Unexpected non-monotonic changing in the heterogeneity of glasses during annealing

The heterogeneity of glasses has critical influences on the properties. How heterogeneity evolves during annealing is an intriguing cutting-edge question. In this work, the heterogeneity of the annealed metallic and polymer glasses has been studied systematically by using stress relaxation and nanoindentation tests. The stress relaxation processes are analyzed using Kohlrausch-Williams-Watts (KWW) equation. We surprisingly find that the heterogeneous factor β_KWW in KWW equation does not change monotonously but increases first and then decreases along with the annealing time. The two-stage process implies that glasses do not evolve as expected toward the more homogeneous glassy state but become homogeneous first and then more heterogeneous. This is further verified by the evolution of critical shear stress for local plasticity. It is revealed that the two-stage process is correlated with different relaxation modes, which is further interpreted using a phenomenological model. These findings not only give insights into understanding the nature of glasses, but also are useful for designing glasses with superior properties.