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Influence of Microstructure and Second Phases on Fracture Toughness

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Final technical rept. 1 Jun 71-31 May 74

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The microstructure, second phases, and micromechanisms contributing to fracture toughness of AISI 4340 steel, maraging steel, Ti-6Al-4V, and 7000- series aluminum alloys are examined. The study focuses on high strength alloys and the component of toughness arising from strain localization and shear rupture, which is shown to be related to the plane strain ductility. Methods of measuring plane strain ductility are examined, and a new double-bend method which can be applied to both plate and sheet coupons is devised. Successive replicas of the bend specimen surface are used to follow the development of slipping regions and cracks under plane strain biaxial loading, and to measure slip offsets. The alloys display a tendency for a more coordinated slipping of neighboring grains under plane strain flow leading to strain localization a the formation of coplanar arrays of slip bands or sliding grain boundaries called superbands, b the growth of superbands which can accommodate large shears, c initiation of shear microcracks within the bands, and d the linking of the cracks into a macrocrack. The susceptibility to strain localization and shear rupture is traced to a number of microstructural elements, including a fine, coherent precipitate particles cut by dislocations, b inclusions, and c the grain size and character of the grain boundaries. Author

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  • Properties of Metals and Alloys

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