engleski

Structural relaxation in metallic glasses below Tg

Metallic glasses, as well as other glasses, form by continuous “hardening” during undercooling of a melt at a sufficiently high rate, “freezing” at glass transition temperature (Tg) before crystallization has time to nucleate and develop. The formed state is seldom fully relaxed, but incorporates quenched‐in defects associated with a so‐called free volume, vf. During a subsequent heating vf is irreversibly annealed out bellow Tg, (α relaxation), resulting in a meastable amorphous structure with the properties that can be cycled reversibly between the values characteristic of different annealing temperatures (β relaxation). The β relaxation has a strong impact on density, Young’s modulus, creep and creep recovery, free energy and stress‐relief. At least two distinct mechanisms were observed, easily illustrated by so called cross‐over effect showing that the glass state is not uniquely defined by a physical property value, rather thermal history must be taken into account. A spectrum of relaxation times has been used to describe rather complex relaxation kinetics. Some early (1980s) results as well as more recent results are presented in a variety of super‐quenched glassy (tapes) systems like: Cu‐Zr, Cu‐Ti, Cu‐ Hf, Cu‐Hf‐Ti, Cu‐Zr(Ti, Hf), Fe‐Ni‐B, Al‐Y‐Ni. The appearance of β relaxation is similar to that observed in short range ordering of crystalline alloys and it is proposed that similar changes in short range order occur in metallic glasses. Differences in the kinetics of initial relaxation and subsequent ordering and disordering are accounted for in terms of vf loss.

Structural relaxation; metallic glasses

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