Abstract
Structure stability study on Alloy 71 8 disks with four different forginglheat treatment process combinations, i.e. Standard7 18, Super I 7 18,Super I1 7 1 8 and DA7 1 8, has been conducted with long time thermal exposure at 593 C (1 1 00°F),649 C (1 200°F ) and 677 C (1 250°F ) till 2000hrs. A b.c.c chromiunl enriched phase a -Cr has been identified by XRD,SEM,TEM and SAD methods. The existence of a -Cr in Alloy 718 is also confirmed by Thermo-Calc method. The weight fraction ofa -Cr has been determined by special designed electrolytic isolation and micro-chemical analyses. a -Cr precipitates are often in small globular particles (-0.3 P m) and form at the vicinity of 6 -Ni,Nb phase. More a -Cr particles will precipitate in Alloy 71 8 at higher exposure temperatures and longer times. Retained stress can promote a -Cr precipitation . The fractions of a -Cr in 4 different processed Alloy 7 18 disks are not very high. only less than l%wt. The highest fraction of a -Cr in DA718 after 677C/2000hrs is about 0.62%wt and the a -Cr fraction in Standard 718 after 677C/2000hrs is only 0.19%wt. Small amount of a -Cr in Alloy 718 does not has serious deleterious effect on 650°C creep crack propagation rate. However, the impact energy does decrease with the thermal exposure times at 649 C and 677 C .
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