Alpha chromium formation in Alloy 718 and its effect on creep crack propagation

Structure stability study on Alloy 718 disks with four different forging/heat treatment process combinations, i.e. Standard718, Super I 718,Super II 718 and DA718, has been conducted with long time thermal exposure at 593 degreesC(1100 degreesF),649 degreesC (1200 degreesF) and 677 degreesC(1250 degreesF) till 2000hrs. A b.c.c chromium enriched phase alpha -Cr has been identified by XRD,SEM,TEM and SAD methods. The existence of alpha -Cr in Alloy 718 is also confirmed by Thermo-Gale method. The weight fraction of alpha -Cr has been determined by special designed electrolytic isolation and micro-chemical analyses. alpha -Cr precipitates are often in small globular particles (similar to0.3 mu m) and form at the vicinity of delta -Ni3Nb phase. More alpha -Cr particles will precipitate in Alloy 718 at higher exposure temperatures and longer times. Retained stress can promote alpha -Cr precipitation. The fractions of alpha -Cr in 4 different processed Alloy 718 disks are not very high, only less than 1%wt. The highest fraction of alpha -Cr in DA718 after 677 degreesC/2000hrs is about 0.62%wt and the alpha -Cr fraction in Standard 718 after 677 degreesC/2000hrs is only 0.19%wt. Small amount of alpha -Cr in Alloy 718 does not has serious deleterious effect on 650 degreesC creep crack propagation rate. However, the impact energy does decrease with the thermal exposure times at 649 degreesC and 677 degreesC.