The Ottosen-Stenstrom-Ristinmaa (OSR) incremental fatigue damage model is adapted for fatigue-life as-sessment of integral airframes milled from 7050-T7451 aluminum plates. For validation, variable-amplitude high-cycle fatigue experiments are conducted for circumferentially notched, axisymmetric specimens, and for a geometry similar to an aircraft fuselage frame, with flanges, stiffeners, and web panels. We also describe how the parameters of the OSR model can be modified to account for surface roughness, and for setting acceptable failure probability.

A NiAl alloy/ Ni3Al-based superalloy diffusion couple with an electroplated NiRe layer was prepared. High-temperature interdiffusion performance of the diffusion couple was analyzed at 1100 degrees C under argon at-mosphere and compared with that of the blank group without NiRe layer. A Re-based diffusion barrier formed during the diffusion. The microstructural instability of the superalloy was found to be alleviated by the Re-based diffusion barrier, and the detailed microstructure of the diffusion barrier was identified to be Re-rich a phases and Mo-rich << phase detected by TEM. The chemical diffusion coefficient of Al was reduced by about two orders of magnitude with the modification of the Re-based diffusion barrier. A microstructural degradation of the Re-based barrier layer was observed after high-temperature diffusion of 300 h, which can be attributed to the internal diffusion of Mo.(c) 2023 Elsevier B.V. All rights reserved.

In laser powder bed fusion (L-PBF), most powders are not melted in the chamber and collected after the printing process. Powder reuse is appreciable without sacrificing the mechanical properties of target components. To understand the influences of powder reuse on mechanical performance, a nickel-based superalloy, IN738LC, was investigated. Powder morphology, microstructure and chemical compositions of virgin and reused powders were characterized. An increase in oxygen content, generally metallic oxides, was located on the surface of powders. Monotonic tensile and cyclic fatigue were tested. Negligible deterioration in strength and tensile ductility were found, while scattered fatigue performance with regard to fatigue life was shown. Deformation and fatigue crack propagation mechanisms were discussed for describing the powder degradation effects.

To attain the desired mechanical properties of an additively manufactured component, robust post-processing in term of thermal treatment is highly required to reduce the crystallographic anisotropy. However, the microstructure appearance with respect to the post-treatment temperature is not well understood mechanistically. In this study, the microstructural evolution of grains of a laser-powder bed fused (L-PBF) nickel-base superalloy, CM247LC, during post-processing heat treatment is investigated systematically. Recrystallization barely happens below the gamma solvus temperature leading to a remaining unhomogenized dendritic(cellular) structure. However, recrystallization is introduced above the gamma solvus temperature. By considering the grain boundary (GB) migration mechanisms and supported by experimental observations, the sluggish recrystallization behavior of this gamma-strengthened nickel-based superalloy has been understood. Owing to lack of the difference in stored energy between adjacent grains, this primary driving force is constrained. The GB migration is majorly driven by capillarity force (1-10 MPa) before the recrystallization occurrence, which is evident by the evolution of GB curvatures. On the other hand, the Zener pinning force generated from GB precipitates including carbides and gamma precipitates provides the dragging force in the comparable scale (1-10 MPa) against the GB migration.

SiC multilayer coatings were deposited via thermal chemical vapor deposition (CVD) using silicon tetrachloride (SiCl₄) and various hydrocarbons under identical growth conditions, i.e. at 1100 °C and 10 kPa. The coatings consisted of layers whose preferred growth orientation alternated between random and highly 〈111〉-oriented. The randomly oriented layers were prepared with either methane (CH₄) or ethylene (C₂H₄) as carbon precursor, whereas the highly 〈111〉-oriented layers were grown utilizing toluene (C₇H₈) as carbon precursor. In this work, we demonstrated how to fabricate multilayer coatings with different growth orientations by merely switching between hydrocarbons. Moreover, the success in depositing multilayer coatings on both flat and structured graphite substrates has strengthened the assumption proposed in our previous study that the growth of highly 〈111〉-oriented SiC coatings using C₇H₈ was primarily driven by chemical surface reactions.

