Refractory Superalloys Research Articles

Microstructure and Phase Equilibria in BCC-B2 Nb-Ti-Ru Refractory Superalloys.

Refractory superalloys (RSAs) are promising candidates for high-temperature, high-strength applications. Two-phase RSAs containing body-centered cubic (BCC) and ordered B2 phases are among the more promising candidates. Systems containing Ru-based B2 precipitates exhibit stable two-phase microstructures at temperatures in excess of 1600 °C. The present study experimentally investigated one potential foundational ternary system for these alloys, Nb-Ti-Ru. Two alloys, (Nb3Ti)0.85Ru0.15 and (Nb4Ti)0.85Ru0.15, were studied to determine phase equilibria and properties at temperatures between 900 °C and 1300 °C. The B2 phase was found to be dominated by RuTi ordering, although considerable Nb solubility was observed up to 18 mol %. The Nb-rich BCC matrix contained up to 15 mol % Ru and 20 mol % Ti. Although a two-phase microstructure of B2 precipitates in a BCC matrix was confirmed, the distribution of elements in the two phases resulted in a larger lattice misfit than expected. The results obtained in this investigation provide valuable information for the future development of RSAs utilizing Ru-based B2 strengthening precipitates.

Mechanical properties of an Al10Nb20Ta15Ti30V5Zr20 A2/B2 refractory superalloy and its constituent phases

Refractory superalloys (RSAs) are a new class of high-temperature structural materials consisting of a nanometer-sized mixture of two coherent A2 (disordered BCC) and B2 (ordered) phases. While these RSAs typically exhibit superior mechanical properties at elevated temperatures, they suffer from limited plasticity at lower temperatures, often attributed to the continuous ordered B2 matrix in these two-phase alloys. To knowledgably control the microstructure and mechanical properties of these alloys, the properties of the constitutive phases in RSAs should be known. Currently this information is absent. In the present work, the microstructure and mechanical properties (at 20–1200 °C) of a candidate Al0.5NbTa0.8Ti1.5V0.2Zr RSA and two single-phase alloys, which compositions correspond to the compositions of the A2 and B2 phases in the selected RSA, are reported. Interestingly, and quite unexpectedly, the intrinsic mechanical behavior of the constituent A2 and B2 phases, deciphered for the first time in the present study, revealed that they are softer and substantially more ductile than the parent two-phase RSA. Furthermore, the ordered B2 phase exhibits a deformation twinning mechanism leading to twinning induced plasticity (TWIP) coupled with a low APB energy and activity of 1/2<111> dislocations. These mechanisms are thought to be responsible for the high plasticity of the ordered B2 phase. The results presented here question the current assumption that the ordered B2 phase in these RSAs has limited plasticity and strongly indicate that the nature of the B2/A2 interface boundaries and coherency strengthening play an important role in controlling the mechanical properties of the two-phase RSA.

Microsegregation and homogenization behavior of a Ni-Co based refractory superalloy for turbine discs

Microsegregation and homogenization behavior of a Ni-Co based refractory superalloy for turbine discs

On the rate of microstructural degradation of Al-Ta-Ti-Zr refractory metal high entropy superalloys

Refractory metal high entropy superalloys (RSA) are being developed for high temperature structural applications. However, recently the microstructures of several RSA have been shown to be unstable, precipitating potentially deleterious intermetallic phases during prolonged exposure at elevated temperatures. For such instability to be acceptable it is necessary for the intermetallic phase formation to be sufficiently sluggish so as to not compromise alloy performance over the time scales likely to be experienced in service. However, the rate of intermetallic phase formation in RSA are not yet known. To address this issue, here, two quaternary Al-Ta-Ti-Zr refractory superalloys that contain all of the key microstructural features of more compositionally complex RSA have been studied following exposures of 1, 10 and 100 h at temperatures between 700 and 900 °C and compared to those previously obtained following 1000 h exposures. Critically, needles of an Al-Zr-rich intermetallic phase were observed following only 1 h exposure at 900 °C, demonstrating rapid formation kinetics. Furthermore, the microstructures continued to coarsen from 100 to 1000 h, indicating low morphological stability. Higher Al content increased both the prevalence of the Al-Zr-rich intermetallic phase and raised the solvus of the desirable B2 phase. Consequently, this work highlights additional challenges in RSA development for elevated temperature applications.

