We present a brief review of the microstructures and mechanical properties of selected metallic alloys processed by additive manufacturing (AM). Three different alloys, covering a large range of ...technology readiness levels, are selected to illustrate particular microstructural features developed by AM and clarify the engineering paradigm relating process-microstructure-property. With Ti-6Al-4V the emphasis is placed on the formation of metallurgical defects and microstructures induced by AM and their role on mechanical properties. The effects of the large in-built dislocation density, surface roughness and build atmosphere on mechanical and damage properties are discussed using steels. The impact of rapid solidification inherent to AM on phase selection is highlighted for high-entropy alloys. Using property maps, published mechanical properties of additive manufactured alloys are graphically summarized and compared to conventionally processed counterparts.
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High-entropy alloys (HEA) are of high current interest due to their unique and attractive combination of structural, physical, chemical or magnetic properties. HEA comprise multiple ...principal elements, unlike conventional alloys. The composition space of HEA is enormous and only a minuscule fraction has been studied. Magnetic HEA are a promising alternative to conventional soft magnetic metallic materials, which typically exhibit poor mechanical properties. We review the progress in the development of magnetic HEA. The influence of alloy composition, crystal structure, phase fraction and processing parameters on the magnetic properties are discussed. Magnetic HEA processed by advanced experimental high throughput techniques such as additive manufacturing, co-sputtering, diffusion multiples, rapid prototyping, and designed via combinatorial computational techniques, such as thermodynamic and phase diagram calculations, density functional theory, machine learning etc. are reviewed. Conventional processing techniques are also discussed. Future trends in magnetic HEA are outlined.
AlxCoCrFeNi is a prominent high entropy alloy system with varying crystal structure from FCC to BCC depending on aluminum content. The mechanical behavior of Al0.7CoCrFeNi with dual phase FCC+BCC ...microstructure has been compared with that of single phase FCC Al0.3CoCrFeNi. Both quasi-static and dynamic strain rate regimes were investigated. Hypo-eutectic Al0.7CoCrFeNi showed much higher strength due to fine lamellar microstructure with a large number of FCC-BCC interphase boundaries. But this also leads to lower strain rate sensitivity due to the long-range nature of these interfaces, overcoming them is indifferent with temperature elevation to assist slip, thus making them athermal barriers. Both these precipitation hardenable alloys were aged to induce precipitation of ordered L12 in the FCC phase. This coherent nano-scale L12 precipitate caused a significant increase in the yield strength of both single-phase and dual phase structures while reducing the strain rate sensitivity (SRS) only slightly. L12 precipitation in FCC matrix greatly enhanced twinning during dynamic deformation. Large-scale deformation twins were observed in coarse Al0.3CoCrFeNi FCC and FCC + L12 microstructures. The scale of deformation twins was much smaller in the dual phase Al0.7CoCrFeNi whose refined lamellae width retarded twinning. The lamellar structures, nevertheless, had higher work hardening due to their higher dislocation density storage capability.
Often the experimentally-observed, single-phase high entropy alloy (HEA) is the result of second-phase precipitation constrained by thermodynamic and kinetic factors. Using Al0.3CoCrFeNi as a ...candidate HEA, this paper demonstrates the strong influence of thermo-mechanical processing on the transformation pathway adopted for isothermal second-phase precipitation. A traditional thermo-mechanical processing route comprised of homogenization cold-rolling solution treatment in the single fcc phase region, followed by a precipitation anneal at a lower temperature, results in a homogeneous distribution of nanometer scale-ordered L12 (gamma prime-like) precipitates within the fcc matrix. In contrast, if cold-rolling is followed directly by annealing at the precipitation temperature, then the resulting microstructural evolution pathway changes completely, with concurrent recrystallization of the matrix fcc grains and precipitation of B2 and sigma phases, largely at the grain boundaries. These experimentally observed variations in transformation pathway have been rationalized via the competition between the thermodynamic driving force and activation barrier for second-phase nucleation in this alloy, coupled with the kinetics of the process. The microstructural variations that result from these dramatically different phase transformation pathways can lead to some rather exceptional mechanical properties that can be varied over a large range even for a single Al0.3CoCrFeNi HEA composition.