Additive manufacturing (AM), e.g., laser powder bed fusion (LPBF) technique, has become a powerful manufacturing process for producing metallic components with the advantages of design freedom, net-shape-forming flexibility, product customization, and reduced lead time to market. Nickel-based superalloys is one of the most significant alloy families used at elevated temperatures. Nickel-based superalloys commonly contain up to 10 more alloying elements like chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. The great capacity of the nickel-based superalloys for high-temperature operation is ensured by the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed corresponding manufacturing processes. However, high-performance nickel-based superalloys generally suffer from structural integrity issues during AM process, i.e., this class of superalloys is highly susceptible to crack. Therefore, new nickel-based superalloys adapted to AM process with tailored chemical composition are under the urgent call. Meanwhile, high-temperature performance is another prioritized target for the new superalloys.The first topic is the chemical composition-dependent cracking mechanisms. The interdendritic region formed at the last-stage solidification has been found as the cracked spaces. Owing to the suppression of precipitate formation, the cracking mechanism is generalized as (1) the large mismatch of the solidification steps accounting for the crack initiation, and (2) the large mismatch of load-bearing capacity accounting for the crack propagation, between the dendritic and interdendritic regions. To quantitatively formulate the additive manufacturability of nickel-based superalloys, herein a two-parameter-based, heat resistance, and deformation resistance (HR-DR) model, has been successfully proposed to predict the printability on accounting for the relation between chemical composition (both major and minor elements) and cracking susceptibility. The concept of this model is formulated as that if the interdendritic region obtains both higher heat and deformation resistances than the rest dendritic region, this alloy is expected to be crack resistant. Validated by the experimental results and hitherto reported literature data, the HR-DR model provides an excellent sound prediction on the crack susceptibility of nickel-based superalloy during AM process. By considering the combination of additive manufacturability and high-temperature performance, a novel high-strength nickel-based superalloy, MAD542 has been developed based on the materials selection procedure from 921,600 candidate compositions. In addition, another precipitation-strengthened nickel-based superalloy, Alloy738+ has been developed based on the modification of the composition of heritage superalloy IN738LC, aiming for improving the additive manufacturability, creep, and oxidation resistance.

The second topic is the post-processing treatments related to microstructural evolution and mechanical properties. Owing to the thermal history during the LPBF process, the as-built microstructure commonly consists of columnar grains nearly parallel to the building direction with strong crystallographic texture. Subjected to the post-processing treatments, the solution treatment is the key to controlling the grain evolution. It has been shown for both LPBF MAD542 and heritage LPBF CM247 superalloys, the high crystallographic texture is maintained at the sub-γ′-solvus temperatures because of the grain boundary pinning effect from grain boundary precipitates. Whilst the crystal anisotropy is highly reduced by the treatment at super-γ′-solvus temperatures driven by the means of recrystallization. However, fully recrystallized microstructure with low texture largely reduced the mechanical properties by the embrittlement manner at elevated temperatures accordingly.

The third topic is the examination of creep and oxidation performance of various LPBF superalloys. A strong building direction-dependent creep performance is found for an LPBF IN738LC superalloy fabricated by the vertical and horizontal build. Vertically built samples show 7-40 times longer rupture life and approximately 2 times longer elongation at fracture than the horizontally produced samples, for the creep at 150-300 MPa at 850 °C. To evaluate the short-term creep performance, constant displacement rate-controlled slow strain rate tensile (SSRT) testing was carried out. The constant load-controlled creep and SSRT are correlated by deformation rate-based power-law type analysis. The new superalloy LPBF MAD542 generally displays a 5 times slower deformation rate than the LPBF IN738LC superalloy at 850 °C. The new superalloy Alloy738+ shows a comparable creep performance to LPBF IN738LC. Oxidation tests were conducted at 850/950/1050 °C. The new superalloy Alloy738+ presents an excellent oxidation resistance at 850 and 950 °C. By comparison, for example, Alloy738+ has 3 times slower oxidation kinetics than IN738LC at 950 °C.

