Effect of cooling rate on microstructure and glass-forming ability of a (Ti_(33)Zr_(33)Hf_(33))_(70)(N_(50)Cu_(50))_(20)Al_(10) alloy

摘要

The alloy (Ti_(33)Zr_(33)Hf_(33))_(70)(N_(50)Cu_(50))_(20)Al_(10) developed by equiatomic substitution was prepared by using different quenching rates, i.e. rapid quenching by melt-spinning and relatively slow cooling using suction casting. The microstructural comparison between the as-spun ribbons and the as-cast bulk specimens reveals a different length scale of an icosahedral phase that formed upon solidification. The as-spun sample exhibits a mixture of nano-scale icosahedral, Zr_2Cu-type and amorphous phases after complete crystallization at 973 K indicating a high stability of the nano-scale icosahedral phase. Due to its small size (30-50 nm) there is no significant strain in the icosahedral phase. On the other hand, in the as-cast bulk sample Zr_2Cu-type and amorphous phases form in the micro-scale modulated icosahedral phase without any discernible exothermic reaction. The structural modulation of the icosahedral phase indicates the occurrence of strains during the growth. The different local accumulation of strain during the growth of the icosahedral phase induces a phase selection between the Zr_2Cu-type or amorphous phases in this alloy.