The Relationship Between Serrated Flow And Shear Banding At Ambient Temperature In Zr-based Bulk Metallic Glasses

Bulk metallic glasses(BMGs)are obtained by rapid cooling,which keep the liquid structure.The BMGs show a series of excellent properties at room temperature,such as high strength,high hardness,and good resistance of wear and corrosion properties etc.The stress fluctuates repeatedly with the increase of strain after yielding during the quasi-static compression test at ambient temperature,which manifests the serrated flow behavior on stress-strain curve.Investigating the serration behavior is beneficial to revealing the evolution of shear banding and further exploring the deformation mechanism of the BMGs.During the jerky flow behavior,the loading time is longer than the unloading time and there is a lack of periodicity at some strain rates.From the microscopic morphology,the intense interactions can be found between profuse shear bands.Furthermore,the cumulative probability densities of normalized stress drop for Zr-based BMG and normalized strain energy density for Pd-based BMG follow the characteristics of power-law distribution.The self-organized critical states occur in the serrated flow behavior of Zr52.5Cu17.9Ni14.6Al10Ti5 BMG at the strain rate of 2 × 10-3 s-1 and Pd75Si15Ag3Cu7 BMG at the strain rate of 2 × 10-4 s-1.The serrated flow is unstable and some small amplitude serrations can be observed during the ascending stage at high strain rates during the quasi-static compression tests in Zr-based BMGs.Decreasing strain rate,the stress presents stable fluctuation and the amplitudes of serrations are larger than those at high strain rates.It means that at high strain rates,because of the slow process of atomic arrangement,if the local stress is high enough,the shear banding happens continuously.However,at low strain rate,the jerky flow keeps stable due to slow loading,which means the atoms have enough time to rearrangement during the steady-state plastic flow.The initial stress drop is very small and a sharp drop is observed immediately increasing with time during the stress-drop stage of the stable serrated flow in Zr-based BMGs.The amplitudes of small stress drops are comparable for different serrations,which is the same with the trend in large amplitude of stress drops.Eventually,in mathematical statistics,we found that the stress decays exponentially with the time,i.e.,the stress follows exponential decay law with the time during the stress-drop process,? ~ Ae-Bt.In Zr-Cu-Al ternary BMG system,the serration behavior is studied by tuning the rates of Zr and Cu atoms when the strain rate is constant during the quasi-static compression tests.With the increase of zirconium content from Zr47.5 to Zr60,the average stress drop and average strain energy display a trend of increasing first and then decreasing.The fundamental reason is that the internal free volume follows the increasing and descreasing law.Furthermore,the results of molecular dynamics is in good agreement with the theoretical results.Finally,according to the effect on serration by controlling the loading condition,mediated composition,and sample size,the predicted model of stress drop amplitude is proposed during the serrated flow behavior.Based on the conversion relation of plastic work and heat,the amplitude of stress drop can be derived as (?).It is obvious that the amplitude of stress is not only related to inherent parameters,such as the Yong's modulus,density,heat capacity,glass transition temperature,but also affected by aspect ratio,shear angle,thickness of shear band and some other outside conditions.