The Effect Of Cu Addition On Creep Deformation Of CoFe-based Metallic Glasses At Room Temperature

Due to the excellent mechanical,soft magnetic,and chemical properties,as well as environmental friendliness,CoFe-based metallic glasses(MGs)are promising for using as functional materials applicated in the field of industry.However,CoFe-based MGs exhibit poor plastic deformation ability at room temperature,which limits their engineering applications.Recently,some studies show that the addition of a tiny Cu element in CoFe-based MGs significantly improves their macroscopic plasticity.Therefore,it is important to investigate the effect of Cu addition on the mechanical behavior of CoFe-based MGs and its underlying mechanism.Combined with various advanced techniques for structural analysis,nanoindentation technology has been used to establish the relationship between plastic deformation behavior and microstructure of MGs.Based on these studies,[(Co_0.7Fe_0.3)_0.68B_0.219Si_0.051Nb_0.05]_100-xCu_x(x=0,0.5)MGs were chosen as a system to investigate the creep behavior as well as the intrinsic mechanism of plastic deformation by using nanoindentation,systematically.Firstly,the creep behavior of[(Co_0.7Fe_0.3)_0.68B_0.219Si_0.051Nb_0.05]_100-xCu_x(x=0,0.5)MGs was studied by using nanoindentation.It is found that the addition of a tiny Cu element leads to the pronounced creep displacement accompanied by the decrease of stress exponents.The addition of Cu also significantly reduces hardness,as well as the loading rate sensitivity of(Co_0.7Fe_0.3)_0.68B_0.219Si_0.051Nb_0.05 MGs.Combined with the atomic force microscope,it is found that the addition of Cu decreases the maximum pile-up and extend the pile-up region around indents,demonstrating a more uniform plastic flow behavior of the Cu-doped MG.It means the addition of Cu can increase the creep deformation ability of CoFe-based MGs and result in more uniform plastic deformation at the macroscopic scale.Secondly,the micro-scale structure and mechanical response of[(Co_0.7Fe_0.3)_0.68B_0.219Si_0.051Nb_0.05]_100-xCu_x(x=0,0.5)MGs as well as their correlation were studied.It is found that the addition of Cu element enhances the sub-nanometer-to nanometer-length-scale structural heterogeneity due to the precipitation of Co(Fe)-rich clusters,which results in the variation of mechanical behavior of MGs at the micro-scale.This short-range ordered structure effectively manipulates elastic modulus E and hardness H of the glassy matrix,locally.The H demonstrates a typical bimodal statistical distribution and a continuous weakly bonded soft region in the Cu-doped MG.It is also beneficial for accelerating the generation rate of free volume during deformation process,which can improve the plastic deformation of Cu-doped CoFe-based MGs.Finally,Maxwell-Kelvin model was utilized to analyze the creep deformation of[(Co_0.7Fe_0.3)_0.68B_0.219Si_0.051Nb_0.05]_100-xCu_x(x=0,0.5)MGs.It is found that the doping of minor Cu element promotes the activation of internal defects in the glassy matrix during deformation.In a quasi-static loading mode with a low loading rate,the characteristic relaxation time of the MG with 0.5 at.%Cu is shortened,implying that these defects with a small size prone to be activated in the glassy matrix with the addition of Cu.With further increasing loading rates,more defects with large size are activated at both hard and soft regions in[(Co_0.7Fe_0.3)_0.68B_0.219Si_0.051Nb_0.05]_99.5Cu_0.5 MGs.It promotes the propagation of multiple shear bands during deformation,thereby,enhances energy dissipation at the tip of microcracks.In conclusion,the creep deformation behavior of a Cu-doped MG and its structural evolution during creep deformation were investigated systematically in this work,which sheds new light on understanding the impact of microalloying on MGs,and might also guide for designing other commercial MGs or glassy composites with large plasticity.