Stress relaxation of shot-peened UDIMET 720Li under solely elevated-temperature exposure and under isothermal fatigue
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INTRODUCTION
NICKEL-BASED superalloys are technologically important materials for turbine engine applications due to their excellent mechanical and corrosion-resistance properties at high temperatures.[1,2] One can distinguish between two major types, namely, single-crystal and polycrystalline-nickel–based superalloys. The temperature range of operation is different for the two types: single-crystal superalloys are used typically above 1000 °C, while traditional polycrystalline superalloys generally operate at around 600 °C to 650 °C. UDIMET* 720Li is a relatively new polycrystalline nickel*UDIMET is a trademark of Special Metals Corporation, New Hartford, NY.
base superalloy, conceived in the late 1990s.[3] It has been microstructurally characterized thoroughly by several authors.[4–7] In its standard heat-treatment condition, it is characterized by the presence of primary, secondary, and tertiary phase, the first precipitating at grain boundaries and having typical size of about 1 to 5 m and the others precipitating inside the grains and having sizes of about 100 and 20 nm, respectively. This alloy finds widespread use in aeroengine turbine
S.-B. KIM, Postdoctoral Student, J. SHACKLETON, Senior Experimental Officer, M. PREUSS, Lecturer, and P.J. WITHERS, Full Professor, are with the Materials Science Centre, University of Manchester, Manchester, M1 7HS, United Kingdom. A. EVANS, Postdoctoral Student, and G. BRUNO, Instrument Scientist, are with the Materials Science Centre, University of Manchester, and the Institut Laue-Langevin, 38042 Grenoble, Cedex 9, France. Contact e-mail: [email protected] Manuscript submitted January 5, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
discs.[7,8] It is currently employed at temperatures as high as 600 °C to 700 °C. Thus, thermomechanical fatigue and creep are important mechanisms of failure. The fatigue life of components can be increased by introducing a compressive surface residual stress, typically by shot peening. Unfortunately, residual stresses tend to relax during service, through a combination of creep and fatigue. These mechanisms are also associated with the loss of the increased work hardening introduced by the peening process.[9] This article examines the resistance to relaxation of shot-peening residual stresses in the Ni-based superalloy UDIMET 720Li when exposed to high-temperature, isothermal strain-controlled low-cycle fatigue (LCF). The two strain-controlled conditions were chosen as being representative of those experienced in turbine discs in service and as a result of a serious overload. Shot peening is a mechanical surface treatment whereby small metal balls impinge on the surface of the component.[10] This generates a work-hardened surface layer as well as a misfit strain between the surface region and the bulk. An isotropic in-plane compressive residual-stress state is created in the near-surface region (typically a few hundred microns). The compressive residual stresses and the associated surface hardening are due to the plastic misfit (Eigens
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