf, reduction of C and addition of Nb eliminated the segregation zone but these changes were associated with an increased susceptibility to solidification and liquation cracking.

An assessment is made of the Wagner transition criteria for predicting the formation of a continuous Al2O3 scale in Ni-based superalloys. Predictions are compared with data from an experimental Ni-based superalloy as well as commercial superalloys for which published data are available. The methodology was generally successful in predicting the transition temperature of the commercial superalloys but underpredicted the transition temperature of the experimental superalloy by approximately 50–100 °C. The difference in the transition temperature of the experimental superalloy to form a continuous Al2O3 scale is primarily attributed to a complex oxide subscale that increased the internal volume fraction of oxide and led to reduced oxygen ingress. The sensitivity and limitations of the methodology are discussed, and recommendations are made to refine the methodology to facilitate the interpretation of oxidation behaviour in polycrystalline Ni-based superalloys.

Ni-based superalloys with enhanced oxidation resistance at high temperatures are crucial for next-generation gas turbine engines. A new polycrystalline Ni-based superalloy (C19) that combines improved microstructural stability with environmental resistance has been developed. Its oxidation resistance has been determined through measurements of the specific mass change and morphological evolution of the formed oxides following furnace exposures at 800 °C in air for up to 1000 hours; the results of which were benchmarked against Nimonic 105. C19 showed hybrid Type II/Type III behavior as a marginal Al2O3 former and performed similarly to established superalloys at 750 °C. The Wagner model for the transition from internal to external oxide formation predicted that C19 should form a continuous Al2O3 scale at higher temperatures. A pre-oxidation treatment at 1100 °C for 1 hour was, therefore, selected and shown to dramatically improve the oxidation resistance during subsequent exposure at 800 °C. Oxide cross-sectional analysis showed that C19 formed a continuous and protective Al2O3 scale after the pre-oxidation treatment, whereas Nimonic 105 retained discontinuous Al2O3 finger-like intrusions beneath a Cr2O3 overscale.

The additive manufacturing (AM) of components through laser-blown-powder directed-energy-deposition (LBP-DED) is highly applicable to the repair of aerospace components. Fabrication of superalloys with this technique, as with other AM methods, often encounters complications that include the formation of undesired phases, irregular microstructure and texture leading to anisotropic elastic properties. Heat treatments and other post-processing techniques can be used to mitigate these issues. The collected data demonstrates the effects of different heat treatment protocols on the microstructure, elastic properties, and hardness of LBP-DED IN718. In this study eight different heat treatment were used to investigate the effects of treatment time and temperature. The microstructure was investigated through SEM, with XRD and EDX used for phase analysis. The texture was characterised using SEM coupled with EBSD and the elastic properties were determined from resonant ultrasound spectroscopy.

The use of thermodynamic predictions of alloy stability to assist with the alloy design process is widespread. Therefore, it is important to understand how accurate these predictions are currently. To this end, a series of model alloys with systematically varying Mo contents have been designed to be unstable with respect to the precipitation of the topologically close-packed phase, sigma. Increasing the Mo content was found to increase the quantity of sigma phase precipitation and alter the precipitate morphology from a plate-like to a more blocky morphology following the same thermal exposure conditions. Differential Scanning Calorimetry has been used to measure the sigma solvus temperatures of these alloys and the results compared with thermodynamic predictions made using Thermocalc software. It was found that the observed trend of increasing solvus temperature with increasing Mo content is reproduced by the predictions, but that the temperatures are significantly under- predicted. This has implications for how we use these values to influence the design of new alloy compositions.

Challenges are encountered with the laser-blown-powder directed-energy-deposition (LBP-DED) of high-performance alloys such as IN718. Complications that arise include the formation of undesired phases, irregular microstructure, and pronounced texture leading to anisotropic elastic properties. To mitigate these issues, deposition parameters and post-processing treatments must be carefully selected to achieve the desired properties in the final product. In this study it is shown that a Brass texture component ({110} ) can be achieved in LBP-DED IN718. EBSD analyses indicate that this texture is enhanced through partial recrystallisation following selected heat treatments, increasing the degree of microstructural anisotropy. Interestingly, resonant ultrasound spectroscopy (RUS) results indicate a decrease in the elastic anisotropy with recrystallisation. This is hypothesised to be a result of the pronounced Brass texture leading to pseudo-isotropic elastic properties along the cubic axes. Further insight into the kinetics of this process have been obtained with in-situ high temperature RUS measurements.

