Effect of the atomic size distribution on glass forming ability of amorphous metallic alloys

A topological approach based on analysis of atomic size distributions has b een developed and applied to multicomponent amorphous alloys with different glass-forming ability. The atomic size distributions were obtained by plot ting atomic concentrations versus atomic radii of constitutive elements. Or dinary amorphous alloys with high critical cooling rates were found to have single-peak distributions with a concave downward shape. These amorphous s ystems have at least one alloying element with a smaller radius, and at lea st one alloying element with a larger radius relative to the base element. The concentration of an alloying element decreases rapidly as the differenc e in the atomic sizes of the base element and the alloying element increase s. Atomic size distributions of Zr, Pd, or Ln-based bulk amorphous alloys, which have a critical cooling rate in the range of 1-100 K/s, have a comple tely different, concave upward shape with a minimum at an intermediate atom ic size. The base alloying element in these alloys has the largest atomic s ize and the smallest atom often has the next-highest concentration. A model that explains the concave upward shape of atomic size distributions for th e bulk amorphous alloys is suggested. This model takes into account that al l alloying elements in bulk glass formers are smaller than the matrix eleme nt, and some of them are located in interstitial sites while others substit ute for matrix atoms in a reference crystalline solid solution. The interst itial and substitutional atoms attract each other and produce short-range o rdered atomic configurations that stabilize the amorphous state. According to this model, the critical concentration of an interstitial element requir ed to amorphize the alloy increases with increasing size difference from th e matrix atom. (C) 2001 Elsevier Science Ltd. All rights reserved.