Accelerating structural relaxation of shear band at ambient conditions through cryogenic thermal-cycling

Cryogenic thermal-cycling (CTC) is a promising method for the rejuvenation of metallic glasses (MGs). Although seemingly arbitrary, the direction of energy change in MGs following CTC sometimes results in relaxation rather than rejuvenation. By demonstrating enthalpy relaxation in a shear band (SB) and enthalpy rejuvenation in the metallic-glass matrix, the current work demonstrates that the initial state of the specimen matters. Micro-hardness, nano-indentation loading curves, and the shapes of indents all support the bidirectional trends. Notably, after subjecting the specimen to 100 cryogenic thermal-cycles, the enthalpy and hardness tend to converge into an equilibrium value. It is discovered that CTC can accelerate the structural relaxation of SB at room temperature, a phenomenon that thermal annealing at the upper temperature (353 K) of CTC cannot achieve. Additionally, the experimental results have been elucidated by adapting the free-volume model. The work provides new insights into the functionalities of CTC and illuminates the initial state of the metallic-glass sample upon reversing the direction of enthalpy change.