Size-dependent Thermoplastic Behavior Of A Zr-based Bulk Metallic Glass In The Supercooled Liquid Region

The size-dependent thermoplastic behavior of Zr55Cu30Al10N15 bulk metallic glass in the supercooled liquid region(SCLR)has been systematically investigated in this dissertation.A variety of analysis approaches were employed in this study,such as differential scanning calorimetry(DSC),X-ray diffraction analysis(XRD),metallographic microscopy measurement system,in-situ nano-indentation test,thermoplastic cylinder compression experiment,micro double cup extrusion test and finite element method(FEM)simulation experiment.Different sized metallic glass specimens with consistent initial free volume percentage were prepared by arc-melting and copper casting method.The glass transition temperature,crystallization temperature and incubation time of different sized specimens were analyzed by the differential scanning calorimetry.The size-dependent thermoplastic flow behavior of Zr55Cu30Al10Ni5 metallic glass in the supercooled liquid region was analyzed through the cylindrical high-temperature compression experiments.The experimental results show the steady-state flow stress,overshoot peak stress,elastic modulus and equilibrium viscosity all decrease with specimen size increasing.On the contrary,the peak strain decreases with size increasing.Under relatively low temperature and high strain rate,as the specimen size increasing,the stress overshoot phenomenon decreases gradually and disappears finally.It implies that there is a mechanism transformation from non-Newtonian to Newton flow.According to the transition state theory,the activation volume and defect concentration of Zr55Cu30Al10Ni5 metallic glass both gradually rise with specimen size increasing.Based on the combination of differential scanning calorimetry experiments and finite element modeling simulations,the internal structural relaxation behavior of Zr55Cu30Al10Ni5 metallic glass was investigated.Recovery enthalpy increases with specimen size decreasing.It is clearly illustrated that the differences of temperature field distributions caused by the specific surface area discrepancy would lead to varying degrees of internal structural relaxation during the actual pre-heating process.Thus,the free volume annihilation percentage of the larger specimen is lower than that of the smaller one.It demonstrates that under the precondition of the similar initial free volume content,the residual free volume content of the smaller specimen is lower compared with the larger one.Therefore,the plastic flow behavior of the larger specimen is easier to occur.According to the viscosity theory and the free volume model,the steady-state flow stress,elastic modulus,overshoot peak stress and strain size-dependent factors were proposed to construct a size-dependent constitutive relationship of Zr55Cu30Al10Ni5 metallic glass in the SCLR.The comparison results indicate that the size-dependent constitutive relationship can accurately describe the size-dependent flow behavior,overshoot phenomenon and the transition from non-Newtonian to Newton flow.Based on in-depth analysis of free volume variations inside different sized specimens,it demonstrates that the steady-state free volume increase with temperature and strain rate rising,as well as specimen size increasing.According to the temperature field gradient distribution,structural relaxation theory and stress-induced effect,specific quantitative equations of free volume saturated content,annihilation and generation were proposed to construct a size-dependent free volume prediction model.This model can provide a reliable quantitative analysis of the size-dependent free volume annihilation and generation throughout the thermoplastic forming process.The influence of internal material flow behavior inside different sized specimens on the thermoplastic forming was analyzed through micro double cup extrusion test,together with FEM simulation experiment and nano-indentation test.It is clearly illustrated that the ratio of upper/lower cup height reduces with specimen size and deformation temperature rising.The micro-hardness decreases as size increasing.In addition,the micro-hardness of the upper cup bottom region is obviously lower than that of the lower part.It demonstrates that the free volume content of the upper cup bottom region is higher than that of the lower part.The average free volume content of the larger specimen is higher.As a consequence,the plastic deformation ability of metallic glass enhances with size increasing during actual thermoplastic forming process.In conclusion,Zr55Cu30Al10Ni5 metallic glass exhibits a 'larger is softer' trend in the supercooled liquid region.Under the high temperature deformation condition,the different free volume annihilations caused by the specific surface area discrepancy are considered as the main cause of size-dependent thermoplastic flow behavior.The experimental results verify that the developed size-dependent constitutive relationship and free volume prediction model can effectively predict the forming load,component geometry,material flow behavior,internal structural evolution and free volume variation during the size-dependent thermoplastic forming process.Thus,the reasonable theoretical analysis and two models are believed to provide effective approaches to understanding the size-dependent thermoplastic flow behavior,as well as the further research of micro structural evolution mechanism of metallic glasses.