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Age hardening; Aluminum alloys; Fatigue of materials; High strength alloys; Magnesium alloys; Microstructural evolution; Plastic deformation; Precipitation (chemical); Silicon alloys; Strain energy; Weibull distribution; X ray diffraction, Aa6063 alloys; Dynamic precipitation; Dynamic structural changes; Fatigue response; Low cycle fatigues; Over-aged alloys; Peak aged; Plastic strain energy; Strain amplitude; XRD, Low-cycle fatigue
Abstract:
Understanding the dynamic structural changes during cyclic deformation of Al-alloys is a discipline of great interest to material technologists. The primary objective of this report is to assess the low cycle fatigue (LCF) behaviour of AA6063 alloy in under-aged (UA), peak-aged (PA) and over-aged (OA) states and to understand the energetic aspects of cyclic plastic behaviour in the background of the nature and density of the dislocations. Two parameters Weibull distribution analyses, for the first time, establish that despite inferior monotonic strength, LCF life of UA alloy is twice compared to PA alloy at higher (≥0.5) strain amplitudes. This corroborates well with the fact that the cumulative dissipated plastic strain energy density (CPSED) values exhibit reverse variation with applied strain amplitude in the case of UA alloy as compared to PA and OA alloys. TEM analyses confirm that strain-induced vacancy-driven dynamic precipitation causes dramatic cyclic hardening in UA alloy resulting in improved fatigue life. One can also evidence this from the fivefold increase in the plastic strain energy dissipation and 2-times increased of post-fatigue hardness. In contrast, fracture surface analyses and XRD-based dislocation density measurements coupled with TEM examinations reveal that the shearing or by-passing of precipitates by the dislocations in the PA or OA alloys, respectively, lead to strain localization and cyclic softening, and consequently, reduce the plastic strain energy dissipation and lower fatigue life. The study provides important guidelines for the development of high strength Al-alloys with improved fatigue performance. © 2021
