Microstructure, creep properties, and rejuvenation of service-exposed alloy 713C turbine blades
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INTRODUCTION
AERO engine
turbine blades are exposed to high temperatures and stresses during service and, for this reason, are usually made from heat-resistant investment cast nickel-base superalloys. Polycrystalline versions of these alloys derive their high-temperature strength through solid solution strengthening of the austenitic 3' matrix and through precipitation of intermetallic T' phase within the grain interiors. They also derive resistance to creep deformation through precipitation of 3" and MC and/or M23C6 carbides at grain boundaries, formation of a serrated grain boundary structure, and segregation of trace elements, such as B and Zr, to the grain boundaries, tu There have been several studies of the effects of service exposure on the microstructure and mechanical properties of turbine blades made from nickel-base superalloys, ]2-sJ and it is recognized that during service, these blades undergo a series of time-, temperature-, and stress-dependent microstructural changes. In particular, the 3" precipitates tend to agglomerate, while carbide reactions lead to the formation of undesirable continuous carbide films along grain boundaries. Under certain conditions, undesirable embrittling phases, such as 0-, can also form. These changes can result in the loss of important mechanical properties, such as creep strength and resistance T.M. MACCAGNO, Research Associate, A.K. KOUL, Senior Research Officer, and J.-P. IMMARIGEON, Deputy Laboratory Head-Materials, are with the Structures and Materials Laboratory, Institute for Aerospace Research (formerly National Aeronautical Establishment), National Research Council of Canada, Ottawa, ON K1A 0R6. L. CUTLER, R. ALLEM, Research Associates, and G. L'ESPI~RANCE, Professor, are with the Department of Metallurgical Engineering, l~cole Polytechnique de Montrral, Montrral, PQ H3C 3A7, Canada. Manuscript submitted March 26, 1990. METALLURGICAL TRANSACTIONS A
to cracking, which, in turn, can lead to accelerated creep deformation in the form of excessive untwisting, lengthening, and bend back of blade airfoil sections, as well as to blade fracture. It has been suggested that the mechanical properties can be "rejuvenated" by heat treatment, using hot isostatic pressing (HIP) to eliminate creep-induced internal cavitation, t2] This paper studies the effects of service exposure on aero engine turbine blades made from investment cast Alloy 713C and attempts to correlate the evolution of microstructure, creep properties, and blade distortion with the service history of the blades. The paper also explores the possibility of rejuvenating the microstructure and creep properties of service-exposed blades through HIP and heat treatments. II.
E X P E R I M E N T A L DETAILS
A. Material and Treatments
Alloy 713C has the nominal composition (in weight percent): 74Ni, 12.5Cr, 6.1A1, 4.2Mo, 2.0Nb, 0.8Ti, 0.10Zr, 0.12B, and 0.12C. New and service-exposed fLrStstage turbine blades made from Alloy 713C were obtained from an engine repair and overhaul facility. The blades are protected b
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