COMPOSITION DESIGN OF Ni–Nb–(Zr, Ta, Ag) TERNARY BULK METALLIC GLASSES BASED ON CLUSTER FORMULA OF Ni–Nb EUTECTIC

A cluster-plus-glue atom model was employed to design Ni-Nb based ternary bulk metallic glasses.The binary eutectic point Ni_(59.5)Nb_(40.5) was first interpreted by the model in form of a cluster formula[(Ni_(0.5)Nb_(0.5))-Ni_6Nb_6]Ni_3,where the cluster is(Ni_(0.5)Nb_(0.5))-centered icosahedron derived from a eutectic phase Ni_6Nb_7(Fe_7W_6 type).It was then pointed out that the best binary glass former Ni_(62)Nb_(38) could be interpreted based on the eutectic cluster formula by replacing the cluster center Nb_(0.5) with Ni_(0.5,namely[Ni-Ni_6Nb_6]Ni_3=Ni_(62.5)Nb_(37.5).To further improve the glass-forming ability,Zr,Ta and Ag are selected as alloying additions to partially replace Nb in the[Ni-Ni_6Nb_6]Ni_3 cluster formula,and glassy rods with a critical size of 3 mm are achieved at appropriate ternary compositions by copper-mould suction-casting.DTA measurements indicate these bulk metallic glasses exhibit high thermal stabilities,among which the[Ni-Ni_6Nb_5Ta]Ni_3 alloy has the highest T_g(glass transition temperature)of 935 K and T_x(crystallization temperature) of 952 K. Room-temperature compressive curves of[Ni-Ni_6Nb_5Zr]Ni_3 and[Ni-Ni_6Nb_5Ta]Ni_3 alloys show they have limited plasticity with a elongation of about 0.3%,fracture strength of the[Ni-Ni_6Nb_5Zr]Ni_3 and [Ni-Ni_6Nb_5Ta]Ni_3 BMGs are about 3.2 GPa and 3.4 GPa,respectively.