GROWTH MECHANISMS OF varies directly as -Cr//2O//3 SCALES ON Y//2O//3 - DISPERSED ALLOYS.

The growth kinetics of Cr//2O//3 scales on the Y//2O//3 dispersion containing Ni-base alloy MA754 at low oxygen partial pressures have been compared with the corresponding growth rates on Ni-30Cr and on Ni-30Cr-0. 5 Ti alloys in the 1000-1125 degree C temperature range. In the dispersion containing alloy, the growth kinetics are slower by a factor of 12-14, the Ni-30Cr and Ni-30Cr- 0. 5 Ti alloys exhibiting similar growth rates. Microstructural evidence does not support mechanisms for growth rate reduction suggested by earlier investigators based on blocking effects of dispersoid particles or dispersoid-assisted generation of a fine-grained Cr//2O//3 scale. Rather, the present evidence supports a model based on yttrium doping. It is shown that in undoped Cr//2O//3 at low oxygen partial pressures, chromium interstitials are the most probable cationic defects, yttrium doping suppresses the concentration of chromium interstitials.