https://dc.engconfintl.org/cgi/viewcontent.cgi?article=1032&context=superalloys_ii

A critical review of high entropy alloys and related concepts

Microstructures and properties of high-entropy alloys

High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m(1/2). Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening.

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A fracture-resistant high-entropy alloy for cryogenic applications

Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase) This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys

Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off

https://zenodo.org/record/1258680/files/article.pdf

The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

A new approach for the design of alloys is presented in this study. These high-entropy alloys with multi-principal elements were synthesized using well-developed processing technologies. Preliminary results demonstrate examples of the alloys with simple crystal structures, nanostructures, and promising mechanical properties. This approach may be opening a new era in materials science and engineering.

Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes

A new approach for the design of alloy systems with multiprincipal elements is presented in this research. The Al
 x
 CoCrCuFeNi alloys with different aluminum contents (i.e., x values in molar ratio, x=0 to 3.0) were synthesized using a well-developed arc-melting and casting method. These alloys possessed simple fcc/bcc structures, and their phase diagram was predicted by microstructure characterization and differential thermal analyses. With little aluminum addition, the alloys were composed of a simple fcc solid-solution structure. As the aluminum content reached x=0.8, a bcc structure appeared and constructed with mixed fcc and bcc eutectic phases. Spinodal decomposition occurred further on when the aluminum contents were higher than x=1.0, leading to the formation of modulated plate structures. A single ordered bcc structure was obtained for aluminum contents larger than x=2.8. The effects of high mixing entropy and sluggish cooperative diffusion enhance the formation of simple solid-solution phases and submicronic structures with nanoprecipitates in the alloys with multiprincipal elements rather than intermetallic compounds.

Microstructure characterization of AlxCoCrCuFeNi high-entropy alloy system with multiprincipal elements

Two promising high-entropy alloys (HEAs) Al x CrFe 1.5 MnNi 0.5 ( x  = 0.3 and 0.5) were designed from Al–Co–Cr–Cu–Fe–Ni alloys by substituting Mn for expensive Co and excluding Cu to avoid Cu segregation. Microstructures and properties were investigated and compared at different states: as-cast, as-homogenized, as-rolled and as-aged states. Al 0.3 CrFe 1.5 MnNi 0.5 alloy in the as-cast, as-homogenized and as-rolled states has a dual-phase structure of BCC phase and FCC phase, in which Al, Ni-rich precipitates of B2-type BCC structure disperse in the BCC phase. Al 0.5 CrFe 1.5 MnNi 0.5 alloy in the corresponding states has a matrix of BCC phase in which Cr-rich particles of BCC structure and Al, Ni-rich precipitates of B2-type BCC structure disperse. These three BCC phases have the same lattice constant. Both alloys are workable and show a hardness range of Hv 300–500 in the as-cast, as-forged, as-homogenized and as-rolled states. Al 0.5 CrFe 1.5 MnNi 0.5 alloy has a higher hardness level than Al 0.3 CrFe 1.5 MnNi 0.5 one because of its full BCC phase. Both alloys thus can be used as structural parts requiring stronger strength. Both alloys display a significant high-temperature age-hardening phenomenon. As-cast Al 0.3 CrFe 1.5 MnNi 0.5 alloy can attain the highest hardness, Hv 850, at 600 °C for 100 h, and Al 0.5 CrFe 1.5 MnNi 0.5 can get even higher hardness, Hv 890. The aging hardening is resulted from the formation of ρ phase (Cr 5 Fe 6 Mn 8 -like phase). Prior rolling on the alloys before aging could significantly enhance the age-hardening rate and hardness level due to introduced defects. Al 0.5 CrFe 1.5 MnNi 0.5 alloy exhibits excellent oxidation resistance up to 800 °C, which is better than Al 0.5 CrFe 1.5 MnNi 0.5 alloy. Combining this merit with its high softening resistance and wear resistance as compared to commercial alloys Al 0.5 CrFe 1.5 MnNi 0.5 alloy has the potential for high-temperature structural applications.

Microstructure and properties of age-hardenable AlxCrFe1.5MnNi0.5 alloys

The (Al, Cr, Ti)FeCoNi alloy thin films were deposited by PVD and using the equimolar targets with same compositions from the concept of high-entropy alloys. The thin films became metal oxide films after annealing at vacuum furnace for a period; and the resistivity of these thin films decreased sharply. After optimum annealing treatment, the lowest resistivity of the FeCoNiOx, CrFeCoNiOx, AlFeCoNiOx, and TiFeCoNiOx films was 22, 42, 18, and 35 μ-cm, respectively. This value is close to that of most of the metallic alloys. This phenomenon was caused by delaminating of the alloy oxide thin films because the oxidation was from the surfaces of the thin films. The low resistivity of these oxide films was contributed to the nonfully oxidized elements in the bottom layers and also vanishing of the defects during annealing.

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The Microstructures and Electrical Resistivity of (Al, Cr, Ti)FeCoNiOx High-Entropy Alloy Oxide Thin Films

In this study, we investigated the electrical properties and microstructures of FeCoNi, Ti0.5FeCoNi, and TiFeCoNi thin films, with the aim of identifying new oxygen-deficient oxides with low resistivity. The resistivity values of as-deposited FeCoNi, Ti0.5FeCoNi, and TiFeCoNi films were 1089, 2883, and 5708 μΩ-cm, respectively. After vacuum annealing at 1000°C for 30 min, the resistivity values plummeted to 20.4, 32.1, and 45.4 μΩ-cm, respectively. Transmission electron microscope (TEM) analysis revealed that all of the as-annealed films were in the form of an oxygen-deficient oxide/metal composite with layered structure. The resistivity of these oxides is lower than that of indium tin oxide (ITO). They represent a new category of low-resistivity oxides.

Chun-Huei Tsau

Papers

Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes

Microstructure characterization of AlxCoCrCuFeNi high-entropy alloy system with multiprincipal elements

Microstructure and properties of age-hardenable AlxCrFe1.5MnNi0.5 alloys

The Microstructures and Electrical Resistivity of (Al, Cr, Ti)FeCoNiOx High-Entropy Alloy Oxide Thin Films

Low-resistivity oxides in TixFeCoNi thin films after vacuum annealing