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A wrought Ni-base superalloy, Waspaloy, with four nominal levels of P additions (0, 0.01, 0.02, and 0.04 wt pct) was studied in order to better understand how P influences the grain boundary microstructures as well as creep resistance. Results from this investigation revealed that P additions promoted the precipitation of M23C6 carbides at grain boundaries. The effect of P additions on the creep properties was assessed by both creep rupture and interrupted creep tests at 816 °C/180 MPa and 650 °C/760 MPa. Changes in the fraction and morphology of the M23C6 carbides at grain boundaries due to P additions were found to result in a notable reduction of the creep ductility when tested at 650 °C/760 MPa where the creep strain was localized along the grain boundaries. However, the additions of P and associated changes in the M23C6 carbides did not affect the creep behavior of samples tested at 816 °C/180 MPa where the creep strain was largely transgranular. This study reveals that P affects the creep resistance of the polycrystalline Ni-base superalloys by accelerating the nucleation and growth kinetics of M23C6 along the grain boundaries.

The effect of varying amounts of CoAl2O4 inoculant ranging from 0 to 2 wt pct on the microstructure evolution of Inconel 718(IN718) fabricated by selective laser melting (SLM) was evaluated. Characterization of the as-built microstructure revealed that addition of CoAl2O4 resulted in a modest degree of grain refinement with a slight increase in microstructural anisotropy. Increasing the total CoAl2O4 content beyond 0.2 wt pct resulted in severe agglomeration of the non-metallic particles and the formation of slag inclusions measuring up to 100 μm in size present in the as-built microstructure. In addition to large agglomerates, the inoculant was chemically reduced to form a fine dispersion of submicron-sized Al2O3 particles throughout the IN718 matrix. The fine dispersion of oxides significantly hindered grain recrystallization during the post-fabrication heat treatment due to a Zener pinning effect. The findings from this study indicate in order to effectively utilize CoAl2O4 as a grain refining inoculant for additive manufacturing, the process parameters need to be optimized to avoid agglomeration of the non-metallic particles and other process-related defects.

This paper reviews the status of technology in design and manufacture of new wrought polycrystalline Ni-base superalloys for critical engineering applications. There is a strong motivation to develop new alloys that are capable of operating at higher temperatures to realize improvements in thermal efficiency, which are necessary to achieve environmental targets for reduced emissions of harmful green-house gases. From the aerospace sector, the development of new powder metallurgy and ingot metallurgy alloys is discussed for disk rotor and static applications. New compositions for powder metallurgy contain about 50 to 55 pct of gamma prime (γ′) strengthening precipitates to ensure components operate successfully at temperatures up to 788 °C (1450 °F). In contrast, new compositions for ingot metallurgy aim to occupy a design space in temperature capability between Alloy 718 and current powder alloys that are in-service, and show levels of γ′ of about 30 to 44 pct. The focus in developing these alloys was design for manufacturability. To complement the aerospace developments, a review of work to understand the suitability of candidate alloys for multiple applications in Advanced-Ultra Supercritical (AUSC) power plants has been undertaken by Detrois, Jablonski, and Hawk from the National Energy Technology Laboratory. In these power plants, steam temperatures are required to reach 700 °C to 760 °C. The common thread is to develop alloys that demonstrate a combination of high-temperature properties, which are reliant on both the alloy composition and microstructure and can be produced readily at the right price. For the AUSC applications, the emphasis is on high-temperature strength, long-term creep life, phase stability, oxidation resistance, and robust welding for fabrications. Whereas for powder disk rotors in aircraft engines, the priority is enhanced resistance to time-dependent crack growth, phase stability, and resistance to environmental damage, while extending the current strength levels, which are shown by existing alloys, to higher temperatures.

The effect of P on the phase stability of a high refractory content powder-processed Ni-base superalloy was systematically studied with a variation of P addition. The microstructural evolution of nominally identical alloys with three levels of P additions (0.013, 0.026, and 0.041 wt pct) were investigated in the as-solutioned state and after thermal exposures at 800 °C for up to 1000 hours. Additions of P were found to segregate strongly at grain boundaries and depress the incipient melting temperature. The presence of P contributed to the formation of a C14 Laves as resolidification of incipiently melted liquid occurred. P additions also affected the solid-state phase stability of the alloys as aging at 800 °C led to the formation of C36 Laves phase precipitates. An increase in the fraction of C36 Laves phase correlated directly to the increases in the P content of the alloy. The extensive precipitation of C36 Laves phase eventually led to the formation of a basket-weave structure composed of an intertwined mixture of C14 Laves and sigma phase during the long-term thermal exposure at 800 °C. The stabilization of the Laves phase structure due to the minor additions of P was found to be consistent with density functional theory calculations and could be rationalized through structure maps that relate the valence electron concentration and relative size differences.

