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, Al_0.5CoCrCuFeNi, and AlCoCrCu_0.5FeNi 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.