High Entropy Materials: Processing, Properties,...
Bulk equiatomic (Hf-Ta-Zr-Ti)C and (Hf-Ta-Zr-Nb)C high entropy Ultra-High Temperature Ceramic (UHTC) carbide compositions were fabricated by ball milling and Spark Plasma Sintering (SPS). It was found that the lattice parameter mismatch of the component monocarbides is a key factor for predicting single phase solid solution formation. The processing route was further optimised for the (Hf-Ta-Zr-Nb)C composition to produce a high purity, single phase, homogeneous, bulk high entropy material (99% density); revealing a vast new compositional space for the exploration of new UHTCs. One sample was observed to chemically decompose; indicating the presence of a miscibility gap. While this suggests the system is not thermodynamically stable to room temperature, it does reveal further potential for the development of new in situ formed UHTC nanocomposites. The optimised material was subjected to nanoindentation testing and directly compared to the constituent mono/binary carbides, revealing a significantly enhanced hardness (36.1 1.6 GPa,) compared to the hardest monocarbide (HfC, 31.5 1.3 GPa) and the binary (Hf-Ta)C (32.9 1.8 GPa).
High Entropy Materials: Processing, Properties,...
High-entropy alloys (HEAs) are alloys that are formed by mixing equal or relatively large proportions of (usually) five or more elements. Prior to the synthesis of these substances, typical metal alloys comprised one or two major components with smaller amounts of other elements. For example, additional elements can be added to iron to improve its properties, thereby creating an iron-based alloy, but typically in fairly low proportions, such as the proportions of carbon, manganese, and others in various steels. Hence, high-entropy alloys are a novel class of materials. The term "high-entropy alloys" was coined by Taiwanese scientist Jien-Wei Yeh because the entropy increase of mixing is substantially higher when there is a larger number of elements in the mix, and their proportions are more nearly equal. Some alternative names, such as multi-component alloys, compositionally complex alloys and multi-principal-element alloys are also suggested by other researchers.
Although HEAs were considered from a theoretical standpoint as early as 1981 and 1996, and throughout the 1980s, in 1995 Taiwanese scientist Jien-Wei Yeh came up with his idea for ways of actually creating high-entropy alloys, while driving through the Hsinchu, Taiwan, countryside. Soon after, he decided to begin creating these special alloys in his lab, being in the only region researching these alloys for over a decade. Most countries in Europe, the United States, and other parts of the world lagged behind in the development of HEAs. Significant research interest from other countries did not develop until after 2004 when Yeh and his team of scientists built the world's first high-entropy alloys to withstand extremely high temperatures and pressures. Potential applications include use in state-of-the-art race cars, spacecraft, submarines, nuclear reactors, jet aircraft, nuclear weapons, long range hypersonic missiles, and so on.
A few months later, after the publication of Yeh's paper, another independent paper on high-entropy alloys was published by a team from the United Kingdom composed of Brian Cantor, I. T. H. Chang, P. Knight, and A. J. B. Vincent. Yeh was also the first to coin the term "high-entropy alloy" when he attributed the high configurational entropy as the mechanism stabilizing the solid solution phase. Cantor did the first work in the field in the late 1970s and early 1980s, though he did not publish until 2004. Unaware of Yeh's work, he did not describe his new materials as "high-entropy" alloys, preferring the term "multicomponent alloys". The base alloy he developed, equiatomic FeCrMnNiCo, has been the subject of considerable work in the field, and is known as the "Cantor alloy", with similar derivatives known as Cantor alloys. It was one of the first HEAs to be reported to form a single-phase FCC (face-centred cubic crystal structure) solid solution.
Before the classification of high-entropy alloys and multi-component systems as a separate class of materials, nuclear scientists had already studied a system that can now be classified as a high-entropy alloy: within nuclear fuels Mo-Pd-Rh-Ru-Tc particles form at grain boundaries and at fission gas bubbles. Understanding the behavior of these "five-metal particles" was of specific interest to the medical industry because Tc-99m is an important medical imaging isotope.
The Hume-Rothery rules have historically been applied to determine whether a mixture will form a solid solution. Research into high-entropy alloys has found that in multi-component systems, these rules tend to be relaxed slightly. In particular, the rule that solvent and solute elements must have the same crystal structure does not seem to apply, as Fe, Ni, Cr, Co, and Mn have four different crystal structures as pure elements (and when the elements are present in equal concentrations, there can be no meaningful distinction between "solvent" and "solute" elements).