The AlMo0.5NbTa0.5TiZr refractory high entropy superalloy: Experimental findings and comparison with calculations using the CALPHAD method

Detailed microstructural characterization of the AlMo0.5NbTa0.5TiZr refractory high entropy superalloy in the as-cast state is reported for first time and compared with the state annealed at 1400 °C for 24 h. The former shows a dendritic structure, with a mixture of A2/B2 phases < 20 nm in both the dendritic and interdendritic regions. A mostly amorphous phase, rich in Al and Zr, is found within the interdendritic region. The annealed state reproduced the combination of A2/B2/Al-Zr-rich phases reported previously. Calculations from two relevant ThermoCalc databases were compared with the experimental results. Equilibrium calculations were compared with results for the annealed alloy, whereas solidification paths calculated using Scheil-Gulliver model were used for comparison with the as-cast alloy. A previously hypothesized spinodal decomposition during cooling as the mechanism responsible for the patterned A2/B2 microstructure is confirmed via the CALPHAD calculations, pointing to its use as an efficient design tool for such alloys. Finally, the comparison between the experimental and computational findings allowed better understanding the solidification path and equilibrium stability of this alloy, giving a base to make better decisions on the field of new refractory superalloy design.

The Almo0.5nbta0.5tizr Refractory High Entropy Superalloy: Experimental Findings and Comparison with Calculations Using the Calphad Method

The Almo0.5nbta0.5tizr Refractory High Entropy Superalloy: Experimental Findings and Comparison with Calculations Using the Calphad Method

Microstructural aspects of the protective ceramic coatings applied on the surfaces of refractory alloys produced by additive manufacturing

The possibility of depositing multi layers made of metals/alloys and high temperature ceramics by electron beam physical evaporation process under high vacuum (EB-PVD) on the surface of a parallelepiped sample made by selective laser melting (SLM) from a Ni base refractory super alloy was experimentally tested. The SEM-EDAX micro structural analysis revealed the morphology and thickness of the coatings consisting of a NiCrAlY base alloy as bonding layer and three successive YSZ, LZO and GZO ceramic layers on the Ni-based super alloy substrate obtained by additive manufacturing. The adhesion of the layer deposited was evaluated by the scratch test method. The analysis highlighted the importance of pre-preparing the surface of the additive manufactured substrates, in order to control the adhesion and uniformity of the deposited layers.

Elastic, Electronic and Vibrational Properties of Ir-based Refractory Superalloys

The mechanical, electronic and vibrational properties of Ir-based refractory superalloys (Ir3Hf and Ir3Nb) in the L12 structure were studied in the framework of ab initio calculations. The obtained equilibrium lattice constants and bulk modulus were reported and compared with the existing data. The elastic constants of alloys were determined using energy strain method. The results were utilised to evaluate mechanical stability of alloys in the crystal structure of L12. Both alloys were found to be mechanically stable based on the Pugh’s criteria. Subsequently, electronic band structures and partial and total densities of states have been obtained for Ir3Hf and Ir3Nb. The band structures of alloys demonstrated metallic behaviour whilst the conductivity was mostly governed by Ir 5d states. Moreover, phonon distribution curves of both alloys were obtained by employing the linear response technique within the density functional theory. Both alloys are found to be dynamically stable based on phonon modes evaluation.

