Abstract
A silver-based alloy containing about 8 wt% Y (Ag-8Y) has been oxidized under 1 atm and 10(-20) atm O-2 at 700 degreesC. The alloy contains a mixture of a solid solution of Y in Ag (alpha phase) with an eutectic mixture of the alpha phase with the intermetallic compound Ag51Y14 (beta phase). The oxidation rate in 1 atm O-2 is very fast and follows the parabolic rate law to a good approximation. Oxidation under low P(O-2) is much slower and quite- irregular with large deviations from an average parabolic behavior. Under 1 atm O-2 the alloy undergoes only an internal oxidation of Y without any significant outward Y diffusion. as a result of a fast oxygen penetration. Under low P(O-2) the alloy forms an almost continuous Y2O3 layer containing Ag particles, plus a region of internal oxidation of Y much thinner than in 1 atm O-2. In bath cases the alloy forms also an external layer of pure Ag metal, more continuous for the oxidation under 1 atm O-2. The Y2O3 particles within the alloy change shape with depth, being small and globular close to the alloy surface but larger and acicular, elongated nearly perpendicularly to the alloy surface, ai larger depths. For samples oxidized under 10(-20) atm O-2 the volume fraction of internal oxide increases with depth in the alloy, tending to form an almost continuous oxide layer close to the front of internal oxidation, except over regions very rich in Ag. (C) 2001 Elsevier Science B.V. All rights reserved.