Evolution of the mechanical performance of a non-equiatomic Zr-Hf-Ti-Al-Co-Ni-Cu High Entropy Metallic Glass through crystallization

The effectiveness in tuning the mechanical performance of a novel fully amorphous Zr 35.7 Hf 19.1 Ti 5.5 Al 12.3 Co 7 Ni 11 Cu 9.4 at. % High Entropy Metallic Glass (HEMG) upon crystallization, has been studied. The activation energy for nucleation (E p = 416.61 kJ/mol) is greater than for growth (E x = 361.51 kJ/mol), thus indicating high thermal stabililty. When heating up to 1000 K at 20 K/min, two partially overlapping DSC transformations are observed. The initial transformation, at 781 K, is associated with the formation of Zr 6 Al 2 Co dendrites embedded within an amorphous matrix. This results in a hardness increase from 7.69 GPa in the amorphous state, to 9.11 GPa, while the wear resistance, assessed from the H/E r and H 3 /Er 2 ratios, increases from 0.071 and 0.038 to 0.084 and 0.064, respectively. For the second transformation at 870 K, a metastable black spot phase characterized by a hierarchical structure and larger Zr 6 Al 2 Co dendrites, due to the continuous growth, is observed. This microstructure results in a maximum hardness of 10.08 GPa, but the wear resistance decreases slightly. Crystallization to a higher temperature, 1000 K for 2 h, results in a decrease of both, the hardness and of the wear resistance. These results show that for the initial stages of crystallization, the hardness and wear resistance of Zr 35.7 Hf 19.1 Ti 5.5 Al 12.3 Co 7 Ni 11 Cu 9.4 at. % HEMG is enhanced. The large variation in mechanical properties upon crystallization of this alloy presents considerable potential to modify the local mechanical properties of microengineering components.