The Study Of The Serrated Flow Behavior In Zr-based Bulk Metallic Glasses At Room Temperature

Bulk metallic glasses(BMGs)possess high strength,large elastic limit,excellent corrosion resistance and other excellent performances.The study on BMGs is a hot topic in the materials science.The structure of BMGs is arranged with a long-range disorder.In addition,BMGs are a kind of metastable materials so that the use is limited under a certain conditions.Due to the rapid cooling during the preparation process for BMGs,inner atoms still keep the state like liquid phase and there is no crystal structure defect so that the BMGs present a lot of unique properties.However,BMGs manifest a low plasticity upon loading at room temperature,which make the practical application severely limited.Now,there are still lack a lot of theories for the study of plastic deformation mechanism of BMGs.Based on this situation for BMGs,different loading strain rates and sample sizes are adopted as a experiment method and the date analysis is obtained from the engineering stress-strain curves.Besides,through the mathematical methods and the surface topographies,the deformation mechanism for BMGs is discussed in detail.The results are as follows :1.For different strain rates,the number of serrations presents linear relationship with the strain rate.Calculated from the cumulative probabilitydistribution,the cut-off values can reflect the interaction between shear bands,and predict the plastic deformation for BMGs.Moreover,the prediction of maximum stress drop and elastic energy density at different strain rates has been realized by the parameters adjustment fitting in mean field theory.At the same time,the maximum stress drop and elastic energy density present a power-law distribution at different strain rates.Besides,there exists an obvious negative strain rate sensitivity for BMGs,and the parameter of strain rate sensitivity can be obtained by measuring the curve slope in the experiment.2.Under a same strain rate,plastic deformation shows a decreasing trend gradually with the increase of sample ratio,and the shape of serrations present a change and the amplitude of serration increase obviously with the strain.Besides,stress drop below 10 MPa present a self-organization critical state characterized by a power-law distribution,and other bigger stress drops present chaotic state represented a gaussian distribution.In addition,it is found that the main shear plane is formed at the moment of 2% strain.3.With the increase of sample ratios,maximum stress drop show a firstly increase and then descend trend at a same strain rate,and the amplitude is up to a peak at the sample ratio of 2.5:1.This is because that shear slip is unstable and material has fractured at the formation moment of main shear plane for the BMGs of sample ratio 3:1.4.At a certain strain rate,the stress drops for different sample ratios can realize normalization through the parameter adjustment in mean field theory,which can achieve the prediction of maximum stress drop at other sample ratios.In addition,the maximum stress drops at different strain rates for a same sample ratio also can realize prediction in mean field theory.