THERMAL STABILITY OF CAST AND WROUGHT ALLOY RENE 65

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1 Superalloys 2016: Proceedings of the 13th International Symposium on Superalloys Edited by: Mark Hardy, Eric Huron, Uwe Glatzel, Brian Griffin, Beth Lewis, Cathie Rae, Venkat Seetharaman, and Sammy Tin TMS (The Minerals, Metals & Materials Society), 2016 THERMAL STABILITY OF CAST AND WROUGHT ALLOY RENE 65 1 Wessman, Andrew; 2,3 Laurence, Aude; 2 Cormier, Jonathan; 2 Villechaise, Patrick; 3 Billot, Thomas; 4 Franchet, Jean-Michel 1 GE Aviation; One Neumann Way, Cincinnati, OH, 45215, USA 2 Institut Pprime, UPR CNRS 3346, CNRS Université de Poitiers ENSMA, Physics and Mechanics of Materials Department, ISAE- ENSMA, 1 avenue Clément Ader, BP 40109, Futuroscope - Chasseneuil, France 3 Snecma-Safran Group, Technical Department, 171 boulevard de Valmy - BP 31, Colombes Cedex, France 4 Safran SA, SafranTech- Materials and Process Department, Magny-Les-Hameaux Cedex, France Keywords: Rene 65, Cast and Wrought, Turbine Disks, Microstructure, Gamma Prime, Mechanical Properties, Stability Abstract The cast and wrought nickel superalloy Rene 65 has been recently introduced to provide a gamma prime strengthened alloy with high performance at temperatures exceeding 700 C at a lower cost than powder metallurgy alloys. While similar in chemistry to the PM alloy Rene 88DT, Rene 65 is typically used in a subsolvus form with finer grain size and a very different gamma prime distribution. This study provides an overview of the structure of Rene 65, and the effects high temperature exposures over long times have on the structure and mechanical properties of the alloy. Introduction In the last 50 years, Alloy 718 has found prolific use in turbine engines, particularly those used in aerospace propulsion. The alloy provides a desirable combination of corrosion resistance and high strength, but the use temperature of the alloy is limited to approximately 649 C [1] due to the metastable nature of the gamma double prime (Ni 3 Nb) phase. Increased operating temperatures required for increased gas turbine engine thermodynamic efficiency have led to increased use of powder metallurgy (PM) alloys such as René 88DT and René 104 for the manufacture of critical rotating aircraft engine components. While providing the required strength, creep, fatigue, and dwell time fatigue crack growth capability at elevated temperatures, their application comes with a cost penalty due to the expensive powder processing route required. Industry efforts have continued to develop a cast and wrought alloy to bridge the gap between Alloy 718 and PM alloys. This has resulted in the development of such alloys as 718Plus TM, Alloy 720 [2], and AD730 TM [3] for high temperature disk applications. GE s approach to fill this need was to start with a successful powder alloy and adapt its chemistry and processing to facilitate cast and wrought (C&W) manufacture. In 2007, GE entered into a Joint Technology Development Agreement with ATI Specialty Materials to develop this approach for Rene 88DT. The resulting C&W alloy, now designated as René 65, is currently in full production across multiple GE engine lines [4]. The alloy has also been adopted by GE s partner in the CFM engine consortium, the SNECMA division of SAFRAN. Rene 65 is a gamma prime (Ni 3 Al) strengthened alloy, and so does not suffer from the coarsening and transformation of the metastable gamma double prime (Ni 3 Nb) phase in the way that Alloy 718 does. As Rene 65 has been developed off of Rene 88DT s chemistry, one would expect similar temperature capability, which was shown by Wlodek et al. [5] to be appropriate for use at temperatures up to ~650 ºC for indefinite times, and up to 760 C for exposure times up to 1000 hours. While Rene 65 is very similar to Rene 88DT in terms of chemistry, there are important structural differences between the alloys that necessitate an examination of its performance during long time at high temperatures. Rene 65 is used in a subsolvus heat treated condition, as opposed to a typical supersolvus Rene 88DT, and thus retains a significant fraction of primary gamma prime in the fully heat treated structure, typically around 15 vol.