Characterization of the Microstructure, Fracture, and Mechanical Properties of Aluminum Alloys 7085-O and 7175-T7452 Hol
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INTRODUCTION
LOW density combined with high strength have made heat-treatable aluminum alloys the major choice for applications in naval ships and in aviation to achieve faster speeds, carry bigger payloads, and travel longer ranges. AA7085 is the seventh-generation structural forging alloy with improved thick section properties. ALCOA first developed AA7085 in the T7452 temper. The T7452 temper of AA7085 was optimized to improve strength. However, many applications require an improvement of fracture toughness.[1–4] This is especially true for 7000 series aluminums that have very low fracture toughness. ALCOA developed the Optimized Corrosion Resistance 3 temper of 7085 in order to improve its fracture toughness and corrosion resistance at the small expense of yield. There are several studies in the literature reporting on microstructural, mechanical, and fracture properties of aluminum alloys. Ludtka and Laughlin[5] discussed the SAMUEL G. BENOIT, Manufacturing Engineer, is with Cheer Pack North America, West Bridgewater, MA, 02379. VIJAYA B. CHALIVENDRA, Associate Professor, is with the Department or Mechanical Engineering, University of Massachusetts, North Dartmouth, MA, 02747. Contact e-mail: [email protected] MATTHEW A. RICE, Chemical Engineer, and ROBERT F. DOLESKI, Mechanical Engineer, are with the Naval Undersea Warfare Center, Newport, RI, 02841. M.A. Rice and R.F. Doleski are employed by the Naval Undersea Warfare Center. U.S. Government work is not protected by U.S. Copyright. Manuscript submitted November 21, 2015. Article published online June 30, 2016 4476—VOLUME 47A, SEPTEMBER 2016
effects of microstructure and strength on fracture toughness of ultrahigh-strength aluminum alloys. Those authors used Kahn-type tear tests to determine fracture toughness values; however, center-cracked tension tests were conducted to determine R-curve and Jc measurements. It has been demonstrated that the toughness decreases as the yield strength is increased. With the decrease in toughness, a transition in fracture mode was observed from predominantly transgranular dimpled rupture to predominantly intergranular dimpled rupture. Dumont et al.[6] presented the effects of process parameters such as quench rate, precipitation heat treatment, and anisotropy associated with rolling plane on the relation between the fracture toughness and the yield strength of AA7050 aluminum alloy. It was identified that toughness is confirmed to be minimum at peak strength, and lower for an overaged material than for an underaged material of the same yield strength. Liu et al.[7] discussed the effect of type of quench solution and treatments of Al-Cu-Mg and Al-Mg-Si alloys on yield strength, ductility, and fracture toughness. The materials treated with an enhanced solution or a stepped solution (in which the solution treatment is performed at temperatures which are gradually elevated with time) were found to display a significant increase in above properties. Csontosa and Starkeb[8] investigated the influence of alloy composition, grain
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