Creep-rupture behavior of a directionally solidified nickel-base superalloy
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I. INTRODUCTION
IN advanced gas turbine engines, some critical components, especially blades and nozzle guide vanes, have been widely fabricated in directionally solidified (DS) or single crystal (SC) superalloys for enhancing the turbine inlet temperature resulting in improved thermal efficiency. The excellent high-temperature properties, especially creep strength and thermal fatigue resistance, of these materials are mainly due to the marked reduction (DS) or elimination (SC) of the grain boundaries normal to the stress axis, which usually are the failure initiation sites in the conventional cast superalloys.[1–4] At high temperature, the life of turbine blades and vanes is controlled by creep deformation and damage processes of the materials.[5] However, most of the past investigations on creep and rupture properties have been focused on SC superalloys[6–15] due to their superior temperature capability compared to DS superalloys. In the 1990s, DS superalloys have received more attention because of their lower process and inspection costs. Meanwhile, the second generation DS superalloys reached equivalent high-temperature properties to the first generation SC superalloys.[16–19] DZ17G is a DS superalloy developed recently in China for the low-pressure blade applications in gas turbine engines. The purpose of the present article is to explore in detail creep and fracture behaviors under constant load and gain a deeper understanding of high-temperature creep deformation and fracture mechanisms of DZ17G superalloy.
J.T. GUO, Professor, and C. YUAN, Doctor, are with the Institute of Metal Research, Chinese Academy of Sciences (IMR-CAS), Shenyang 110015, People’s Republic of China. H.C. YANG, Professor, is with Department of Materials and Metallurgy, Northeastern University (NEU), Shenyang 110006, China. V. LUPINC and M. MALDINI, Doctors, are with the Instituto per la Tecnologia dei Materiali e dei Processi Energetici, Consiglio Nazionale delle Ricerche (TMEPE-CNR), 53-1-20125 Milano, Italy. Manuscript submitted May 9, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
II. MATERIAL AND EXPERIMENTAL PROCEDURE The DZ17G alloy was produced by the withdrawal process at a speed of 7 mm/min in the vacuum furnace for directional solidification, to get round bars of 16-mm diameter and 130mm length. The chemical composition is given in Table I. The bars were solution treated at 1493 K for 4 hours and air-cooled; they were then aged at 1253 K for 16 hours. The fully heat-treated microstructure consists of a bimoded distribution of cubidal ␥ ⬘ particles of 0.2 m (aging precipitates) and 0.6 m (primary precipitates) size and 65 pct volume fraction, a small amount of ␥-␥ ⬘ eutectic, MC (M mainly Ti), and M23C6 (M mainly Cr and Mo) carbides. Creep tests were done in two different laboratories. The specimens of 8- and 5.6-mm gage diameter and of 100- and 28-mm gage length were machined from the heat-treated bars with the loading axis parallel to the 具001典 crystalline direction. The constant load tensile creep tests were conducted
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