High-entropy alloys: a critical assessment of their founding principles and future prospects

High-entropy alloys (HEAs) are a relatively new class of materials that have gained considerable attention from the metallurgical research community over recent years. They are characterised by their unconventional compositions, in that they are not based around a single major component, but rather comprise multiple principal alloying elements. Four core effects have been proposed in HEAs: (1) the entropic stabilisation of solid solutions, (2) the severe distortion of their lattices, (3) sluggish diffusion kinetics and (4) that properties are derived from a cocktail effect. By assessing these claims on the basis of existing experimental evidence in the literature, as well as classical metallurgical understanding, it is concluded that the significance of these effects may not be as great as initially believed. The effect of entropic stabilisation does not appear to be overarching, insufficient evidence exists to establish the strain in the lattices of HEAs, and rapid precipitation observed in some HEAs suggests their diffusion kinetics are not necessarily anomalously slow in comparison to conventional alloys. The meaning and influence of the cocktail effect is also a matter for debate. Nevertheless, it is clear that HEAs represent a stimulating opportunity for the metallurgical research community. The complex nature of their compositions means that the discovery of alloys with unusual and attractive properties is inevitable. It is suggested that future activity regarding these alloys seeks to establish the nature of their physical metallurgy, and develop them for practical applications. Their use as structural materials is one of the most promising and exciting opportunities. To realise this ambition, methods to rapidly predict phase equilibria and select suitable HEA compositions are needed, and this constitutes a significant challenge. However, while this obstacle might be considerable, the rewards associated with its conquest are even more substantial. Similarly, the challenges associated with comprehending the behaviour of alloys with complex compositions are great, but the potential to enhance our fundamental metallurgical understanding is more remarkable. Consequently, HEAs represent one of the most stimulating and promising research fields in materials science at present.One of the authors (NGJ) would like to acknowledge the EPSRC/Rolls-Royce Strategic Partnership for funding under EP/M005607/1

Rapid precipitation in an Al<inf>0.5</inf>CrFeCoNiCu high entropy alloy

The effect of cooling rate on the microstructural evolution of Al0.5CrFeCoNiCu has been studied using differential scanning calorimetry and scanning electron microscopy. As-cast Al0.5CrFeCoNiCu contained three phases; Cr-Fe-Co-Ni solid solution dendrites, Cu-rich interdendritic material and L12 precipitates. During cooling at rates between 10 and 50˚C.min-1 , an additional exothermic event, at ~1010˚C, was observed in the heat flow curves. Microstructural examination after cooling revealed the presence of two distinct populations of intragranular precipitates not present in the as-cast material. Energy dispersive X-ray spectroscopy indicated that Cu-rich precipitates formed within the dendrites, whilst a Cr-Fe-Co rich phase formed in the interdendritic constituent. Precipitation during cooling at rates approaching 1˚C.s-1 indicates that the diffusion kinetics of Al0.5CrFeCoNiCu are not, as previously suggested, sluggish.authors would like to acknowledge support from the EPSRC / Rolls-Royce Strategic Partnership under EP/H500375/1, EP/M005607/1 (NGJ & HJS) and EP/H022309/1 (KAC).This is the final version of the article. It first appeared from Maney via http://dx.doi.org/10.1179/1743284715Y.000000000

Phase equilibria of an Al<inf>0.5</inf>CrFeCoNiCu high entropy alloy

The phase equilibria of an Al0.5CrFeCoNiCu High Entropy Alloy has been studied following 1000 h exposures at 700, 850 and 1000 °C. Above1000 °C, the material comprised of two fcc solid solutions, one a multi-element phase and the other a Cu rich phase. Below 1000 °C, the fcc phases persisted, but were accompanied by the formation of two intermetallic compounds. In contrast to previous reports, the L12 phase was also found to precipitate through a solvus at ~850 °C. The results indicated that a solid state single phase field does not exist in this material at any temperature and all of the observed phases could be rationalised with reference to existing phase diagrams. This suggests that configurational entropy does not overcome the enthalpic contribution to the Gibbs energy, which governs phase equilibria of this alloy.This is the final published version of the paper. It was originally published by Elsevier in Materials Science & Engineering: A here: http://dx.doi.org/10.1016/j.msea.2014.07.059

Fine-scale precipitation in the high-entropy alloy Al<inf>0.5</inf>CrFeCoNiCu

The high-entropy alloy Al0:5CrFeCoNiCu has been shown to consist of two stable, face-centred cubic solid solutions at temperatures approaching its solidus; one rich in Cr, Fe, Co & Ni (dendritic) and the other rich in Cu (interdendritic). Whilst some studies have suggested that the high-temperature microstructure may be metastably retained to room temperature through rapid cooling, evidence of phase decomposition has also been reported. In this study, fine-scale precipitation has been observed in samples of Al0:5CrFeCoNiCu that have been rapidly cooled after casting, and water quenched following ageing for 1000 h at 1000°C. Contrary to previous reports, in the as-cast state, the two face- centred cubic phases, as well as an L12 phase, were found in both dendritic and interdendritic areas, with the dendritic areas having undergone a spinodal decomposition. After ageing and quenching, L12 precipitates were found in both dendritic and interdendritic areas, and precipitates of the Cr-, Fe-, Co- and Ni-enriched face-centred cubic phase were found in the Cu-rich interdendritic regions. Given the nature of the heat treatments applied, the results suggest that precipitation in the alloy is rapid and cannot be avoided, even when the material is cooled quickly to room temperature.The authors acknowledge funding from Rolls-Royce plc and the EPSRC under the Rolls-Royce/EPSRC Strategic Partnership (EP/H022309/1).This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.msea.2015.08.01