A novel Ni-based superalloy was designed with γʹ precipitate strengthening, controlled γ/γʹ lattice misfit and high configurational entropy of the γ matrix for improved high temperature performance up to 800 °C. The alloy contains nano-sized γʹ precipitates, MC and other grain boundary carbides. Three variants of the alloy were fabricated using vacuum induction melting, a computational-based homogenization cycle for reducing solidification segregation, and thermomechanical processing steps of forging followed by hot rolling to produce plates from which ASTM standard test specimens were extracted. Tensile testing at room temperature and elevated temperatures up to 800 °C revealed superior yield stress when compared to Nimonic 105 with good tensile strength values. Furthermore, the three alloys are machinable with maximum stresses comparable to standard practices as determined using deformation mechanisms maps obtained from Gleeble testing and EBSD analysis. Due to the composition of the experimental alloys falling outside the typical range used to populate thermodynamic databases, differences in phase predictions and related temperatures were observed between the experiment and Thermo-Calc predictions. The γʹ forming elements Ti and Nb had a similar effect on the γʹ precipitates and indirectly contributed to changing the entropy of the γ matrix. Based on the results of this study, these alloys have the potential for use at 800 °C in energy structural applications. Definitions related to this novel class of alloys are discussed.

To minimize the deviation of the predicted creep curves obtained under constant load conditions by the original θ projection model, a new modified version that can be expressed by e = θ 1 1 - e - θ 2 t + θ 3 e θ 4 e θ 5 e t - 1 , was derived and experimentally validated in our last study. In the present study, the predictive capability of the modified θ projection model was investigated by comparing the simulated and experimentally determined creep curves of K465 and DZ125 superalloys over a range of temperatures and stresses. Furthermore, the linear relationship between creep temperature and initial stress was extended to the 5-parameter model. The results indicated that the modified model could be used as a creep life prediction method, as it described the creep curve shape and resulted in predictions that fall within a specified error interval. Meanwhile, this modified model provides a more accurate way of describing creep curves under constant load conditions. The limitations and future direction of the modified model were also discussed. In addition, this modified θ projection model shows great potential for the evaluation and assessment of the service safety of structural materials used in components governed by creep deformation.

The size, morphology and fraction of each of the γ′ precipitate populations in polycrystalline γ- γ′ nickel-base superalloys are highly sensitive to the cooling rate from the solution temperature. As a result, thermal processing parameters are carefully selected to control precipitate populations in an effort to modify specific microstructural features that can impact resulting properties. The present investigation reports on how a stepped cooling rate was able to modify the microstructure of two experimental Ni-base superalloys. Cooling rates of 0.1 °C/s and 0.01 °C/s were maintained through the γ′ solvus temperature to a transition temperature ∼10 °C to 15 °C below the solvus. Following the transition temperature, a controlled cooling rate of 1 °C/s was applied to control the size and distribution of the secondary γ′ precipitates. For both experimental alloys, slow cooling rates through the γ′ solvus temperature resulted in the formation of coarse grain boundary precipitates while secondary γ′ precipitates formed upon the subsequent faster cooling rate of 1 °C/s. Coarse grain boundary γ′ precipitates resulted in higher amplitudes and wavelengths of serration along the boundary. In addition to cooling rate, the grain boundary misorientation was also found to influence the precipitation of grain boundary γ′ with larger precipitate populations found along high angle grain boundaries. Results from this study appear to support the concept that meso-scale engineering of grain boundary structures may potentially be used to enhance and optimize the high temperature properties of Ni-base superalloys.

A series of six high-entropy alloys in non-equiatomic proportions were produced to study the phase stability across a range of compositions within the Al-Co-Cr-Fe-Nb-Ni system. Phase identification was performed using SEM, TEM, and XRD and computational tools were employed for comparison to the experimental findings. Complex microstructures were obtained following exposure at 900 °C with precipitation of Laves, NiAl, γ′ and δ phases. Primary and secondary Laves populations were identified and found to become more stable with increasing Fe and Nb content. Additions of Nb promoted the formation of thin, plate-like, δ phase at the expense of the secondary Laves. The NiAl phase was observed to precipitate in the alloys with high Fe content, above the γ′ solvus temperature. Overall, the thermodynamic predictions using Thermo-Calc with the TCNi8 database were accurate with respect to identifying the phase composition, while TTNi8 provided good phase stability results. However, significant differences were observed with respect to the solvus temperatures of the precipitate phases.

Estimating long-term creep deformation and life of materials is an effective way to ensure the service safety and to reduce the cost of long-term integrity evaluation of high temperature structural materials. Since the 1980s, the θ projection model has been widely used for predicting creep lives due to its ability to capture the characteristic transitions observed in creep curves obtained under constant true stress conditions. However, the creep rupture behavior under constant load or engineering stress conditions cannot be simulated accurately using this model because of the different stress states. In this paper, creep curves obtained under constant load conditions were analyzed using a modified θ projection model by considering the increase in true stress with creep deformation during the creep tests. This model is expressed as e = θ 1 1 − e − θ 2 t + θ 3 e θ 4 e θ 5 e t − 1 , and was validated using the creep curves of K465 and DZ125 superalloys tested at a range of temperatures and engineering stresses. Moreover, it was shown that the predictive capability of the modified θ projection model was significantly improved over the original one, as it reduces the prediction uncertainty from a range of 10% to 20% to below 5%. Meanwhile, it was shown that the model can be reasonably used for predicting constant stress creep conditions, when appropriate parameters are used. The prediction performance of the modified model will be discussed in another paper. The results of this study show great potential for the evaluation and assessment of the service safety of structural materials used in applications where designs are limited by creep deformation.

