Thu, 01 April, 2021
Since the Bronze Age, humans have been developing metallic materials based on the ‘one-principal-element’ strategy, e.g. iron-based, aluminium-based, magnesium-based alloys. It was only in the recent decade, two independent research groups (Yeh  and Cantor ) proposed the ‘multi-principal-elements’ alloy design strategy, which involves the mixing of multiple elements with equiatomic or near-equiatomic compositions. This leads to a new class of alloys: compositionally complex alloys (CCAs) with each element contributing 5-35 at.% of composition. These alloys are also termed as medium entropy alloys (three principal elements) or high entropy alloys (at least four principal elements). One of the most studied CCAs is the Co20Cr20Fe20Ni20Mn20 (at.%) alloy, also known as Cantor alloy, which has a single phase face-centred-cubic (FCC) structure. Yeh, et al.  proposed that the single phase is stabilized by the increase in configurational entropy.
Despite its popular acceptance, it was soon realised that entropy term alone could not determine the phase formations. Other factors such as atomic size difference, valence electron concentration, and electronegativity can also influence the phase selections. In addition to the CCAs prepared by arch melting and casting, CCAs produced using additive manufacturing (AM) and coating approaches, such as physical vapor deposition (PVD) and high-velocity oxy-fuel (HVOF), have attracted tremendous interests in the recent years. The major advantages of AM and coating approaches include easy fabrication of near-net-shape parts, and recyclability of the unused feedstock powders, which minimize the tooling or material waste.
The near-infinite composition map provided by compositionally complex alloy strategy enables the design of alloys with various properties for specific applications. George and Ritchie  found that equiatomic CrCoNi CCA possesses extraordinary fracture toughness of 275 MPa√m at cryogenic temperature. The unusual toughness is attributed to the low stacking fault energy activating twinning induced plasticity (TWIP) in this alloy. Hydrogen that normally causes alloy embrittlement surprisingly improves the ductility of the equiatomic CoCrFeNiMn Cantor alloy at cryogenic condition . Raabe, Li, and Tasan  established that phase transformation from face-centered cubic (fcc) to hexagonal close-packed (hcp) in a metastable Fe50Mn30Co10Cr10 (at.%) CCA delivers a yield strength and ductility that outperformed most of the structural metallic materials. Furthermore, the hcp phase in this CCA has a c/a ratio of 1.616 that is below the ideal value of 1.633, which is fundamentally different from the dilute hcp alloys or element hcp metals . Intermetallic compounds have also been used to improve the mechanical properties of CCAs, for instance, intermetallic nanoparticles reinforced (FeCoNi)86-Al7Ti7 (at.%) achieves an ultrahigh tensile yield strength of 1 GPa and ductility of 50% .
In addition to the fcc Cantor alloy and its derivatives described above, body-centered cubic (bcc) structured CCAs, especially refractory CCAs, have also attracted tremendous interests in recent decade. Senkov et al.  developed the first equiatomic TiZrHfNbTa refractory CCA on 2011, which has a single-phase bcc structure and reveals high deformation ability in compression test at ambient temperature. The equiatomic TiZrHfNbTa CCA was later tested in compression from 296 K (ambient temperature) to 1473K to explore its potential high temperature applications . One promising approach towards high-performance CCAs is through interstitial solid solutioning. Intriguingly, the addition of 2 at.% oxygen, one of the most abundant elements on Earth, into an equiatomic TiNbZrHf CCA increases the yield strength by 48% and ductility by 95%, owing to the formation of ordered oxygen complexes . Due to the ability for composition tailoring and structure design, CCAs have become one of the most attractive alloys in scientific study and practical applications.
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Article courtesy: Max-Planck-Institut für Eisenforschung GmbH (MPIE)