Relationship between fracture toughness, fracutre path, and microstructure of 7050 aluminum alloy: Part II. Multiple mic
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I.
INTRODUCTION
FRACTURE processes in partially recrystallized commercial wrought 7XXX aluminum alloys involve multiple micromechanisms. Fracture of coarse constituent particles, intergranular fracture, and microvoid-induced transgranular fracture are the three dominant fracture micromechanisms. Consequently, the fracture toughness of these alloys depends on the relative contributions of these fracture micromechanisms to the overall fracture, which are in turn governed by the microstructure. The majority of the existing models for fracture toughness of 7XXX alloys have been developed for microstructures, where a single fracture micromechanism is operative,[2,3,4] and therefore, such models are not applicable to partially recrystallized commercial 7XXX alloys. All the existing models assume that the microstructure is isotropic, the grain-size distribution is uniform (unimodal), and the coarse constituent particles are not present.[2–5] Microstructures of partially recrystallized hot-rolled commercial 7XXX alloys do not satisfy these conditions. Furthermore, the existing theoretical models do not provide quantitative relationships between the fracture toughness and the attributes of partially recrystallized microstructure, such as degree of recrystallization, recrystallized grain size, anisotropy of microstructure, etc. Therefore, there is a need to develop a multiple fracture micromechanism-based model that quantitatively relates the fracture toughness of partially recrystallized 7XXX alloys to their microstructural attributes. In this article, such a multiple micromechanismbased model is developed, and it is verified using the stereological and fractographic data on partially recrystallized 7050 alloy presented in the companion article.[1] The model is then used to obtain a quantitative relationship between microstructural parameters of the partially recrystallized A.M. GOKHALE, Professor, is with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 303320245. N.U. DESHPANDE, Engineer, is with Cessna Aircraft Co., Wichita, KS 67277-7704. 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
microstructure and its plane-strain fracture toughness. A brief review of the existing models follows, to point out their underlying assumptions. Hornbogen and Garf [2] used the critical strain to fracture concept to arrive at the following relationship for the planestrain fracture toughness, (KIC)i, for a completely intergranular fracture, where the strain localization is in the precipitate-free zones (PFZ): (KIC)i 5 [Y s PFZ « f PFZ (w PFZ /CDg )]1/2
[1]
The term εf PFZ is the critical fracture strain in the PFZ; Y is the Young’s modulus; C is a constant with dimensions of the reciprocal of the length; sPFZ and wPFZ are the yield strength and the width of PFZ, respectively; and Dg is the average grain size. Equation [1] is app
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