The recently developed refractory metal high entropy superalloys (RSA) have been proposed as novel materials for high temperature service. A key requirement of any material used in such applications is to retain its properties throughout the life of a component. Consequently, it is critical that the microstructures that give rise to these properties are stable across the temperature ranges experienced in service, yet, at present, very little is known about the microstructural stability of RSA. To address this issue, here we report on the microstructural evolution of two alloys from the AlMoNbTaTiZr RSA system following long duration thermal exposures of 1000 h at 1200, 1000 and 800 ˚C. At these temperatures, the initial microstructures were found to be unstable, forming new intragranular precipitates. In AlNbTa0.5TiZr0.5, both Al-Zr hexagonal and Al-Nb rich orthorhombic phases were observed following exposure at 1200 and 1000˚C, whilst additional fine scale phases were found to have formed following exposure at 800˚C. In AlMo0.5NbTa0.5TiZr0.5, the Al-Zr phase was observed to precipitate within the grains following exposure at both 1200 and 1000˚C. A refractory metal-rich secondary bcc phase was observed following exposure at 1000˚C and 800˚C and a Zr-rich phase precipitated at 800˚C. The formation of these additional phases would undoubtedly affect the properties of these alloys, altering their in-service performance. Consequently, these results highlight the need to consider microstructural stability in the future development of RSA and to gain an enhanced understanding of the phase equilibria of these systems.

The effect of Co:Ni ratio on the oxidation performance of γ-γ' alloys based on the (Ni,Co)75Al5Ti5Cr15 system (at.%) was investigated. Seven model superalloys of Co concentration 0, 9, 19, 28, 38, 47 and 56 at.% underwent isothermal oxidation in air at 800°C using both box-furnace exposure (1000 hours) and thermogravimetric analysis (100 hours). Following 1000 hours oxidation at 800°C, alloys containing 28, 38 and 47 at.% Co exhibited a flatter, more compact external scale and Cr2O3 layer as well as reduced oxygen ingress compared with alloys of lower Co content. The alloy containing 47 at.% Co exhibited two distinct morphologies.

Synchrotron X-ray imaging has been used to examine in situ the deformation of dendrites that takes place during the solidification of a nickel-based superalloy. By combining absorption and diffraction contrast imaging, deformation events could be classified by their localization and permanence. In particular, a deformation mechanism arising from thermal contraction in a temperature gradient was elucidated through digital image correlation. It was concluded that this mechanism may explain the small misorientations typically observed in single crystal castings.

Degradation of the mechanical integrity of cobalt-based superalloys can occur as the carbide network is progressively oxidised during high temperature service. In this study, a heat-treatment aimed at redistributing the carbides was tested on two similar commercial Co-based superalloys, one with high C content (Co-101) and one with low C content (Stellite-21), to determine its influence on oxidation resistance. It was found that the carbide phases in the lower C-containing alloy could be solutioned more readily than the higher C-containing alloy, enabling the continuity of the carbide network to be reduced. This resulted in a reduced attack of the carbides down the interdendritic channels during oxidation testing, but increased thickness of the oxide overscale.

The development of refractory metal high entropy superalloys has received great interest due to their potential for high temperature structural applications. The formation of a two phase nanoscale microstructure, comprising bcc and ordered B2 phases, is thought to be fundamentally linked to the presence of Al. As such, determining the influence of Al concentration on the microstructural formation and thermal stability of these novel materials is critical for future alloy development. To elucidate this effect, the microstructural evolution of a series of alloys with systematically varying compositions from the Ti-Ta-Zr-Alx system have been studied following homogenisation and long term exposures at 1100, 900 and 700˚C. Nanoscale cuboidal morphologies were observed in all alloys, but ordering was only observed when the bulk Al content was > 5 at.%. These results indicated that a common formation mechanism, thought to be spinodal decomposition, occurred prior to any B2 ordering. The results also showed that the B2 phase was only stable to relatively moderate temperatures, < 900˚C, eliminating order hardening as a high temperature strengthening mechanism. In addition, for the alloy with the greatest Al content, deleterious intragranular Al-Zr-rich intermetallics were observed following the long duration thermal exposures.

The recently developed Refractory Metal High Entropy Superalloys have the potential to replace Ni-based alloys in very high temperature structural applications. However, the microstructures of these new alloys typically consist of refractory metal based solid solution precipitates within an ordered superlattice structured matrix, which is likely to compromise key properties such as toughness. As such, there is significant interest in inverting this arrangement, such that superlattice precipitates form within a disordered refractory metal matrix. Yet the mechanisms by which these microstructures form and how they might be modified with compositional variations are currently unclear. To elucidate these mechanisms, the microstructural evolution of a series of compositionally simpler alloys from the Ti–Ta–Zr system have been studied following long term exposures at 700, 900 and 1000 °C. Exposures of up to 1000 h were used as a proxy to equilibrium and the resulting microstructures were analysed using advanced scanning and transmission electron microscopy methods. The microstructures of these alloys were found to predominantly contain one or two bcc phases, the lengthscale and morphology of which changed with exposure temperature. From these results it is established that the fine-scale microstructure, which is very similar to that widely reported in the more compositionally complex refractory metal high entropy superalloys, forms via spinodal decomposition during cooling. It is also shown, for the first time, how compositional modification can lead to a refractory metal solid solution based matrix. It is believed that these results provide key insights that can guide further development in the more complex systems that will be required for commercial applications.