International audience; We have investigated the partitioning of solutes at crystal defects in intergranular Cr-rich M2B borides after creep at 850°C/185MPa and annealing at 850°C for approximately 3000 hours in a polycrystalline nickel-based superalloy. Borides were found to coarsen in both cases, with the borides after creep to be the thickest (800-1100nm), compared to borides annealed in the absence of external applied load (400-600nm). Transmission electron microscopy revealed that the coarsened borides have either a tetragonal I4/mcm structure, or an orthorhombic Fddd, with those two structures coexisting in a single particle. The presence of a very high density of planar faults is systematically 2 observed within the coarsened borides. The faults were correlated with chemical fluctuations of B and Cr, revealed by atom probe tomography. In addition, partitioning of Ni and Co was observed at dislocations within the borides after creep providing insights into the deformation of borides.

This work investigates two nominally similar polycrystalline alloys, with a subtle difference in Nb content, intended to elucidate its effect on local phase transformation strengthening during high temperature creep. Tests were conducted at 750 °C and 600 MPa to target the creep regime dominated by superlattice intrinsic and extrinsic stacking faults, as well as microtwinning. Alloy A, with higher Nb and lower Al, was found to be superior in creep strength to Alloy B, with lower Nb and higher Al, as well as previously investigated ME3 and LSHR. Atomic resolution scanning transmission electron microscopy and energy-dispersive spectroscopy found that this increased creep strength was due to a novel local phase transformation occurring along microtwin boundary interfaces as a result of the Nb increase. Complementary density functional theory calculations helped to confirm that this was χ phase formation. It is hypothesized that this transformation was the cause of the increased creep strength exhibited by Alloy A.

The stress relaxation behavior of two experimental and one commercial powder-processed Ni-base were assessed using a servo-hydraulic frame under strain control at 700 °C. In addition to quantifying the effect of composition on the stress relaxation behavior, the effect of microstructure and initial strain were also evaluated. The magnitude and rate of stress reduction for the various samples was measured during testing and an apparent activation model was used to normalize the magnitude of the stress drop with the initial stress. Stress relaxation tests with an initial strain of 0.6% exhibited characteristic behaviors that could be correlated to alloy chemistry and microstructure. The extent of stress relaxation in highly alloyed RRHT5P samples possessing high volume fractions and a high number density of intragranular γ′ precipitates was limited. Although P additions were not observed to exert any significant effect, processing of these alloys with lower cooling rates from solution to coarsen the γ′ precipitates was shown to effectively increase the degree of stress relaxation. Reducing the degree of alloying and maintaining a lower overall fraction of γ′ precipitates effectively confers a higher degree of stress relaxation at 700 °C. Stress relaxation testing and the application of an apparent activation volume model may be effectively used for characterizing the notch sensitivity and crack growth behavior of high temperature structural materials.

Additive manufacturing has enabled the production of highly complex designs that are not producible using traditional manufacturing techniques. While superalloys such as IN718 have been used in these processes for the manufacture of turbine engine structural components, applications requiring higher service temperatures necessitate the development of alloys with increased capability. Rene 65 was developed as a cast and wrought alloy with increased capability relative to wrought IN718, and characteristics of that alloy, including temperature stability and thermal crack-resistance, made Rene 65 an appealing candidate to withstand the extreme temperature gradients that are characteristic of direct metal laser melting (DMLM) additive manufacturing. The as-built DMLM microstructure is very different from as-forged microstructure, and this work will examine the effect of heat treatments both below (sub-) and above (super-) the gamma prime (γ′) solvus on the grain and precipitate structure of AM Rene 65 material. Tensile, fatigue and creep behavior of the alloy in these different heat treatment conditions is reported. Relative to AM IN718, AM Rene 65 shows the desired improvement in temperature capability analogous to that in the cast and wrought versions of the alloys. Differences in the balance of properties are noted between AM Rene 65 and cast and wrought Rene 65, which are attributable to the differences in grain size and precipitate distribution and which may provide benefits for certain applications.

This article presents the effect of carbide inoculants additions on microstructure evolution and mechanical properties of Inconel 718 superalloy fabricated by selective laser melting (SLM) process. Flakes of titanium carbide (TiC) and niobium carbide (NbC) were mixed with the Inconel 718 powder and acted as nucleating agents to induce heterogenous nucleation in order to eliminate anisotropic grain structure. By increasing fraction of carbide inoculants, more isotropic grain texture was detected. Furthermore, significant improvements on creep properties have been observed with minor carbide additions. With 0.5 wt% TiC and NbC addition, creep rupture life could be increased from 198.5 h to 449.5 h and 371.8 h, respectively. Moreover, creep strain rate was dramatically decreased from 0.513 × 10−8 to 0.12 × 10−8 s−1 by 0.5 wt% TiC additions. This study has demonstrated that minor carbide addition can have profound impact on the microstructure and property of Inconel718 processed by SLM.

