Experimental and Numerical Analysis of Microstructures and Stress States of Shot-Peened GH4169 Superalloys
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Nickel-based superalloy GH4169, the microstructures and mechanical behaviors of which are very much similar to those of IN 718, is widely used in gas turbines of aero-engines owing to its good mechanical properties and structural stability at elevated temperatures.[1] It comprises of the c¢¢ phase, a metastable intermetallic compound Ni3Nb of the centered tetragonal crystal structure, and c¢ phase, a Ni3(Al, Ti) compound of the simple cubic crystal structures. These two phases are the main strengthening precipitates contributing to the high-temperature strength of GH4169.[2–4] Meanwhile, another phase, i.e., the d phase (intermetallic Ni3Nb) can also appear in GH4169 under different heat treatments, which helps improve the stress rupture ductility[5] and inhibit the growth of large grains during the forging process.[6,7]
DIANYIN HU and RONGQIAO WANG are with the School of Energy and Power Engineering, Beihang University, Beijing 100191, China, and with the Collaborative Innovation Center of Advanced Aero-Engine, Beijing 100191, China, and also with the Beijing Key Laboratory of Aero-Engine Structure and Strength, Beijing 100191, China. Contact e-mail: [email protected] YE GAO is with the School of Energy and Power Engineering, Beihang University. FANCHAO MENG and JUN SONG are with the Department of Mining and Materials Engineering, McGill University, Montreal, QC H3A 0C5, Canada. Contact e-mail: [email protected] Manuscript submitted September 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Because of these good properties mentioned above, GH4169 is often considered as constituent material for components used in conditions of high tensile stress and thermal loading. However, components under such conditions are prone to fatigue failure,[8] a serious threat to their reliability. To mitigate this threat, surface treatments such as shot peening, ultrasonic peening (USP), laser shock peening (LSP), and hammer peening are often applied to induce a compressive residual stress layer in the near-surface region in order to improve the fatigue performance. Nonetheless, besides bringing the benefit of the compressive residual stress layer, those surface treatments necessarily modify various surface characteristics, including surface roughness, surface morphology, and near-surface microstructure.[9–11] In recent years, numerous studies have demonstrated the phenomenon of surface nanocrystallization (SNC) in various alloys when they are subjected to USP or LSP treatment. Yin et al.[12] demonstrated that the depth of SNC layer increases from 2 lm to about 10 lm without grain size change when the surface of AISI-1018 was treated by USP for a duration of 0.5-1 hour. Trdan et al.[13] showed that the LSP treatment resulted in high dislocation density on the surface of the Al-Mg-Si alloy. Similar experimental observations were also reported by other studies.[14,15] However, most of the materials studied above are of low-strength alloys, and for the high-strength nickel superalloy GH4169, different experimental results
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