The ability to quantify accurately the formation of topologically close-packed phases in nickel-based superalloys is key to assessing their thermal stability and ensuring that their performances will not deteriorate during long-term exposure at high temperatures. To investigate the effectiveness of synchrotron XRD for the detection of such minority phases in Ni-based superalloys, the commercial polycrystalline alloy RR1000 was analyzed following exposures of varying times at 800 °C. Data were collected from both solid samples and extracted residues, and additional laboratory X-ray diffraction was performed on the residues. The minor phases were successfully detected in solid samples using synchrotron radiation, and a comparison of the results from these quantification methods shows that the extraction method gives results of the right order of magnitude to reflect the phase quantities that are present in the alloy. However, the results indicate that the synchrotron route is not a suitable method for the quantification of phases present in quantities less than approximately 0.3 wt pct.

The room temperature deformation behaviour of a series of model polycrystalline Ni-based superalloys with varying Mo content has been studied in compression using in situ neutron diffraction. Initially, it was found that intergranular load partitioning was operative, followed by interphase partitioning at higher applied loads, with yield of the γ phase and associated strain redistribution to the γ′ phase. The initiation of interphase load partitioning was found to be dependent on the lattice misfit, occurring at lower applied stress in alloys with larger lattice misfit, and was influenced by the sign of the lattice misfit. Notably, deformation behaviour was found to be contingent on the complex relationship between lattice misfit and the strength of each phase.

Titanium is commonly added to nickel superalloys but has a well-documented detrimental effect on oxidation resistance. The present work constitutes the first atomistic-scale quantitative measurements of grain boundary and bulk compositions in the oxide scale of a current generation polycrystalline nickel superalloy performed through atom probe tomography. Titanium was found to be particularly detrimental to oxide scale growth through grain boundary diffusion.

The lattice parameters and misfit of the &gamma, and &gamma, &prime, phases in a series of model quaternary Ni-based superalloys with and without Mo additions have been determined using neutron diffraction between room temperature and 700 &deg, C. Despite the fact that Mo is typically expected to partition almost exclusively to the &gamma, phase and thereby increase the lattice parameter of that phase alone, the lattice parameters of both the &gamma, phases were observed to increase with Mo addition. Nevertheless, the effect on the &gamma, lattice parameter was more pronounced, leading to an overall decrease in the lattice misfit with increasing Mo content. Alloys with the lowest Mo content were found to be positively misfitting, whilst additions of 5 at.% Mo produced a negative lattice misfit. A general decrease in the lattice misfit with increasing temperature was also observed.

Refractory metal high entropy superalloys (RSAs) have been heralded as potential new high temperature structural materials. They have nanoscale cuboidal bcc+B2 microstructures that are thought to form on quenching through a spinodal decomposition process driven by the Ta-Zr or Nb-Zr miscibility gaps, followed by ordering of one of the bcc phases. However, it is difficult to isolate the role of different elemental interactions within compositionally complex RSAs. Therefore, in this work the microstructures produced by the Nb-Zr miscibility gap within the compositionally simpler Ti-Nb-Zr constituent system were investigated. A systematic series of alloys with compositions of Ti5NbxZr95−x (x = 25–85 at.%) was studied following quenching from solution heat treatment and long duration thermal exposures at 1000, 900 and 700 °C for 1000 h. During exposures at 900 °C and above the alloys resided in a single bcc phase field. At 700 °C, alloys with 40–75 at.% Nb resided within a three phase bcc + bcc + hcp phase field and a large misfit, 4.7–5%, was present between the two bcc phases. Evidence of nanoscale cuboidal microstructures was not observed, even in slow cooled samples. Whilst it was not possible to conclusively determine whether a spinodal decomposition occurs within this ternary system, these insights suggest that Nb-Zr interactions may not play a significant role in the formation of the nanoscale cuboidal RSA microstructures during cooling.