Spheroidal chip in micromilling

In a manufacturing process, studying the removed material called chip, swarf or debris, is the key to understand the process. In micro cutting process, the size effect is a dominant factor for material removal mechanism and chip generation physics. There are different types of chips but a particular type, stands out from the others due to its specificities: the spheroidal chip. It is often found in grinding and tribological tests. This type of chip has no specific classification and there are no reports of it being found in milling or micro milling research. This paper makes a review of this not so well known type of chip, to classify it by its characteristics and to present some examples of spheroidal chips obtained from micro milling operation. Spheroidal chips were obtained when micro milling slots using a 381 µm diameter TiN coated WC tool. The workpiece material was Inconel 718, a high strength refractory superalloy. SEM images were obtained to analyze and classify the chips and observe the machined surface of the slots. In the proposed classification of spherical chip, there are eight major types: Fatigue Sphere, Layer-like Sphere, Thin Plate Sphere, Dendritic-like Sphere, Partially Formed Sphere, Tadpole, Melted sphere and Hollow sphere. The thin chip is submitted to high energy, which is one of the main aspects of size effect that occurs in micro milling. Inconel 718 has low thermal conductivity and high oxidation rates, which makes it an ideal environment for the formation of spherical chip. The spheres formed can be classified as Dendritic Spheres, defined by extremely high rates of oxidation in a thin chip which leads to high heat energy causing its spherical form and dendritic microstructure.

Creep properties of a powder metallurgy disk superalloy at 700 °C

Abstract

Simulation domain size requirements for elastic response of 3D polycrystalline materials

A fast Fourier transform (FFT) based spectral algorithm is used to compute the full field mechanical response of polycrystalline microstructures. The field distributions in a specific region are used to determine the sensitivity of the method to the number of surrounding grains through quantification of the divergence of the field values from the largest simulation domain, as successively smaller surrounding volumes are included in the simulation. The analysis considers a mapped 3D structure where the location of interest is taken to be a particular pair of surface grains that enclose a small fatigue crack, and synthetically created statistically representative microstructures to further investigate the effect of anisotropy, loading condition, loading direction, and texture. The synthetic structures are generated via DREAM3D and the measured material is a cyclically loaded, Ni-based, low solvus high refractory (LSHR) superalloy that was characterized via 3D high energy x-ray diffraction microscopy (HEDM). Point-wise comparison of distributions in the grain pairs shows that, in order to obtain a Pearson correlation coefficient larger than 99%, the domain must extend to at least the third nearest neighbor. For an elastic FFT calculation, the stress–strain distributions are not sensitive to the shape of the domain. The main result is that convergence can be specified in terms of the number of grains surrounding a region of interest.

Grain size effects in a Ni-based turbine disc alloy in the time and cycle dependent crack growth regimes

The fatigue crack growth (FCG) behaviour in a Ni-based turbine disc alloy with two grain sized variants, in a low solvus high refractory (LSHR) superalloy has been investigated under a range of temperatures (650–725°C) and environments (air and vacuum) with trapezoidal waveforms of 1:1:1:1 and 1:20:1:1 durations at an R=0.1. The results indicate that a coarse grained structure possesses better FCG resistance due to the enhanced slip reversibility promoted by planar slip as well as the reduction in grain boundary area. The fatigue performance of the LSHR superalloy is significantly degraded by the synergistic oxidation effect brought about by high temperature, oxidising environment and dwell at the peak load, associated with increasingly intergranular fracture features and secondary grain boundary cracking. Secondary cracks are observed to be blocked or deflected around primary γ′, carbides and borides, and their occurrence closely relates to the roughness of the fracture surface, FCG rate and grain boundary oxidation. The apparent activation energy technique provides a further insight into the underlying mechanism of the FCG under oxidation–creep–fatigue testing conditions, and confirms that oxidation fatigue is the dominant process contributing to the intergranular failure process. At high enough crack growth rates, at lower temperatures, cycle dependent crack growth processes can outstrip crack-tip oxidation processes.

Comparison of fatigue crack propagation behaviour in two gas turbine disc alloys under creep–fatigue conditions: evaluating microstructure, environment and temperature effects

Gas turbine disc materials should possess excellent fatigue and creep performance due to the severe in service conditions experienced. In this study, a comparison of fatigue crack propagation behaviour in two turbine disc alloys, i.e. N18 and low solvus high refractory (LSHR) superalloy, has been made in terms of the propagation rate and fractography observed under equivalent testing conditions. Temperatures of 650 and 725°C are compared for a trapezoidal dwell fatigue cycle (1–20–1–1) in both air and vacuum at an R ratio of 0·1. It is found that coarse grained LSHR superalloy has better fatigue crack propagation resistance than fine grained N18 in vacuum, which is ascribed to its better creep performance. Oxidation causes significant degradation of fatigue performance of these two alloys, especially in the LSHR superalloy at higher temperature (725°C), resulting in its inferior fatigue performance compared with N18. In the LSHR superalloy, it seems that oxidation is the principal contributor to the deterioration of fatigue resistance. This is supported by observations of transgranular fracture in vacuum and intergranular fracture in air. In contrast, creep is a greater contributor to the deterioration in fatigue performance of N18 (as indicated by the intergranular failure modes observed in vacuum). An apparent activation energy analysis is able to provide further insight into the underlying mechanisms of fatigue crack propagation under creep–oxidation–fatigue conditions in these two alloys.