%. This primary gamma prime particles serve to pin the grain boundaries during heat treatment, retaining a fine grain size, and this gamma prime pinning eliminate the need for intentional carbon additions to form pinning carbides as are typically used in Rene 88DT for grain size control in supersolvus heat treatments. Thus, Rene 65 has a much different starting gamma prime structure, and lacks the MC carbides noted in Rene 88DT examinations. This study will examine the behavior of the alloy in terms of gamma prime thermal stability, TCP phase formation, and resulting information regarding coarsening rates and mechanical properties that can be gleaned from such examination. Experimental Setup: Material Processing Both GE Aviation and SNECMA have implemented Rene 65 in producing aero engine components, and so this study will also show results from independent analyses performed at each partner and compare those results. Work was performed by both GE Aviation and SNECMA researchers, and also by researchers at the Pprime Institute under the sponsorship of SNECMA. The partners carried out isothermal aging studies of Rene 65 at a variety of times and temperatures, and the test conditions examined are shown in Figure 1. Figure 1. Rene 65 isothermal aging conditions. 793

2 The starting structure of Rene 65 is shown in Figure 2. The microstructure consists of gamma grains having an approximately 10 micron grain size, interspersed with micron-scale primary gamma prime particles which serve to pin grain boundaries during forging and solution heat treatment. At a finer scale, the subgrain structure consists of a secondary gamma prime distribution of nm particles formed on cooling from solution heat treatment, and a tertiary gamma prime distribution of nm formed during a lower temperature event on cooling from solution heat treatment and further developed during aging. The size of the secondary and tertiary gamma prime distributions are dependent upon the cooling rate of the material, with faster cooling rates resulting in finer particles. The material in Figure 2 shows a secondary gamma prime distribution with a roughly 70 nm average size, corresponding to a cooling rate of ~83 C/min. observed even for times up to 5000 hours at 732 C. Increasing the temperature to 760 C drove an increased coarsening rate. Micrographs of the secondary gamma prime after various thermal exposures are shown in Figure 3. a) b) c) d) 500nm Figure 3. Effect of moderate temperature exposures on gamma prime, a) unexposed, b) 5000 hours at 704 C, c) 5000 hours at 732 C and d) 500 hours at 760 C. a) The sizes of the secondary gamma prime particles in the micrographs in Figure 3 were measured by manual measurement of a minimum of 48 particles for each exposure. After 5000 hours at 704 C, the measured average secondary gamma prime particle diameter was 71.7 nm, with a 13.1 nm standard deviation, which is essentially unchanged from the unexposed condition. After 5000 hours at 732 C, the average size of the particles has increased to 93.4 nm, with a standard deviation of 16.5 nm. While this represents a coarsening of the material, the distribution of particles still appears to be a relatively narrow, normally distributed population. Exposure for less time at a higher temperature, 500 hours at 760 C, causes the distribution of particle sizes to become a much broader distribution, and even though the average particle size is approximately that of the starting material, at 68.3 nm, the standard deviation of the population is substantially larger at 24.3 nm. This distribution broadening is shown clearly in Figure 4. b) Figure 2. Microstructure of fully heat treated Rene 65 prior to isothermal exposure at a) 5,000 x and b) 50,000 x. Effect of Exposure on gamma prime Structure The microstructures of Rene 65 samples were examined after the isothermal exposures shown in Figure 1. At temperatures below 730 C, no change in gamma prime distribution is observed. At temperatures from 732 ºC and above, long time exposures lead to the complete dissolution of the tertiary gamma prime particles. Coarsening of the secondary gamma prime is also observed visually, although only a modest amount change in size is 794