The selective laser melting of high temperature alloys is of great interest to the aerospace industry as it offers the prospect of producing more complex geometries than can be achieved with other manufacturing methods. In this study, the microstructure of the nickel-based superalloy, CM247LC, has been characterised following selective laser melting and after a post deposition heat treatment below the γʹ solvus temperature. In the as-deposited state, scanning electron microscopy with electron backscatter diffraction revealed a fine, cellular microstructure with preferential alignment of 〈001〉 along the build direction. A high dislocation density was seen at the periphery of the cells, indicating substantial localised deformation of the material. Fine primary MC carbides were also observed in the inter-cellular regions. High-resolution transmission electron microscopy identified the occurrence of very fine γʹ precipitates, approximately 5 nm in diameter, dispersed within the gamma phase. After heat treatment, the elongated cell colonies were observed to partially coalesce, accompanied by a decrease in dislocation density, producing columnar grains along the build direction. Cuboidal γʹ precipitates approximately 500 nm in diameter were observed to form in the recrystallised grains, accompanied by larger γʹ precipitates on the grain boundaries.

The almost limitless variations in potential compositions of high entropy alloys necessitates the use of computational methods when attempting to optimise for any given application. However, the accuracy of the current thermodynamic approaches commonly being used for this purpose remains under debate, as relatively few validatory studies have been performed. Within the CrMnFeCoNi family of alloys, the formation of the σ phase and how it is influenced by compositional variations is of particular interest for elevated temperature structural applications. Here, the role of Ni on the formation of the σ phase has been studied through a systematic series of CrMnFeCoNix alloys, 0 ≤ x ≤ 1.5, following 1000 hour exposures at temperatures typically found to promote σ formation. Ni was found to have a significant effect on the phase stability of these alloys, suppressing the σ phase such that a single solid solution phase was the only stable phase in the CrMnFeCoNi1.5 alloy, whilst the CrMnFeCo alloy formed the σ phase during solidification. The corresponding thermodynamic predictions varied dramatically from the experimentally observed microstructures, indicating that the underlying databases require further optimisation. Interestingly, it was found that a relatively simple electronic structure based approach, New PhaComp, provided much more accurate predictions of the observed σ phase formation in the CrMnFeCoNix and CrMnxFeCoNi systems and could be manipulated to obtain σ formation temperatures. As such, this method could be extremely useful to those wanting to design CrMnFeCoNi high entropy alloys that are free from the σ phase.

The data presented in this article are related to the research article entitled "a new beta titanium alloy system reinforced with superlattice intermetallic precipitates" (Knowles et al., 2018) [1]. This includes data from the as-cast alloy obtained using scanning electron microscopy (SEM) and x-ray diffraction (XRD) as well as SEM data in the solution heat treated condition. Transmission electron microscopy (TEM) selected area diffraction patterns (SADPs) are included from the alloy in the solution heat treated condition, as well as the aged condition that contained < 100 nm B2 TiFe precipitates [1], the latter of which was found to exhibit double diffraction owing to the precipitate and matrix channels being of a similar width to the foil thickness (Williams and Carter, 2009) [2]. Further details are provided on the macroscopic compression testing of small scale cylinders. Of the micropillar deformation experiment performed in [1], SEM micrographs of focused ion beam (FIB) prepared 2 µm micropillars are presented alongside those obtained at the end of the in-situ SEM deformation as well as videos of the in-situ deformation. Further, a table is included that lists the Schmidt factors of all the possible slip systems given the crystal orientations and loading axis of the deformed micropillars in the solution heat treated and aged conditions.

A new computational tool has been developed to model, discover, and optimize new alloys that simultaneously satisfy up to eleven physical criteria. An artificial neural network is trained from pre-existing materials data that enables the prediction of individual material properties both as a function of composition and heat treatment routine, which allows it to optimize the material properties to search for the material with properties most likely to exceed a target criteria. We design a new polycrystalline nickel-base superalloy with the optimal combination of cost, density, γ′ phase content and solvus, phase stability, fatigue life, yield stress, ultimate tensile strength, stress rupture, oxidation resistance, and tensile elongation. Experimental data demonstrates that the proposed alloy fulfills the computational predictions, possessing multiple physical properties, particularly oxidation resistance and yield stress, that exceed existing commercially available alloys. Keywords: Neural network, Materials design, Nickel-base superalloy

An artificial intelligence tool is exploited to discover and characterize a new molybdenum-base alloy that is the most likely to simultaneously satisfy targets of cost, phase stability, precipitate content, yield stress, and hardness. Experimental testing demonstrates that the proposed alloy fulfills the computational predictions, and furthermore the physical properties exceed those of other commercially available Mo-base alloys for forging-die applications.

