High-Entropy Alloys(HEA)

Product Information

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. Alloys that have been maturely melted include W, Ta, Mo, Nb, V, Cr, Zr, Ti, Hf, and other metals with equal atomic ratios and non-equal atomic ratios in high-entropy alloys. Ordinary components of Co, Cr, Fe, Ni, Cu, Al can be melted and cut by high entropy alloy of various systems according to the customer requirements of components, and processe These alloys are currently the focus of significant attention in materials science and engineering because they have potentially desirable properties. Furthermore, research indicates that some HEAs have considerably better strength-to-weight ratios, with a higher degree of fracture resistance, tensile strength, and corrosion and oxidation resistance than conventional alloys.

For those alloys that do form solid solutions, an additional empirical parameter has been proposed to predict the crystal structure that will form. HEAs are usually FCC (face-centred cubic), BCC (body-centred cubic), HCP (hexagonal close-packed), or a mixture of the above structures, and each structure have their own advantages and disadvantages in terms of mechanical properties. There are many methods to predict the structure of HEA. Valence electron concentration (VEC) can be used to predict the stability of the HEA structure. The stability of physical properties of the HEA is closely associated with electron concentration (this is associated with the electron concentration rule from the Hume-Rothery rules). When HEA is made with casting, only FCC structures are formed when VEC is larger than 8. When VEC is between 6.87 and 8, HEA is a mixture of BCC and FCC, and while VEC is below 6.87, the material is BCC. In order to produce certain crystal structure of HEA, certain phase stabilizing elements can be added. Experimentally, adding elements such as Al and Cr and help the formation of BCC HEA while Ni and Co can help forming FCC HEA

High-Entropy Alloys Manufacture

High-entropy alloys are difficult to manufacture using extant techniques ， and typically require both expensive materials and specialty processing techniques. High-entropy alloys are mostly produced using methods that depend on the metals phase – if the metals are combined while in a liquid, solid, or gas state. Most HEAs have been produced using liquid-phase methods include arc melting, induction melting, and Bridgman solidification. Solid-state processing is generally done by mechanical alloying using a high-energy ball mill. This method produces powders that can then be processed using conventional powder metallurgy methods or spark plasma sintering. This method allows for alloys to be produced that would be difficult or impossible to produce using casting, such as AlLiMgScTi. Gas-phase processing includes processes such as sputtering or molecular beam epitaxy (MBE), which can be used to carefully control different elemental compositions to get high-entropy metallic or ceramic films. Additive manufacturing, can produce alloys with a different microstructure, potentially increasing strength (to 1.3 gigapascals) as well as increasing ductility. Other techniques include thermal spray, laser cladding, and electrodeposition.

Properties and potential uses

• 1.      The crystal structure of HEAs has been found to be the dominant factor in determining the mechanical properties. bcc HEAs typically have high yield strength and low ductility and vice versa for fcc HEAs. Some alloys have been particularly noted for their exceptional mechanical properties. A refractory alloy, VNbMoTaW maintains a high yield strength (>600 MPa (87 ksi)) even at a temperature of 1,400 °C (2,550 °F), significantly outperforming conventional superalloys such as Inconel 718. However, room temperature ductility is poor, less is known about other important high temperature properties such as creep resistance, and the density of the alloy is higher than conventional nickel-based superalloys.

• 2.      CoCrFeMnNi has been found to have exceptional low-temperature mechanical properties and high fracture toughness, with both ductility and yield strength increasing as the test temperature was reduced from room temperature to 77 K (−321.1 °F). This was attributed to the onset of nanoscale twin boundary formation, an additional deformation mechanism that was not in effect at higher temperatures. At ultralow temperatures, inhomogenous deformation by serrations has been reported.As such, it may have applications as a structural material in low-temperature applications or, because of its high toughness, as an energy-absorbing material.However, later research showed that lower-entropy alloys with fewer elements or non-equiatomic compositions may have higher strength or higher toughness. No ductile to brittle transition was observed in the bcc AlCoCrFeNi alloy in tests as low as 77 K.

• 3.      Al0.5CoCrCuFeNi was found to have a high fatigue life and endurance limit, possibly exceeding some conventional steel and titanium alloys. But there was significant variability in the results, suggesting the material is very sensitive to defects introduced during manufacturing such as aluminum oxide particles and microcracks.

• 4.      A single-phase nanocrystalline Al20Li20Mg10Sc20Ti30 alloy was developed with a density of 2.67 g cm−3 and microhardness of 4.9 – 5.8 GPa, which would give it an estimated strength-to-weight ratio comparable to ceramic materials such as silicon carbide,though the high cost of scandium limits the possible uses.

• 5.      Rather than bulk HEAs, small-scale HEA samples (e.g. NbTaMoW micro-pillars) exhibit extraordinarily high yield strengths of 4 – 10 GPa — one order of magnitude higher than that of its bulk form – and their ductility is considerably improved. Additionally, such HEA films show substantially enhanced stability for high-temperature, long-duration conditions (at 1,100 °C for 3 days). Small-scale HEAs combining these properties represent a new class of materials in small-dimension devices potentially for high-stress and high-temperature applications.

• 6.      New types of HEAs based on the careful placement of ordered oxygen complexes, a type of ordered interstitial complexes, have been produced. In particular, alloys of titanium, hafnium, and zirconium have been shown to have enhanced work hardening and ductility characteristics.

• 7.      Bala et al. studied the effects of high-temperature exposure on the microstructure and mechanical properties of the Al5Ti5Co35Ni35Fe20 high-entropy alloy. After hot rolling and air-quenching, the alloy was exposed to a temperature range of 650-900 °C for 7 days. The air-quenching caused γ′ precipitation distributed uniformly throughout the microstructure. The high-temperature exposure resulted in growth of the γ′ particles and at temperatures higher than 700 °C, additional precipitation of γ′ was observed. The highest mechanical properties were obtained after exposure to 650 °C with a yield strength of 1050 MPa and an ultimate tensile yield strength of 1370 MPa. Increasing the temperature further decreased the mechanical properties.

• 8.      Liu et al. studied a series of quaternary non-equimolar high-entropy alloys AlxCo15xCr15xNi70−x with x ranging from 0 to 35%. The lattice structure transitioned from FCC to BCC as Al content increased and with Al content in the range of 12.5 to 19.3 at%, the γ′ phase formed and strengthened the alloy at both room and elevated temperatures. With Al content at 19.3 at%, a lamellar eutectic structure formed composed of γ′ and B2 phases. Due to high γ′ phase fraction of 70 vol%, the alloy had a compressive yield strength of 925 MPa and fracture strain of 29% at room temperature and high yield strength at high temperatures as well with values of 789, 546, and 129 MPa at the temperatures of 973, 1123, and 1273K.

• 9.      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 studie.

• 10.   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

• 11.   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 Species

Packing of High-Entropy Alloys

Standard Packing:

Typical bulk packaging includes palletized plastic 5 gallon/25 kg. pails, fiber and steel drums to 1 ton super sacks in full container (FCL) or truck load (T/L) quantities. Research and sample quantities and hygroscopic, oxidizing or other air sensitive materials may be packaged under argon or vacuum. Solutions are packaged in polypropylene, plastic or glass jars up to palletized 440 gallon liquid totes Special package is available on request.

ATTs’ High-Entropy Alloys is carefully handled to minimize damage during storage and transportation and to preserve the quality of our products in their original condition.