Effects of Ru additions on the microstructure and phase stability of Ni-base superalloy, UDIMET 720LI
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I. INTRODUCTION
THE nickel-base superalloy UDIMET* 720LI, also known *Udimet is a trademark of Special Metals Corp.
as U720LI, is widely used in aircraft and in land-based turbine discs, such as in BMW-Rolls Royce BR700 aircraft engines and AE3007, because of its high creep strength and low crack-growth rates at high temperatures.[1,2] The changes in the microstructural and mechanical properties have shown that the formation of deleterious, topological closed-packed (TCP) phase, i.e., the phase, occurs during the in-service thermal exposure.[1] Recent studies[3,4] on the formation of phase in U720LI have shown that its composition was (Cr0.5,Mo0.1), (Ni0.2,Co0.2); the phase is easily formed above 700 °C, especially at 750 °C. When the phase is present, the tensile yield strength from room temperature to 750 °C is reduced by 5 to 10 pct, and the creep resistance at 625 °C is also reduced by a factor of 5.[5] The loss in mechanical properties is due to the deletion of Cr from the matrix, which is a strong solid-solution element in U720LI. Fortunately, U720LI possesses good thermal stability at temperatures below 700 °C.[3,4,6] For example, during annealing at 700 °C for up to 3000 hours,[6] the size, shape, and volume fraction of the phase are stable, and the phase is not formed.[3,4] Therefore, in practice, the temperature required for a long-term use of U720LI is below 700 °C. However, the need for higher-temperature capability of the rim sections of high-pressure turbine discs has continued to challenge material and design engineers, as the operating temperatures now approach 760 °C and even 815 °C, for military applications. If the required component lives cannot be achieved using the present alloy U720LI, either improvements will need to be made on U720LI or a new disc alloy will need to be introduced. Recently, additions of platinum-group metals (PGMs), such as Ru, have been conducted in Ni-base alloys[7,8,9] to improve their microstructural stability and to suppress the formation C.Y. CUI, M. OSAWA, and A. SATO, Research Fellows, Y.F. GU and D.H. PING, Senior Researchers, and H. HARADA, Group Leader, are with the National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047, Japan. Contact e-mail: [email protected] Manuscript submitted April 27, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
of the TCP phase, which improves the high-temperature creep properties. For example, the Ni-base superalloy TMS-162, containing 6 wt pct Ru, has good microstructural stability during high-temperature creep, because the formation of the TCP phase is successfully suppressed by the Ru additions. Thus, TMS-162 has reached a temperature capability of 1100 °C which is the highest temperature capability to be reported,[9] at 137 MPa, and a creep-rupture life as long as 1000 hours. However, up to now, there have been few studies on the effect of Ru additions on microstructural stability in Ni-base disc alloys. In the present study, the effects of Ru additions on the microstructure and phase stabil
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