The microstructural stability of nickel-based superalloys is critical for maintaining alloy performance during service in gas turbine engines. In this study, the precipitate evolution in a model polycrystalline Ni-based superalloy during aging to 1000 hours has been studied via transmission electron microscopy, atom probe tomography and neutron diffraction. Variations in phase composition and precipitate morphology, size and volume fraction were observed during aging, whilst the constrained lattice misfit remained constant at approximately zero. The experimental composition of the γ matrix phase was consistent with thermodynamic equilibrium predictions, whilst significant differences were identified between the experimental and predicted results from the γʹ phase. These results have implications for the evolution of mechanical properties in service and their prediction using modeling methods.

Recent research has identified compositions of nickel-based superalloys with microstructures containing appreciable and comparable volume fractions of γ′ and γ″ precipitates. In this work, an alloy capable of forming such a dual-superlattice microstructure was subjected to a range of thermal exposures between 873 and 1173 K (600 and 900 ˚C) for durations of 1 to 1000 hours. The microstructures and nature of the precipitating phases were characterised using synchrotron X-ray diffraction and electron microscopy. These data have enabled the construction of a T-T-T diagram for the precipitating phases. Hardness measurements following each thermal exposure have identified the age-hardening behaviour of this alloy and allowed preliminary mechanical properties to be assessed.

The influence of sulfur contamination on the corrosion-fatigue behavior of a polycrystalline superalloy used in aero-engines is considered. Samples tested under a variety of environmental conditions (including exposures to air, SO x gas, and salt) are characterized through a suite of high-resolution characterization methods, including transmission electron microscopy (TEM), secondary ion mass spectroscopy (nanoSIMS), and atom probe tomography (APT). The primary effect of sulfur contamination is to accelerate the crack growth rate by altering the failure mechanism. The SIMS and TEM analyses indicate Cr-Ti sulfide particle formation at grain boundaries ahead of and around oxidized cracks. The APT analysis suggests that these particles then oxidize as the crack propagates and are enveloped in chromia. The chromia is surrounded by a continuous layer of alumina within the cracks. All of the sulfur detected was confined within the particles, with no elemental segregation found at grain boundaries.

Improving the efficiency of gas turbine engines requires the development of new materials capable of operating at higher temperatures and stresses. Here, we report on a new polycrystalline nickel-base superalloy that has exceptional strength and thermal stability. These properties have been achieved through a four-element composition that can form both gamma prime and gamma double prime precipitates in comparable volume fractions, creating an unusual dual-superlattice microstructure. Alloying studies have shown that further property improvements can be achieved, and that with development such alloys may be suitable for future engine applications.

The oxidation response and microstructural evolution of an Inconel 625 alloy exhaust manifold exposed to an automobile racing environment has been examined using a range of advanced electron microscopy-based techniques, atom probe tomography and high-sensitivity laser ablation mass spectrometry. The dynamic, corrosive gas conditions result in accelerated oxidation, with the inner exhaust surface also heavily contaminated by multiple species including Zn, P, K and Na. Nb carbides and Ti nitrides identified in stock control samples evolve into mixed (Ti, Nb)N species during exposure, decorated by smaller Mo, Si-rich precipitates. The exposed alloy component therefore reveals unique surface and subsurface features following in-service use.

Alloys with fine-scale eutectic microstructures comprising Ti-based A2 and TiFe B2 phases have been shown to have excellent mechanical properties. In this study, the potential of alloys with further refined A2-B2 microstructures formed through solid-state precipitation has been explored by analyzing a series of six alloys within the Fe-Mo-Ti ternary system. Partial isothermal sections of this system at 1173 K (900 °C) and 1023 K (750 °C) were constructed, from which the ternary solubility limits of the A2 (Ti, Mo), B2 TiFe, D8$_5$ Fe$_7$Mo$_6$ , and C14 Fe$_2$Ti phases were determined. With these data, the change in solubility of Fe in the A2 phase with temperature, which provides the driving force for precipitation of B2 TiFe, was determined and used to predict the maximum potential volume fraction of B2 TiFe precipitates that may be formed in an A2 (Ti, Mo) matrix.

Atom probe tomography has been used to characterise the effect of varying Ni:Co ratio on elemental phase partitioning at 800 °C in γ-γ′ alloys derived from the Ni-Co-Al-Ti-Cr system. In all alloys tested, Al and Ti were found to partition preferentially to the γ′ phase, whereas Co and Cr partitioned preferentially to the γ phase. However, above a critical Co content (~19 at.%), the extent of partitioning of Al and Ti to the γ′ phase reduced. Conversely, Cr partitioned more strongly to the γ phase with Co additions of up to ~19 at.%, above which this preferential segregation was less pronounced. This non-monotonic trend of elemental partitioning behaviour with increasing Co concentration was attributed to a transition in the chemistry of the L1$_2$ γ′ phase from Ni$_3$(Ti, Al) to (Ni, Co)$_3$(Ti, Al) and thus to a change in its solute solubility.

