Abstract
The phase evolution of an Al0.5CrFeCoNiCu High Entropy Alloy has been characterised following isothermal exposures between 0.1 and 1000 h at temperatures of 700, 800 and 900 °C. The NiAl based B2 phase formed extremely quickly, within 0.1 h at the higher exposure temperatures, whilst the Cr-rich σ phase formed more slowly. The solvus temperatures of these two phases were found to be ∼975 and ∼875 °C respectively. Compilation of the data presented here with results previously reported in the literature enabled the production of a time-temperature-transformation diagram, which clearly indicates that the diffusion kinetics of this material cannot be considered sluggish.
Highlights
High Entropy Alloy (HEA) systems have been the subject of extensive research since their inception just over a decade ago
The as-cast bars were encapsulated in evacuated, Ar-backfilled glass ampoules and homogenised at 1100 C for 96 h. This temperature was selected based upon previous reports, which indicated that the first melting event observed in this alloy occurred at ~1150 C [20,21]
Similar features were not observed in the water quenched material, suggesting that diffusion in the interdendritic phase was sufficiently fast to allow elemental redistribution, leading to precipitation of the B2 phase during air cooling
Summary
High Entropy Alloy (HEA) systems have been the subject of extensive research since their inception just over a decade ago. Due to the compositionally complex nature of these materials, the development of optimal alloy chemistries will only be realistically possible via high-throughput computational and experimental techniques [7e10] Efficient use of such techniques requires an improved understanding of the fundamental mechanisms behind the behaviour of these materials, which can only be achieved through systematic studies of the compositional and processing dependence of key material properties. The multi-element basis of HEAs has been reported to give rise to a number of key benefits over their conventional, single element based counterparts [11e14] These effects include; increased entropies of mixing, which improve the structural stability, at elevated temperatures, suppressing the formation of undesirable embrittling intermetallic phases [11,12]; severely distorted crystal lattices, which are thought to be responsible for the low thermal and electrical conductivity of these materials, as well as providing strengthening by impeding dislocation movement, and; sluggish diffusion kinetics, which gives rise to low grain growth rates and enhanced creep resistance. Despite the considerable number of research studies published to date, the validity of these underlying principles has not yet been conclusively established
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