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.
Two definitions of high-entropy alloys (HEAs), based on composition and entropy, are reviewed. Four core effects, i.e., high entropy, sluggish diffusion, severe lattice distortion, and cocktail ...effects, are mentioned to show the uniqueness of HEAs. The current state of physical metallurgy is discussed. As the compositions of HEAs are entirely different from that of conventional alloys, physical metallurgy principles might need to be modified for HEAs. The thermodynamics, kinetics, structure, and properties of HEAs are briefly discussed relating with the four core effects of HEAs. Among these, a severe lattice distortion effect is particularly emphasized because it exerts direct and indirect influences on many aspects of microstructure and properties. Because a constituent phase in HEAs can be regarded as a whole-solute matrix, every lattice site in the matrix has atomic-scale lattice distortion. In such a distorted lattice, point defects, line defects, and planar defects are different from those in conventional matrices in terms of atomic configuration, defect energy, and dynamic behavior. As a result, mechanical and physical properties are significantly influenced by such a distortion. Suitable mechanisms and theories correlating composition, microstructure, and properties for HEAs are required to be built in the future. Only these understandings make it possible to complete the physical metallurgy of the alloy world.
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.
•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.
Strength through high slip-plane density Yeh, Jien-Wei
Science (American Association for the Advancement of Science),
2021-Nov-19, 2021-11-19, 20211119, Letnik:
374, Številka:
6570
Journal Article
Recenzirano
Subjecting a multicomponent alloy to cyclic torsion can create a strong, ductile material.
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.
The HfNbTaTiZr refractory high-entropy alloy was investigated on the grain growth kinetics and tensile properties. Grain growth at 1200–1350°C is rather slow. The activation energy is 389kJ/mol and ...the growth exponent is 3.5. The HfNbTaTiZr alloy has high strength, small work hardening and high ductility. Grain refining is found to enhance the tensile strength and ductility simultaneously.
•The HfNbTaTiZr alloy exhibits low rate and high activation energy of grain growth.•The slow grain boundary migration is a result of the solute-drag mechanism.•Grain refinement simultaneously increases tensile strength and ductility•The alloy with a small grain size has excellent tensile yield strength and ductility.
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.
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.
PDF