Features and effect factors of creep of single-crystal nickel-base superalloys
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SINGLE-CRYSTAL nickel-base superalloys have been widely used because they possess a high fraction of ␥ ⬘ strengthening phase and good high-temperature properties. Some experimental results show that the deformation behavior of single-crystal nickel-base superalloys depends on their chemical composition as well as on microstructure, such as the average size and the distribution of ␥ ⬘ precipitates.[1–4] Better mechanical properties may be obtained when the ␥ ⬘ phase occupies a higher volume fraction, and the cuboidal ␥ ⬘ precipitates (about 0.5 m in edge size) are coherently embedded in the ␥ matrix. It is recognized that refractory metal additions can also have significant effects on the mechanical properties of superalloys.[4–11] The creep lives at 980 ⬚C and 234 MPa exhibit a steep peak as a function of Mo content with the maximum in life occurring at about 14 wt pct Mo.[4] According to a matrix of compositions based on MAR-M247, substitution of Ni for Ta causes large decrease in ␥ ⬘-solvus temperature, ␥ ⬘-volume fraction, and ␥ ⬘-␥ lattice mismatch, whereas, substitution of W for Ta results in smaller decreases in these features.[5] Substitution of 2 wt pct W for 3 wt pct Ta results in decreased creep life at high stress but improved life at low stress.[6] Murakami et al.[12] investigated the Ni-base superalloys containing 2 at. pct Ir and indicated that Ir atoms are expected to act as solid-solution hardeners of both the ␥ ⬘ and ␥ phases without SUGUI TIAN, Professor, is with the Department of Materials Science and Engineering, Shenyang Polytechnic University, Shenyang 110023, People’s Republic of China. JINGHUA ZHANG, Associate Professor, and YONGBO XU and ZHUANGQI HU, Professors, are with the State Key Laboratory for Fatigue and Fracture of Materials Institute of Metal Research, Chinese Academy of Science, Shenyang 110015, P.R. China. HONGCAI YANG, Professor, is with the College of Materials and Metallurgy, Northeastern University, Shenyang 110006, P.R. China. XIN WU, Professor, is with the Department of Mechanical Engineering, Wayne State University, MI 48202. Manuscript submitted January 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
reducing the microstructural stability. The reason that the refractory metal additions affect the creep features and resistance of superalloys is that they have the potential of influencing a number of properties, such as solid-solution hardening, ␥ ⬘-␥ lattice mismatch, ␥ ⬘-volume fraction, and diffusion rate. However, knowledge of the relative effectiveness of refractory elements W and Ta on the creep resistance of alloys separately and the mechanisms responsible for their effects is lacking. In addition, it may be considered that the creep resistance of alloys depends on the volume fraction of ␥ ⬘ precipitates and the internal friction stress of dislocation motion. The internal friction stress (0/MPa) is the average internal stress due to neighboring dislocations and other obstacles determining the behavior of mobile dislocations, which is expressed as the differe
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