Distribution Of Serrated Flow Characteristic Parameters In Zr-based Bulk Metallic Glasses At Room Temperature

The short-range ordered and long-range disordered structures in bulk metallic glasses(BMGs)contribute to their excellent mechanical properties,but structural engineering applications are severely limited because of their poor room-temperature plasticity.It is difficult for BMGs to capture their plastic deformation via existing instruments and microscopic characterization methods.And the relationship between the internal structure and mechanical properties is still unclear.Intermittent serrations or slip avalanches appearing on the stress-strain or time curves under slow compression are bridges to understand the microplastic deformation mechanism in BMGs,also provide the possibility to improve their plasticity.There are hundreds of serrations,whose statistical results may reflect some information during the plastic deformation.The paper analyses systematically about serrated flow and shear bands at different strain rates and cryogenic thermal cycling(CTC),through the statistics of stress drops,durations and stress drop rates.In addition,the failure threshold theory of BMGs is established based on the free volume and shear transition zone theory.According to the simple mean-field model,two distinct types of slip avalanches during serrated plastic flows in bulk metallic glasses are extracted to distinguish the slipping modes of shear bands under various strain rates.Small avalanches follow a power-law scaling,which propagate progressively,while large avalanches follow a simultaneous propagation and appear periodically.A weakening parameter and the critical size are defined for characterizing the difference on a small avalanche-mediated modes.At higher strain rate(5×10-3 s-1),the number of small avalanches is not sufficient to form the power-law scaling,and the large avalanches play a leading role,forming a large avalanche-induced mode.The smallest critical size at the strain rate of 5 × 10-5 s-1 means largest weakening,at which the applied stress is more likely to reach the local failure threshold and activate the STZs with slipping events.Small avalanche-mediated mode is the mechanism at strain rates of 5×10-4 s-1 and 5×10-5 s-1.Small avalanches are beneficial to the plastic shear,while large avalanches may cause the risk of failure of the materials.Therefore,the failure threshold theory is proposed to predict the maximum stress drop that causes the fracture of BMGs.A linear relation between the increment of free volume and strain rate at small time intervals is found to describe inhomogeneous flow.A theory,bridging the free-volume and shear-transformation zone(STZ)concepts through mean field theory(MFT),is established to predict the failure threshold of bulk metallic glasses(BMGs).The room-temperature theoretical failure threshold,Smax,follows a universal material-dependent criterion Smax?Tg?,where Tg is the glass-transition temperature,and ? is a constant related to the serrated flow.This criterion is in good accordance with experimental results,and provides a quantitative understanding of the failure mechanisms highly dependent on the serrations in BMGs,which is helpful to enhance plasticity via tuning the failure threshold.In order to verify the validity of the failure threshold of BMGs,the samples were subjected to CTC treatment.After CTC,the volume of STZ inside the sample increased obviously,and some characteristic parameters of the serrations were changed,which showed the rejuvenating behavior.The plasticity of CTC100 increases with the decrease of strain rate and the stress drops decrease gradually.By calculation,the experimental failure threshold of CTC100 sample at 5×10-3 s-1 strain rate is far lower than the predicted value,indicating that the CTC treatment can change the characteristic parameters of the serrations and delay the failure of BMGs.In addition,if the samples were treated with the most favorable number of CTC and then compressed at an optimal strain rate may obtain the largest plasticity for BMGs.By combining the advantages of CTC treatment and strain rates,the BMGs with high plasticity can be realized.