Pronounced Seconary Beta Relaxation Behaviors In La-based Bulk Metallic Glasses

Relaxation behaviors of supercooled liquids and glassy solids are still intriguing issues in condensed matter physics. The β relaxation is intrinsic and universal to the supercooled liquids and of importance for understanding the nature and properties of glasses. However, for most known MGs, the β relaxations are rather obscure since MGs belong to relatively strong supercooled liquids. Recently, the secondary β relaxation has been focused due to its scientific significance for understanding the nature of glass transition and plastic deformation of metallic glasses.La-based bulk metallic glasses (BMGs) have superior glass forming ability, which can form a BMG over30mm in diameter by copper mold casting. To understand the structure and property relationship in such bulk samples is important for industrial applications. In this paper, a pronounced secondary β-relaxation is observed in both a traditional BMG and a newly-developed La-based BMG. It is demonstrated that the β-relaxation is sensitive to the sub-Tg annealing, causing the atomic rearrangement in local cages. By using nanoindentation, we find that the shear-transformation-zone (STZ) volume and involved atoms are linear related with the coupling angle between β-relaxation and the a relaxation in the annealed samples, implying that the decoupling of the β-relaxation from the a relaxation can promote the movement of local atoms. However, it exhibits little correlations with the ambient compressive plasticity.