Oxidation of Zr-TM Metallic Glasses

摘要

Oxidation of Zr-based metallic glasses is of utmost interest since some of these materials can be used as structural materials, e.g. as a golf club or the matrix material for penetrators, or due to their catalytic activity. For some applications good oxidation resistance is necessary; in other cases fast oxidation is required. Oxidation of Zr_(69.5)Cu_(12)Ni_(11)Al_(7.5) as well as related binary Zr-TM metallic glasses was studied by means of thermogravimetric analysis (TGA) in dry air, X-ray diffraction, SEM and TEM. In all glasses oxidation kinetics was observed to follow a parabolic law indicating a reaction kinetic controlled by oxygen diffusion through the scale towards the ZrO_2/glass interface. For Zr-Pd and especially Zr-Au TGA revealed oxidation kinetics by orders of magnitude faster than in comparable Zr-Pt or Zr-Ni, Zr-Cu as well as related ternary or quaternary glasses. The quite different stability against oxidation of the Zr-based metallic glasses will be discussed in detail taking into account for example atomic size, valency or oxidation potential of the alloying elements, the formation of protecting oxide layers as well as a reduced driving force for oxidation. On the as-cast surfaces of some glassy ribbons, e.g. Zr-Au as well as Zr-Pd, nucleation of oxidation starts at defects like scratches. The continuous scales formed on ground surfaces consist mainly of tetragonal and some monoclinic ZrO_2, but as indicated by EDX-profiles all the TM metals are incorporated in the same ratio as in the glass. The scales formed consist mainly of ZrO_2 nodules and nanocrystalline TM solid solution. Overall there is neither evidence for preferred oxidation of Zr nor for the formation of a Zr-depleted zone underneath the scale. Single ZrO_2 nodules are assumed to grow until the resulting TM enrichment around them leads locally to nanocrystallization. The observed results are also expected to provide significant conclusions on the potential use of these new metastable materials at elevated temperatures.