•Disordered BCC phase in Al0.9–Al1.0 alloys transforms into FCC and σ phases at 873K.•FCC and σ phases form in an associated manner from the Fe,Cr-rich BCC phase.•The σ phase connected with the FCC ...phase are formed from disordered BCC phase.•The TT range of the Alx alloys is at 810–930K which is about 0.5Tm of the alloys.•The Alx alloys with x⩾0.5 is slip below the TT and creep deformation above the TT.
The Alx–Co–Cr–Fe–Ni high-entropy alloy system (x=0–1.8 in molar ratio) was prepared by vacuum arc melting and casting method. Variations of temperature on crystal structure, microstructure and mechanical properties were investigated. The evolution of structure with temperature can be classified into four types: Al0–Al0.3: FCC structure; Al0.5–Al0.7: mixed structure (FCC+spinodal A2+B2)→FCC+B2 structure; Al0.9–Al1.2: spinodal A2+B2 structure (<873K)→FCC+σ+B2 structure (⩾873K)→FCC+B2 structure (⩾1235K); and Al1.5–Al1.8: spinodal A2+B2 structure→B2 structure. The hot hardness transition temperature (TT) range of this alloy system was at 810–930K. The Al0.5 alloy exhibited the highest TT/Tm value. Above TT, the Al0 and Al0.3 alloys possessed the highest softening coefficient and the Al0.9 and Al1.0 alloys exhibited the maximum softening coefficient amongst the Alx alloys. Differences of constituent phases, phase distribution and morphology could account for the softening difference. The mechanism for high softening resistance was also discussed.
The concept of high-entropy alloys has been extended to ceramics, polymers, and composites. “High-entropy materials (HEMs)” are named to cover all these materials. Recently, HEMs has become a new ...emerging field through the collective efforts of many researchers. Basically, high mixing entropy can enhance the formation of solution-type phases for alloys, ceramics, and composites at high temperatures, and in general leads to simpler microstructure. Large degrees of freedom in composition design as well as process design have been found to provide a wide range of microstructure and properties for applications. There are many opportunities for HEMs to overcome the bottlenecks of conventional materials. In this article, several possible breakthrough applications are pointed out and emphasized for turbine blades, thermal spray bond coatings, high-temperature molds and dies, sintered carbides for cutting tools, hard coatings for cutting tools, hardfacings, and radiation-damage resistant materials. In addition, more possible breakthrough examples are briefly described.
Refractory alloys without Cr, Al, and Si additions exhibit very poor high temperature oxidation resistance and thus significantly limit their applications. With an aim to improve the poor oxidation ...resistance of strong and ductile refractory TiZrNbHfTa high‐entropy alloys (HEAs), this study investigates the effect of Al additions on the oxidation behavior and mechanisms for Al0‐1TiZrNbHfTa HEAs. Higher Al content renders the alloy more resistant to oxidation. The AlTiZrNbHfTa alloy exhibits a mass gain twice of that of conventional Ni‐based alloys at 1100 °C for 1 h, but much less than Nb refractory alloys because an Al‐containing oxide on the surface layer provides a partial barrier against oxidation between 700 and 1100 °C. But, at 1300 °C the Al‐containing alloys exhibit poor oxidation resistance because the less dense oxide layers provide oxygen with an effective diffusional channel and enable oxygen to penetrate the substrate more easily.
This study investigates the effect of Al additions on the oxidation behavior and mechanisms for strong and ductile Al0‐1TiZrNbHfTa HEAs. Higher Al content provides the alloy more resistance to oxidation and pesting. The authors speculate that a partial barrier against oxidation is established between 700 and 900 °C; however, at 1300 °C the Al‐containing alloys still exhibit poor oxidation resistance.
Fatigue behavior of a cold-rolled two-phase Al0.5CoCrCuFeNi high-entropy alloy (HEA) was studied. Some specimens were fabricated, using commercial-purity raw materials, while others were manufactured ...with high-purity components. Scatter in the fatigue life of the commercial-purity samples was found in the stress vs. lifetime plot (S–N curve). However, the high-purity samples showed less scatter, and fatigue life is predictable using fatigue statistics. The fatigue property of the alloy is comparable with and may even outperform many commercial alloys. Fatigue cracking is promoted by shrinkage pores with a size of ∼5μm, while mechanical nanotwinning was found to be the main deformation mechanism before crack-initiation and during crack propagation by transmission electron microscopy (TEM). Two orientations of dense nanotwins were found at the crack-initiation site, while less-dense nanotwins were found away from the crack initiation site. The nanotwinning behavior resulted in strengthening of the alloy and, consequently, high fatigue strength (383±71MPa). Moreover, statistical models were applied to predict fatigue life, suggesting that using improved fabrication processes and/or high-purity raw materials may enhance the fatigue behavior and scatter by reducing the number of fabrication microcracks and pores in the test samples.
Understanding the effect of temperature variation on the microstructural evolution is particularly important to refractory high-entropy alloys (RHEAs), given their potential high-temperature ...applications. Here, we experimentally investigated the grain-growth behavior of the HfNbTaZrTi RHEAs during recrystallization at temperatures from 1,000 to 1,200 °C for varied durations, following cold rolling with a 70% thickness reduction. Following the classical grain-growth kinetics analysis, two activation energies are obtained: 205 kJ/mol between 1,000 and 1,100 °C, and 401 kJ/mol between 1,100 and 1,200 °C, which suggests two mechanisms of grain growth. Moreover, the yield strength – grain size relation was found to be well described by the Hall-Petch relation in the form of σy=942+270D−0.5. It was revealed that the friction stress, 942 MPa, in the HfNbTaZrTi HEA is higher than that of tungsten alloys, which indicates the high intrinsic stress in the BCC-RHEA. The coefficient, 270 MPa/μm −1/2, is much lower than that in the 316 stainless steel and Al0.3CoCrFeNi HEAs, which indicates low grain-boundary strengthening.
