The influence of Mg on creep properties and fracture behaviors of Mar-M247 superalloy under 1255 K/200 MPa
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I. INTRODUCTION
THE Mar-M247 superalloy was first introduced in 1972 by Martin Metals, a Division of Martin Marietta (Baltimore, MD). The merits of Mar-M247 include good castability, as well as excellent creep and oxidation resistance at elevated temperatures.[1–4] Over the past 2 decades, this alloy has been widely applied in fabricating advanced turbine blades and rotating parts in the aerospace industry. However, based on previous studies,[5,6] Mar-M247 has been found to have a low ductility under creep conditions of moderate temperatures and high stresses. This problem is reported to be related to the presence of elongated or scriptlike MC carbides[7,8,9] which act as crack-initiation sites and as propagation paths.[6,9,10] In order to minimize this detrimental effect, the refinement of the coarse MC carbides through microalloying is thought to be a desirable solution. It has been reported[11–22] that small amounts of Mg added to several wrought superalloys enhance the toughness, hot workability, creep life, and ductility of these alloys. In recent investigations,[9,23–25] it has been shown that the microaddition of Mg in directionally solidified and conventional cast superalloys can also improve the solidification behavior and structure. The Mg segregates to the carbide/matrix interface, leading to a refinement of the primary MC carbide and inhibiting the scriptlike carbide formation.[9,23,26] According to our previous study,[9] the creep life and rupture elongation of a Mar-M247 superalloy containing 30 to 80 ppm Mg could be effectively improved under a moderate temperature/ high stress condition (1033 K/724 MPa). In addition, the tensile elongation obtained at 1172 K reached 14 pct, which is 3 times greater than the engine material specification (EMS-55447) requirement.[5] Thus, the problem of low elongation at moderate temperature appears to be successfully H.Y. BOR and C.Y. MA, Associate Scientists, are with the Materials and Electro-optics Research Division, Chung-Shan Institute of Science and Technology, Taiwan 325, Republic of China. C.G. CHAO, Professor, is with the Institute of Materials Science and Engineering, National ChiaoTung University, Hsin-Chu, Taiwan 300, Republic of China. Manuscript received June 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
resolved. According to EMS-55447,[5] the creep properties of a Mar-M247 superalloy must satisfy requirements under two test conditions, i.e., at a moderate temperature/high stress (1033 K/724 MPa) and a high temperature/low stress (1255 K/200 MPa). However, the influence of microadditions of Mg on the high temperature/low stress properties and fracture mechanism in Mar-M247 superalloy is still equivocal. In this study, the effects of Mg microadditions on the creep behavior of a Mar-M247 superalloy at 1255 K/200 MPa were investigated in detail. The objectives of this investigation were to study the characteristic changes of grain boundary (GB) carbide caused by Mg microadditions to the Mar-M247 superalloy and to determine the suitable contents of
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