Microstructural Features Leading to Enhanced Resistance to Grain Boundary Creep Cracking in ALLVAC 718Plus
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ALLVAC*
INTRODUCTION
718Plus is a newly developed Ni-base
*ALLVAC is a trademark of ATI, Charolette, NC.
superalloy that has the processing and welding characteristics of INCONEL** 718 but with the high-temper**INCONEL is a trademark of Special Metals Corporation, Huntington, WV.
ature capability of WASPALOY. ALLVAC 718Plus WASPALOY is a trademark of United Technologies, Hartford, CT.
was developed in response to the need for an advanced turbine disk alloy that could be processed and welded similar to the current alloy INCONEL 718 but that would also be capable of a 55 C increase in temperature KINGA A. UNOCIC, Research Staff, is with Oak Ridge National Laboratory, Oak Ridge, TN 37831. MICHAEL J. MILLS and GLENN S. DAEHN, Professors, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] ROBERT W. HAYES, Director Research and Development, is with the Metals Technology, Inc., Northridge, CA 91324. Manuscript submitted April 7, 2009. Article published online November 17, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
capability over INCONEL 718 (hereafter 718). A series of articles describing ALLVAC 718Plus and its development can be found in Reference 1. Briefly, the increase in the temperature capability of ALLVAC 718Plus over 718 comes from a significant modification of the composition of ALLVAC 718Plus, as shown in Table I. As can be seen in Table I, ALLVAC 718Plus can be considered a cross between 718 and WASPALOY. The reduction of Fe and the addition of 10 wt pct Co along with the increase in Al content by 1 wt pct were found to promote the formation of the ordered L12 c¢ phase (Ni3(Al,Ti)) as opposed to the DO22 bct c¢¢ phase (Ni3Nb),[2–4] which is the predominant strengthening phase in 718. Also, 1 wt pct tungsten is added in order to increase the high-temperature deformation resistance of ALLVAC 718Plus. It is well established that the instability and coarsening of the c¢¢ phase sets the upper temperature limit of 718 at approximately 650 C. In addition, with long-term exposure of 718 to temperatures of 650 C and higher, the c¢¢ begins to transform to the equilibrium orthorhombic d phase (Ni3Nb).[3] At this point, the resistance of the 718 to deformation decreases and the mechanical properties such as creep and low cycle fatigue (LCF) deteriorates. On the other hand, the L12 c¢ phase has much higher thermal stability due to the very small c c¢ lattice misfit, thus making WASPALOY capable of service at temperatures of up to 700 C. This was the major emphasis behind the development of ALLVAC 718Plus.[1] Another well-recognized feature of Ni-base turbine disk alloys is their tendency toward brittle intergranular fracture and severe loss in ductility when tested in air at intermediate temperatures within the range of VOLUME 41A, FEBRUARY 2010—409
approximately 600 C to 800 C. Recently, Woodford[7] has reviewed the literature on gas phase embrittlement (GPE) and grain boundary cracking in Ni-base superalloys. He
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