EFFECTS OF MICROSTRUCTURE ON HIGH-CYCLE FATIGUE OF AN AL-ZN-MG-CU ALLOY (AL-7055)

ABSTRACTEFFECTS OF MICROSTRUCTURE ON HIGH-CYCLE FATIGUE OF AN AL-ZN-MG-CU ALLOY (AL-7055)Lynn Eugene Oswald, MSUniversity of Pittsburgh, 2003The study of fatigue in aluminum alloys is an area that has been studied for many years. Although proposed processes and mechanisms for fatigue initiation have been reported in various studies, there still exist areas that require improved understanding. The current study examines the role that microstructural features can play with regard to fatigue crack initiation.Alloy 7055 has found wide acceptance by commercial aircraft manufacturers in the use of upper wing skins. This product displays a unique combination of compressive yield strength, fracture toughness and corrosion resistance which are required for use in compression-dominated upper wing structures. Fatigue crack initiation is used by aircraft designers to determine the initial inspection intervals an aircraft must adhere to. By increasing the fatigue initiation resistance of an aluminum alloy, an aircraft can endure additional flights prior to initial inspection. This has an obvious impact on the financial and safety performance of the structure.Constituent particles (Mg2Si and Al7Cu2Fe) play a primary role in the fatigue crack initiation resistance of 7055 alloy. In the current study Mg2Si was determined to be a particularly damaging feature in the microstructure in terms of fatigue crack initiation resistance, as compared to Al7Cu2Fe. The elastic and plastic behavior of these particles, as determined with an atomic force microscope (AFM) equipped with a nanoindenter, revealed that the Fe-bearing particles were more resistant to fracture and exhibit increased plasticity, as compared to the Si-bearing particle