Study Of Polyamorphous Phase Transition In Mg-based Metallic Glasses With Anomalous Exothermal Peak

The glass-forming ability of Mg-Cu-Gd alloys could be promoted with the addition of Ag.A calorimetric anomaly could be observed in the supercooled liquid region of the Mg₆₅Cu₁₅Ag₁₀Gd₁₀ metallic glass,indicating the occurrence of a polyamorphous transition driven by entropy.However,the underlying mechanism of the polyamorphous transition remains unsettled.The polyamorphous transition in Mg-based metallic glasses with an anomalous exothermal peak（AEP）has been studied in this thesis.Structural changes in the atomic and nanoscale range during the phase transition have been revealed.The glass-forming ability parameters T_rg andγof the Mg-based metallic glass and its hardness,modulus,chemical stability,and other properties were studied.The potential correlation between polyamorphous transition and glass-forming ability and properties will be discussed.The main work of this study is as follows:Thermal analysis shows that the glass-forming ability criterion T_rg=0.601 andγ=0.404 of Mg₆₅Cu₁₅Ag₁₀Gd₁₀ metallic glass are more significant than the reference system Mg₆₅Cu₂₅Gd₁₀metallic glass T_rg=0.557,γ=0.399,indicating that Mg₆₅Cu₁₅Ag₁₀Gd₁₀ metallic glass has a better glass-forming ability.In-situ scattering techniques were employed to reveal multiscale structure evidence during the polyamorphous transition in Mg₆₅Cu₁₅Ag₁₀Gd₁₀ metallic glass with an anomalous exothermal peak above the glass transition temperature.The experimental results show that the peak position of the first sharp diffraction peak in the structure factor slowly decreases when heating above the glass transition,reaching the lowest value at the anomalous exothermal temperature,and then the peak position increases until crystallization.The intensity of the first sharp diffraction peak increases between the glass transition and the AEP temperature region first and then decreases when heating above the AEP temperature.The analysis results of synchrotron scattering show that the atomic-scale local structure tends to undergo local structural rearrangement and order when the polyamorphous transition occurs.The square resistance of Mg₆₅Cu₁₅Ag₁₀Gd₁₀ metallic glass decreases linearly in the anomalous exothermal peak temperature area during the heating process,increases after the glass transition temperature,and then decreases sharply until the anomalous exothermal temperature reaches the lowest value,further confirming that the polyamorphous transition occurred in the anomalous exothermal peak temperature area.Furthermore,the small-angle X-ray scattering results also show that the nanoscale structural heterogeneity increases first,and then it reaches the maximum value at the anomalous exothermal peak temperature and indicates the presence of a metastable amorphous phase with an average size of～4.0 nm.Data analysis found that the core-shell model can well fit the supercooled liquid structure during the polyamorphous transition,showing that the average core atomic size of the amorphous phase is about 2.6 nm,and the thickness of the shell is about 1.0nm.Porod analysis shows that the diffusion interface becomes more sharply when the phase transition occurs.The metastable amorphous phase dissolves/reentrants into the matrix when heated to a higher temperature.Both small-angle scattering results and transmission electron microscopy analysis exclude large-scale phase separation.The nanoindentation results observed that the hardness of the metallic glass annealed in the anomalous exothermal temperature region was increased compared to the as-cast state,suggesting that the structural change caused by the polyamorphous transition can improve the mechanical behavior of the Mg-based metallic glass.The corrosion resistance results also show that the metallic glass annealed in the anomalous exothermal temperature region has a higher open-circuit potential than the as-cast sample.Electron microscopy results show that the sample morphology is relatively less pitting after annealing at anomalous exothermal temperature.Moreover,the Mg 1s spectrum of XPS has a lower Mg²⁺ion content,indicating that the annealed samples have higher chemical stability,implying that the structural changes caused by the polyamorphous transition of Mg-based metallic glasses are likely to lead to corresponding changes in chemical stability.In contrast,there is no abnormal change for the reference sample Mg₆₅Cu₂₅Gd₁₀ in the supercooled liquid region.To sum up,the experimental evidence of multiscale structure evolution shows that polyamorphous transition may occur in Mg₆₅Cu₁₅Ag₁₀Gd₁₀ metallic glass with good glass-forming ability near the anomalous exothermal peak,indicating that there may be a possible correlation between polyamorphous transition and glass-forming ability.Moreover,its structural changes may cause corresponding changes in mechanical properties and corrosion resistance.Our finding may shed light on the development of lightweight metallic glass alloys with high-strength layered structures ranging from atomic to nanometer in Mg-based metallic glass.