Mechanical properties of two novel non-equiatomic Zr-Hf-Ti-Cu-Ni-Co-Al High Entropy Alloys with high glass forming ability

Abstract

This manuscript aims to study the microstructure and mechanical properties of two novel non-equiatomic Zr_27.5Hf_11.1Ti_6.2Cu_32.4Ni_10.7Co_5.5Al_6.6 and Zr_29.7Hf_16.8Ti_5.2Cu_6.3Ni_12.1Co_8.4Al_21.5 at. % High Entropy Alloys (HEAs) obtained at two different average cooling rates (∼1000 K/s and ∼250 K/s, for 2 and 4 mm diameter samples, respectively). For each casted sample, the cooling rate also changes with the distance from the centre (lowest) to the edge (fastest) thus enabling to explore the evolution of the microstructures at a wide range of cooling rates. For the Zr_27.5Hf_11.1Ti_6.2Cu_32.4Ni_10.7Co_5.5Al_6.6 alloy, the mechanical properties variation between the highest and lowest cooled regions, from the narrow amorphous ring edge (nanoindentation hardness H = 8.2±0.42 GPa) to the centre of the largest sample (H = 8.8±0.35 GPa), is very small. This is attributed to the small microstructural differences, mostly formation of a solid solution BCC crystalline phase, although with some HCP phase. The amorphous phase is in a very relaxed state, about to crystallize. However, for the Zr_29.7Hf_16.8Ti_5.2Cu_6.3Ni_12.1Co_8.4Al_21.5 alloy, larger microstructural differences, and therefore mechanical properties, between the highest and lowest cooled regions are detected. From a fully amorphous region far from equilibrium (H = 8.5±0.44 GPa) to a solid solution of BCC (∼80% vol.) and HCP (∼20% vol.) crystalline phase (H = 10.8±0.6 GPa) and free from brittle intermetallic phases. This suggests, the latter alloy is a nearer eutectic composition and therefore the microstructure is more sensitive to changes of the cooling rate, something to take into consideration when designing microstructures for engineering applications.