CREEP CRACK GROWTH BEHAVIOR OF ALLOY 718 DURING HIGH TEMPERATURE EXPOSURE

Alloy 718 is a precipitation strengthened nickel-based superalloy based on the precipitation ofγ"-Ni3Nb (DO22 structure) and γ-Ni3(Al, Ti) (Ll2 structure) phases. Creep crack growth rate(CCGR) was investigated after high temperature exposure at 593, 650 and 677℃ for 2000h inAlloy 718. In addition to the coalescence of γ'/ γ" and the amount increasing of δ phase, theexistence of a bcc chromium enriched α-Cr phase was observed by SEM, and the weight fraction of α-Cr and other phases were determined by chemical phase analysis methods. The CCGR behavior and regulation have been analyzed by means of strength and structure analysis approaches. The experimental results show higher the exposure temperature and longer the ex posure time, lower the CCGR. This is probably attributed to the interaction of material softening and brittling due to complex structure changes during high temperature exposure. Therefore,despite α-Cr phase formation and amount enhancement were run in this test range. It seems to us a srnall amount of α-Cr will be not harmful for creep crack propagation resistance, which is critical for disk application in aircraft and land-based gas turbine.