Relationship between fracture toughness, fracture path, and microstructure of 7050 aluminum alloy: Part I. Quantitative
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I.
INTRODUCTION
THE Al-Zn-Mg-Cu–based wrought 7XXX alloys are widely used for structural applications, where fracture toughness and yield strength are of prime concern. Fracture toughness of precipitation-hardened 7XXX alloy products decreases with an increase in the degree of recrystallization.[1] Therefore, for many practical applications of these alloys, recrystallization is undesirable. However, in commercial practice, it is not always possible to suppress partial recrystallization during processing of these alloys. Consequently, some commercial wrought 7XXX alloy products are partially recrystallized. Therefore, it is of interest to understand the effect of the attributes of partially recrystallized microstructure on the fracture toughness and fracture mechanisms in precipitation-hardened 7XXX alloy products. There have been several investigations on the effect of microstructure on the fracture mechanisms and fracture toughness of 7XXX alloys.[1–10] These studies have been extremely useful for developing an understanding of how the microstructure of these materials affects the fracture behavior. However, to the best of the authors’ knowledge, model-based predictive quantitative relationships between microstructural attributes, fracture surface morphology, and fracture toughness, applicable to commercial partially recrystallized 7XXX alloys such as 7050, have not been developed. To develop such quantitative relationships, and to verify them, systematic and in-depth quantitative microstructural and fractographic data are required as a function of the degree of recrystallization, orientation, and other variables. It is the purpose of this contribution to report an in-depth quantitative investigation of the effect of microstructural and fractographic parameters on the fracture mechanisms and fracture toughness of partially recrystalliN.U. DESHPANDE, Engineer, is with Cessna Aircraft Co., Wichita, KS 67277-7704. A.M. GOKHALE, Professor, is with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245. D.K. DENZER, Staff Engineer, and JOHN LIU, Senior Technical Specialist, are with the Alcoa Technical Center, Alcoa Center, PA 15069. Manuscript submitted June 10, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
zed wrought 7050 alloy, which is a typical alloy of the 7XXX series. Fracture toughness and fracture path are characterized as a function of degree of recrystallization and specimen orientation. Quantitative fractography and stereology are utilized for characterization of fracture path and microstructural attributes. In the companion article,[11] these quantitative data are used to develop and verify a multiple micromechanisms-based model for fracture toughness of partially recrystallized 7050 alloy. With appropriate modifications, this model should be applicable to other alloys of the 7XXX series as well. A brief background on the microstructure and fracture mechanisms in 7050 alloy plates is given in Section II. This is followed by a description of experimenta
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