Effects of temperature and shot peening on S-N behavior of a PM Ni-base superalloy UDIMET 720
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2/12/04
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Effects of Temperature and Shot Peening on S–N Behavior of a PM Ni-Base Superalloy UDIMET 720 J. LUO and P. BOWEN Fatigue S–N (stress vs life) curve behavior for a powder metallurgy (PM) Ni-base superalloy UDIMET 720 in both polished and shot-peened surface conditions was investigated at room temperature and 600 °C in air. Tests were carried out under four-point bending at a load ratio of 0.1 and a frequency of 10 Hz. At 600 °C, an offset S–N curve was found for both polished and shot-peened surface conditions. This offset S–N curve is deduced to be controlled by the location of crack nucleation sites. Observations and deductions of crack nucleation sites, crack nucleation life, crack closure, environmental attack, and cyclic softening and hardening have been used to explain the experimental results.
I. INTRODUCTION
WHEN airplanes take-off and land, the components of the turbine engines experience large loading-unloading cycles. For discs, differential thermal expansion superimposed on centrifugal forces means that repetition of taking-off, landing, and reverse-thrust subjects the disc to an “on-off,” high amplitude loading over a range of temperatures.[1] Indeed, the stresses on discs in service can approach the proof stress of the material. Therefore, it is very important to understand the low-cycle fatigue (LCF) behavior of the materials used, and particularly with respect to the influence of elevated temperature. For this reason, the LCF behaviors of Ni-base superalloys, which are used for discs, have been studied extensively.[2–11] One interesting result is that a typical Coffin–Manson curve at room temperature for conventional superalloys is represented by a bilinear curve,[3,11–15] and this bilinear behavior was attributed to a change of LCF deformation mechanisms.[3] It is also well known that surface residual stresses introduced by a variety of surface treatment methods such as shot peening, fillet rolling, induction hardening, laser treatment, and simply machining can affect the fatigue performance of various metallic materials.[16–26] However, the effects of residual stress induced by surface treatment on the LCF behavior of Ni-base superalloys have been studied only rarely at elevated temperatures. Chang et al.[27] reported that at room temperature, the fatigue life of M17 superalloys is increased by higher compressive residual stresses induced by creep-feed grinding. Elsewhere, Forget et al.[28] studied the effect of residual compressive stresses induced by laser shock treatment on the LCF life of powder metallurgy (PM) Astroloy at a temperature of 550 °C. The large increase of fatigue life in specimens surface treated by laser shock treatment shows a beneficial effect of surface compressive residual stresses. This advantage is attributed to a protection against surface defects by compressive residual stresses. However, when the applied stress exceeds the yield stress of the material, any beneficial effects of compressive residual stresses can disapJ. LUO, Research Fellow
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