Investigation Of A Hidden Liquid-Liquid Phase Transition And Its Correlation With Glass-Forming Ability In Zr-Cu-Based Metallic Glasses

It has been reported that the glass-forming ability（GFA）of ZrCuAl metallic glasses could be significantly improved with the addition of appropriate Y element.An anomalous exothermal peak（AEP）could be observed in the supercooled liqiuid region of the Zr₄₂Cu₄₆Al₇Y₅ metallic glass of the best GFA in Zr Cu Al alloys by the differential scanning calorimetry（DSC）,suggesting that the occurrence of a liquid-to-liquid phase transition（LLPT）.In-situ neutron and synchrotron high-energy X-ray diffraction results reveal that there is a new metastable amorphous phase in the anomalous exothermal peak temperature region of the Zr₄₄Cu₄₆Al₇Y₃ and Zr₄₂Cu₄₆Al₇Y₅ metallic glasses.While the reference Zr₄₇Cu₄₆Al₇ glassy alloy did not hava abnormal structural changes upon heating before crystallization.A new broad shoulder peak could be observed at the first sharp diffraction peak in the structure factor S（Q）curves for the glasses of an AEP,and its peak intensity and position possess noticeable slope change with the temperature rising,indicatingthat the structural phase transition may occur and accompanied by the packing density change.Moreover,the analysis of the synchrotron reduced pair distribution function（PDF）reveals that the coorpratively rearrangement of the medium-range order（MRO）structure plays a more essential role in liquid-liquid phase transition compared to the role of local topological or chemical short-range order（SRO）structures.The results of small-angle neutron scattering（SANS）revealed that the Zr₄₄Cu₄₆Al₇Y₃ and Zr₄₂Cu₄₆Al₇Y₅ metallic glasses possess nanoscale heterogeneous structures after LLPT compared with the reference alloy,and model fitting proves that the metastable amorphous phase is core-shell structure.The TEM observation indicates that the sample is still amorphous after heating to the AEP temperature region,and there is no contrast resulting from the second phase precipitates,reducing the possibility of phase separation.Our experimental evidence suggest that the liquid-liquid phase transition may be an entropy-drivenphase transition.The TMA results show that Zr₄₂Cu₄₆Al₇Y₅ metallic glasses will produce abnormal themal expansion near the AEP upon heating,furtherconfirming the occurrence of LLPT.Transformation kinetics for the hidden metastable amorphous phase were also sdutied by in-situ neutron and synchrotron X-ray diffraction during annealing the Zr₄₄Cu₄₆Al₇Y₃and Zr₄₂Cu₄₆Al₇Y₅alloys in the AEP temperature region.Our experimental results reveal that the structure of the metastable phase is different from that of the initial amorphous phase and its crystalline counterpart.Moreover,the metastable phase formed in Zr₄₂Cu₄₆Al₇Y₅ is much more stable than that in Zr₄₄Cu₄₆Al₇Y₃.The integrated intensity curves of diffraction peaks for metastable phase and crystalline phase exhibits that metastable phase and crystalline phase always present the oppsite trend,suggesting that the metastable phase can improve the stability of supercooled liquid and inhibit the following crystallization process.The formation of unique medium-range order and its capability to against crystallization would be one of the reasons of the excellent GFA for Zr-Cu-based metallic glasses with the addition of Y element.To sum up,in this article,a suite of neutron and synchrotron high-energy X-ray diffraction techniques were employed to reveal the microscopic mechanism of LLPT in Zr-Cu-Al-Y metallic glasses at multiple length scales.Moreover,the possible reason for the excellent GFA of Zr₄₂Cu₄₆Al₇Y₅ could be correlated with the occurrence of structural phase transition.Our work may provide a reference for developing Zr-Cu-based bulk metallic glasses with excellent GFA and tunable properties in the future.