International audience; Predicting the mechanical failure of parts in service requires understanding their deformation behavior, and associated dynamic microstructural evolution up to the near-atomic scale. Solutes are known to interact with defects generated by plastic deformation, thereby affecting their displacement throughout the microstructure and hence the material's mechanical response to solicitation. This effect is studied here in a polycrystalline Ni-based superalloy with two different Nb contents that lead to a significant change in their creep lifetime. Creep testing at 750°C and 600 MPa shows that the high-Nb alloy performs better in terms of creep strain rate. Considering the similar initial microstructures, the difference in mechanical behavior is attributed to a phase transformation that occurs along planar faults, controlled by the different types of stacking faults and alloy composition. Electron channeling contrast imaging reveals the presence of stacking faults in both alloys. Microtwinning is observed only in the low-Nb alloy, rationalizing in part the higher creep strain rate. In the high-Nb alloy, atom probe tomography evidences two different types of stacking faults based on their partitioning behavior. Superlattice intrinsic stacking faults were found enriched in Nb, Co, Cr and Mo while only Nb and Co was segregated at superlattice extrinsic stacking faults. Based on their composition, a local phase transformation occurring along the faults is 2 suggested, resulting in slower creep strain rate in the high-Nb alloy. In comparison, mainly superlattice intrinsic stacking faults enriched in Co, Cr, Nb and Mo were found in the low-Nb alloy. Following the results presented here, and those available in the literature, an atomic-scale driven alloy design approach that controls and promotes local phase transformation along planar faults at 750°C is proposed, aiming to design superalloys with enhanced creep resistance.

Critical rotating components used in the hot section of gas turbine engines are subject to cyclic loading conditions during operation, and the life of these structures is governed by their ability to resist fatigue. Since it is well known that microstructural parameters, such as grain size, can significantly influence the fatigue behavior of the material, the conventional processes involved with the manufacture of these structures are carefully controlled in an effort to engineer the resulting microstructure. For a commercial Ni-base superalloy, RR1000, the development of process models and deformation mechanism maps has enabled not only control of the resultant grain size but also the ability to tailor and manipulate the resulting grain boundary character distribution. The increased level of microstructural control was coupled with a physics-based fatigue model to form an integrated computational materials engineering framework that was used to guide the design of damage-tolerant microstructures. Simulations from a 3D crystal plasticity finite element model were used to identify microstructural features associated with strain localization during cyclic loading and to guide the design of polycrystalline microstructures optimized for fatigue resistance. Conventionally processed and grain boundary engineered forgings of a commercial Ni-based superalloy, RR1000, were produced to validate the design methodology. For nominally equivalent grain sizes, high-resolution strain maps generated via digital image correlation confirmed that the high density of twin boundaries in the grain boundary engineered material were desirable microstructural features as they contribute to limiting the overall length of persistent slip bands that often serve as precursors for the nucleation of fatigue cracks.

Microstructural features, including γ′ volume fraction and size, γ-γ′ lattice misfit, γ channel width, and dislocation substructure, are known to significantly influence the creep performance in Ni-base single-crystal superalloys. In this study, the microstructural characteristics of Ru-containing single-crystal superalloys with different levels of Co, Mo, and Ru additions were quantitatively investigated after ruptured and interrupted creep tests conducted at 1223 K (950 °C) and 400 MPa. The creep lifetime was slightly increased with the high level of Co addition and significantly increased with the coadditions of Mo and Ru. A minor effect of Co content on the γ channel width and γ′ volume fraction was found in experimental alloys. The alloy with high levels of Mo and Ru additions was determined to possess a more negative γ-γ′ lattice misfit, and a high density of stacking faults (SFs) was formed in the γ channels during creep. The combined effects of the SFs in the γ matrix serving as the barriers to dislocation movement, as well as the dense interfacial dislocation networks preventing dislocation to shear the γ′ phase, were considered as the main mechanism responsible for the improvement of creep resistance. Results from this study are helpful to understand the effect of microstructural features on creep performance and contribute to the knowledge of physical metallurgy in Ru-containing single-crystal superalloys.

Detailed ex-situ electron microscopy and atom probe tomography (APT) were combined with in-situ synchrotron diffraction to systematically quantify the chemical, morphological, and lattice instabilities that occur during aging of a polycrystalline high-refractory content Ni-base superalloy. The morphological changes and splitting phenomenon associated with the secondary γ′ precipitates were related to a combination of discrete chemical composition variations at the secondary γ′/γ interfaces and additional chemical energy arising from γ precipitates that form within the secondary γ′ particles. The compositional phase inhomogeneities led to the precipitation of finely dispersed tertiary γ′ particles within the γ matrix and secondary γ particles within the secondary γ′ precipitates, which, along with surface grooving of the secondary γ′ particles, likely due to a spike in the lattice misfit at the particle interfaces, contributed to the splitting of the precipitates during aging.

The influence of varying fractions of primary gamma prime precipitates on the hot deformation and annealing behavior of an experimental Nickel-based superalloy containing 24 wt pct. Co was investigated. Billets heat treated at 1110 °C or 1135 °C were subjected to hot compression tests at temperatures ranging from 1020 °C to 1060 °C and strain rates ranging from 0.001 to 0.1/s. The microstructures were characterized using electron back scatter diffraction in the as-deformed condition as well as following a super-solvus anneal heat treatment at 1140 °C for 1 hour. This investigation sought to quantify and understand what effect the volume fraction of primary gamma prime precipitates has on the dynamic recrystallization behavior and resulting length fraction ∑3 twin boundaries in the low stacking fault superalloy following annealing. Although deformation at the lower temperatures and higher strain rates led to dynamic recrystallization for both starting microstructures, comparatively lower recrystallized fractions were observed in the 1135 °C billet microstructures deformed at strain rates of 0.1/s and 0.05/s. Subsequent annealing of the 1135 °C billet microstructures led to a higher proportion of annealing twins when compared to the annealed 1110 °C billet microstructures.