Cupronickel alloys, reinforced by L1$_2$ precipitates, offer a high strength, corrosion resistant and anti-bio fouling material for marine engineering applications. These alloys exhibit complex precipitate nucleation and growth mechanisms that must be fully understood in order to optimise the mechanical properties. In this work, the microstructural characteristics of the Cu-Ni-Al based alloy, Hiduron$^®$ 130, have been assessed in the as-received condition and following 100-h thermal exposures at 500, 600, 700 and 900 °C and after a two-stage exposure at 900 °C for 100 h followed by 500 °C for 100 h. The L1$_2$ phase was observed to precipitate both discontinuously and continuously and its subsequent coarsening was characterised alongside measurements of the lattice misfit. The hardness of the alloy was found to decrease with increasing exposure temperature up to the L1$_2$ solvus and correlated with the evolution of the L1$_2$ precipitates, the alloy grain size and the fraction of the discontinuously formed L1$_2$ phase.

© 2016 Acta Materialia Inc. Cu 3 Au is often cited as a case example of a metallic system exhibiting both short-range order in the solid solution phase and a long-range order-disorder transition. In this work, X-ray total scattering data obtained from the in situ heating of a gas-atomised powder sample of Cu 3 Au are used to demonstrate the suitability of total scattering, in conjunction with large-box modelling, for the analysis of short-range order in alloys. The existence of an ordering transition at c. 400° is confirmed, and the development of short-range order reminiscent of the L1 2 long-range ordered structure is observed prior to this transition. Furthermore, it is found that a degree of short-range order is present even in quenched samples (usually assumed to be completely random) which throws into question the identification of short-range order in previous ex situ studies. It is demonstrated that total scattering can be used successfully to identify the type and degree of ordering, differences in the bond length distributions in the first coordination shell and to suggest a likely mechanism for the formation of order in the system.

In spite of its influence on a number of physical properties, short-range order in crystalline alloys has received little recent attention, largely due to the complexity of the experimental methods involved. In this work, a novel approach that could be used for the analysis of ordering transitions and short-range order in crystalline alloys using total scattering and reverse Monte Carlo (RMC) refinements is presented. Calculated pair distribution functions representative of different types of short-range order are used to illustrate the level of information contained within these experimentally accessible functions and the insight into ordering which may be obtained using this new method. Key considerations in the acquisition of data of sufficient quality for successful analysis are also discussed. It is shown that the atomistic models obtained from RMC refinements may be analysed to identify directly the Clapp configurations that are present. It is further shown how these configurations can be enhanced compared with a random structure, and how their degradation pathways and the distribution of Warren-Cowley parameters, can then be used to obtain a detailed, quantitative structural description of the short-range order occurring in crystalline alloys.

The phase evolution of an Al₀.₅CrFeCoNiCu High Entropy Alloy has been characterised following isothermal exposures between 0.1 and 1000 hours at temperatures of 700, 800 and 900˚C. The NiAl based B2 phase formed extremely quickly, within 0.1 hours at the higher exposure temperatures, whilst the Cr-rich σ phase formed more slowly. The solvus temperatures of these two phases were found to be ~ 975 and ~ 875˚C respectively. Compilation of the data presented here with results previously reported in the literature enabled the production of a time-temperature-transformation diagram, which clearly indicates that the diffusion kinetics of this material cannot be considered sluggish.

The effects of Ni:Co and Al:Ti ratios on the room temperature microstructure, hardness and lattice parameter of twenty-seven quinary Ni-Co-Al-Ti-Cr alloys have been evaluated. All of the alloys exhibited a uniform $\gamma$-$\gamma ^\prime$ microstructure. Differential scanning calorimetry (DSC) showed that the liquidus and solidus temperatures of the alloys increased with greater Al:Ti ratios, decreased with Cr concentration and remained largely unchanged with respect to the Ni:Co ratio. Neutron diffraction measurements of the $\gamma$ and $\gamma ^\prime$ lattice parameters revealed that the lattice misfit in all of the alloys was positive and increased with Ti concentration (i.e. lower Al:Ti ratio) regardless of the concentration of Cr, or the ratio of Ni:Co. Importantly, alloys with a Ni:Co ratio of 1:1, were found to have consistently greater lattice misfits than alloys with Ni:Co ratios of either 1:3 or 3:1. The measured lattice misfits were found to be strongly correlated with the Vickers hardness of the alloys, suggesting that lattice misfit plays a key role in the strengthening of $\gamma$-$\gamma ^\prime$ alloys of this type.

