Topological instability and glass forming ability of Al-Ni-Sm alloys

The thermal crystallization of Al-based metallic glasses can be described in association with the topological instability lambda criterion. In the present work, we report on the crystallization behavior and glass forming ability of Al-rich, Al-Ni-Sm alloys, designed with compositions corresponding to the same topological instability condition of lambda approximate to 0.1. Amorphous melt-spun alloys were prepared with the following compositions, varying the ratio of Ni andSmelements: Al87.5Ni4Sm8.5, Al83.5Ni10Sm6.5, Al80.5Ni14.5Sm5 and Al76.5Ni20.5Sm3. The glass forming ability of each alloy composition was evaluated based on the thermal parameters obtained from DSC runs and on X-ray diffraction patterns. Better glass forming ability was observed in compositions whose Sm content was increased and Ni content reduced. Thermal crystallization of the alloys with low Sm content showed only one crystallization peak and no glass transition event. In alloys with higher rare-earth content, a glass transition event was clearly detected before the crystallization event. The results are interpreted considering the different types and proportions of Sm-Al and Ni-Al clusters that can be formed in the alloys along the lambda approximate to 0.1 line. They also emphasize the relevance of these different types of clusters in the amorphous phase in defining the stability of the glass and the types of thermal crystallization. (C) 2011 Elsevier B. V. All rights reserved.