Exploration Of The Plastification And Concomitant Effects Of The High Rheological Rate Forming Method On Zr-based Bulk Metallic Glasses

Bulk metallic glasses?BMGs?possess superior mechanical and physical properties,whereas,their room-temperature brittleness and the difficulty in forming parts are two major hurdles for their widespread applications.In order to reverse this situation,this group,in light of Johnson's fast Joule heating method,innovated and developed the high rheological rate forming method?HRRF?before,which helped to significantly intensify the microstructural heterogeneity and achieve the improving plasticity of Zr₄₈Cu₃₆Al₈Ag₈ BMG.However,the plastification universality and functional mechanism of HRRF on Zr-based BMGs are still unclear,meanwhile,the function and law of HRRF on ?-relaxation,thermal stability,internal friction?IF?and relaxation behaviors of Zr-based BMGs are still lack of research.This paper,taking Zr₄₈Cu₃₀Al₈Ag₈Pd₆,Zr45(Cu₅/6Ag_1/6)48Al7 and Zr₄₈Cu₃₆Al₈Ag₈ BMGs as research objects,makes research on the plastification,law and mechanism of HRRF on Zr-based BMGs by compressive tests on the as-cast & formed BMGs and observing their surface shear bands & fracture appearance by scanning electron microscope?SEM?;and then,the function and law of HRRF on BMGs' ?-relaxation,thermal stability,IF and relaxation behaviors are investigated by adopting the differential scanning calorimetry?DSC?,IF and resistance techniques,for revealing the plastification functional laws of HRRF and its corresponding phenomena on BMGs,which can serve for providing the scientific and technical basis for conquering BMGs' fragility and expanding its engineering application range.The main results and conclusions are summarized as follows:1.Compressive stress-strain curves of the as-cast & formed Zr₄₈Cu₃₀Al₈Ag₈Pd₆ and Zr45(Cu₅/6Ag_1/6)48Al BMGs indicate that,the compressive plasticity of formed samples improve remarkably,and the curves both show more distinct serrated waveform,reflecting that the HRRF method enhances the plasticity of BMGs and changes its flow behaviors.SEM pictures of compressive samples reveal that,shear bands on the formed samples' surface increase in number and present obvious intersection & branching features,besides,dense shear vein patterns and plentiful liquid-like particles are seen in their fracture surfaces,which manifests that HRRF changes the formation & evolution behaviors of shear bands and BMGs' microstructure nonuniformity.DSC results show that,HRRF improves sample's exothermic peak area,indicating the enhancement of HRRF on BMGs' free volume.These results reveal the plastification essence of HRRF on BMGs from the perspective of plastic rheological behavior and micromechanism.2.Combining the DSC method,this paper creatively adopts the resistivity method to investigate the effects of HRRF on ?-relaxation and thermal stability of Zr₄₈Cu₃₆Al₈Ag₈ BMG.Verification tests at a heating rate of 10K/min show that,characteristic parameters calibrated by the resistivity method,like ?-relaxation onset temperature T?,first crystallization onset temperature T_x-onset & its related interval,are consistent with DSC results perfectly,which manifests the effectiveness of the resistivity method on exploring ?-relaxation and thermal stability of Zr₄₈Cu₃₆Al₈Ag₈ BMG.For more detailed information,normalized resistivity curves at a heating rate of 1K/min are obtained,which uncover a sharp T? advance and a significant increase in the resistivity change value ??rel contributed by the ?-relaxation of the formed BMG,indicating an intensified ?-relaxation of the formed BMG;furthermore,obviously lagging T_x-onset and broadening ?Tx of the formed BMG evince significantly enhanced thermal stability of the formed BMG,which gets further confirmation by XRD results of the quenching samples under different temperatures.3.The IF temperature spectra reveal that,with the temperature increasing,the IFs of as-cast and formed Zr₄₈Cu₃₆Al₈Ag₈ BMG successively exhibits four trends of low-speed linear increasing,middle-speed nonlinear increasing,quickly increasing and an IF peak occurring;accordingly,the IFs on IF frequency spectra successively exhibits four trends of the linear increasing,first quickly decreasing and then slowly increasing,first quickly decreasing and then slowly decreasing,and an IF peak occurring.Comparing the IF temperature spectra of the as-cast and formed samples,the IF peak values & temperatures under nine loading frequencies are all higher than the corresponding values of the formed;and comparing the as-cast and formed temperature intervals owning the same frequency spectrum feature,the onset ?-relaxation and transformtion from ?-to ?-relaxation of the formed sample occur at lower temperatures,indicating that the related atom or atom group movements become easier,what't more,the atom structure rearrangements corresponding to the crystallization become more difficult.