High-entropy Matrix Research Articles

Hot-corrosion of refractory high-entropy ceramic matrix composites synthesized by alloy melt-infiltration

Carbon fiber-containing refractory high-entropy ceramic matrix composites (C/RHECs) were fabricated through a reaction with carbon powders, transition metal carbides, and Zr–Ti alloys as a novel heat resistant material used for components of hypersonic vehicles cruising at Mach 7–10. With the infiltration of alloys at 1750 °C into a composite preform containing carbon and carbide powders for 15 min, a high-entropy matrix was successfully formed in situ. Arc-jet tests were conducted in the temperature range of 1800–1900 °C. Results showed the formation of an oxidized region composed of complex oxides, such as (Zr, Hf)O2, (Nb, Ta)2(Zr, Hf)6O17, (Zr, Hf)TiO4, and Ti(Nb, Ta)2O7, with an average thickness of ~600 μm, under which an unoxidized region remained. The porous oxidized region resulted from the evolution of CO(g) during oxidation, while a dense oxide region formed as the outermost region. This indicates that the dense oxide region acted as a barrier to oxygen diffusion for the unoxidized region during oxidation.

A novel in-situ exothermic assisted sintering high entropy Al2O3/(NbTaMoW)C composites: Microstructure and mechanical properties

In this paper, we have designed a Al2O3/(NbTaMoW)C composite by in-situ exothermic reaction, which opened a new field to explore HEC matrix composites. The Al2O3/(NbTaMoW)C composite was synthesized at 1600 °C using Nb, Ta, W, MoO3, Al and graphite powders as raw materials, and the thermite of Al + MoO3 as heat source provides additional energy for the formation of composites. As-prepared composites with relative density 98.5% only exhibited two phases including a high entropy carbide of (NbTaMoW)C and Al2O3 phases, and Al2O3 randomly distributed within high entropy matrix. Phase boundary between (NbTaMoW)C and Al2O3 was a noncoherent interface, resulting in a moderate interfacial bonding strength which was beneficial to both flexural strength and fracture toughness. Moreover, Al2O3 and HEC form an interesting interlocking structure and a grain size varies from 1 μm to 10 μm due to the inhomogeneous temperature distribution introduced by the in-situ exothermic heat. By taking advantages of these special structures, the composites exhibited significantly enhanced mechanical properties compared to (NbTaMoW)C. The flexural strength of composites was up to 530 MPa, and the fracture toughness was 4.5 MPa m1/2. The main strengthening mechanism is second phase strengthening and the presence interlocking structure releases stress intensity at crack tip.

Precipitation-hardened high entropy alloys with excellent tensile properties

To achieve balance of high strength and high ductility remains a difficult problem for metallic materials, especially in as-cast alloys. We combined the design concepts of high-entropy alloys (HEAs) and Ni-based superalloys, and designed two new Ni48.7Co17.5Cr10Fe9.3Al7.5Ti7 and Ni47.7Co17.5Cr10Fe9.3Al7.5Ti7Mo1 (at%) precipitation-hardened cast HEAs. Both two alloys have face-centered cubic (fcc) high-entropy matrix + nano γ′ (Ni3(Al,Ti)-type) particles structure. Fine γ′ particles with the average diameter less than 70 nm and 130 nm are homogeneously distributed in dendritic regions and inter-dendritic regions, respectively. The yield strength of Ni47.7Co17.5Cr10Fe9.3Al7.5Ti7Mo1 alloy is 899 MPa, ultimate tensile strength is 1064 MPa and elongation is 14.3%. The strengthening theory is discussed. Attributable to substantial γ′ precipitates, shearing strengthening mechanism plays an important role. Fracture is caused by micron-sized eutectic γ′ phases that lead to stress concentration.

Pressure Influence on the AlCrFeNiMn/Graphite High Entropy Composite Microhardness

During the last years, mechanical alloying technique for high entropy alloys (HEAs) has been more often approached due to the good homogenous chemical distribution and near net shape technology provided by the respectively process. A new composite material having the matrix as HEA reinforced with graphite particles was designed. The graphite particles addition in the high entropy matrix (AlCrFeNiMn) improves the particles weldability during mechanical alloying and assures a good creep behavior for the final product. The aim of this paper is to investigate the pressure influence on the microhardness as dependence of sintering parameters which can be reflected also on the microstructure. The high entropy composite was completely alloyed after 40 hours of milling. The obtained composite was pressed using different pressures values in order to investigate the pressure influence on the microhardness and microstructure. The samples were investigated using optical microscopy, scanning electron microscopy, X-rays diffraction and microhardness tests. The microhardness values for all the samples were between 300 – 700 HV.