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1. Additive Manufacturing of High-Entropy Alloys: A Review

   https://doi.org/10.3390/e20120937  

   Chen, Shuying ; Tong, Yang ; Liaw, Peter ( December 2018 , Entropy)

   Owing to the reduced defects, low cost, and high efficiency, the additive manufacturing (AM) technique has attracted increasingly attention and has been applied in high-entropy alloys (HEAs) in recent years. It was found that AM-processed HEAs possess an optimized microstructure and improved mechanical properties. However, no report has been proposed to review the application of the AM method in preparing bulk HEAs. Hence, it is necessary to introduce AM-processed HEAs in terms of applications, microstructures, mechanical properties, and challenges to provide readers with fundamental understanding. Specifically, we reviewed (1) the application of AM methods in the fabrication of HEAs andmore »(2) the post-heat treatment effect on the microstructural evolution and mechanical properties. Compared with the casting counterparts, AM-HEAs were found to have a superior yield strength and ductility as a consequence of the fine microstructure formed during the rapid solidification in the fabrication process. The post-treatment, such as high isostatic pressing (HIP), can further enhance their properties by removing the existing fabrication defects and residual stress in the AM-HEAs. Furthermore, the mechanical properties can be tuned by either reducing the pre-heating temperature to hinder the phase partitioning or modifying the composition of the HEA to stabilize the solid-solution phase or ductile intermetallic phase in AM materials. Moreover, the processing parameters, fabrication orientation, and scanning method can be optimized to further improve the mechanical performance of the as-built-HEAs.« less
2. Investigating the Precipitation Kinetics and Hardening Effects of γ” in Inconel 625 Using a Combination of Meso-Scale Phase-Field Simulations and Macro-Scale Precipitate Strengthening Calculations

   https://doi.org/10.1115/IMECE2020-23328  

   Yenusah, Caleb O. ; Ji, Yanzhou ; Liu, Yucheng ; Stone, Tonya W. ; Horstemeyer, Mark F. ; Chen, Lei ( November 2020 , Proceedings of the ASME 2020 International Mechanical Engineering Congress and Exposition)

   Precipitation strengthening of alloys by the formation of secondary particles (precipitates) in the matrix is one of the techniques used for increasing the mechanical strength of metals. Understanding the precipitation kinetics such as nucleation, growth, and coarsening of these precipitates is critical for evaluating their hardening effects and improving the yield strength of the alloy during heat treatment. To optimize the heat treatment strategy and accelerate alloy design, predicting precipitate hardening effects via numerical methods is a promising complement to trial-and-error-based experiments and the physics-based phase-field method stands out with the significant potential to accurately predict the precipitate morphologymore »and kinetics. In this study, we present a phase-field model that captures the nucleation, growth, and coarsening kinetics of precipitates during isothermal heat treatment conditions. Thermodynamic data, diffusion coefficients, and misfit strain data from experimental or lower length-scale calculations are used as input parameters for the phase-field model. Classical nucleation theory is implemented to capture the nucleation kinetics. As a case study, we apply the model to investigate γ″ precipitation kinetics in Inconel 625. The simulated mean particle length, aspect ratio, and volume fraction evolution are in agreement with experimental data for simulations at 600 °C and 650 °C during isothermal heat treatment. Utilizing the meso-scale results from the phase-field simulations as input parameters to a macro-scale coherency strengthening model, the evolution of the yield strength during heat treatment was predicted. In a broader context, we believe the current study can provide practical guidance for applying the phase-field approach as a link in the multiscale modeling of material properties.

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3. Processing, Microstructures and Mechanical Properties of a Ni-Based Single Crystal Superalloy

   https://doi.org/10.3390/cryst10070572  

   Ding, Qingqing ; Bei, Hongbin ; Zhao, Xinbao ; Gao, Yanfei ; Zhang, Ze ( July 2020 , Crystals)

   A second-generation Ni-based superalloy has been directionally solidified by using a Bridgman method, and the key processing steps have been investigated with a focus on their effects on microstructure evolution and mechanical properties. The as-grown microstructure is of a typical dendrite structure with microscopic elemental segregation during solidification. Based on the microstructural evidence and the measured phase transformation temperatures, a step-wise solution treatment procedure is designed to effectively eliminate the compositional and microstructural inhomogeneities. Consequently, the homogenized microstructure consisting of γ/γ′ phases (size of γ′ cube is ~400 nm) have been successfully produced after a two-step (solid solution and aging)more »treatment. The mechanical properties of the resulting alloys with desirable microstructures at room and elevated temperatures are measured by tensile tests. The strength of the alloy is comparable to commercial monocrystalline superalloys, such as DD6 and CMSX-4. The fracture modes of the alloy at various temperatures have also been studied and the corresponding deformation mechanisms are discussed.« less
4. Selective laser melting of metal structures onto graphite substrates via a low melting point interlayer alloy

   https://doi.org/https://doi.org/10.1016/j.apmt.2021.101334  

   Arad Azizi, Xiaobo Chen ( March 2022 , Applied materials today)

   We demonstrate a process to selective laser melt a metal alloy directly onto graphite. The heat trans- fer applications of metal features printed onto annealed pyrolytic graphite are compelling, as pyrolytic graphite has the second highest in-plane thermal conductivity ( > 1500 W/m-K at room temperature) of any bulk material. While the bonding of metal alloys commonly used in selective laser melting (SLM) with graphite is relatively weak, the proper interlayer material drastically improves the wetting and bond- ing. The challenge is the alloys that typically bond to graphite require extended bonding times at elevated temperatures (minutes to hours), whilemore »the SLM process delivers only brief exposures to high tempera- tures ( ∼100 μs). In this paper, we employ a Sn3Ag4Ti alloy that rapidly forms a nanometer-thin layer of TiC, as verified by transmission electron microscopy. The influence of graphite thermal properties on interfacial bond strength is shown by mechanical testing and simulations of selective laser melting.« less
5. Supersolvus Hot Workability and Dynamic Recrystallization in Wrought Co–Al–W-Base Alloys.

   https://doi.org/10.1007/978-3-030-51834-9_84  

   Wertz, K. ; Weaver, D. ; Wen, D. ; Titus, M.S. ; Shivpuri, R. ; Niezgoda, S.R. ; Mills, M.J. ; Semiatin, S.L. ( January 2020 , Superalloys 2020)

   Gamma-prime strengthened Co–Al–W-based superalloys offer a unique combination of weldability, mechanical strength, creep resistance, and environmental resistance at temperature—leading many to consider the system as an alternative to nickel-base superalloys for future generation turbine engine hardware. However, little information exists regarding the deformation processing required to turn these novel alloys into useable product forms with appropriate microstructure refinement. Supersolvus thermomechanical processing sequences were successfully demonstrated using right-cylindrical upset specimens for two wrought γ′-strengthened cobalt-base superalloys at industrially relevant temperatures and deformation rates. Hot flow behavior and microstructure evolution were quantitatively characterized and compared to available information on a legacy nickel-basemore »system, Waspaloy. Further, density functional theory was used to explore the compositional dependency of the intrinsic material properties influencing single-phase hot working behavior of model Ni–Al binary and Co–Al–W ternary systems. The apparent similarity in the supersolvus thermomechanical processing behavior of Co–Al–W-base systems and their two-phase γ–γ′ Ni-base counterparts suggests conventional pathways, models, and equipment may be leveraged to speed transition and implementation of wrought Co–Al–W-base alloys for components where their properties may be advantageous.« less