The Dynamics Of Serrations In Zr-based Bulk Metallic Glasses

The poor room-temperature plasticity of bulk metallic glasses(BMGs)severely limits the application in structural engineering.For BMGs,it is difficult to characterize their amorphous structure for understanding the plastic deformation mechanisms.Intermittent serrations or slip avalanches are ubiquitous in slowly compressed BMGs.The stress bursts and sudden stress drops on the stress-strain or stress-time curves compose serrations.The dynamics of serrations relates to the shear banding,providing an effective tool to investigate the plastic deformation mechanisms in BMGs.There are hundreds of serrations,which are of scale-free feature and appear randomly.The statistical methods including the probability density function,complementary cumulative distribution function,Bi test,two-sided K-S(Kolmogorov-Smirnov)test,and earthquake laws(consisting of the productivity law,the Bath law,and the Omori law),are borrowed to analysis the size dynamics and time correlation of serrations in BMGs that are compressed at different strain rates and with different aspect ratios.The amplitudes of serrations decrease as strain rate increases.The mean stress drops linearly decrease with logarithmic strain rates.The probability density function of the elastic density changes from the Gauss to the power-law pattern with the increase of strain rates from 5×10-5 s-1 to 5×10-3 s-1.The specimen with the power-law scaling exhibits a large plastic strain.The appearance of power-law scaling depends on the applied strain rates,which is inconsistent with the basic conditions of self-organized criticality.For each strain rate,the complementary cumulative distribution function of stress drops in Zr61.88Cu18Ni10.12Al10 BMGs follows the power-law coupled with the exponentially decaying scaling.Both power-law cutoff stress drops and maximum stress drops decrease as strain rate increases.For each applied condition,both power-law cutoff stress drops and maximum stress drops increase as applied stress increases.All theses results show that the serration in BMGs are tuned by applied conditions and agree with the prediction of a simple mean-field theory,suggesting that the mean-field theory is effective in explaining the statistical properties of serrations in BMGs.Based on the results that the sizes of serrations are tuned by strain rates and the maximum stress drop usually happens close to failure,the maximum temperature rises during large serrations in Zr62Cu15.5Ni12.5Al10BMGs compressed at three different strain rates(3×10-5,3×10-4,and 3×10-3 s-1)are estimated.Two parameters including the total elastic energy released during the load drop and the elapsed time of the load drop are critical for the calculation of temperature rises.The elastic energies are decreased with the increase of strain rates.The elapsed time of the load drop are on the order of tens of milliseconds.The simultaneous propagation of shear banding is chosen based on the prediction of the mean-filed theory.The maximum temperature rises are decreased from 2.6,2.3,to 1.9 K as strain rate increases.The maximum temperature rise for each strain rate is lower than 3 K.Moderate heat induced by shearing indicates that thermal softening has a small effect on shear-banding instability,further forwarding the importance of structural softening.The dynamics of serrations in Zr52.5Cu17.9Ni14.6Al10Ti5 BMGs with four different aspect ratios(3:1,2:1,1:1,and 1:2)are analyzed,by using the complementary cumulative distribution functions of stress drops,Bi test,and two-sided K-S test,and by analogy with the productivity law,the Bath law,and the Omori law.As aspect ratio decreases,a transition from brittle to ductile appears when the aspect ratio is decreased to 1:1.The dynamic properties of serrations change when the aspect ratio is further decreased to 1:2.It is found that the serrations of BMGs have the seismic-like dynamics and agree with the prediction of the mean-field theory for the cases of different softening parameters.For the specimens with aspect ratios 3:1,2:1,and 1:1,the complementary cumulative distribution functions of stress drops follow the power-law coupled with exponentially decaying scaling with a power-law exponent of 0.5,and the power-law exponents of the productivity law also approach to 0.5.As aspect ratio decreases,the lateral confining effect on shear banding are increased,resulting in the increase of confidence level for rejecting Poissonian process from 70%,99.5%,to 99.99%.The confidence-level increase is due to the time extension of serration clusters.For the case of brittle materials with large positive softening parameters?w,1,the increase of lateral effect causes a small decrease of ?w,1,leading to the decay of the appearance of large serrations with serious volume dilation.As a result,the time window of serration clusters is prolonged and the number of large serrations is decreased.As aspect ratio decreases to 1:2,the effect of lateral confinement vastly increases,triggering the formation of multiple primary shear bands.In this case,the amplitudes of stress drop are quite small,and the complementary cumulative distribution function of stress drops follow a simple power-law scaling.The power-law exponent is close to 1.The system-spanning large serrations and the serration cluster disappear.The serrations proceed in a Poissonian manner at the confidence level of 50%.The exponent of productivity law is close to 1 as well.All these results are consistent with the prediction of the mean-field theory for the case of near-zero positive softening parameter ?w,2.