Ir–Hf–Zr ternary refractory superalloys for ultra-high temperatures—Phase and microstructural constitution

A phase and microstructural evaluation of Ir–Hf–Zr ternary alloys with a composition below 30 mol% (Hf + Zr) was conducted by microstructural observation using scanning electron microscopy (SEM), composition analysis using electron probe microscopy analysis (EPMA), and phase identification using X-ray diffraction analysis (XRD). Partial isothermal sections of the Ir–Hf–Zr ternary system close to the Ir corner at 1800 °C and 2000 °C were determined. Research revealed that the f.c.c. and L12–Ir3(Hf, Zr) two-phase regions, which are shown in the Ir–Hf and Ir–Zr binary systems, were connected from the Ir–Hf side to Ir–Zr side in the Ir–Hf–Zr ternary system at Hf + Zr contents of less than 25 mol%. The L12–Ir3Hf and L12–Ir3Zr phases were fully soluble with each other. An Ir3(Hf, Zr)/Ir(Hf, Zr) two-phase structure was found in the Ir–15Hf–15Zr alloy with the Hf + Zr contents of 30 mol%. The potential of the Ir–Hf–Zr ternary alloys as ultra-high-temperature structural materials is discussed from the viewpoints of the microstructure and the lattice misfit between the f.c.c. and the L12 phases.

Influence of Ru on the Microstructure of Ni-Base Single Crystal Superalloys

Addition of Ru in Ni-base single crystal superalloys had been used to improve the elevated temperature strength and other multiple properties. Significant decreases in stacking fault energy of the γ phase and the volume fraction of γ′ phase are observed with the addition of Ru. As well as serving as an effective solid-solution strengthening element in high refractory content Ni-base single crystal superalloys, Ru additions are able to effectively strengthen both the γ and γ′ phases and suppress the formation of TCP phases. Due to the changes in the partitioning behavior of elements and the slight decrease in the supersaturation of refractory elements in γ phase associated with Ru additions, high temperature creep resistance and the microstructural stability of the alloy are improved remarkable. The influence of Ru on the microstructure of Ni-base single crystal superalloys is reviewed.

Combined ab initio/CALPHAD modeling of fcc-based phase-equilibria in the Ir–Nb system

A preliminary thermodynamic assessment of the Ir–Nb system, one of the key binary systems of the Ir-based refractory superalloys, has been performed by combining ab initio calculations and the CALPHAD (CALculation of PHAse Diagrams) technique. The ground-state formation enthalpies have been calculated by the full-potential linearized augmented plane wave method. The free energies at finite temperatures have been estimated using the cluster variation method, where the effective cluster interaction energies have been extracted from the formation enthalpies by the cluster expansion method. The liquid and A1 phases are modeled as substitutional solutions. The L1 0 and L1 2 phases are described using the four-sublattice model with the formula (Ir,Nb) 1/4(Ir,Nb) 1/4(Ir,Nb) 1/4(Ir,Nb) 1/4, while other solid phases are not considered in the present assessment. The obtained parameter set reproduces well the characteristic features of the experimental phase diagram and thermodynamic quantities.

