Microstructure Control And Deformation Mechanism Of Cu-Fe-Zr-based Phase Separated Nanoglasses

Metallic glasses(MGs),completely lacking long-range periodic order,is often known as amorphous alloys.The liquid-liquid phase separation(LLPS)could take place during solidification in phase separated alloy and liquid separateds into two liquids.By skillfully controlling the solidification process of these phase separated alloys,phase separated metallic materials with different microstructure and phases can be obtained.Phase separated MGs are vitrified solids consisting of two glassy phases induced by the LLPS in the miscibility gap and the subsequent liquid-to-glass transitions in the two separated coexistent liquids.Although the developed phase separated MGs may be plastically deformed to some extent,it is still a challenge to realize macro-plastic deformation in MGs.The nanoglasses,consisting of nanoscale glassy particles connected by glassy interfaces,show excellent properties.However,it is difficult to fabricate nanoglasses by traditional rapid cooling method.In this work,utilizing the metastable LLPS phenomenon of Cu-Fe system,through rapid solidification we develop a new-type bulk metallic nanoglasses consisting of a high-population density of glassy spherical grains in nano-meter scale.The new Cu-Fe-Zr-(Al)alloy system is designed.The effects of composition and cooling rate on the solidification structure have been investigated.The competition between the metastable LLPS and the liquid-to-glass transition has been discussed.In order to build a valid route to adjust and control the microstructure of the phase separated nanoglasses,the effects of the microalloying and the mechanism of LLPS on the solidification microstructure have been clarified.In addition,the local microstructure evolution during deformation of the phase separated nanoglasses is studied,and the relationship between microstructure and mechanical properties and the deformation mechanism are investigated.The main research work and results are shown as follows:The microstructural evolution and competitive mechanism of phase formation in the rapidly solidified Cu-Fe-Zr system were studied experimentally and by thermodynamic calculations.The results show that the metastable miscibility gap of the binary Cu-Fe system can be extended into the Cu-Fe-Zr ternary system.The LLPS into Cu-rich and Fe-rich liquids takes place in the as-quenched Cu-Fe-Zr alloy.The solidification and phase structure are tunable by modifying the atomic ratio of Cu to Fe and Zr contents.A glassy structure with nanoscale phase separation is obtained in the as-quenched(Cu₀.5Fe₀.5)₄0Zr₆0 alloy sample,exhibiting a homogeneous distribution of glassy Cu-rich nanoparticles in glassy Fe-rich matrix.Moreover,the electrical property of the as-quenched Cu-Fe-Zr alloy samples was examined.It displays an abnormal change of electrical resistivity upon temperature in the phase separated nanoglass.This may be attributed that the formation of high population density nanocrystals with slow growth in the glassy Fe-rich matrix results in the enhancement of electron scattering.The effect of nanoscale phase separation on the shear transformation zone(STZ)was analyzed.It is found that the volume of STZ decreases obviously in phase separated nanoglass due to the large free volume introduced by nanoscale phase separation.By using microalloying elements to tailor the microstructure in Cu-Fe-Zr MGs,the formation of heterogeneous structure and the correlation between the heterogeneous structure and the deformation behavior were discussed.It is found that the addition of Nb,Ta and Au would enhance the LLPS,but conversely Ag minor addition inhibits the occurrence of the LLPS.A honeycomb-like structure is formed in the(Cu₀.5Fe₀.5)₄0Zr₅9X₁(X=Nb and Ta)alloys.The kinetic analysis indicates that the addition elements of Nb and Ta causes a decrease of the interfacial energy between the two liquid phases and thus an increase in the nucleation rate of the droplets,which is favorable for the formation of high-population density of droplets.The deformation behavior of(Cu₀.5Fe₀.5)₄0Zr₅9X₁ MGs with a high population of nanoscale glassy particles was investigated by nanoindentation and bending test.The results suggest that STZ volumes decrease with the increase of volume fraction of nanoscale glassy patricles,which is favorable for the formation of SBs.These glassy particles play an interesting role in the internal record of the local structure evolution.It is found that Fe-rich nanoscale glassy patricles contiguous to SBs coarsen by Ostwald ripening,while those within SB would dissolve due to direct mechanical shearing and mixing.The Cu-Fe-Zr-Al alloy has been designed and the effect of the Al content and atomic ratio of Cu to Fe on the glass forming ability(GFA),thermal stability and microstructure was studied.The relationship between structural heterogeneity and serrated flow in Cu-Fe-Zr-Al phase separated nanoglasses with different plasticity was investigated.The results show that the GFA of(Cu₀.5Fe₀.5)_41-xZr₅9Al_x and(Cu₀.5Fe₀.5)₃3Zr_67-xAl_x alloy firstly increases and then decreases with the increase of the Al content.For(Cu_1-xFe_x)₃3Zr₅9Al₈ alloy,the GFA is better when the atomic of Cu to Fe is between 5:5 and 8:2.The ? parameter and crystallization temperature indicate that(Cu₀.7Fe₀.3)₃3Zr₅9Al₈ alloy exhibits the excellent GFA and high thermal stability.Moreover,the results show that LLPS would occur during rapid cooling,resulting in the formation of amorphous FeZr-rich nanoparticles embedded in the amorphous CuZr-rich matrix in(Cu_1-xFe_x)₃3Zr₅9Al₈(x=0.4 and 0.5)alloys.The phase separated bulk metallic nanoglasses exhibits obvious macroscopic plasticity during compression deformation,and the plastic strain of(Cu₀.6Fe₀.4)₃3Zr₅9Al₈ alloy could reach about 15%.The statistical analysis indicate that the serration dynamics transforms from a chaotic state with a characteristic length scale of stress drop to a self-organized critical(SOC)state with the increase of the atomic ratio of Cu to Fe.This transformation is ascribed to the increase of the number of nucleation sites for STZs with the increase of structural heterogeneity,which is conducive to the formation of multiple SBs.