The transferability of diatoms to clothing and the methods appropriate for their collection and analysis in forensic geoscience

AbstractForensic geoscience is concerned with the analysis of geological materials in order to compare and exclude environmental samples from a common source, or to identify an unknown provenance in a criminal investigation. Diatom analysis is currently an underused technique within the forensic geoscience approach, which has the potential to provide an independent ecological assessment of trace evidence. This study presents empirical data to provide a preliminary evidence base in order to be able to understand the nature of diatom transfers to items of clothing, and the collection of transferred diatom trace evidence from a range of environments under experimental conditions. Three diatom extraction methods were tested on clothing that had been in contact with soil and water sites: rinsing in water (RW), rinsing in ethanol (RE), and submersion in H2O2 solution (H). Scanning electron microscopy (S.E.M.) analysis was undertaken in order to examine the degree of diatom retention on treated clothing samples. The total diatom yield and species richness data was recorded from each experimental sample in order to compare the efficacy of each method in collecting a representative sample for analysis. Similarity was explored using correspondence analysis. The results highlight the efficiency of H2O2 submersion in consistently extracting high diatom counts with representative species from clothing exposed to both aquatic and terrestrial sites. This is corroborated by S.E.M. analysis. This paper provides an important empirical evidence base for both establishing that diatoms do indeed transfer to clothing under forensic conditions in a range of environments, and in identifying that H2O2 extraction is the most efficient technique for the optimal collection of comparative samples. There is therefore potentially great value in collecting and analysing diatom components of geoforensic samples in order to aid in forensic investigation

Superelastic load cycling of Gum Metal

The superelastic beta titanium alloy, Gum Metal, has been found to accumulate plastic strain during tensile load cycling in the superelastic regime. This is evident from the positive drift of the macroscopic stress vs. strain hysteresis curve parallel to the strain axis and the change in its geometry subsequent to every load-unload cycle. In addition, there is a progressive reduction in the hysteresis loop width and in the stress at which the superelastic transition occurs. In situ synchrotron X-ray diffraction has shown that the lattice strain exhibited the same behaviour as that observed in macroscopic measurements and identified further evidence of plastic strain accumulation. The mechanisms responsible for the observed behaviour have been evaluated using transmission electron microscopy, which revealed a range of different defects that formed during load cycling. The formation of these defects is consistent with the classical mathematical theory for the bcc to orthorhombic martensitic transformation. It is the accumulation of these defects over time that alters its superelastic behaviour

On the time-temperature-transformation behaviour of a new dual-superlattice nickel-base superalloy

Recent research has identified compositions of nickel-based superalloys with microstructures containing appreciable and comparable volume fractions of γ′ and γ″ precipitates. In this work, an alloy capable of forming such a dual-superlattice microstructure was subjected to a range of thermal exposures between 873 and 1173 K (600 and 900 ˚C) for durations of 1 to 1000 hours. The microstructures and nature of the precipitating phases were characterised using synchrotron X-ray diffraction and electron microscopy. These data have enabled the construction of a T-T-T diagram for the precipitating phases. Hardness measurements following each thermal exposure have identified the age-hardening behaviour of this alloy and allowed preliminary mechanical properties to be assessed.The authors would like to thank K. Roberts and S. Rhodes for experimental assistance, and acknowledge funding through the EPSRC/Rolls-Royce strategic partnership EP/M005607/1 and EP/H022309/1 as well as from the Diamond Light Source for the provision of beam time (EE9270)

The influence of Al: Nb ratio on the microstructure and mechanical response of quaternary Ni-Cr-Al-Nb alloys

The influence of Al:Nb ratio on the microstructure and properties of Ni–Cr–Al–Nb alloys has been investigated following long-term exposure at elevated temperatures. The γ′ volume fraction, size and lattice misfit were seen to increase with a larger Al:Nb ratio, although these changes resulted in reduced hardness. The change in the critical resolved shear stress (CRSS) associated with strong dislocation coupling was determined to be the dominant strengthening mechanism and increased with decreasing Al:Nb ratio. A distribution of tertiary γ′ was observed to be necessary in maximising the mechanical properties of these alloys.This work was supported by the EPSRC/Rolls-Royce Strategic Partnership (EP/H022309/1 and EP/H500375/1).This is the final published version, which can also be found on the Elsevier website at: http://www.sciencedirect.com/science/article/pii/S0921509314007369