Evolution path of metallic glasses under extensive cryogenic thermal cycling : Rejuvenation or relaxation?

Rejuvenation of metallic glasses (MGs) under Cryogenic thermal cycling (CTC) has aroused attention due to its prospect of fundamental and practical interest in recent years. However, not all MGs could be rejuvenated by CTC. For some system, obvious rejuvenation is achieved under few number of cycles. While for other system, rejuvenation is hard to achieve and relaxation to lower energy states occurs. The responses of MGs to CTC are so complicated and the inherent dominat factors leading to this situation remains unclear. By extensive CTC (∼800 cycles), the present work discovered quasi periodic characteristics of two typical evolution paths (rejuvenation and relaxation) in MGs, and revealed the different structural origins between them by experimental and simulation. It has been verified that the rejuvenation process induced by CTC comes mainly from local atomic motion in loosely packed regions (LPRs), while the cooperative motion of atoms in densely packed regions (DPRs) is responsible for the other evolution path, i.e., relaxation. Besides, the relaxation path induced by CTC could stabilize the supercooled liquids, with a mechanism different from that underlying the general effect of annealing on marginal MGs. Our finding explains why MGs (even with the same composition) have diverse responses to CTC. It also provides a comprehensive understanding for the mechanism of rejuvenation and a new strategy for modulating the properties of MGs by controlling microstructures by CTC.