Lioba Jastrow

Environmental properties of Zr-based metallic glasses and nanocrystalline alloys

Abstract Zr-Cu-Ni-Al belongs to the best glass forming systems known; these glasses are suitable as precursor material for nanocrystalline alloys. For an application as hydrogen storage materials for example it is of great interest to know more about these metastable materials in regard to their environmental properties. Corrosion as studied by a salt spray test or anodic polarization in aqueous solutions exhibit a rather high sensitivity with no significant differences between the amorphous and nanocrystalline state. Hydrogen charging was performed electrochemically in a glycerine-phosphoric acid electrolyte. In Zr-Cu-Ni-Al alloys absorption kinetics and storage capacity were found to be very similar for the amorphous and the nanocrystalline phase. In the nanocrystalline alloy consisting mainly of a fcc (big cube) phase with a NiTi 2 type structure a hydrogen induced amorphization was observed. Oxidation of metastable Zr-based materials in air was studied below the glass transition temperature at 360°C by thermogravimetry. Oxidation resistance was found to improve very significantly from the amorphous to the nanocrystalline microstructure. The scales formed on both materials consist mainly of columnar ZrO 2 with diameter in the nanometer range; Probably including the other metals as a nanocrystalline solid solution.

Absorption/desorption behavior of hydrogen and deuterium in a Pd-coated Zr-based amorphous alloy

Palladium (Pd) coating was found to enhance significantly the absorption/desorption behavior of hydrogen in amorphous Zr69.5Cu12Ni11Al7.5, thereby improving the applicability of this alloy to hydrogen storage systems. This paper demonstrates the different characteristics of deuterium/hydrogen absorption/desorption in this Pd-coated metallic glass. Pd-coated amorphous ribbons were electrochemically charged with hydrogen or deuterium to various concentrations. Absorption and desorption characteristics were determined by means of a hydrogen determinator and thermal desorption spectroscopy (TDS), respectively. Hydrogen effects on the thermal stability and crystallization characteristics were studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The hydrogen uptake was found to be twice faster than that of deuterium. Hydrogen-induced phase transformations, however, might prevent the use of this alloy for hydrogen storage applications.

Mutual Influences of Oxidation and Crystallization in Zr-Based Metallic Glasses

In order to reveal the potential application of Zr-based metallic glasses their stability against oxidation and crystallization is of utmost importance. Recent detailed studies on the crystallization as well as oxidation of these materials, however, indicated some mutual influences of these reactions. Crystallization of the metastable metallic glasses usually reduces the driving force for an oxidation process. Whereas nanocrystallization seems to improve the oxidation resistance of many Zr-based metallic glasses, a coarse crystalline structure exhibits fast oxidation along grain boundaries, at least in Zr2Pd. The reason for this is not understood as yet. In melt-spun metallic glasses a native oxide layer may be formed thus avoiding sublimation or selective oxidation during further annealing; in particular in metal-metalloid glasses sublimation as well as selective oxidation are known to accelerate or modify crystallization. In some Zr-based metallic glasses such a native oxide layer was found to protect against further oxidation of the ribbon. Oxygen diffusion into the Zr-based metallic glasses ahead of the proceeding oxidation front seems to be too slow; any effect of an increased oxygen content could be neglected.

Oxidation of amorphous Zr 70 Pd 30 and coarse crystalline Zr 2 Pd

Zr-based bulk metallic glasses are of increasing interest due to their excellent properties, e.g. high elastic limit or catalytic activity. For some applications (golf clubs, hydrogen storage) a good oxidation resistance is necessary; in other cases (catalysis) fast oxidation is required. Zr-Pd melt-spun glasses are known to exhibit “catastrophic” oxidation even at temperatures far below their glass transition. However, the surface of as-cast amorphous Zr 70 Pd 30 was found to be protected by a thin native ZrO 2 layer, thus allowing only localized nucleation of the oxidation reaction at preferred nucleation sites activated by destruction of the native oxide layer. Detailed cross-sectional SEM and TEM revealed a rather complicated microstructure of the oxide islands consisting of a lamellar structure of tetragonal and moniclinic ZrO 2 with different Pd contents and embedded nanocrystalline Pd. There is also evidence for a Pd enriched reaction zone between the oxide cone and the glassy matrix. In order to understand the influence of the structure on the oxidation, the behavior of the metallic glass was compared with that of a crystallized alloy or even coarse crystalline Zr 2 Pd. Coarse crystalline Zr 2 Pd was found to be rather stable against oxidation, but exhibit very fast oxygen diffusion along the grain boundaries followed by oxide formation deep inside the material. Grain boundaries in the crystallized Zr 70 Pd 30 does not exhibit such a behavior.