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8 but differently processed alloy, Rene 88DT. This study will serve as a useful guide in future applications of Rene 65. Acknowledgements The authors wish to thank GE Aviation and SNECMA for their support of this research. Pr. F. Pettinari-Sturmel, Dr. J. Douin, Dr. M. Hantcherli and Dr. F. Mompiou (CEMES, CNRS, Toulouse, France) are gratefully acknowledged for several TEM phase identifications. References [1] R. L. Kennedy, "Allvac 718Plus, Superalloy for the Next 40 Years," in Superalloys 718, 625, 706 and Various Derivatives, [2] A. Devaux, E. Georges and P. Heritier, "Properties of New C&W Superalloys for High Temperature Disk Applications," in Superalloys 718, 625, 706 and Various Derivatives, [3] A. Devaux, B. Picque, M. F. Gervais, E. Georges, T. Poulain and P. Heritier, "AD730- A New Nickel-Based Superalloy for High Temperature Engine Rotative Parts," in Superalloys 2012, [4] J. Heaney, M. Lasonde, A. Powell, B. Bond and C. O'Brien, "Development of a New Cast and Wrought Alloy (Rene 65) for High Temperature Disk Applications," in Superalloys 718, 625, 706 and Various Derivatives, [5] S. T. Wlodek, M. Kelly and D. A. Alden, "The Structure of Rene 88DT," in Superalloys 1996, [6] I. Slyozov et al., "The Kinetics of Precipitation from Supersaturated Solid Solutions," J. Phys. Chem. Solids, vol. 19, no. 1/2, pp , [12] S. Simonetti and P. Caron, "Role and Behavior of mu Phase During Deformation of a Nickel Base Single Crystal Superalloy," Materials Science and Engineering A, vol. 254, pp. 1-12, [13] C. M. F. Rae, M. S. Hook and R. C. Reed, "The Effect of TCP Morphology on the Development of Aluminide Coated Superalloys," Materials Science and Engineering A, vol. 396, pp , [14] C. M. F. Rae and R. C. Reed, "The Precipitation of Tolologically Close Packed Phases in Rhenium Containing Superalloys," Acta Materialia, vol. 49, pp , [15] J.-B. Le Graverend, J. Cormier, P. Caron, S. Kruch, F. Gallerneau and J. Mendez, "Numerical Simulation of Gamma/Gamma Prime Microstructural Evolutions Induced by TCP-phase in the MC2 Nickel Base Single Crystal Superalloy," Materials Science and Engineering A, vol. 528, pp , [16] A. Laurence, J. Cormier, P. Villechaise, T. Billot, J.-M. Franchet, F. Petinari-Sturmel, M. Hantcherli, F. Mompiou and A. Wessman, "Impact of the Solution Cooling Rate and of Thermal Aging on the Creep Properties of the New Cast & Wrought Rene 65 Ni-based Superalloy," in 8th Symposium on Superalloys 718 and Derivatives, [17] B. Alexandrov et al., "Non-equilbrium Phase Transformation Diagrams in Enginneering Alloys," Trends in Welding Research, vol. VIII, pp , [18] B. Alexandrov et al., "Single Sensor Differntial THermal Analysis of Phase Transformation and Structural Changes during Welding and Postweld Heat Treatment," Welding in the World, vol. 51, pp , [7] B. Flagolet, P. Villechaise, M. Jouiad and J. Mendez, "Aging Characterization of the Powder Metallurgy Superalloy N18," in Superalloys 2004, [8] W. Huther and B. Reppich, "Interaction of Disclocactions with Coherent, Stress-Free, Ordered Particles," Z. Metallk, vol. 69, p. 628, [9] E. I. Galindo-Nava, L. D. Connor and C. M. F. Rae, "On the Prediction of Yield Stress of Unimodal and Multimodal Gamma Prime Nickel-Base Superalloys," Acta Materialia, vol. 98, pp , [10] S. Sondhi, D. Wei and M. Henry, "A Physics Based Yield Strength Model for Nickel-Based Superalloys," Private Correspondence of Unpublished Manuscript. [11] Mishima et al., "Transactions of the Japan Institute of Metals," vol. 27, p. 656,