A detailed analysis of the response of as-cast third-generation single-crystal nickel-based superalloy CMSX-10K® to solution heat treatment (SHT) has been carried out, alongside an SHT optimization exercise. The analysis was conducted through microstructural characterization, differential scanning calorimetry, and compositional homogeneity measurements, quantifying (i) the dissolution and microstructural evolution of the inter-dendritic constituents, (ii) the shift in thermo-physical characteristics of the material, and (iii) the change in compositional homogeneity across the microstructure, in order to gain further understanding of these phenomena during the progression of the SHT. During the early stages of SHT, the coarse cellular γ′/narrow γ channel inter-dendritic constituents which were the last areas to solidify during casting, progressively dissolve; homogenization between these inter-dendritic areas and adjacent dendritic areas leads to a rapid increase in the incipient melting temperature T IM. The fine γ/γ′ morphology which were the first inter-dendritic constituents to solidify after primary γ dendrite solidification were found to progressively coarsen; however, subsequent dissolution of these coarsened γ/γ′ inter-dendritic areas did not result in significant increases in the T IM until the near-complete dissolution of these inter-dendritic areas. After the final SHT step, residual compositional micro-segregation could still be detected across the microstructure despite the near-complete dissolution of these remnant inter-dendritic areas; even so the T IM of the material approached the solidus temperature of the alloy.

An isothermal section of the Fe-Mo-Ti ternary system at 1000 °C has been constructed using data acquired from a series of seven alloys. The limit of solubility of Fe in the continuous A2 phase field between Ti and Mo has been determined, as have the extents to which Mo may be accommodated in the B2 TiFe phase, and Ti in the D8$_5$ Fe$_7$Mo$_6$ phase. The B2, D8$_5$ and C14 Fe$_2$ (Ti, Mo) intermetallics were found to have limited tolerance for non-stoichiometric compositions. The positions of the A2 + B2 + C14 and A2 + C14 + D8$_5$ three-phase fields were determined, along with the extents of the A2 + B2, A2 + D8$_5$, A2 + C14, C14 + B2 and C14 + D8$_5$ two-phase fields. No ternary phases were observed in any of the alloys studied.

The next generation of aerospace gas turbine engines need to operate at higher temperatures and stresses to improve their efficiency and reduce emissions. These operating conditions are beyond the capability of existing nickel-base superalloys requiring the development of new high temperature materials. Controlling the microstructures of these new materials is key to obtaining the required properties and therefore, it is critical to understand how these alloys respond to processing and heat treatment. Here, the microstructural evolution of V207M, a new δ containing, nickel-base superalloy, has been investigated following heat treatment and forging. The solvus temperatures of the γ′ and δ phases, determined by differential scanning calorimetry and microscopy, were found to be ~ 985 and ~ 1060 ˚C respectively. Isothermal forging of the alloy was conducted at 1000, 1050 and 1100 ˚C, corresponding to different volume fractions of retained δ. Considerable softening was observed prior to steady state flow when forging at 1000 ˚C, whilst only steady state flow occurred at 1050 and 1100 ˚C. The steady state flow process was believed to be dominated by dynamic recovery in the γ phase, with an activation energy of 407 kJ.mol-1. Samples that exhibited flow softening also showed a significant change in the orientation of the δ precipitates, preferentially aligning normal to the forging axis, and this reorientation was thought to be the cause of the observed flow softening.

High angle annular dark field (HAADF) images of the omega phase in metastable beta titanium alloys exhibit tri-layered periodicity. However, it is unclear if this indicates preferential site occupation, or is related to the structural modification of omega formation. Here, the periodicity was studied using a combination of HAADF imaging and electron energy loss spectroscopy. The results show that there is no preferential site occupancy or ordering and that the observed intensity variations are related to the imaging conditions.

The effect of cooling rate on the microstructural evolution of Al0.5CrFeCoNiCu has been studied using differential scanning calorimetry and scanning electron microscopy. As-cast Al0.5CrFeCoNiCu contained three phases; Cr-Fe-Co-Ni solid solution dendrites, Cu-rich interdendritic material and L12 precipitates. During cooling at rates between 10 and 50˚C.min-1 , an additional exothermic event, at ~1010˚C, was observed in the heat flow curves. Microstructural examination after cooling revealed the presence of two distinct populations of intragranular precipitates not present in the as-cast material. Energy dispersive X-ray spectroscopy indicated that Cu-rich precipitates formed within the dendrites, whilst a Cr-Fe-Co rich phase formed in the interdendritic constituent. Precipitation during cooling at rates approaching 1˚C.s-1 indicates that the diffusion kinetics of Al0.5CrFeCoNiCu are not, as previously suggested, sluggish.

