Papers by Keyword: High-Entropy Alloy

Abstract: This study aimed to smelt high-entropy alloys (HEAs) composed of inexpensive and commonly available metallic elements using iron-based alloys, ligatures, and commercially pure metals, and to investigate their phase structure and heat-resistance. High-entropy alloys of the FeNiCrCuAl system were smelted in air using an induction furnace with a crucible lined with rammed neutral aluminum and magnesium oxides. The elements Fe, Ni, Cr, Cu, and Al were introduced via high-alloy cast iron, stainless steel grade GX10CrNiMn-18-9-1 (1.4541), industrial-grade low-carbon ferrochrome (FeCr70C1), binary Cu-33Al ligature, tough-pitch copper, and semi-finished nickel. Samples of the investigated alloys were prepared using lost foam and sand mold casting methods. Microstructural analysis revealed the presence of rounded dendritic branches, copper-rich interdendritic regions, and high-chromium carbides. The phase composition of the as-cast FeNiCrCuAl alloys consisted of multiple phases: solid solutions with a BCC structure ordered in the B2 type, an FCC structure, and complex carbides (FeCr)₇C₃. High-entropy alloys of the FeNiCrCuAl system, with increased aluminum and chromium content, can significantly outperform standard heat-resistant stainless steels in terms of oxidation resistance indicators – surface oxidation rate, and oxidation stability at 900°C and 1000°C. The specific oxidation of the high-entropy alloy FeNiCrCuAl, which contains at least 18 at. % chromium, was 0.1627 mg/cm² after a 4-hour exposure at 1000 °C. Under the same conditions, the specific oxidation of X2CrNi19-11 stainless steel (1.4306) was 0.6689 mg/cm².

Abstract: The microhardness of CoCrFeNiMnV_x (х = 0-2) high-entropy alloys (HEAs) was measured in the temperature range 77-293 K. At x ≤ 0.4, a significant monotonic increase in microhardness occurs with decreasing temperature, which indicates the thermally-activated character of plastic deformation of the material under the indenter. At x = 0.5, as well as at x = 0.75 and 0.85, athermal behavior of microhardness was detected in the ranges of 200-293 K and 150-293 K, respectively. The latter is apparently associated with the appearance in the indicated alloys, along with the FCC phase, of precipitates of the hard intermetallic sigma phase, which are athermal obstacles to the motion of dislocations. For the first time the microhardness of the sigma phase in the range of 77-293 K was measured; at 293 K and 77 K it was about 9.5 GPa and 12.5 GPa, respectively, which is approximately 5 times higher than the microhardness of the FCC alloy with x = 0.25.

Abstract: High-entropy alloys (HEAs) have excellent properties that are being explored for potential applications in many engineering fields. Their excellent properties strongly depend on their phases. The vastness of alloy compositions that can be synthesized makes it extremely challenging to experimentally investigate all the possible HEA types. To mitigate these challenges, more efficient and systematic computational techniques can be applied to the existing experimental data to accelerate HEA design and discovery. Therefore, this study developed three soft computing classification models based on artificial neural network, k-nearest neighbor (kNN), and support vector machine (SVM) to classify solid solution, amorphous and intermetallic phases in HEAs. Empirical studies showed that hyperparameter optimization improved classification accuracies of the classifiers with kNN (92%) outperforming ANN (86%) and SVM (90%) using all five predictive features. Feature selection did not improve the classification accuracy of any of the model. This studied demonstrated the importance of applying soft computing techniques and hyperparameter optimization for enhancing the classification accuracies of models to predict the phases in HEAs.

Abstract: The mechanical properties and microstructural evolution of a medium-entropy alloy Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₅ (at%) in a low temperature range (including the record low temperatures region down to 0.5 K) were investigated. It has been established that low-temperature plastic deformation initiates martensitic phase transformations in this alloy, and the values of the dynamic modulus of elasticity correlate with the degree of phase transformations.

