Abstract High-aluminum refractory compositionally complex alloys are intrinsically brittle due to ordering and grain-boundary intermetallics. We show that extreme thermal gradients during selective laser melting kinetically suppress these transformations in NbMoCrTiAl, stabilizing a disordered A2 solid solution despite strong thermodynamic driving forces for B2 ordering and C14/A15 formation. Multi-scale characterization and kinetic simulations reveal how rapid liquid-solid and solid-state transformations dissolve intermetallic phases and prevent reordering during cooling. The resulting precipitate-free A2 microstructure exhibits substantially enhanced room-temperature plasticity, overcoming brittleness without compositional modification. Our results establish non-equilibrium processing as a route to access otherwise inaccessible phase states in refractory complex alloys, expanding the design space of high-temperature structural materials.

This study investigates the influence of hydrogen charging on the mechanical behaviour and deformation mechanisms of a nanoparticle-reinforced CoCrFeNi medium-entropy alloys fabricated via laser powder bed fusion (LPBF). Changing the precursor nanoparticles from TiN to TiO2, resulted in a change in the microstructure from columnar to equiaxed, while all the printing parameters remained the same. Nanoindentation mapping was conducted on both the microstructures before and after electrochemical hydrogen charging to quantify any variation in mechanical behaviour. The columnar matrix grain morphology reveals progressive matrix softening with hydrogen exposure, with hardness decreasing from 3.53 ± 0.16 GPa to 2.84 ± 0.19 GPa after 24 h charging, attributed to hydrogen-enhanced localized plasticity (HELP). In contrast, the same alloy with an equiaxed microstructure exhibited minimal softening, demonstrating that grain morphology governs hydrogen susceptibility. Scanning transmission electron microscopy analysis from beneath the nanoindents revealed that while the uncharged state exhibited predominant deformation twinning, the hydrogen-charged specimens displayed spatially heterogeneous deformation features characterized by well-defined subgrains and deformation twins, indicating a net softening effect where HELP is partially offset by twin-mediated hardening. Thermal desorption spectroscopy yielded a binding energy of 24 kJ/mol, which is indicative of dislocations acting as the dominant hydrogen traps given the extremely high defect density inherent to LPBF processing. The dispersed nanoparticles maintain their strengthening effect while showing no evidence of preferential hydrogen accumulation at particle-matrix interfaces. These findings establish the critical role of grain boundary tortuosity and interfacial trap density in controlling embrittlement resistance in additively manufactured nanocomposite alloys.

Additive manufacturing of bimetallic parts has gathered significant interest in recent years. Of particular relevance is the combination of Ni-based superalloys and Cu-based alloys suitable for high temperature applications, such as those found in aerospace rocket engines. To provide confidence in the production of such bimetallic parts, a detailed understanding of the process-structure relationship is required. This study investigates the effect of deposition sequence and recoating direction on the interfacial microstructure evolution in samples where material is graded through horizontal, vertical and angled interfaces. The samples in this study are produced through laser powder bed fusion of IN718 and GRCop-42, using a Schaeffler Aerosint selective powder deposition recoater. Analysis was conducted through a combination of backscatter electron imaging, x-ray diffraction, energy dispersive spectroscopy, and electron backscatter diffraction. For a horizontal interface, the deposition sequence has a significant effect on the presence of defects and phase formation at the interface. The recoating direction is shown to have an influence on the mixing behaviour for vertical and angled interfaces, with a gradual transition of the alloys produced when the interface is aligned parallel to the recoating direction. In contrast either a significant crossing of the first-deposited material across the interface occurs, or a sudden change in composition develops when deposition is perpendicular to the interface. On the basis of these findings, design rules can be developed to ensure the fabrication of suitably designed bimetallic parts for high temperature applications such as rocket engines in the aerospace sector.

Nanoparticle reinforced metallic composites manufactured using laser powder bed fusion (LPBF) provide an economically viable avenue to obtain high strength near-net shaped critical components in automotive and aviation industry. In this study, the equiatomic compositionally complex alloy (CCA) CoCrFeNi is manufactured by LPBF with two types of reinforcing particles, titanium nitride (TiN) and titanium oxide (TiO2). The reinforcing particles are introduced with varying size and volume concentration to the CCA powder fulfilling two purposes – improving the flowability of the feedstock and to cause nanoparticle strengthening, as demonstrated here using nanoindentation. We focus on the microstructure and texture evolution of all the alloys, and the phase transitioning of the particles. TiN nanoparticles of all sizes dissolve in the melt pool and uniformly precipitate as TiO2. We observed distinctive core-shell nanoparticle formation with higher TiN content that also resulted in 45 % higher tensile strength. The successful integration of homogenously distributed nanoparticles without inducing cracks or defects and with superior mechanical properties signifies a leap forward in the fabrication of high-performance metal matrix composites like the commercial oxide dispersion strengthened (ODS) alloys.