The multi-component alloys that Yeh developed also consisted mostly or entirely of solid-solution phases, contrary to what had been expected from earlier work in multi-component systems, primarily in the field of metallic glasses. Yeh attributed this result to the high configurational, or mixing, entropy of a random solid solution containing numerous elements. The mixing entropy for a random ideal solid solution can be calculated by:
where R is the ideal gas constant, N is the number of components, and ci is the atomic fraction of component i. From this it can be seen that alloys in which the components are present in equal proportions will have the highest entropy, and adding additional elements will increase the entropy. A five-component, equiatomic alloy will have a mixing entropy of 1.61R.
The atomic-scale complexity presents additional challenges to computational modelling of high-entropy alloys. Thermodynamic modeling using the CALPHAD method requires extrapolating from binary and ternary systems. Most commercial thermodynamic databases are designed for, and may only be valid for, alloys consisting primarily of a single element. Thus, they require experimental verification or additional ab initio calculations such as density functional theory (DFT). However, DFT modeling of complex, random alloys has its own challenges, as the method requires defining a fixed-size cell, which can introduce non-random periodicity. This is commonly overcome using the method of "special quasirandom structures", designed to most closely approximate the radial distribution function of a random system, combined with the Vienna Ab initio Simulation Package. Using this method, it has been shown that results of a four-component equiatomic alloy begins to converge with a cell as small as 24 atoms. The exact muffin-tin orbital method with the coherent potential approximation has also been employed to model HEAs. Other techniques include the 'multiple randomly populated supercell' approach, which better describes the random population of a true solid solution (although is far more computationally demanding). This method has also been used to model glassy and amorphous systems without a crystal lattice (including bulk metallic glasses).
Simulations have highlighted the preference for local ordering in some high-entropy alloys and, when the enthalpies of formation are combined with terms for configurational entropy, transition temperatures between order and disorder can be estimated, allowing one to understand when effects like age hardening and degradation of an alloy's mechanical properties may be an issue.
In general, refractory high-entropy alloys have exceptional strength at elevated temperatures but are brittle at room temperature. The HfNbTaTiZr alloy is an exception with plasticity of over 50% at room temperature. However, its strength at high temperature is insufficient. With the aim of increasing high temperature strength Chien-Chuang et al. modified the composition of HfNbTaTiZr, and studied the mechanical properties of the refractory high-entropy alloys: HfMoTaTiZr and HfMoNbTaTiZr. Both alloys have simple BCC structure. Their experiments showed that the yield strength of HfMoNbTaTiZr had a yield strength 6 times greater than HfNbTaTiZr at 1200 C with a fracture strain of 12% retained in the alloy at room temperature.
CoCrCuFeNi is an fcc alloy that was found to be paramagnetic. But upon adding titanium, it forms a complex microstructure consisting of fcc solid solution, amorphous regions and nanoparticles of Laves phase, resulting in superparamagnetic behavior. High magnetic coercivity has been measured in a BiFeCoNiMn alloy. There are several magnetic high-entropy alloys which exhibit promising soft magnetic behavior with strong mechanical properties. Superconductivity was observed in TaNbHfZrTi alloys, with transition temperatures between 5.0 and 7.3 K.
The high concentrations of multiple elements leads to slow diffusion. The activation energy for diffusion was found to be higher for several elements in CoCrFeMnNi than in pure metals and stainless steels, leading to lower diffusion coefficients.Some equiatomic multicomponent alloys have also been reported to show good resistance to damage by energetic radiation. High-entropy alloys are investigated for hydrogen storage applications. Some high-entropy alloys such as TiZrCrMnFeNi show fast and reversible hydrogen storage at room temperature with good storage capacity for commercial applications. The high-entropy materials have high potential for a wider range of energy applications, particularly in the form of high-entropy ceramics.
High entropy alloys (HEAs) and the more broadly defined multi-principal-elements alloys (MPEAs) represent a major paradigm shift in alloy design. HEAs focus on the compositions near the center of a multicomponent phase diagram. By contrast, traditional alloys focus compositions on the boundaries (vertices, edges, or faces) of a phase diagram and typically consist of one principal element. Therefore, HEAs represent a tremendously vast compositional space that is largely unexplored by science. As an emerging field, research on HEAs has attracted rising worldwide attention and interest from both academia and industry since 2004. The number of published papers has increased rapidly each year, and there have been many dedicated conferences, symposia and workshops on HEAs. Traditional physical metallurgy principles as well as novel processing methods have all been applied to HEAs, and new materials with extraordinary properties have been reported. New results also show that traditional materials science concepts are inadequate to explain some of the newly observed behaviors, fueling intense development of new models for complex, concentrated alloys. The high-entropy concept has now been extended to ceramics, semiconductors, polymers, and a broad range of functional materials. As a result, the whole field has advanced dynamically and rapidly in almost every aspect of materials science and engineering. 041b061a72