Selective External Oxidation of the Intermetallic Compound, BaAg_5

The selective oxidation of BaAg_5 has been examined at 650–680°C in flowing 3%H2/Ar (po2 ≤ 1.1×x×10^–19atm). Under these conditions, a continuous external barium oxide scale formed. Depletion of Ba from the underlying BaAg_5 led to the formation of a continuous Ag layer between the oxide scale and the BaAg_5. Ba was only detected along grain boundaries in the continuous Ag layer, which was consistent with the negligible solubility reported for Ba in bulk Ag. The local thickness of the continuous Ag layer was inversely correlated to the local Ag grain size. Subsequent experiments with Ag-clad BaAg_5 revealed that surface oxide formation commenced at exposed Ag grain boundaries. BaAg_5 specimens clad with fine grained Ag foil exhibited more extensive oxide formation in a given time than specimens clad with coarse grained Ag foil. These observations confirmed that outward Ba migration through the continuous Ag layer occurred preferentially along Ag grain boundaries. This work demonstrates that an intermetallic compound may undergo external oxidation even when a continuous metallic (or intermetallic) layer, that possesses a low solubility for the oxidizable element, forms under the oxide scale

Growth of 1-D TiO 2

The growth of titania nanowires by a simple metal oxidation process was investigated for both commercially pure α-Ti and Ti alloys including Ti64 and β-Ti under a limited supply of oxygen. The effects of processing variables including heat treatment temperature, gas flow rate, and process duration on the growth of nanowires were explored. Similarities and differences in the growth of nanowires on pure Ti versus Ti alloys were observed. While the growth window in terms of temperature and flow rate is narrow in pure Ti, the window is much wider in the alloys. However, the trend towards high temperature is similar in all the samples promoting faceted oxide crystal growth rather than nanowires