The several investigations associated with the composition/processing/property in multiple precipitation-strengthened nickel-based superalloys fabricated by AM in this thesis have proven that the materials development requires comprehensive in-depth considerations. The presented results can contribute to the fundamental understanding and/or serve as the reference data for other superalloys by AM from the properties perspective.

In this study, Alloy 247LC samples were built with different laser powder bed fusion (L-PBF) process parameters. The samples were then subjected to solution heat treatment at 1260 degrees C for 2 h. The grain size of all the samples increased significantly after the heat treatment. The relationship between the process parameters and grain size of the samples was investigated by performing a design of experiment analysis. The results indicated that the laser power was the most significant process parameter that influenced the grain height and aspect ratio. The laser power also significantly influenced the grain width. The as-built and as-built + heat-treated samples with high, medium, and low energy densities were characterized using a field emission gun scanning electron microscope equipped with an electron backscatter diffraction detector. The micrographs revealed that the cells present in the as-built samples disappeared after the heat treatment. Isolated cases of twinning were observed in the grains of the as-built + heat-treated samples. The disappearance of cells, increase in the grain size, and appearance of twins suggested that recrystallization occurred in the alloy after the heat treatment. The occurrence of recrystallization was confirmed by analyzing the grain orientation spread of the alloy, which was lower and more predominantly <1 degrees in the as-built + heat-treated conditions than in the as-built conditions. The microhardness of the as-built + heat-treated samples were high which was plausible because gamma precipitates were observed in the samples. However, the L-PBF process parameters had a very low correlation with the microhardness of the as-built + heat-treated samples.

The additive manufacturability of nickel-based superalloys for laser powder bed fusion (LPBF) technologies is studied by considering the in-process cracking mechanisms. The additive manufacturability of nickel-based superalloys largely depends on the resistance to the liquid and solid-state cracking. Herein, we propose a two-parameter-based, heat resistance and deformation resistance (HR-DR) model, accounting for the relation between chemical composition (both major and minor elements) and cracking susceptibility, which is generalized from the elemental microsegregation behavior and mechanisms of LPBF process induced cracking. The proposed model is validated by the LPBF experiments in this study and by the hitherto reported data in LPBF superalloys community. The HR-DR-model is found to be a theoretically acceptable and easy-to-use approach for the prediction of in-process cracking of nickel-based superalloys during LPBF. The influence of alloying elements and the gamma precipitates on the additive manufacturability is discussed. The model provides a path for designing not only new solid solutioning, but also and more importantly gamma strengthened nickel-based superalloys for LPBF applications. (c) 2022 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Nickel-based superalloys, an alloy system bases on nickel as the matrix element with the addition of up to 10 more alloying elements including chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. Through the development and improvement of nickel-based superalloys in the past century, they are well proved to show excellent performance at the elevated service temperature. Owing to the combination of extraordinary high-temperature mechanical properties, such as monotonic and cyclic deformation resistance, fatigue crack propagation resistance; and high-temperature chemical properties, such as corrosion and oxidation resistance, phase stability, nickel-based superalloys are widely used in the critical hot-section components in aerospace and energy generation industries.

The success of nickel-based superalloy systems attributes to both the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed manufacturing processes. The microstructure of the modern nickel-based superalloys consists of a two-phase configuration: the intermetallic precipitates (Ni,Co)3(Al,Ti,Ta) known as γ′ phase dispersed into the austenite γ matrix, which is firstly introduced in the 1940s.  The recently developed additive manufacturing (AM) techniques, acting as the disruptive manufacturing process, offers a new avenue for producing the nickel-based superalloy components with complicated geometries. However, γ′ strengthened nickel-based superalloys always suffer from the micro-cracking during the AM process, which is barely eliminated by the process optimization.

