Macro-response And Micro-mechanism Of Metallic Glasses Under Cryogenic Thermal Cycling

Metallic glasses(MGs)have become a key research target in the field of materials science because of their unique structure and excellent properties.Due to their non-equilibrium metastable state in thermodynamics,MGs will spontaneously transform from high energy state to low energy state under the effect of energy difference.This physical aging tendency is called relaxation.The relaxation process is usually accompanied with the structural ordering and deterioration of deformability at room temperature,which eventually accelerates the macroscopic brittle failure of MGs in service.Therefore,overcoming the relaxation behavior and alleviating the caused embrittlement is one of the key issues to promote the practical application of MGs.The research shows that the cryogenic thermal cycling(CTC)can raise the energy state of MGs and make them show the rejuvenation effect contrary to the relaxation behavior,which can improve the room-temperature plasticity of MGs to a certain extent.However,the performance regulation of MGs under CTC is very complicated.The specific performance is as follows:the evolution path and rejuvenation capacity of different alloy systems under the same CTC treatment varies from one to another.Even for the same system,the rejuvenation degree moves randomly to higher or lower values with increasing the number of cycles,showing irregularity.The current understanding of the origin behind this complexity is not clear.It is necessary to systematically study the performance regulation and structural evolution of MGs during CTC,and further improve the rejuvenation mechanism picture of MGs.It is also of great guiding significance to modulate the energy state,structure and properties of MGs.In this paper,by experiment and simulation,we systematically studied the macroscopic response and microstructure evolution of La-Ni-Al and Fe-Si-B MGs under CTC treatment,and revealed the structural origin behind the two typical evolution paths(rejuvenation and relaxation).The influence of initial energy state or fictive temperature Tf on the rejuvenation behavior in La-Ni-Al MGs is systematically studied,and the range of energy states in which rejuvenation can occur is determined.The liquid structural origin behind rejuvenation is revealed.The main conclusions are as follows:(1)By expanding the number of cycles(up to 800),the quasi-periodic characteristics of the two typical evolution paths are found in La-based and Fe-based MGs.This periodicity is the result of the competition between rejuvenation and relaxation.It is verified that the rejuvenation process of La-based MGs mainly comes from the local movement of atoms in soft regions(loosely-packed regions,LPRs),which is related to β relaxation.The relaxation process of Fe-based MGs involves the cooperative movement of atoms in hard regions(densely-packed regions,DPRs),which is associated with a relaxation.The increased proportion of soft regions or flow units is conducive to the occurrence of rejuvenation behavior.(2)The relaxation path induced by CTC is different from the general effect of sub-Tg annealing on Fe-based MGs.For Fe-based metallic glasses with insignificant glass transition,CTC can stabilize the supercooled liquid and make the glass transition temperature Tg visible.This abnormal behavior which is distinguished from sub-Tg relaxation is related to the transformation from crystal-like structure to icosahedron-like structure.(3)The energy state or fictive temperature Tf has a significant influence on the rejuvenation capacity of MGs.For La-Ni-Al MGs,the fictive temperature Tf range that rejuvenation can occur is between Tn(～1.04Tg)and Tf,c(～1.1 Tg).In this range,the rejuvenation degree decreases with the increase of Tf.Relaxation occurs when Tf is higher than the value of Tf,c.This transition can be attributed to the preferential response of local structures smaller than β relaxation to CTC.In addition,CTC can induce MGs to achieve a similar effect of supercooled liquid freezing at higher Tf.This phenomenon clarifies the important role of cooling path on rejuvenation behavior,and reflects the memory effect of MGs on liquid history during rejuvenation process.