EFFECT OF LONG-TERM 1093-K EXPOSURE TO AIR OR VACUUM ON THE STRUCTUREOF SEVERAL WROUGHT SUPERALLOYS

Long-term 1093 K heat treatments of three commercial superalloy sheet materials were undertaken in air and vacuum. With either exposure, sig nificant precipitation of second phases occurred in the Co-base Haynes (R) Alloy 188 (HA 188) and the Ni-base Haynes(R) Alloy 230 (HA 230); h owever, much less precipitation was found in the exposed Ni-base alloy Inconel(R) 617 (IN 617). Although some grain growth occurred in HA 18 8, no changes in the grain size of either RA 230 or IN 617 were observ ed after 22,500 h at temperature. Oxidation during air heat treatments led to weight gain due to the formation of chromia + spinel scales an d surface-connected grain boundary pits/oxides in all three superalloy s. Both the weight gain and depth of intergranular attack were depende nt on the square root of time, which is indicative of diffusion-contro lled phenomena. Because many alloy samples had neighbors in close prox imity, most vacuum heat treated specimens did not suffer significant l oss of volatile elements. However, some exposed samples were subjected to unrestricted vacuum heat treatments, allowing estimates of volatil ization to be made. Based on the data for HA 188, the weight loss duri ng 1093 K vacuum exposure was diffusion controlled once the inhibiting effects of surface films on the as-received alloys were broken down.