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ystallization (RX) induced by residual strains in directionally solidified (DS) Ni-base superalloys is a well-known issue in the investment casting industry.[1] It is generally accepted that the newly formed RX grain boundaries, particularly the transverse grain boundaries, are weak sites during service and may degrade the mechanical properties of the DS alloys. Therefore, strict specifications of RX grains have been proved in foundry manufacturing of the DS components. On the other hand, however, the data concerning the effect of RX on the mechanical properties of DS alloys is surprisingly limited in the open literature. Studies on different alloy systems under different conditions have not yet produced conclusive results.[2–4] The purpose of the present investigation was to study the effect of local RX on the high-temperature tensile and creep rupture properties of an experimental DS Nibase superalloy and to explore the role of the RX based on the microstructural observation. The nominal composition of the DS alloy studied is 9Cr, 10Co, 7W, 2Mo, 5Al, 3.5Ti, 4Ta, 0.1C, 0.01B, and balance Ni, in wt pct. The alloy was directionally solidified into a plate with the size of 220 · 70 · 16 mm using a Bridgman furnace. Details of the DS process were reported elsewhere.[5] The resulted DS slab has a primary dendrite arm spacing around 300 lm.

G. XIE and L. WANG, Postdoctoral Candidates, and J. ZHANG and L.H. LOU, Professors, are with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted December 1, 2006. Article published online November 22, 2007 206—VOLUME 39A, JANUARY 2008

Plates were cut by electrodischarge machining and indented on one side using a Brinell hardness tester. After full heat treatment (1220 C/2 h + 1080 C/ 4 h + 900 C/16 h) during which the RX occurred, the indentation was carefully removed by grinding. Based on the relationship between the depth of the local RX and the applied load obtained from our previous work,[6,7] specimens with different depths of local RX were obtained by applying different indentation loads and controlling the subsequent grinding process. (The depth of RX was also cross-checked by cutting and measuring some of the machined specimens after full heat treatment.) The plates were subsequently machined on the opposite side to the final dimension (35 · 8 · 5 mm gage section for tensile tests, and 15 · 5 · 2 mm gage section for creep rupture tests). To check the effect of sample thickness, two specimens with 15 · 5 · 0.8 mm gage section (with and without local RX) were also prepared for creep tests. After macroetching the machined specimens, six to eight grains were found for the 2-mm specimens and three to four grains for the 0.8-mm specimens within the gage section. As for specimens with a different number of local RX, up to ten indentations were uniformly made, so that the resulting local RX regions were isolated within the gage section. As a comparison, the shot peening specimen wa