CONTROLLING THE DEFORMATION MECHANISM IN DISK SUPERALLOYS AT LOW AND INTERMEDIATE TEMPERATURES Y. Yuan 1 , Y.F. Gu 2 , Z.H. Zhong 2 , T. Osada 2† , T. Yokokawa 1 , H. Harada 1 1 Environment and Energy Materials Division, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan 2 High Temperature Materials Unit, National Institute for Materials Science, Sengen 1-2-1, Tsukuba 305-0047, Japan †now at Research Center for Green Materials Innovation, Yokohama National University, 79-1 Tokiwadai, Hodogaya-ku,Yokohama 240- 8501, Japan Keywords：Ni-Co-base superalloy; Tensile; Microstructure; Deformation mechanism; TMW-4M3 Abstract The newly developed TMW ® disk superalloy exhibits better overall mechanical properties in the intended service temperature region (650-750 ℃) than the most advanced cast & wrought disk alloy U720Li. Clarification of the underlying mechanisms is beneficial for designing advanced superalloys. Among the TMW alloys, TMW-4M3 has the best tensile strength and creep resistance. In this study, tensile tests of U720Li and TMW-4M3 were conducted at temperatures ranging from 25 ℃ to 750 ℃. The deformation microstructures have been investigated by transmission electron microscopy (TEM). Dislocation activity, involving anti-phase boundary (APB), was the dominant mechanism in U720Li up to 725 ℃. However, the transition of deformation mechanisms from dislocation pairs cutting to stacking fault (SF) shearing and deformation twinning was observed in TMW-4M3. A concise model related to the increased surface energy is put forward to describe the competing mechanisms. It is found that APB energy, SF energy (SFE), and volume fraction of tertiary γ΄ have important influence on the transition of deformation mechanisms. The controlling of deformation mechanism by alloy design is discussed. Introduction Ni-base superalloys are extensively used in aircraft engines and land-base gas turbines, owing to their unique high temperature mechanical properties. Super-dislocations play an important role during the deformation of disk superalloys, which usually interact with γ΄ precipitates in two different ways at low (25-600 ℃) to intermediate temperatures (600-800 ℃ ), i.e., cutting the precipitates by coupled a/2<110> (a is the lattice constant) dislocation pairs or dissociating into super-partial dislocations [1]. The former creates anti-phase boundary (APB) in the γ΄ precipitates, and the latter forms stacking fault (SF). The mechanism favored by the specific alloy depends on the energy status in the system, which is related to microstructure, deformation condition and temperature. In disk superalloys, usually, dislocation pairs cutting mechanism prevails at temperatures below 600 ℃ [2-4]. Planar defects, such as SF and deformation twin, occur in the temperature range of 600-800 ℃. Above 800 ℃, dislocation climb is the dominant mechanism [1]. Kolbe reported the transition of deformation mechanisms from dislocation pairs cutting to deformation twinning at 780 ℃ in NIMOIC 105 and NIMONIC PE 16 [5]. However, the transition of deformation mechanisms associated with APB energy and SF energy is not yet fully understood. At National Institute for Materials Science, Japan, a new cast & wrought Ni-Co-base disk superalloy, TMW-4M3, has been developed for applications at elevated temperatures up to 725 ℃ [6-12]. It has been reported that TMW-4M3 has comparable yield strength with U720Li at room temperature, but superior yield strength in the service temperature region from 650 ℃ to 750 ℃ [6]. TMW-4M3 has higher APB energy and lower SF energy than U720Li. The relationship between these energies and higher yield strength at intermediate temperatures need further investigation. In the present study, the deformation microstructures of U720Li and TMW-4M3 after tensile tests at low (25 ℃) to intermediate temperature (750 ℃ ) have been investigated by transmission electron microscopy (TEM). A concise model related to the increased interface energy is put forward to describe the transition of deformation mechanisms. The controlling of deformation mechanism by alloy design is discussed. Experimental The detailed processes of preparing two alloy ingots can be found elsewhere [6]. All specimens were cut from the forged pancakes (440 mm in diameter and 65 mm in thickness), then heat treated as follows: 1100 ℃/4 h/oil quenching (OQ), then aging at 650 ℃ /24 h/OQ +760 ℃/16 h/OQ. Tensile tests were performed at 25 ℃ (room temperature, RT), 400 ℃, 650 ℃, 700 ℃, 725 ℃, and 750 ℃ according to ASTM E8/E21. Besides the ruptured tests, three interrupted tests with 1.0% plastic strain for TMW-4M3 at 25 ℃, 650 ℃, and 725 ℃ were selected. After the mechanical tests, TEM discs with thickness of around 300 μm were cut from the deformed samples perpendicular to the stress axis. Then the discs were manually ground to 50 μm and perforated by twin-jet electro-polisher at 40 V/18 mA and -10 ℃ . The electrolyte consisted of 225 ml acetic acid, 225 ml butylcellosolve, and 50 ml perchloric acid. The microstructures of specimens were investigated using a Tecnai 20 microscope operated at 200 kV. Burgers vectors of dislocations were determined using g·b criterion. In our observations, the deviation vector (s) was always kept positive and the deviation parameter ( s g ⋅ ξ ) was maintained to be larger than 0.7. For different values of g·b, the visibility and invisibility of partial dislocation on the one side of SF are summarized as follows: 35 Superalloys 2012: 12 th International Symposium on Superalloys Edited by: Eric S. Huron, Roger C. Reed, Mark C. Hardy, Michael J. Mills, Rick E. Montero, Pedro D. Portella, Jack Telesman TMS (The Minerals, Metals & Materials Society), 2012