The high-entropy alloy Al0:5CrFeCoNiCu has been shown to consist of two stable, face-centred cubic solid solutions at temperatures approaching its solidus; one rich in Cr, Fe, Co & Ni (dendritic) and the other rich in Cu (interdendritic). Whilst some studies have suggested that the high-temperature microstructure may be metastably retained to room temperature through rapid cooling, evidence of phase decomposition has also been reported. In this study, fine-scale precipitation has been observed in samples of Al0:5CrFeCoNiCu that have been rapidly cooled after casting, and water quenched following ageing for 1000 h at 1000°C. Contrary to previous reports, in the as-cast state, the two face- centred cubic phases, as well as an L12 phase, were found in both dendritic and interdendritic areas, with the dendritic areas having undergone a spinodal decomposition. After ageing and quenching, L12 precipitates were found in both dendritic and interdendritic areas, and precipitates of the Cr-, Fe-, Co- and Ni-enriched face-centred cubic phase were found in the Cu-rich interdendritic regions. Given the nature of the heat treatments applied, the results suggest that precipitation in the alloy is rapid and cannot be avoided, even when the material is cooled quickly to room temperature.

Discontinuous precipitation of chromium-rich Co3V lamellae has been found in a Co-based alloy containing 2 wt% V after prolonged ageing at 800 {\degr}C. This discontinuous precipitation is associated with a noticeable redistribution of alloying elements in the alloy relative to those parts of the aged alloy that preserve the c.c.p.-L12 microstructure found in the as-cast and homogenized condition. The orientation relationship between the c.c.p. Co-rich matrix and these hexagonal phase chromium-rich Co3V precipitates is shown to be || and [1 1 1]Co || , i.e. || and [1 1 1]Co || in the four-index notation. 3 × 3 transformation matrices relating directions and planes in the two phases have been established. The observed orientation relationship between the two phases is consistent with low lattice misfit between the two phases.

The extent to which configurational entropy can stabilise a single solid solution in an Al0.5CrFeCoNiCu high entropy alloy has been assessed through characteristion of samples following casting and heat treatment at 1000 C. At temperatures between 1000 C and the onset of melting, the alloy was shown to be within a two phase field and these phases were stable following prolonged exposure at elevated temperature. X-ray and transmission electron diffraction indicated that both constituent phases had an fcc structure. Therefore, these phases share a Gibbs energy curve that must contain two local minima at the solidus temperature, rather than the single minimum required for a continuous solid solution. These observations indicate that there is no temperature at which this material is in a stable, solid state single phase field and that therefore, the configurational complexity is insufficient to stabilise a solid solution phase against enthalpic effects.

The influence of Al:Nb ratio on the microstructure and properties of Ni–Cr–Al–Nb alloys has been investigated following long-term exposure at elevated temperatures. The γ′ volume fraction, size and lattice misfit were seen to increase with a larger Al:Nb ratio, although these changes resulted in reduced hardness. The change in the critical resolved shear stress (CRSS) associated with strong dislocation coupling was determined to be the dominant strengthening mechanism and increased with decreasing Al:Nb ratio. A distribution of tertiary γ′ was observed to be necessary in maximising the mechanical properties of these alloys.

The phase equilibria of an Al0.5CrFeCoNiCu High Entropy Alloy has been studied following 1000 h exposures at 700, 850 and 1000 °C. Above1000 °C, the material comprised of two fcc solid solutions, one a multi-element phase and the other a Cu rich phase. Below 1000 °C, the fcc phases persisted, but were accompanied by the formation of two intermetallic compounds. In contrast to previous reports, the L12 phase was also found to precipitate through a solvus at ~850 °C. The results indicated that a solid state single phase field does not exist in this material at any temperature and all of the observed phases could be rationalised with reference to existing phase diagrams. This suggests that configurational entropy does not overcome the enthalpic contribution to the Gibbs energy, which governs phase equilibria of this alloy.

An assessment is made of the Wagner transition criteria for predicting the formation of a continuous Al2O3 scale in Ni-based superalloys. Predictions are compared with data from an experimental Ni-based superalloy as well as commercial superalloys for which published data are available. The methodology was generally successful in predicting the transition temperature of the commercial superalloys but underpredicted the transition temperature of the experimental superalloy by approximately 50–100 °C. The difference in the transition temperature of the experimental superalloy to form a continuous Al2O3 scale is primarily attributed to a complex oxide subscale that increased the internal volume fraction of oxide and led to reduced oxygen ingress. The sensitivity and limitations of the methodology are discussed, and recommendations are made to refine the methodology to facilitate the interpretation of oxidation behaviour in polycrystalline Ni-based superalloys.