Formation and Disruption of W-Phase in High-Entropy Alloys

High-entropy alloys (HEAs) are single-phase systems prepared from equimolar or near-equimolar concentrations of at least five principal elements. The combination of high mixing entropy, severe lattice distortion, sluggish diffusion and cocktail effect favours the formation of simple phases—usually a bcc or fcc matrix with minor inclusions of ordered binary intermetallics. HEAs have been proposed for applications in which high temperature stability (including mechanical and chemical stability under high temperature and high mechanical impact) is required. On the other hand, the major challenge to overcome for HEAs to become commercially attractive is the achievement of lightweight alloys of extreme hardness and low brittleness. The multicomponent AlCrCuScTi alloy was prepared and characterized using powder X-ray diffraction (PXRD), scanning-electron microscope (SEM) and atomic-force microscope equipped with scanning Kelvin probe (AFM/SKP) techniques. Results show that the formation of complex multicomponent ternary intermetallic compounds upon heating plays a key role in phase evolution. The formation and degradation of W-phase, Al2Cu3Sc, in the AlCrCuScTi alloy plays a crucial role in its properties and stability. Analysis of as-melted and annealed alloy suggests that the W-phase is favoured kinetically, but thermodynamically unstable. The disruption of the W-phase in the alloy matrix has a positive effect on hardness (890 HV), density (4.83 g·cm−3) and crack propagation. The hardness/density ratio obtained for this alloy shows a record value in comparison with ordinary heavy refractory HEAs

The Effect of Scandium Ternary Intergrain Precipitates in Al-Containing High-Entropy Alloys

We investigate the effect of alloying with scandium on microstructure, high-temperature phase stability, electron transport, and mechanical properties of the Al2CoCrFeNi, Al0.5CoCrCuFeNi, and AlCoCrCu0.5FeNi high-entropy alloys. Out of the three model alloys, Al2CoCrFeNi adopts a disordered CsCl structure type. Both of the six-component alloys contain a mixture of body-centered cubic (bcc) and face centered cubic (fcc) phases. The comparison between in situ high-temperature powder diffraction data and ex situ data from heat-treated samples highlights the presence of a reversible bcc to fcc transition. The precipitation of a MgZn2-type intermetallic phase along grain boundaries following scandium addition affects all systems differently, but especially enhances the properties of Al2CoCrFeNi. It causes grain refinement; hardness and electrical conductivity increases (up to 20% and 14% respectively) and affects the CsCl-type → fcc equilibrium by moving the transformation to sensibly higher temperatures. The maximum dimensionless thermoelectric figure of merit (ZT) of 0.014 is reached for Al2CoCrFeNi alloyed with 0.3 wt.% Sc at 650 °C

Scandium metal processing for aerospace application

The use of scandium has been conventionally restricted to minor additions in alloys for structural applications. The term ‘scandium effect’ came thus to indicate the properties improvements caused by the precipitation of scandium intermetallics in the matrix.The development of High-Entropy Alloys (HEA, multi-principal component alloys combining compositional complexity with simple crystal structures) opened the door for the development of new systems, and therefore for new applications for lightweight metals such as scandium.This work is a thorough investigation of the potential of scandium-based in- termetallics in the growing field of HEAs.The synthesis and characterisation of multi-principal component alloys contain- ing scandium as active alloying element illustrate the compound-forming ability of this element. The high mixing entropy of the studied systems (comprising alloys of scandium with first raw metals or with hcp-structured elements) cannot efficiently inhibit the precipitation of stable intermetallic compounds. Among them, of in- terest is the so-called W -phase, so-far only reported for Al-basedCu-containing commercial alloys.Small scandium additions to Al2CoCrFeNi, Al0.5CoCrCuFeNi and AlCoCr- Cu0.5FeNi cause the segregation of a secondary phase along grain boundaries. This hexagonal Laves phase, formed by scandium in combination with Al, Co, Cr, Cu, Fe and Ni, does not disrupt the HEA matrix and is extremely stable. Moreover, it deeply affects microstructure and mechanical properties – for ex-ample, by enhancing the HEA stability with the postponement of a T-dependent phase exsolution by roughly 150 °C. Furthermore, a synergistic effect in the main phase stabilisation takes place when the Sc-doped sample is pressed at 9.5 GPa: no transition occurs and the intermetallic dissolves in the matrix.Preliminary investigations performed by spark-plasma sintering of different additives (among which Sc2O3, used as a source of scandium metal) in combination with the Al CoCrFeNi alloy led to the discovery of a promising nanodiamond HEA composite

An Alternative Route to Pentavalent Postperovskite

Two different high-pressure and -temperature synthetic routes have been used to produce only the second-known pentavalent CaIrO₃-type oxide. Postperovskite NaOsO₃ has been prepared from GdFeO₃-type perovskite NaOsO₃ at 16 GPa and 1135 K. Furthermore, it has also been synthesized at the considerably lower pressure of 6 GPa and 1100 K from a precursor of hexavalent Na₂OsO₄ and nominally pentavalent KSbO₃-like phases. The latter synthetic pathway offers a new lower-pressure route to the postperovskite form, one that completely foregoes any perovskite precursor or intermediate. This work suggests that postperovskite can be obtained in other compounds and chemistries where generalized rules based on the perovskite structure may not apply or where no perovskite is known. One more obvious consequence of our second route is that perovskite formation may even mask and hinder other less extreme chemical pathways to postperovskite phases

High-pressure high-temperature tailoring of High Entropy Alloys for extreme environments

The exceptional performance of some High Entropy Alloys (HEAs) under extreme conditions holds out the possibility of new and exciting materials for engineers to exploit in future applications. In this work, instead of focusing solely on the effects of high temperature on HEAs, the effects of combined high temperature and high pressure were observed. Phase transformations occurring in a pristine HEA, the as-cast bcc–Al$_2$CoCrFeNi, are heavily influenced by temperature, pressure, and by scandium additions. As-cast bcc–Al$_2$CoCrFeNi and fcc–Al$_{0.3}$CoCrFeNi HEAs are structurally stable below 60 GPa and do not undergo phase transitions. Addition of scandium to bcc–Al$_2$CoCrFeNi results in the precipitation of hexagonal AlScM intermetallic (W-phase), which dissolves in the matrix after high-pressure high-temperature treatment. Addition of scandium and high-pressure sintering improve hardness and thermal stability of well-investigated fcc- and bcc- HEAs. The dissolution of the intermetallic in the main phase at high pressure suggests a new strategy in the design and optimization of HEAs