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

点击翻译按钮获取中文摘要

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.