On this basis, the new compositions of γ′ strengthened nickel-based superalloy adapted to the AM process are of great interest and significance. This study sought to design novel γ′ strengthened nickel-based superalloys readily for AM process with limited cracking susceptibility, based on the understanding of the cracking mechanisms. A two-parameter model is developed to predict the additive manufacturability for any given composition of a nickel-based superalloy. One materials index is derived from the comparison of the deformation-resistant capacity between dendritic and interdendritic regions, while another index is derived from the difference of heat resistant capacity of these two spaces. By plotting the additive manufacturability diagram, the superalloys family can be categorized into the easy-to-weld, fairly-weldable, and non-weldable regime with the good agreement of the existed knowledge. To design a novel superalloy, a Cr-Co-Mo-W-Al-Ti-Ta-Nb-Fe-Ni alloy family is proposed containing 921,600 composition recipes in total. Through the examination of additive manufacturability, undesired phase formation propensity, and the precipitation fraction, one composition of superalloy, MAD542, out of the 921,600 candidates is selected.

Validation of additive manufacturability of MAD542 is carried out by laser powder bed fusion (LPBF). By optimizing the LPBF process parameters, the crack-free MAD542 part is achieved. In addition, the MAD542 superalloy shows great resistance to the post-processing treatment-induced cracking. During the post-processing treatment, extensive annealing twins are promoted to achieve the recrystallization microstructure, ensuring the rapid reduction of stored energy. After ageing treatment, up to 60-65% volume fraction of γ′ precipitates are developed, indicating the huge potential of γ′ formation. Examined by the high-temperature slow strain rate tensile and constant loading creep testing, the MAD542 superalloy shows superior strength than the LPBF processed and hot isostatic pressed plus heat-treated IN738LC superalloy. While the low ductility of MAD542 is existed, which is expected to be improved by modifying the post-processing treatment scenarios and by the adjusting building direction in the following stages of the Ph.D. research.

MAD542 superalloy so far shows both good additive manufacturability and mechanical potentials. Additionally, the results in this study will contribute to a novel paradigm for alloy design and encourage more γ′-strengthened nickel-based superalloys tailored for AM processes in the future.

The fabrication of gamma prime (γ′) strengthened nickel-based superalloys by additive manufacturing (AM) techniques is of huge interest from the industrial and research community owing to their excellent high-temperature properties. The effect of post-AM-processing heat treatment on the microstructural characteristics and microhardness response of a laser powder bed fused (LPBF) γ′ strengthened nickel-based superalloy, MAD542, is systematically investigated. Post-processing heat treatment shows the significant importance of tailoring the γ′ morphology. With insufficient solutioning duration time, coarse γ′ formed in the interdendritic region heterogeneously, due to the lack of chemical composition homogenization. The cooling rate from the super-solvus solutioning plays an important role in controlling the γ′ size and morphology. Spherical γ′ is formed during the air cooling while irregularly shaped γ′ formed during the furnace cooling. The following aging heat treatment further tunes the γ′ morphology and γ channel width. After two-step aging, cuboidal γ′ is developed in the air-cooled sample, while in contrast, bi-modally distributed γ′ is developed in the furnace cooled sample with fine spherical γ′ embedded in the wide γ channel between coarse irregular shaped secondary γ′. More than 90% of the grains recrystallized during solutioning treatment at the super-solvus temperature for 30 min. The rapid recrystallization kinetics are attributed to the formation of annealing twins which significantly reduced the stored energy. Microhardness responses from different heat-treated conditions were examined.

The post-processing on the additively manufactured component is of huge interest as the key to tailor the microstructure to obtain certain mechanical properties. In this present study, the effects of hot isostatic pressing, as well as heat treatment on the microstructure, phase configuration and mechanical properties of laser powder bed fused (LPBF) IN718 superalloy were systematically investigated. Three different post-processes were studied such as hot isostatic pressing (HIP), heat treatment (HT), and HIP followed by HT (HIP+HT). The HIP process effectively eliminated the Laves phase remained in the as-built microstructure and brought uniformly distributed super fine γ″ precipitates in nano-meter size. In the heat-treated microstructure, larger γ″ precipitates were promoted directly from the as-built material. In comparison the HIP+HT process caused a moderate growth of γ″. In the latter two cases, the developed γ″ significantly strengthened the material. Yield strength of IN718 was increased from 738 MPa in as-built condition to 1015 MPa and 1184 MPa after HT and HIP+HT, respectively. On the contrary the ductility in the as-built IN718 condition was reduced by more than 40% after HT and HIP+HT. This can be compared to an increase in the ductility by almost 30% when subjected the as-built specimens to only HIPping. Finally, the correlation between microstructure evolution and mechanical properties is discussed in detail.

