"Aging" is the natural evolution law of non-equilibrium materials such as amorphous/glass driven by energy. High temperature annealing can accelerate the rate of amorphous aging until it is reduced to an equilibrium state. In industry, proper aging is widely used to improve the soft magnetic properties of amorphous alloys or to improve the uniformity of optical glass.
However, in 1963, Professor Kovacs of the University of Wisconsin-Madison found that if an amorphous material undergoes a two-step annealing process at low temperature and then high temperature, its volume or enthalpy will not monotonically "age", but will increase first, and then later. reduce. The "rejuvenation" phenomenon caused by this abnormal increase in enthalpy is called the Kovacs memory effect (memory effect).
Memory effect is widely present in various glassy materials such as metallic glass, polymer glass, oxide glass, spin glass, electronic glass; shape memory alloys, pleated paper balls, friction surfaces, and complex mechanical systems also have memory effects. The memory effect is closely related to the non-equilibrium state of thermodynamics. Once the material or system reaches the thermal equilibrium state, the memory of the initial state and history will be completely forgotten.
However, for half a century, people's understanding of the memory effect is still limited to the phenomenological level such as the TNM model, and the physical origin of the memory effect is still unclear.
Dr. Song Lijian and Associate Researcher Xu Wei from the Amorphous Alloy Functional Characteristics Team of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, under the guidance of researchers Huo Juntao and Wang Junqiang, have carried out in-depth systematic research on the relaxation behavior of amorphous alloys in recent years. The research found that isothermal annealing is not a single kinetic process as traditionally thought, but two-step relaxation (Intermetallics 93, 101 (2018).), and clarified the influence of the two relaxation processes on crystallization and magnetism And mechanism (Acta Mater. 185, 38 (2020). Phys. Rev. Mater. 2, 063601 (2018).).
On this basis, they cooperated with Professor Ediger of the University of Wisconsin-Madison, Professor Wang Limin of Yanshan University, and Professor Li Fushan of Zhengzhou University (Phys. Rev. Lett. 125, 135501 (2020).), using high-precision flash speed difference The scanning calorimeter studied the effect of two-step annealing temperature and time on the memory effect of amorphous alloys, and calculated the evolution of enthalpy and activation entropy in the relaxation process based on the absolute rate theory. They found that only when the enthalpy of the amorphous alloy enters the heavily-aged stage, the memory effect will appear, which is like when a person is old and will often recall things when he was young.
They found that activation entropy plays an important role in the memory effect: when jumping from low temperature to high temperature, the memory effect will only appear when the activation entropy in the high temperature annealing stage is relatively large; if the activation entropy is small, the memory effect cannot be detected. Large activation entropy means that the material has more evolution paths in the relaxation process, just like a person suddenly becomes retreated when faced with too many complicated choices.
Although these results are based on thermally activated disordered systems such as amorphous alloys, since the concept of entropy is applicable to all disordered complex systems, the relevant conclusions are also helpful for understanding the memory effect in non-thermally activated systems.