The effect of strain on the resultant microstructure of an experimental low stacking fault energy Nickel based superalloy containing 24 wt. pct. Co was investigated. Billets subjected to a preliminary heat treatment at 1110 °C were compressed to strain limits of 0.15 and 0.5 at strain rates ranging from 0.1/s to 0.01/s and temperatures at 1020 °C and 1060 °C. The as-deformed microstructures were assessed and characterized using electron backscatter diffraction, as were microstructures corresponding to a super-solvus anneal heat treatment at 1160 °C for one hour. This study sought to identify a critical strain limit at which conditions indicative of Strain induced boundary migration (SIBM) could be effectively triggered for the experimental Ni-based superalloy over a set range of thermal-mechanical parameters. Microstructures corresponding to SIBM were then compared to more extensively deformed billets which contained notable fractions of dynamically recrystallized grains to quantify differences in the length fraction and density of ∑3 twin boundaries of the respective microstructures. Though billet samples deformed to both 0.15 and 0.5 contained notable magnitudes of stored strain energy, microstructures deformed to 0.15 were noted as having maintained larger length fractions of ∑3 twins due to a predominant absence of dynamic recrystallization. Annealed samples originally deformed to 0.15 yielded annealing twin length fractions as high as 59% when compared a sample deformed to the 0.5 strain limit under equivalent thermal-mechanical conditions that resulted in a twin length fraction of 50%. Although samples deformed to the lower strain limit exhibited higher length fractions of annealing twins, samples deformed to the higher strain limit of 0.5 were noted to yield ∑3 densities as high as 0.65 μ m − 1 , whereas the annealed sample deformed under equivalent thermal-mechanical parameters to the 0.15 strain limit produced ∑3 densities as low as 0.32 μ m − 1 .

The investigation aims to clarify the influence of scan parameters on the morphology and phase constitution of CoAl2O4 inoculants in Inconel 718 (IN718) superalloy processed by selective laser melting (SLM). Gas atomized IN718 powder feedstock was uniformly blended with 0.2 wt% of CoAl2O4 flakes and processed with a range of laser scan parameters to understand the effect of the inoculant additions on microstructure. Similar to the reactions observed in investment cast Ni-superalloys, the CoAl2O4 particles were found to chemically react with elemental Al, Cr, and Ti present in the melt pool of IN718 and become reduced into Al2O3, Cr2O3, and TiO2 oxides after the SLM process. Analyses of single melt pool beads revealed the existence of Co-rich metallic particles distributed along the surface of the oxides. However, since SLM processing involves remelting of the previously deposited layers to minimize the occurrence of porosity, this results in the dissolution of the Co-rich metallic particles that form when the CoAl2O4 particles are reduced in the melt pool. As such, the ability of the metallic Co particles to contribute to grain refinement becomes limited during SLM processing. Furthermore, the inoculant particles were found to agglomerate along the scan direction. The underlying mechanisms can be attributed to the Marangoni convection following the moving center of laser beam. It was found that decreasing energy density and increasing scan speed can effectively reduce the size and number density of the agglomerated oxide particles. Faster laser scan velocities lead to reduced convective flow within the melt pool and reduce the tendency for agglomeration. However, the reduced magnitude of energy input limited the time over which the CoAl2O4 particles could react with the melt pool. This led to the formation of lack of fusion defects and incomplete chemical reaction between CoAl2O4 and IN718 after the SLM process. The effectiveness of CoAl2O4 inoculant particles on grain refinement in as-built SLM IN718 microstructures is strongly dependent on the melt pool physics and laser scan conditions.

A series of non-equiatomic Al-Co-Cr-Fe-Nb-Ni high-entropy alloys, with varying levels of Co, Nb and Fe, were investigated in an effort to obtain microstructures similar to conventional Ni-based superalloys. Elevated levels of Co were observed to significantly decrease the solvus temperature of the γ′ precipitates. Both Nb and Co in excessive concentrations promoted the formation of Laves and NiAl phases that formed either during solidification and remained undissolved during homogenization or upon high-temperature aging. Lowering the content of Nb, Co, or Fe prevented the formation of the eutectic type Laves. In addition, lowering the Co content resulted in a higher number density and volume fraction of the γ′ precipitates, while increasing the Fe content led to the destabilization of the γ′ precipitates. Various aging treatments were performed which led to different size distributions of the strengthening phase. Results from the microstructural characterization and hardness property assessments of these high-entropy alloys were compared to a commercial, high-strength Ni-based superalloy RR1000. Potentially, precipitation-strengthened high-entropy alloys could find applications replacing Ni-based superalloys as structural materials in power generation applications.

Traditionally, material design and property modifications are usually associated with compositional changes. Yet, subtle changes in the manufacturing process parameters can also have a dramatic effect on the resulting material properties. In this work, an integrated computational materials engineering (ICME) framework is adopted to tailor the fatigue performance of a Ni-based superalloy, RR1000. An existing fatigue model is used to identify microstructural features that promote enhanced fatigue life, namely a uniform, fine grain size distribution, random orientation, a distinct grain boundary distribution (specifically high twin boundary density and limited low-angle grain boundaries). A deformation mechanism map and process models for grain boundary engineering of RR1000 are used to identify the optimal thermo-mechanical processing parameters to realize these desirable microstructural features. For validation, small-scale forgings of RR1000 were produced and heat-treated to attain fine grain and coarse grain microstructures that represent the conventionally processed and grain boundary engineered (GBE) conditions, respectively. For each of the four microstructural variants of RR1000, the twin density and grain size were characterized and were in agreement with the desired microstructural attributes. In order to validate the deformation mechanisms and fatigue behavior of the material, high-resolution digital image correlation was performed to generate strain maps relative to the microstructural features. The high density of twin boundaries was confirmed to inhibit the length of slip bands, which is directly attributed to extended fatigue life. Thus, this study demonstrated the successful role of models, both process and performance, in the design and manufacture of Ni-based superalloy disk forgings.