Complex components for high-temperature gas turbine applications require materials that offer a combination of excellent high-temperature strength and oxidation resistance. Nickel-based superalloys with high gamma prime (gamma)' volume fractions are particularly suited for these applications, especially combined with additive manufacturing for intricate geometries. Despite the complex thermal history that these materials experience during laser powder bed fusion (LPBF) processing, gamma'formation is suppressed when manufacturing IN738LC, which has a medium equilibrium gamma'content of about 40-50 vol%. This study follows gamma'formation in LPBF IN738LC during subsequent annealing treatments at temperatures ranging from 745 C-degrees to 865 C-degrees, creating an experimentally determined TTT (temperature-time-transformation) diagram. This diagram is largely based on scanning electron microscopy (SEM) imaging supported by Vickers hardness measurements and scanning transmission electron microscopy (STEM) bright field imaging. Atom probe tomography (APT) of the as-built material indicated nm-sized regions depleted in Cr and enriched in Ni, Al, and Ti, but show no characteristic superlattice patterns in TEM diffraction. APT and TEM diffraction analysis of material annealed at 850 C for 3 min confirmed the presence of the gamma'phase but indicated that gamma'had formed through spinodal decomposition instead of precipitation.

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Additive manufacturing (AM) by laser powder bed fusion (LPBF) involves melting of layers of powder onto a substrate, called a building platform.Due to cost or convenience considerations, building platform materials rarely match the LPBF material, especially for high temperature materials.To ensure tolerances in component geometries, AM components are often stress-relieved/heat-treated while still attached to the building platform.It is therefore important to understand the effect of dissimilar building platform materials on the properties of the built-up material.These effects may be particularly important for high performance materials such as Ni-base superalloys used for critical applications in the aerospace and energy industries.To investigate this effect, samples of a Ni-base superalloy HAYNES® 282® were built onto a carbon steel building platform in several configurations.The samples were removed from the building platform after heat treatment and subjected to detailed composition analysis and microstructural characterization to investigate the effect of the building platform material on the properties of the additively manufactured part.Room temperature and high temperature tensile testing were used to characterize the material.Results showed no risk of large-scale chemical composition change, or mechanical property degradation of built-up material from on-platform heat treatment.

The demand for manufacturing increasingly complex geometries for high temperature applications drives the increasing interest into additive manufacturing of nickel-based superalloys. Of particular interest are superalloys with high contents of the strengthening phase y' such as IN738LC. Previous research suggests that especially B and Zr have a detrimental influence on crack formation during the laser powder bed fusion (LPBF) process. The present study investigates solidification cracks in an IN738LC derivative with increased B (0.03 wt.%) and Zr (0.07 wt.%) in more detail using high resolution techniques such as transmission electron microscopy (TEM) and atom probe tomography (APT). Analysis of the bulk material shows a high number of MC carbides containing Ti and Cr. The concentration profiles indicate non-equilibrium carbide compositions by suggesting that Cr is pushed out of these particles. The carbides are surrounded by a thin B-rich layer at the metal/carbide interface. Analysis of the fracture surface shows both Zr and small amounts of B in the formed oxide layer. The presence of these elements together with thermodynamic calculations and previously reported findings of the same material variant support the hypothesis that low-melting phases are likely reasons for cracking of IN738LC.

One of the factors limiting the use of additive manufacturing, particularly powder bed processes, is their low productivity. An approach to increasing laser powder bed fusion (LPBF) build rate without costly hardware modifications is to alter process parameters. This study evaluates the possibilities to increase build rates through this route without compromising material quality. Equations for productivity are derived based on process parameters and build geometry, and applied on the process window for Hastelloy X in LPBF. It is demonstrated that virtually flaw-free parts can be printed at build rates that differ up to tenfold. To investigate potential variations in the microstructure and performance, Hastelloy X specimens manufactured at varying build rates were characterized. Electron backscattered diffraction (EBSD) analysis revealed that the specimen built at the lowest rate shows strong texture with columnar grains, while the specimen built at the highest rate presents significantly more random orientation and evident melt pool contours with pockets of very fine grains at the bottom. Despite the major differences in microstructure, the tensile properties do not necessarily vary substantially. Thus, the results indicate that the build rate of LPBF Hastelloy X can be significantly varied based on process parameters, still yielding consistent mechanical properties.

