13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Some Microstructural Aspects on Humidity-Enhanced Deterioration in the Fatigue Strength of Age-Hardened 7075 Al Alloy Yuzo Nakamura1,*, Kohji Kariya 2, Norio Kawagoishi 2 1 Department of Mechanical Engineering, Graduate School of Science and Engineering, Kagoshima University Kagoshima 890-0065, Japan 2 Department of Mechanical System Engineering, Daiichi Institute of Technology Kirishima 899-4395, Japan * Corresponding author: nakamura@mech.kagoshima-u.ac.jp Abstract Age-hardened 7075 Al alloys are one of the materials which possess large strength and large toughness. Their weight and large resistance to corrosion are also attractive in practical use. We have recently shown that the fatigue behavior of extruded 7075-T6 Al alloy is strongly influenced by the humidity in atmospheric air, and the fatigue strength decreases markedly at the humidity higher than about 60%. In addition, shear-type fracture with high crack growth rate occurs at high humidity, while tensile-type fracture dominates at low humidity. The occurrence of the change in fracture mode, however, depends on the conditions used in fatigue tests. In the present study, microstructures of the 7075 T6 Al alloy subjected to fatigue tests in various conditions and humidity are examined by using electron backscattering diffraction (EBSD) and X-ray diffraction (XRD). The results of EBSD and XRD analyses indicate that the microstructures depend on the history of metalworking and heat treatment and that the fatigue properties in high humidity environment are influenced by the difference of the microstructures in addition to the effect of humidity. A crytallographic geometry model is also shown to interpret the preferential occurrence of the shear-type fracture based upon the EBSD analyses. Keywords Fatigue, Humidity, Fracture mode, 7075-T6 Al alloy, Microstructure 1. Introduction Age-hardened Al alloys are very attractive from the viewpoints of those large strength, large fracture toughness and light weight. In particular, Al-Mg-Zn(-Cu) alloys (7000 series) are commonly strengthened by the precipitation of spheric GP zone, hexagonal η' phase (MgZn2) and hexagonal η phase (MgZn2), and the degree of strengthening depends on the heat treatment conditions used for aging [1]. It is well established that 7000 series Al alloys are very susceptible to stress corrosion cracking in aggressive corrosion environment [1]. The fatigue properties of age-hardened Al alloys are very sensitive to environment, and a drastic decrease in fatigue strength occurs in the air of relative humidity (RH) higher than 85% [2]. Our previous studies [3-7] have also shown that the fatigue strength of 7075 T6 alloy decreases markedly as RH is increased to more than about 60 %. Anodic dissolution of particles or stress concentration arising from large particles may become the causes of initiating cracks. However, the previous studies strongly suggest that hydrogen-enhanced local plasticity mechanism plays a major role in the deterioration of fatigue properties. The difference in microstructure especially due to under-, peak- and over-aging has been considered to be another key issue to understand the susceptibility of age-hardened Al alloys and improve their resistance to environmental attacks. Commercial Al alloys, however, commonly experienced metalwork processes such as rolling and extrusion in order to obtain products with desired strength, shape and size. Thus, we have to consider the effect of these microstructures produced by manufacturing processes. The previous studies have shown that the extruded rods of 7075 T6 alloy exhibit unique deformation and fracture behaviors in cyclic loading. The initiation of crack and its propagation takes place at angles of about 35o inclined from the loading axis in certain atmospheres and for certain test conditions. In the rotating bend tests with the frequency of 50 Hz, this shear-type cracking is observed in high humidity more than 60% RH at high stress levels, while the decrease in
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