Boron additions to Ni-based superalloys are considered to be beneficial to the creep properties of the alloy, as boron has often been reported to increase grain boundary cohesion, increase ductility, and promote the formation of stable boride phases. Despite the importance, it is not well understood whether these improvements are associated with the presence of elemental boron or stable borides along the grain boundaries. In this investigation, two experimental powder-processed Ni-based superalloys containing elevated levels of Nb were found to exhibit increased solubility for B in the γ matrix when compared to similar commercial Ni-based superalloys. This resulted in an overall lower B concentration at grain boundaries that suppressed boride formation. As the predictive capability of CALPHAD database models for Ni-based superalloys have improved over the years, some discrepancies may still persist around compositionally heterogeneous features such as grain boundaries. Improved quantification of the characteristic partitioning of B as a function of the bulk alloy composition is required for understanding and predicting the stability of borides.

Atom probe tomography was used to quantitatively assess grain boundary phase compositions and determine local segregation along the grain boundary before and after a 1000 h thermal exposure at 800 °C on an experimental powder processed Ni-base superalloy containing elevated levels of Nb. Due to high levels of refractory alloying elements at the grain boundary, a complex network of σ phase precipitates formed and the interfacial segregation profiles were studied. Although elemental B segregates to grain boundaries and secondary phase interfaces, insufficient levels are present to result in boride formation due to an enhanced solubility of B in the matrix phase.

In polycrystalline Ni-base superalloys, grain boundary precipitation of secondary phases can be significant due to the effects they pose on the mechanical properties. As new alloying concepts for polycrystalline Ni-base superalloys are being developed to extend their temperature capability, the effect of increasing levels of Nb alloying additions on long term phase stability and the formation of topologically close packed (TCP) phases needs to be studied. Elevated levels of Nb can result in increased matrix supersaturation and promote the precipitation of secondary phases. Long term thermal exposures on two experimental powder processed Ni-base superalloys containing various levels of Nb were completed to assess the stability and precipitation of TCP phases. It was found that additions of Nb promoted the precipitation of η-Ni 6 AlNb along the grain boundaries in powder processed, polycrystalline Ni-base superalloys, while reduced Nb levels favored the precipitation of blocky Cr and Mo – rich σ phase precipitates along the grain boundary. Evaluation of the thermodynamic stability of these two phases in both alloys using Thermo-calc showed that while σ phase predictions are fairly accurate, predictions of the η phase are limited.

The effects of thermo-mechanical processing parameters on the resulting microstructure of an experimental Nickel-based superalloy containing 24 wt pct Co were investigated. Hot compression tests were performed at temperatures ranging from 1293 K to 1373 K (1020 to 1100 °C) and strain rates ranging from 0.0005 to 0.1/s. The mechanically deformed samples were also subject to annealing treatments at sub-solvus 1388 K (1115 °C) and super-solvus 1413 K (1140 °C) temperatures. This investigation sought to quantify and subsequently understand the behavior and evolution of both the grain boundary structure and length fraction of Σ3 twin boundaries in the low stacking fault energy superalloy. Over the range of deformation parameters investigated, the corresponding deformation mechanism map revealed that dynamic recrystallization or dynamic recovery was dominant. These conditions largely promoted post-deformation grain refinement and the formation of annealing twins following annealing. Samples deformed at strain rates of 0.0005 and 0.001/s at 1333 K and 1373 K (1060 °C and 1100 °C) exhibited extensive grain boundary sliding/rotation associated with superplastic flow. Upon annealing, deformation conditions that resulted predominately in superplastic flow were found to provide negligible enhancement of twin boundaries and produced little to no post-deformation grain refinement.

The stress relaxation behavior at 700 °C of two experimental and one commercial powder-possessed Ni-base superalloys possessing different microstructural characteristics were studied following the application of two different strain levels (0.6% and 2%) in order to investigate how phosphorus additions, secondary γ′ microstructure and tertiary γ′ microstructure affect the stress relaxation behavior, respectively. Analysis of the characteristic stress relaxation behavior using the natural logarithm of plastic strain rate ln( e ˙ ) vs. stress σ curves, revealed that the compositional and microstructural variations impact the stress relaxation behavior differently following deformation to a total strain of 0.6%. No changes in the stress relaxation behavior or γ′ microstructure was observed in nominally identical alloys RRHT5P1 and RRHT5P2 that contained 0.013 wt % P and 0.041 wt % P, respectively. Changing the cooling rate from the solution heat treatment temperature for these alloys from 1 °C/s to 2 °C/s, however, did lead to modest changes in both the γ′ microstructure and the stress relaxation behavior. Significant variations in the stress relaxation behavior were quantified as a function of the tertiary γ′ microstructure in a commercial Ni-base superalloy, RR1000. Reducing the amount of tertiary γ′ precipitates by either a prolonged aging process or multi-step aging process contributed to a greater stress drop during stress relaxation, regardless of the variations in the secondary γ′ microstructure. Following the application of a total strain of 2%, the stress relaxation process for all the samples was observed to be insensitive to compositional and microstructural variations.

