Liquid-liquid Phase Transition And Crystallization Of A Zr₅₆Al₁₆Co₂₈ Bulk Metallic Glass

Zr based bulk metallic glasses（BMGs）are the most widely studied BMG systems due to their large glass-forming ability,stable supercooled liquid states and unique mechanical properties such as high strength,high elasticity and high wear resistance.These alloys have potential applications in the fields of biomedicine and electronic devices to exploiting their attractive glass state properties.The amorphous structures of BMGs are lack of any long-rnage order,while the determination of their local structures by diffraction is a formidable task.Structure phase transition of BMGs however,can occur,which Phase excellent performance comes from the atomic structure,while the phase transition in metallic glass will cause the change of atomic local structure,generally including liquid-liquid phase transition and liquid-solid phase transition.Recent studies have shown the likely existence of liquid-liquid phase transformation in some BMG liquids,but with weak first-order characteristics similar to those observed in high pressure phase transitions of many elemental and alloy liquids,the undercooled liquid of oxide glasses,and undercooled bulk water.In our researcg group,extentively investigation on phase transition have been made on various Zr based BMGs.Here,we report the liquid-liquid phase transition and crystallziaion behaviours of a Zr₅₆Al₁₆Co₂₈BMG alloy.The chief experimental findings are as follows:（1）Zr₅₆Al₁₆Co₂₈BMG exhibits two exothermic peaks in the isochronal differential scanning calorimetry（DSC）curve,the heat release for the first exothermic peak is greater than that of the second one.X-ray diffraction（XRD）and transmission electron microscopy（TEM）indicated that the sample preserved an amorphous structure after the first exothermic event.Chemical phase separation was excluded by 3D atom probe tomography（APT）and small angle X-ray scattering（SAXS）.The elemental distributions at the nanometer scale are uniform before and after the first exothermic peak.We denote the as-cast Zr₅₆Al₁₆Co₂₈metallic glass as G1,and the sample annealed to the end of the first exothermic peak as Glass2.Basd on the high resolution synchrotron X-ray diffraction（HR-SXRD）data,the structure of G2 has changed and becomes more ordered than G1.Upon heating,G1 enters the supercooled liquid at the end of the first exothermic peak,and G2 is formed after immediate cooling down to room temperature.During this exothermic process,liquid-liquid phase transition has occurred in the Zr₅₆Al₁₆Co₂₈metallic glass.（2）The isochronal DSC demonstrates that considerable shifts of the the peak positions of the two exothermic events within a wide temperature range against the heating rate,so the liquid-liquid phase transition and crystallization transition allow large mestability,which is the typical characteristic of metastable first-order phase transition;Calculating the enthalpy of the two exothermic peaks at each heating rate reveals that the heat release of the first peak is greater than that of the second peak,that is,the heat release of liquid-liquid phase transition is greater than that of crystallization.The HR-SXRD spectra of samples isothermal at 783 K for different times indicate the possible coexsitance of G1 and G2,but in an unusual way contrast to crystallization.The finite specific heat peak and the volume jump in thermal expansion during phase transformation confirm the first-order of the phase transformations.The extended X-ray absorption fine structure（EXAFS）shows that the local structure of G2 has changed significantly compared with G1,and G2 has a more ordered local structure.The experimental data of density,elastic modulus and hardness indicate that the ratio of the density of G2 to G1,ρ₁/ρ₀=1.00214,which is consistent with the results measured by thermal expansion test;and that the Young’s modulus,the shear modulus the bulk modulus are increased by 20.7%,24.2%,and 2.6%,respectively,but the Poisson’s ratio decreases by6.25%and the G/B value increases by 19.3%.G1 transforms into G2 through reconstructive liquid-liquid phase transition,and G2 is more ordered and stable.Room temperature compression test results show that G1 has a certain amount of plasticity but G2 is brittle.The above results reveal that the liquid-liquid phase transition is a first-order reconstructive phase transition.（3）The isothermal kinetic analysis by KJMA kinetics showed that at the initial stage of phase transformation with a transformed volume fraction of 0～3%,the Avrami exponent is about 2,and the apparent activation energy Ec=803±57 k J/mol.In the late stage of phase transformation within the volume fraction of 3%～55%,the Avrami index is between 1.5～2.5.It may be tentatively to the diffusion-controled growth G2 associated with an apparent activation energy of Ec=812±56 k J/mol.（4）The second exothermic peak in the continuous heating DSC curve of Zr₅₆Al₁₆Co₂₈metallic glass is due to crystallization.According to XRD,TEM and HR-SXRD analysis,in the crystallization process,h P-Al₂Co Zr₆phase nucleates and grows out.The KJMA kinetic analysis shows that within the crystallization volume fraction of 0～50%,the Avrami exponent is 1.5～2.5,indicating diffusion-controlled nucleation and growth,and a decreased nucleation rate as well.