Microstructural evolution related to the mechanical response from isothermal dwell-fatigue testing at 700 °C of two austenitic steels, Esshete 1250 and Sanicro 25, is reported. Coherent Cu-precipitates and incoherent Nb-carbides were found to impede dislocation motion, increase hardening and improving the high temperature properties of Sanicro 25. Sparsely placed intergranular Cr- and Nb-carbides made Esshete 1250 susceptible to creep damage and intergranular crack propagation, mainly from interaction of the carbides and fatigue induced slip bands. Dynamic recrystallization of the plastic zone at the crack tip appeared to affect crack propagation of Sanicro 25 by providing an energetically privileged path.

Relationships between microstructures and hardening nature of laser powder bed fused (L-PBF) 316 L stainless steel have been studied. Using integrated experimental efforts and calculations, the evolution of microstructure entities such as dislocation density, organization, cellular structure and recrystallization behaviors were characterized as a function of heat treatments. Furthermore, the evolution of dislocation-type, namely the geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs), and their impacts on the hardness variation during annealing treatments for L-PBF alloy were experimentally investigated. The GND and SSD densities were statistically measured utilizing the Hough-based EBSD method and Taylor's hardening model. With the progress of recovery, the GNDs migrate from cellular walls to more energetically-favourable regions, resulting in the higher concentration of GNDs along subgrain boundaries. The SSD density decreases faster than the GND density during heat treatments, because the SSD density is more sensitive to the release of thermal distortions formed in printing. In all annealing conditions, the dislocations contribute to more than 50% of the hardness, and over 85.8% of the total dislocations are GNDs, while changes of other strengthening mechanism contributions are negligible, which draws a conclusion that the hardness of the present L-PBF alloy is governed predominantly by GNDs.

An experimental printable gamma -strengthened nickel-based superalloy, MAD542, is proposed. By process optimization, a crack-free component with less than 0.06% defect was achieved by laser powder bed fusion (LPBF). After post-processing by solution heat treatment, a recrystallized structure was revealed, which was also associated with the formation of annealing twins. After the aging treatment, 60-65% gamma precipitates were obtained with a cuboidal morphology. The success of printing and post-processing the new MAD542 superalloy may give new insights into alloy design approaches for additive manufacturing.

The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail.

Additive manufacturing (AM) of high γ′ strengthened Nickel-base superalloys, such as IN738LC, is of high interest for applications in hot section components for gas turbines. The creep property acts as the critical indicator of component performance under load at elevated temperature. However, it has been widely suggested that the suitable service condition of AM processed IN738LC is not yet fully clear. In order to evaluate the short-term creep behavior, slow strain rate tensile (SSRT) tests were performed. IN738LC bars were built by laser powder-bed-fusion (L-PBF) and then subjected to hot isostatic pressing (HIP) followed by the standard two-step heat treatment. The samples were subjected to SSRT testing at 850 °C under strain rates of 1 × 10−5/s, 1 × 10−6/s, and 1 × 10−7/s. In this research, the underlying creep deformation mechanism of AM processed IN738LC is investigated using the serial sectioning technique, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). On the creep mechanism of AM polycrystalline IN738LC, grain boundary sliding is predominant. However, due to the interlock feature of grain boundaries in AM processed IN738LC, the grain structure retains its integrity after deformation. The dislocation motion acts as the major accommodation process of grain boundary sliding. Dislocations bypass the γ′ precipitates by Orowan looping and wavy slip. The rearrangement of screw dislocations is responsible for the formation of subgrains within the grain interior. This research elucidates the short-creep behavior of AM processed IN738LC. It also shed new light on the creep deformation mechanism of additive manufactured γ′ strengthened polycrystalline Nickel-base superalloys.