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Refractory high-entropy alloys (RHEAs) have recently attracted much attention, primarily due to their mechanical properties at elevated temperatures. However, the equilibrium phase-stability of these ...alloy systems is not well established. The present investigation focuses on the phase stability of Al0.5NbTa0.8Ti1.5V0.2Zr RHEA at temperatures ranging from 600 to 1200 °C. The detailed phase characterization involves coupling of scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The stable phases present at these temperatures are (i) 1200 °C—body-centered cubic (BCC) matrix with nano-B2 precipitates; (ii) 1000 °C and 800 °C—a BCC matrix phase with Al–Zr rich hexagonal closed packed intermetallic precipitates and, (iii) 600 °C—a BCC + B2 microstructure, comprising a continuous BCC matrix with discrete B2 precipitates. These results highlight the substantial changes in phase stability as a function of temperature in RHEAs, and high-entropy alloys in general, and also the importance of accounting for these changes especially while designing alloys for high temperature applications.
Accurate diagnosis, monitoring and treatment decisions in patients with chronic liver disease currently rely on biopsy as the diagnostic gold standard, and this has constrained early detection and ...management of diseases that are both varied and can be concurrent. Recent developments in multiparametric magnetic resonance imaging (mpMRI) suggest real potential to bridge the diagnostic gap between non-specific blood-based biomarkers and invasive and variable histological diagnosis. This has implications for the clinical care and treatment pathway in a number of chronic liver diseases, such as haemochromatosis, steatohepatitis and autoimmune or viral hepatitis. Here we review the relevant MRI techniques in clinical use and their limitations and describe recent potential applications in various liver diseases. We exemplify case studies that highlight how these techniques can improve clinical practice. These techniques could allow clinicians to increase their arsenals available to utilise on patients and direct appropriate treatments.
Significant theoretical efforts have been made to understand the Hall-Petch and inverse Hall-Petch relations of nanocrystalline pure metals, metallic glasses and binary alloy systems. However, only a ...few studies have investigated the Hall-Petch or inverse Hall-Petch relations in high-entropy alloys. In this work, phase stability of single-crystalline CoNiFeAlxCu1-x and uniaxial compression of polycrystalline CoNiFeAlxCu1-x are investigated by molecular dynamics simulation. Calculations of cohesive energies indicate that FCC structured CoNiFeAlxCu1-x is more stable at low Al concentrations (x ≤ 0.4) and BCC structured CoNiFeAlxCu1-x is more stable for high Al concentrations (x > 0.4). Based on the phase stability, FCC structured polycrystalline CoNiFeAl0·3Cu0.7 and BCC structured polycrystalline CoNiFeAl0·7Cu0.3 are constructed to perform uniaxial compression. Hall-Petch and inverse Hall-Petch relations are observed in both FCC and BCC structured polycrystalline CoNiFeAlxCu1-x. The microstructural evolutions of polycrystalline CoNiFeAlxCu1-x reveal that the dominant deformation mechanisms in the Hall-Petch regime of FCC structures are dislocation slip and deformation twinning due to relatively low stacking fault energy and that of BCC structures is phase transformation plasticity. For the inverse Hall-Petch relation, the dominant deformation mechanisms for both FCC and BCC HEAs are the rotation of grains and migration of grain boundaries. It indicates that FCC and BCC HEAs exhibit similar Hall-Petch and inverse Hall-Petch relations with the conventional polycrystalline materials, but its grain size exponent and gradient are quite different from those of pure metals.
The microstructure and magnetic properties of three face-centered cubic (FCC) FeCoNiCrCu(x) high entropy alloys (HEAs) (x = 0, 0.5, 1) are investigated. Interestingly, addition of the nonmagnetic ...element Cu to FeCoNiCr HEA is found to enhance exchange interactions and low temperature saturation magnetization. The paramagnetic to ferromagnetic Curie transition temperature increases from 85 K for FeCoNiCr to 118 K for FeCoNiCrCu. This is counterintuitive since Cu is nonmagnetic; however, atom probe tomography revealed Cu rich clusters containing 5 at% Ni and 1 at% each of Fe, Co, Cr, within FCC matrix, these clusters altered the matrix composition and consequently its magnetic properties.
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