Abstract: High-entropy alloys (HEAs) have led to breakthroughs in materials science due to their superior properties and the challenge of achieving the high strength and high ductility trade-off. Microstructural evolution during cold and warm compression tests of the single-phase Al₈Cr₁₂Mn₂₅Fe₃₅Ni₂₀ high entropy alloy (Fe-HEA) is investigated in the present work. The current study assesses the effect of temperature on the mechanical properties and deformation mechanism of the face-centered cubic structure Fe-HEA. The arc-melted ingot is homogenized at 1473 K and then directly hot-rolled to break the cast structure of the alloy prior to testing procedures. Fe-HEA is tested through uniaxial compressive testing at three different selected temperatures: 293, 473, and 673 K utilizing a Gleeble thermo-mechanical simulator at a strain rate of 0.001 s^-1. The compressive behavior at 673 K showed a higher strain hardening exponent when compared to 293 and 473 K. The deformed microstructural features of the compressed and quenched specimens, deformation mechanism, and phase revolution are investigated with X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). Dislocation densities for the deformed conditions were estimated to be 4.11 × 10¹⁴ and 5.39 × 10¹⁴ m^-2 for the 473 and 673 K deformed conditions, respectively. At a deformation temperature of 673 K, B2 precipitation is observed at the high-angle grain boundaries.

Abstract: In Cr-rich CoCrFeMnNi alloys, the precipitation of the σ phase at grain boundaries during recrystallization is so fast that ultrafine-grained structure is formed due to the pinning effect of the precipitates. The average grain size of the fcc parent phase is found to be consistent with modified Zener-Smith model. If conventional alloys come to equilibrium, volume fraction of precipitates should approach a saturation value. However, it is interesting to note that the volume fraction of the σ phase in Cr-rich CoCrFeMnNi alloys is inversely proportional to the average grain size of the fcc parent phase. For instance, in Co₂₀Cr₂₅Fe₂₀Ni₁₅Mn₂₀ alloys, the volume fraction changes from 6.5% to 1.2% with increasing average gran size from 14 μm to 210 μm even after annealing at 1273 K for 100 h. It is well known that heterogeneous nucleation of precipitates at grain boundary is energetically favorable and fast diffusion through grain boundary can assist the precipitation. However, they cannot account for the grain size dependence of the volume fraction after reaching equilibrium. Based on stereology, the reciprocal of grain size is proportional to grain boundary area. Thus, chemical fluctuation at grain boundaries (e.g. segregation) is considered to be related to the unusual precipitation at the grain boundaries.

Abstract: The work intends to study the microstructure, chemical and phase composition and homogeneity of chemical elements distribution in the Co-Cr-Fe-Mn-Ni high-entropy alloy produced via wire-arc additive manufacturing technology. The study has revealed three structure types in the alloy: (1) a smooth shagreen-type structure (an orange peel), which turns into a stripe-like structure (2) in some areas, and a grain structure (3) to appear as lengthy thin layers with the width of 50-80 μm and an average grain size of 12.5 μm, the most probable size of grains is detected to be in the range from 10 to 15 μm, a preferred number of such grains is 31%. The chemical composition of the produced alloy is assessed using X-ray microspectroscopy. The elements identified rank in descending order of concentration: Fe (38.88 wt. %), Co (26.08 wt. %), Ni (17.34 wt. %), Cr (14.33 wt. %), Mn (3.37 wt. %). The mapping of the alloy structure demonstrates the homogeneous and uniform distribution of chemical elements.

Abstract: The article presents the research results of the plasma jet multiple reflow effect over the multicomponent coating FeCoCrAlTiCuNiMo, obtained by plasma metallization in an open atmosphere, on its wear resistance under dry sliding friction. The research results indirectly confirm the influence of the coating entropy over the wear resistance increasing along of the reflow number growth.

Abstract: The article presents the assessment study of mechanical activation and mechanical alloying effect on the structure, phase state of the powder composition and of the coating based on it. The duration of mechanical activation was varied as follows: 1 h, 2 h, 4 h, 8 h, 16 h, 32 h. It is shown that in order to from a high-entropy FeNiCoAlNb alloy on the workpiece surface the duration of mechanical activation must be at least 4 hours. We described the mechanism of mechanical alloying and ways of accelerating this process.

Abstract: In this work, a high-entropy alloy and CrZrTiNiCu coating were synthesized by mechanical alloying. It is shown that the microhardness of the CrZrTiNiCu coating is not inferior to and in most cases exceeds the hardness of high-entropy equiatomic alloys. The wear resistance of such a coating is 3·10^-4 g/min, which also corresponds to special steels in terms of wear resistance. The high-entropy coating has a low coefficient of friction. It turns out to be antifrictional, which obviously leads to energy savings. For the first time, the surface energy, contact potential difference and work function of electrons for CrZrTiNiCu coating were determined.