Rejuvenation By Enthalpy Relaxation In Metallic Glasses

Relaxation results in a decrease of free volume and in densification of atomic packing,causing the degradation of the plasticity and fracture toughness of the MGs,that is,embrittlement.Rejuvenation,a reversal process of relaxation,can generate more free volume and elevate the energy towards higher,and is of the methods to improve the plasticity of MGs.However,the current rejuvenation methods are not suitable to be employed to rejuvenate the properties of MGs for industrial application due to their limitations,e.g.,the limitation of depth of penetration,destructive and involving multiple steps.Therefore,in the present work,a new simple,effective and nondestructive rejuvenation method,enthalpy relaxation,was developed to revive the relaxation-induced embrittlement so as to restore the plasticity and toughness of MGs.The present work systematically investigated the effect of rejuvenation temperature on the energy,mechanical properties and microstructure of the rejuvenated MGs,and revealed the mechanism of the observed rejuvenation behavior.Additionally,the correlation between the value of β(the Kohlrausch exponent of the KohlraushWilliams-Watts function)and the heterogeneity and rejuvenation ability was established,and the possible microscopic origin of the rejuvenation behavior was revealed.In this work,a simple,effective and non-destructive rejuvenation method,enthalpy relaxation,was employed to revive the relaxation-induced embrittlement in MGs,i.e.,reheating the relaxed MGs to the temperature range of enthalpy relaxation(higher than the pre-annealing temperature,but below glass transition temperature)and then quenching into iced-water.After a short times thermal exposure,rejuvenation rather than structural relaxation occurred in the treated MGs.This rejuvenation lifts the energy state of the relaxed MGs back towards that of the as-cast state.Surprisingly,the plasticity and fracture toughness of the optimally rejuvenated samples are 3 times higher than that of relaxed samples and even exceed those of the as-cast samples.This phenomenon shows that the rejuvenation causes a brittle-to-ductile transition in the relaxed MGs,i.e.,reductilization.Similar energy increase and softening behavior were also achieved in Zr52.5Cu17.9Ni14.6Al10Ti5、Zr65Cu17.5Ni10Al7.5 and La65Cu10Ni5Co5Al25 MGs by enthalpy relaxation,confirming that the rejuvenation method is widespread at least in MGs.This is significant to promote the industrial application of MGs as structural materials.The structural changes of the MGs before and after rejuvenation via synchrotron radiation were investigated.It is found that the rejuvenation process increases the average atomic volume(the Q1 position of the first sharp diffraction peak,FSDP,in the structure factor shifting to lower position),but also increases the degree of atomic ordering(the FSDP becomes sharper)of the MGs.Moreover,during rejuvenation,the evolutions of energy and mechanical properties match that of the atomic volume but are opposite to that of structure ordering,indicating that the energy increase and reductilization induced by enthalpy relaxation may originated more directly from the increase of average atomic volume.A criterion linking the rejuvenation ability and the stretching exponent β for different kinds of MGs was established.This work systemically investigated the rejuvenation ability and the value of β for eight kinds of MGs,and found that the rejuvenation ability is negative proportional to the value of β.In other words,the lower the value of β is,the more pronounced rejuvenation will be achieved using enthalpy relaxation.It is demonstrated that the β value is closely related with the structural heterogeneity in the MGs.The higher degree of structural heterogeneity,the lower value of β,representing a broader distribution of relaxation time and indicating more soft regions with a short relaxation time exist in the MGs.The higher proportion of soft regions in the MGs contributes to the higher rejuvenation ability.The present work proposes an efficient parameter(β)for evaluating the rejuvenation ability for various MGs.