Toughness-Strength Relations in the Overaged 7449 Al-Based Alloy

COST reduction is a key objective of modern airframe The 7449 and 7150 plates, respectively 30and 36-mm manufacture. This can be achieved by the design of materials thick, were provided by Pechiney CRV (Voreppe, France) with improved properties leading to weight reduction of the in a solutionized, quenched, and stretched condition (corresairframe structure and, therefore, reduction in fuel consumpponding to the commercial W51 heat treatment). The nomition. The high-strength 7449 aluminum alloy was particunal composition of the two alloys is given in Table I. larly developed to be used for the upper wing skin of aircraft Specimens were naturally aged for several months before structures replacing the established 7150 aluminum alloy.[1] undergoing three commercial heat treatments, which are preThis relatively new alloy possesses a higher Zn content and sented in Table II. Here, it should be noted that one single slightly less Cu than the 7150 aluminum alloy. However, in T651 treatment was applied to both alloys. This treatment aerospace design, damage tolerant concepts and, therefore, is designed to give peak strength for the 7449 alloy. Five fracture toughness is a critical parameter. As high strength experimental overaged heat treatments, T7A to T7E, have is known to be detrimental for toughness (e.g., Reference also been applied to the 7449 aluminum alloy (Table II). 2), it is relevant to analyze the influence of the strength and Tensile tests on the materials were carried out, according the microstructural conditions related to this high strength to ASTM standard E-8. Specimens were taken in the longituon the toughness of the alloy. The complex microstructure dinal (L) orientation (i.e., parallel to the rolling direction). and multiple failure mechanisms of 7xxx (Al-Zn-Mg-Cu) Toughness tests were performed on compact tension specialloys have undoubtedly hampered attempts to build a commens in accordance with the ASTM standard E-399. Speciprehensive model of plane strain fracture toughness (KIc) mens were taken in L-transverse (T) orientation. Tests were for these materials. Such modeling of toughness has been carried out on an INSTRON* 8801 servohydraulic machine. the subject of numerous studies,[3–6] which show that a range