The shear-compression behavior of four commercial aluminum armor alloys, 2139, 2519, 5083, and 7039, that exhibit enhanced resistance to high-strain-rate deformation, were evaluated using a Split Hopkinson Pressure Bar. Each of the alloys was found to exhibit a characteristic critical equivalent strain beyond which plastic collapse of the material occurred. Microstructural changes were systematically quantified as a function of equivalent strain using electron backscatter diffraction along with the effects of crystallographic orientation, secondary particles, and solid solution strengthening on the accumulation of localized strain within the microstructure. The onset of the plastic collapse was determined to correlate with an equivalent strain where nominally all of the grains within the microstructure exhibited characteristics associated with adiabatic shear band formation. The rapid decline of the flow stress during plastic collapse was found to be enhanced by grain fragmentation and refinement in regions of high stress concentrations. Results from this study suggest that improvements in the performance of these Al armor alloys may potentially be achieved through careful control of their processing, in particular with respect to their texturing and the dispersion of secondary particles in the microstructure.

The application of grain boundary engineering (GBE) techniques to enhance the physical and mechanical properties of Ni-based superalloys could potentially increase the efficiency of turbine engines. Compared to traditional GBE processes that require multiple iterations of room temperature deformation followed by annealing, novel techniques for GBE based on the optimization of the thermal–mechanical processing parameters exhibit more potential for producing complex-shaped Ni-based superalloys components. To date, the formation and microstructural evolution of Σ3 boundaries during thermal–mechanical processing have yet to be fully understood. In this investigation, the effects of deformation texture and strain were systematically investigated in an advanced Ni-based superalloy, RR1000. Using various strains and annealing temperatures, the effects of recrystallization and texturing were quantified. Although texturing was often associated with recrystallization that caused the length fraction of Σ3 boundaries to decrease, the formation of Goss type texture during deformation was found to promote the formation of Σ3 boundaries upon annealing when compared to deformation texturing 〈111〉 parallel to the rolling direction.

This work investigated the effects of CoAl2O4 inoculants on the microstructure and mechanical properties of a superalloy – Inconel 718 (IN718), processed by selective laser melting (SLM). IN718 powder was blended with 0.2 wt. % of CoAl2O4 particles. After the SLM process, the CoAl2O4 particles reacted with IN718 and formed a dispersion of Al-rich nano-oxide particles in the matrix. These inoculants were observed to facilitate the formation of fine, equiaxed grains and reduce the degree of crystallographic texture in the as-built microstructure. The presence of the nano-oxide particles in the microstructure served to restrict the mobility of grain boundaries during heat-treatment and promoted the formation of a bimodal grain structure with comparatively finer average size. Due to the reduction of crystallographic texture, the elastic anisotropy present in as-built specimens was greatly minimized with the addition of CoAl2O4 inoculants. Specimens containing inoculants were also found to possess improved tensile properties following heat-treatment at both room temperature and 650 °C. Furthermore, creep testing at 650 °C/650 MPa revealed that the steady strain rate of IN718 was lowered from 8.8×10−9 s−1 to 4.9×10−9 s−1, and the creep rupture life was extended by 52 hours in samples that were fabricated with CoAl2O4. These results strongly suggest that the decomposition of CoAl2O4 into Al-rich oxides during SLM processing not only contributes to grain refinement and a reduction in the degree of crystallographic texture but also forms an oxide dispersion that restricts the mobility of dislocations and grain boundaries in IN718.

The effect of grain size on the phase stability of two nominally identical high refractory content powder-processed Ni-base superalloy with varying levels of P additions (0.013 wt% for alloy P1 and 0.026 wt% for alloy P3) was studied. Solutioning of the alloys at either sub-solvus or super-solvus temperatures were used to vary the grain size of the starting microstructure prior to long-term thermal exposures at 800 °C for up to 1000 h. EBSD analyses revealed that the sub-solvus solutioned samples had an average grain size of 10 μm and possessed a high angle grain boundary length density that was approximately 2.5 times greater than that found in the super-solvus solutioned samples with an average grain size of 16 μm. Differences in both the initial grain boundary character distribution and P content present in these alloys resulted in varying behavior with respect to the grain boundary precipitation of Laves and sigma phase during aging. Since P additions to Ni-base superalloys are known to promote the formation of Laves phases and segregate predominately to grain boundaries, sub-solvus heat treated samples exhibited a lower susceptibility of Laves phase precipitation compared with super-solvus heat treated samples for both P1 and P3 alloys. Sub-solvus heat treated P1 samples were found to be resistant to the formation of Laves phase after 1000 h exposure while the coarser grained super-solvus heat treated P3 samples exhibited extensive formation of intertwined Sigma-Laves along the grain boundaries just after 100 h exposure. The improvement in phase stability could be attributed to the reduced concentration of P along the grain boundary as increasing the length density of random high-angle grain boundary resulted in a more dilute spatial distribution of segregated P atoms.

