EFFECTS OF CHROMIUM ON CRACK-GROWTH AND OXIDATION IN NICKEL ALUMINIDE

The influence of chromium additions on crack growth and oxidation have been examined in the nickel aluminide, Ni3Al. Crack growth rates were measured in a chromium containing alloy as a function of stress intensity at temperatures between 600 and 760-degrees-C in air, together with rates of oxide film growth, and compared with previous measurements taken from Ni3Al. The mechanisms of crack propagation and oxidation were investigated with a range of analytical techniques, including SEM, AES, XPS, SIMS, TEM and STEM. An addition of 8% chromium had a significantly beneficial effect on both crack growth resistance and oxidation resistance between 600 and 760-degrees-C. Low oxidation rates were associated with the formation of Cr2O3 together with Al2O3 at the metal/oxide interface, consistent with chromium acting initially as an oxygen getter, and promoting the formation of a protective Al2O3 layer, with little internal oxidation. It is proposed that chromium was also responsible for inhibiting oxygen access to and diffusion along grain boundaries at crack tips modifying the mechanism of crack propagation from "step-wise cracking", dominated by oxygen embrittlement (observed in the absence of chromium), to a more conventional creep crack growth process.