•HfNbTaTiZr HEA with a 70% cold-rolled reduction was annealed from 1000 from 1200 °C.•Grain-growth exponent and activation energy were obtained by grain-growth kinetic.•Yield strength – grain size relation was described by the Hall-Petch relation.•A high friction stress is found in the HfNbTaZrTi HEA.
Thermoelectric (TE) research is not only a course of materials by discovery but also a seedbed of novel concepts and methodologies. Herein, the focus is on recent advances in three emerging ...paradigms: entropy engineering, phase‐boundary mapping, and liquid‐like TE materials in the context of thermodynamic routes. Specifically, entropy engineering is underpinned by the core effects of high‐entropy alloys; the extended solubility limit, the tendency to form a high‐symmetry crystal structure, severe lattice distortions, and sluggish diffusion processes afford large phase space for performance optimization, high electronic‐band degeneracy, rich multiscale microstructures, and low lattice thermal conductivity toward higher‐performance TE materials. Entropy engineering is successfully implemented in half‐Huesler and IV–VI compounds. In Zintl phases and skutterudites, the efficacy of phase‐boundary mapping is demonstrated through unraveling the profound relations among chemical compositions, mutual solubilities of constituent elements, phase instability, microstructures, and resulting TE properties at the operation temperatures. Attention is also given to liquid‐like TE materials that exhibit lattice thermal conductivity at lower than the amorphous limit due to intensive mobile ion disorder and reduced vibrational entropy. To conclude, an outlook on the development of next‐generation TE materials in line with these thermodynamic routes is given.
High configurational entropy, phase‐boundary mapping, and liquid–solid ions are thermodynamic routes for designing ultralow thermal conductivity and high‐performance thermoelectrics. These conceptual and methodological breakthroughs provide new perspectives for developing next‐generation thermoelectrics.
High-entropy alloys (HEAs) are newly emerging advanced materials. In contrast to conventional alloys, HEAs contain multiple principal elements, often five or more in equimolar or near-equimolar ...ratios. The basic principle behind HEAs is that solid-solution phases are relatively stabilized by their significantly high entropy of mixing compared to intermetallic compounds, especially at high temperatures. This makes them feasibly synthesized, processed, analyzed, and manipulated, and as well provides many opportunities for us. There are huge numbers of possible compositions and combinations of properties in the HEA field. Wise alloy design strategies for suitable compositions and processes to fit the requirements for either academic studies or industrial applications thus become especially important. In this article, four core effects were emphasized, several misconceptions on HEAs were clarified, and several routes for future HEA research and development were suggested.
This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA). The microstructure of this HEA resembles that of advanced superalloys with a high entropy ...FCC matrix and L1
ordered precipitates, so it is also named as "high entropy superalloy (HESA)". The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures; furthermore, its creep resistance at 982 °C can be compared to those of some Ni-based superalloys. Analysis on experimental results indicate that HESA could be strengthened by the low stacking-fault energy of the matrix, high anti-phase boundary energy of the strengthening precipitate, and thermally stable microstructure. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 °C, and the creep curves of HESA were dominated by tertiary creep behavior. To the best of authors' knowledge, this article is the first to present the elevated temperature tensile creep study on full scale specimens of a high entropy alloy, and the potential of HESA for high temperature structural application is discussed.
High entropy alloys (HEAs) have been considered for applications in nuclear reactors due to their promising mechanical properties, corrosion and radiation resistance. It has been suggested that ...sluggish diffusion kinetics and lattice distortion of HEAs can enhance the annihilation of irradiation-induced defects, giving rise to a higher degree of tolerance to irradiation damage. In order to understand the irradiation effects in HEAs and to demonstrate their potential advantages over conventional austenitic stainless steels (SS), we performed in-situ ion irradiation experiments with 1 MeV krypton at 300 °C on two HEAs and a 316H SS under an identical irradiation condition. The irradiation introduced a high density of dislocation loops in all materials, and the microstructural evolution as a function of dose was similar for HEAs and 316H SS. Nanoindentation tests showed that the degree of irradiation hardening was also comparable between them. The similar microstructural evolution and irradiation hardening behavior between the HEAs and 316H indicate that, at low temperatures (≤300 °C), the irradiation damage of fcc alloys is not sensitive to compositional variation and configurational entropy.
Although refractory high-entropy alloys have exceptional strength at high temperatures, they are often brittle at room temperature. One exception is the HfNbTaTiZr alloy, which has a plasticity of ...over 50% at room temperature. However, the strength of HfNbTaTiZr at high temperature is insufficient. In this study, the composition of HfNbTaTiZr is modified with an aim to improve its strength at high temperature, while retaining reasonable toughness at room temperature. Two new alloys with simple BCC structure, HfMoTaTiZr and HfMoNbTaTiZr, were designed and synthesized. The results show that the yield strengths of the new alloys are apparently higher than that of HfNbTaTiZr. Moreover, a fracture strain of 12% is successfully retained in the HfMoNbTaTiZr alloy at room temperature.
•Both HfMoTaTiZr and HfMoNbTaTiZr alloys have simple BCC structure.•The elevated temperature properties and microstructure evolution of both alloys are investigated.•HfMoNbTaTiZr has better combination of strength and plasticity than HfMoTaTiZr.•The yield strength of HfMoNbTaTiZr is six times that of HfNbTaTiZr at 1200 °C.•HfMoTaTiZr and HfMoNbTaTiZr have great potential in high-temperature applications.
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