Effect and mechanism of similar elements on glass forming abilities of Hf-based bulk metallic glasses

Compared to Zr-based bulk metallic glasses (BMGs), glass forming abilities (GFAs) of Hf-based counterparts limit practical applications. X-ray diffraction and transmission electron microscopy analysis revealed that only HfNiAl and CuHf2 crystal phases existed in Hf47Cu29Ni10Al14 master ingots. However, when partially substituting Ni with Ag or Co, the GFAs were remarkably improved. Particularly, alloying 3 at.% Ag resulted in an almost fully amorphous master ingot with a maximum thickness of 10.75 mm. Our experimental results conflict with conventional theories such as the deep eutectics theory. A large negative heat of mixing (HM) between constituent elements has been widely accepted as a key factor in enhancing GFAs. Our results posed a puzzling problem due to the non-negative HM between two similar elements either Ag and Cu or Co and Ni. To address the contradiction, a similar competition mechanism was proposed to elucidate the apparent difference in GFAs between quaternary and quinary alloys. Competition is triggered between similar elements when interacting with another element due to their analogous properties, which undermines the interaction between each of similar elements and another element and then suppresses crystal phases containing one similar element. To quantitatively describe this mechanism, we introduced a dimensionless parameter, chi.