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.

This study aims to provide a fundamental understanding of the role of microstructural characteristics influencing tool wear when machining Alloy 718 fabricated using Powder Bed Fusion (PBF). The effects of preferred crystallographic orientation (texture), shape and distribution of grains, local misorientation, type and amount of precipitates as well as the type, size and amount of abrasive carbides, nitrides and oxides on tool wear are investigated in as-built condition and after the standard solutionising and double-aging treatment. The microstructures of workpiece materials and the surfaces of worn tools were examined using different material characterisation techniques, including Scanning Electron Microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). A dislocation-based approach was used to reveal the cumulative effects of the microstructural characteristics on deformation behaviour and the thermo-mechanical loads on the tools during cutting. The analyses suggest that texture and the extent of material work-hardening prior to the onset of crack formation markedly influence the amount of plastic work and thus heat generation when machining Electron Beam Powder Bed Fusion (EB-PBF) material. The higher heat generation in the cutting zones provokes thermally-induced wear mechanisms like diffusion-dissolution and oxidation. In addition, the larger amount of hard oxide inclusions present in EB-PBF material leads to higher wear by abrasion. In contrast to the prevailing experimental approaches in this field, the present investigation is built on a physics-based framework to understand the fundamental aspects that govern material deformation and heat generation in cutting and, consequently, tool wear mechanisms. This framework can be used for machinability assessment of any alloy manufactured by different additive manufacturing (AM) technologies and for optimising the process-chain, including printing strategies and thermal post-treatments, to improve the machinability of AM alloys by tailoring their microstructure.

Ni-base superalloys are essential materials for high-temperature applications in the energy and aerospace sectors. Significant benefits in design, function, and manufacture of high-temperature components may be realized from additive manufacturing (AM) of these materials. However, because of cracking issues during AM fabrication, only a handful of materials have been tried and qualified. This article provides an initial evaluation of the processability and properties of Haynes 282 by laser-powder bed fusion (LPBF), which is a relatively new Ni-base superalloy with properties superior to those of many legacy wrought superalloys. The results demonstrated that crack-free Haynes 282 can be manufactured by means of LPBF with full density. The mechanical properties at ambient temperature exceeded the properties of the reference material in the as-built and heat-treated conditions, albeit with significant anisotropy. Mechanical properties at 800 ?C indicated that the yield strength of heattreated Haynes 282 by LPBF was comparable to that of the reference material, however, ductility was significantly reduced. Promising stress rupture performance also indicates that Haynes 282 is an ideal candidate for adoption in additive manufacturing, especially if heat treatments can be re-designed for the additively manufactured as-built microstructure.

The precipitation kinetics of gamma prime in the nickel based superalloy RR1000 has been characterised after solid-solution heat treatments and isothermal aging conditions relevant to service conditions. Multimodal precipitate dispersions are formed within the alloy. Numerical methods are presented for determining the three dimensional size of the particle populations combining information obtained from Scanning Electron microscopy and Transmission Electron microscopy. This information has been used to develop a multicomponent mean-field model descriptive of precipitation kinetics. The smallest particle population increases in mean size during isothermal aging at 700 BC where classical mean-field models of coarsening kinetics suggest that these particles should dissolve. A phenomenological model has been proposed to capture this behaviour within a statistical formulation that is applicable to both processing and service conditions. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

This work investigates coarsening behavior of strengthening precipitates ( γ ′) in coarse-grained RR1000 after isothermal exposure for various times (50 to 500 hours) at 700 °C and 750 °C. The impact of these isothermal treatments is then studied by carrying out dwell (1 hour) fatigue crack growth tests at 700 °C in air. Such dwell periods can increase crack growth rates by nearly two orders of magnitude compared with baseline (0.25 Hz trapezoidal waveform) fatigue crack growth tests. Predominantly, scanning electron microscopy was used for γ ′ analysis. Transmission electron microscopy was also utilized when necessary. Overaging as a thermodynamically driven and diffusion-controlled process strongly affects tertiary γ ′ precipitates for the temperatures and treatment times investigated here. No influence of overaging on baseline fatigue behavior is observed. However, after overaging at 750 °C for 500 hours (which increases the mean tertiary γ ′ size by a factor of two from 17 to 37 nm), dwell crack growth rates at 700 °C are reduced by one order of magnitude. Increased dwell fatigue crack growth resistance is also measured after overaging for 100 hours at 700 °C. The potential influence of γ ′ distributions on dwell fatigue crack growth resistance is discussed in terms of stress relaxation behavior during dwell periods.