Ir–Nb–Si ternary refractory superalloys with a three-phase fcc/L1 2/silicide structure for high-temperature applications

Ir–Nb binary alloys doped with silicon have been used in this work to attain a three-phase fcc/L1 2/silicide structure. Typical Ir–Nb binary alloys, including a hypoeutectic Ir–10Nb, an eutectic Ir–16Nb, and a hypereutectic Ir–25Nb, were used as alloy bases, and Ir was further replaced by 5 at% Si. With the addition of Si, the microstructures of the Ir–(10–25)Nb–5Si ternary alloys contained three phases: fcc, L1 2, and compounds of Ir and Si (referred to silicide hereafter). Compressive tests from room temperature to 1500 °C showed that the Ir–10Nb–5Si alloy, with a predominant fcc microstructure, always had the highest deformation hardening rate, strength, and ductility; on the other hand, the Ir–25Nb–5Si alloy showed the worst performance. With the silicide in the microstructures, the damage sustained by the Ir–Nb–Si alloys at both room and high temperatures was dominated by interface debonding, which occurred between the fcc and the silicide or the L1 2 and the silicide. It is believed that the interface debonding is an instinct failure mechanism of Ir-based alloys. Additionally, a strong solid-solution hardening effect of Si acting on the fcc phase was found to occur without loss of ductility. A principle in the composition and microstructure design is proposed in this paper for further development of Ir-based alloys with Si addition. This principle is to saturate the fcc phase with Si and other alloying elements so as to achieve maximum solid-solution hardening and tie-in fine silicides homogenously distributed within the fcc by elimination of the grain boundary concentration of silicides.

Analyse tribo-énergétique du délaminage des couches de revêtements des outils coupants lors d'un usinage à sec des alliages aéronautiques

The applied thermo-mechanical loading during cutting process lead to an important tool failure especially when machining refractory super-alloys such as titanium and/or nickel based-alloys. Heat removal generated by tool/workpiece friction plays an important role in preserving the structural integrity of the cutting tool. Efficient heat removal in machining depends solely on the thermal properties of the tool and on their evolution with temperature and mechanical parameters (strain, strain rate, etc.). In the present work, the effect of these thermal properties on the produced wear during the machining of refractory alloys is particularly investigated. Finite Element Analysis (FEA) and SEM-imagery were used to map the thermal conductivity within the affected tool zone. The results indicate that depending on the temperature rise, the tool-tip might undergo a severe drop in thermal conduction. This drop may locally restrict the ability of the tool material to dissipate the applied thermal load. This may nucleate thermally congested clusters within the tool-tip where the material completely loses the ability to transport heat. Thermal congestion renders an energetically active zone where the thermal energy available may be used to activate wear through different mechanisms. It is also found that the immediate layer under the surface of the tool a Auteur correspondant :

Influence of Ru and Cr on the Heat-Treated Microstructure of Ni-Based Superalloys

The effects of ruthenium and chromium contents have been investigated on phase transformation temperatures and the morphology of γ’ precipitates as well as microstructural stability in high refractory Ni-base superalloys. The solidus and liquidus temperatures were determined by differential scanning calorimetry (DSC), suggesting that the addition of Cr resulted in a decrease in solidus/liquidus temperatures while the Ru addition (3.5 at.%) had the neutral effect. The morphology of γ’ precipitates in the heat-treated microstructure was changed under the influence of the Cr and Ru additions, suggesting that Ru and Cr contents affected the γ−γ’ lattice misfit through changes in the associated partitioning to the constituent phases. The microstructural instability has been investigated at 1000 oC. High levels of Cr addition (8 at.%) strongly promoted the formation of TCP phases, while Ru improved the microstructural stability to some extent.

Chemical diffusion in the iridium-richA1 andL12 phases in the Ir-Nb system

The diffusion in iridium-rich Ir-Nb alloys has been studied by single-phase interdiffusion experiments. The chemical diffusion coefficient has been measured for the primary fcc solid-solution and theL12 ordered compound Ir3Nb in the temperature range between 1650 and 1950 °C, using Ir/Ir-8Nb and Ir-26Nb/Ir-28Nb diffusion couples, respectively (numbers indicate mol%). While the chemical diffusion coefficient in the solid-solution phase is close to the tracer self-diffusion coefficient of pure iridium, the diffusion in the compound phase is extremely slow: the chemical diffusion coefficient is 1/40 to 1/50 of that in the solid solution. The low diffusion rate in the compound must be beneficial for high-temperature performance of refractory superalloys based on the Ir-Nb system.