FRACTOGRAPHIC STUDY OF TWO DAMAGE TOLERANT ALUMINIUM ALLOYS A.M. Irisarri and G. Atxaga Structural Integrity. Fundación INASMET. C. Portuetxe 12. 20009 San Sebastian ABSTRACT The influence of second phase particles on the fracture behaviour of two high strength aluminium alloys (7475 and 7050) has been analysed. Fractographic study was carried out by scanning electron microscopy and energy dispersive spectrometry was used to identify the particles present on the fracture surfaces. Examination of the failed specimens revealed that the whole fracture surfaces were covered by ductile dimples pointing towards microvoid coalescence as operating mechanism and evidences the important role played by the second phase particles. Most of these particles were iron rich ones, varying the other elements, although mainly corresponded to the aluminium-copper-iron type. These observations showed the significant contribution of the iron content to the failure process and confirmed that the used approach to improve toughness by reducing the percentage of iron is a good practice and even more strict control of this element is recommended. INTRODUCTION The call for improved economy in connection with high reliability is the overriding requirement for the development of aircraft. The structures should have a long life, low weight and minimum maintenance costs. To satisfy such properties, materials providing not only high static and fatigue strength but also greater resistance to crack propagation, high residual strength and good corrosion characteristics are required [1]. Aluminium alloys have satisfied these requirements best for more than 50 years and therefore make up for most of the aircraft structure. One vision of the subsonic commercial jetliner of about 2020 suggests an aircraft with a structure containing almost entirely of polymer matrix composites. However, the Boeing 777 was also predicted to be built of about 70% of polymer matrix composites, and yet is predominantly built in conventional aluminium alloys [2]. Only alloys from two systems, both precipitation hardenable, are serious candidate for aircraft structures; the aluminium-copper-magnesium (2XXX series) and the aluminium-zinc-copper-magnesium (7XXX family) systems. High strength Al-Zn-Cu-Mg based wrought 7XXX alloys are widely used for structural applications in aircraft industry, where fracture toughness and yield strength are of prime concern. Aircraft designers generally prefer 7XXX alloys for lower wing structures because of their high strength even though they have only moderate damage tolerance. The story of the development of aluminium alloys over the greater part of the last decades has been one of early, and generally quite successful, attempts to increase the static strength, followed by the realisation that this property is not necessarily the one of most importance. A high value of static strength is, of course, always desirable and the development procedures applied to any alloy cannot be allowed to sacrifice the mechanical strength to any considerable degree. However, almost all of the research
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