Investigation of the compositional dependence of structural changes of Pt/Pd-Cu-Ni-P bulk glass forming liquids and their connection to thermodynamics, kinetics, dynamics and mechanical properties

金属熔体在被冷却到低于其熔点时阻止结晶并最终形成玻璃的能力是由系统的热力学、动力学、动力学和结构特征产生的。玻璃的形成是由一个低的驱动力结晶，缓慢的动力学和高的界面能之间的液体和结晶相促进。为了系统地研究这些量作为组成的函数的行为，Pt/Pd-Cu-Ni-P系统被选择用于本研究，因为Pt和Pd是可互换的并且几乎拓扑等价。具有最高玻璃形成能力的合金组合物被发现在相同的组成范围内，使它们成为理想的“案例研究”系统。此外，非晶态Pt/PdCuNiP合金尽管贵金属含量低，但具有用于燃料电池的恒星催化性能，使得详细的研究非常有趣，超出了其基础研究的价值。有趣的是，Pt-和Pd-体系在玻璃形成能力（80 mm（PdCuNiP）和20 mm（PtCuNiP））和机械性能（Pt基比Pd基更易延展）方面显示出很大的差异，同时具有非常相似的温度依赖性粘度行为（动力学脆性）。本文对12种不同Pt/Pd比的合金（Pt_（42.5-x）Pd_xCu_（27）Ni_（9.5）P_（21））进行了系统的研究。测定了低温粘度、比热容、α-弛豫时间和热力学函数，并用于模拟由闪热量法获得的实验等温结晶，得到界面能的估计和系统玻璃形成能力的完整描述。此外，不同的敏感性依赖于冷却速度和退火引起的脆化将在临界假想温度模型的框架内进行研究。通过等温退火改变样品的假想温度，最终导致样品的机械脆化。进行三点弯曲弯曲测试以确定样品不表现出任何塑性变形的临界假想温度。在这种情况下，亚Tg弛豫行为将通过量热实验和动态力学分析作为假想温度和组成的函数进行分析，分析它们在Pt-P和Pd-P大块金属玻璃脆化过程中可能的作用。此外，静态和动态结构因素的差异，有关的组成和热历史的检查，使用高能同步辐射X射线衍射。最终，通过对这个独特的模型系统的研究，将对金属玻璃的成分、热历史和相应结构的玻璃形成能力和脆化过程有更深入的了解。

The ability of a metallic melt to impede crystallization when being cooled below its melting point and then eventually forming a glass, is generated by thermodynamic, kinetic, dynamic and structural features of the system. Glass formation is facilitated by a low driving force towards crystallization, sluggish kinetics and a high interfacial energy between the liquid and crystalline phase. To investigate the behavior of these quantities systematically as a function of composition, the Pt/Pd-Cu-Ni-P system is chosen for this study, as Pt and Pd are interchangeable and are almost topologically equivalent. The alloy compositions with the highest glass-forming ability are found in the same compositional range, making them an ideal “case-study” system. Further amorphous Pt/PdCuNiP alloy possess, despite their low noble metal content, stellar catalytic properties for fuel cells, making the detailed investigation highly interesting beyond its value to fundamental research. Interestingly, the Pt- and the Pd-system show a large discrepancy in glass-forming ability (80 mm (PdCuNiP) and 20 mm (PtCuNiP)), and mechanical performance (Pt based more ductile than Pd-based), while having a very similar temperature dependent viscosity behavior (kinetic fragility). A systematic investigation will be carried out on twelve alloy compositions with different Pt/Pd ratios (Pt42.5-xPdxCu27Ni9.5P21). The low temperature viscosity, the specific heat capacity, the α-relaxation times and the thermodynamic functions are determined and used to model the experimental isothermal crystallization, obtained by flash-calorimetry, yielding an estimation of the interfacial energy and a full description of the glass forming ability of the system. Further, the different sensitivity to cooling rate dependent and annealing induced embrittlement will be investigated in the framework of the critical fictive temperature model. The fictive temperature of the samples is altered through isothermal annealing, leading eventually to a mechanical embrittlement of the samples. Three-point bending flexural tests are performed to determine the critical fictive temperature at which the samples do not exhibit any plastic deformation. In this context, the sub-Tg relaxation behavior will be analyzed by calorimetric experiments and dynamic mechanical analysis as a function of fictive temperature and composition, analyzing their possible role in the embrittlement process in Pt-P and Pd-P bulk metallic glasses. Moreover, differences in static and dynamic structure factors, related to composition and thermal history are examined, using high-energy synchrotron X-ray diffraction. Ultimately, deeper insights into the glass forming ability and the embrittlement process of metallic glasses with respect to composition, thermal history and respective structure shall be derived through the work on this unique model-system.