In response to the increasing need for higher operating temperatures in advanced gas turbine engines, new alloying concepts are required to develop novel nickel-base superalloys with enhanced temperature capabilities. Recent studies have shown that polycrystalline Ni-base superalloys containing elevated levels of Nb additions exhibit superior properties at elevated temperatures when compared to existing commercial Ni-base superalloys. In order to design, develop and fully exploit this innovative class of superalloys, an understanding of how alloying elements partition to each phase is essential. Using atom probe tomography (APT), compositions of the constituent phases were measured in four high Nb content γ–γ′ Ni-base superalloys and the results were compared to thermodynamic database models from Thermo-Calc. Results were also used in predicting the solid solution strength behavior of the four alloys. The differences in phase composition predictions from thermodynamic models resulted in dissimilarities between the generated strength behavior curves and those from the experimental work. Keywords: Superalloy, Partition, Atom probe tomography, Thermo-Calc, Strengthening

Ceramics often have high hardness and strength, and good wear and corro- sion resistance, and hence have many important applications, which, however, are often limited by their poor fracture toughness. Carbon nanotubes (CNTs) may enhance ceramic fracture toughness, but hot pressing (which is one typical approach of fabri- cating CNT-ceramic composites) is difficult to apply for applications that require localized heat input, such as fabricating composites as surface coatings. Laser beam may realize localized material sintering with little thermal effect on the surrounding regions. However, for the typical ceramics for hard coating applications (as listed in Ref.(1)), previous work on laser sintering of CNT-ceramic composites with mechanical property characterizations has been very limited. In this paper, research work has been reported on the fabrication and characterization of CNT-ceramic composites through laser sintering of mixtures of CNTs and chromium carbide powders. Under the studied conditions, it has been found that laser-sintered composites have a much higher hardness than that for plasma-sprayed composites reported in the literature. It has also been found that the composites obtained by laser sintering of CNTs and chromium

Enhancement of the physical and mechanical properties of polycrystalline Ni-base superalloys may be achieved through control of the grain boundary structure and is dependent on the optimization of the thermal–mechanical processing parameters. Superalloys containing grain boundary networks that are comprised with a sufficiently high fraction of Σ 3/twin boundaries have been reported to exhibit enhanced creep and fatigue properties. In this report, the density and length fraction of twin boundaries in annealed samples of powder processed Ni-base superalloy RR1000 were quantified and expressed as a function of the average grain diameter. The results were found to be consistent with classical models relating density and length fraction to grain size. The effects of varying hot deformation parameters on twin density and length fraction were also quantified and modified models were derived to describe the relationships.

Recent interest in bulk ultra-fine-grained microstructures has given rise for the necessity to quantify their behavior during heat treatment should any subsequent thermal processing of the material be necessary after forming. The present study showed that the microstructure of 9310 steel forgings containing varying fractions of sub-micron grains retained some degree of stability after 4 hours of heat treatment between the temperatures of 522 K and 866 K (249 °C and 593 °C, respectively). The behavior of the microstructure during heat treatment was largely influenced by both the carbide volume fraction and distribution, which affected the level of Zener Drag present. This in effect controlled the type of growth behavior exhibited by the ferrite grains and the ability to retain the fine-grained structure.

The high strain rate shear-compression behavior of two model Al–Cu and Al–Cu–Mn–Mg alloys was compared to a commercially available AA 2139 (Al–Cu–Mn–Mg–Ag) alloy. All three materials exhibited strain softening after the ultimate stress was reached, followed by a rapid degradation of mechanical properties after a critical strain level had been realized. Detailed microstructural characterization of the alloys at various interrupted strain levels revealed that the formation of shear bands led to the rapid degradation of properties. EBSD analysis subsequently revealed that shear bands typically formed within grains of various crystallographic orientations whose Schmid factor was maximized. Microstructural studies also correlated higher levels of local misorientation with higher concentrations of precipitates within the microstructures; samples with more alloying elements showed an increased amount of precipitation that provided more pinning points for dislocations. Evidence from this study suggests that improvements to the performance of aluminum alloys used for high strain rate applications could be made through careful control of the precipitation of secondary-phase particles as well as the orientation of the grain structure with respect to the loading direction.

Ternary eutectic γ–γ′–δ Ni-base superalloys have been the subject of a number of recent investigations due to the relatively high volume fraction of δ phase providing composite strengthening. However, an additional intermetallic precipitate phase, η, has been observed in Ni-base superalloys containing additions of both Nb and Ti. As both precipitate phases δ and η appear similar and are often mistaken for one another, a better fundamental understanding of their formation is required for developing innovative new classes of Ni-base superalloys. The morphology, formation, and composition of precipitate phases in a number of experimental alloys spanning a broad range of compositions were explored in this investigation. Additionally, compositional relationships were developed to facilitate the design of γ–γ′–(δ/η) Ni-base superalloys. When the alloy chemistry was observed to exhibit a compositional ratio of Al/(Nb + Ta + Ti) less than 0.85, δ and/or η phase precipitates formed while a ratio greater than 0.85 resulted in conventional γ–γ′ microstructures. For alloys in which δ and/or η phase precipitates were formed, the prevalent phase could be determined by evaluating the compositional ratio for (Nb + Ta)/(Al + Ti). Alloys that had ratios greater than 1 were largely comprised of δ phase precipitates, whereas a ratio less than 1 resulted in the predominance of the η phase precipitates.