Correlation of Fractographic Features with Mechanical Properties in Systematically Varied Microstructures of Cu-Strength
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I.
INTRODUCTION
VOID nucleation, growth, and coalescence play a central role during ductile fracture of metallic materials. The halves of these voids are represented by the dimple distribution on fracture surfaces. If void nucleation is delayed or suppressed, there would be an increase in ductility.[1] Therefore, it is important to understand the metallurgical and physical variables that control void nucleation. In this study, geometrical information on ductile fracture features has been experimentally obtained. Further, the correlation between fracture surface features and deformation properties has been explored for a copper-strengthened high-strength lowalloy (HSLA) steel to establish the close association between deformation and fracture. In ductile materials, such as copper-strengthened HSLA steel, fracture occurs through the process of microvoid initiation, growth, and coalescence. Voids initiate and grow at inclusions, precipitates, and other second-phase particulate matter under the influence of plastic strains and hydrostatic stress.[2–7] The role of inclusions and particles in initiating ductile fracture has been demonstrated for a variety of materials: oxidized copper alloys,[8] maraging steels,[9,10] quenched and tempered high-strength steels,[9] low-strength steels,[11] aluminum alloys,[12] etc. The distribution, size, shape, type, and coherency of these constituents of the microstructure play an important role in controlling void formation and eventual fracture.[13–18] ARPAN DAS, Scientist, Fatigue & Fracture Group, SWAPAN KUMAR DAS, Scientist, and SOUMITRA TARAFDER, Dy. Director, are with the Materials Science & Technology Division, National Metallurgical Laboratory (Council of Scientific & Industrial Research), Jamshedpur 831 007 India. Contact e-mail: arpan@nml india.org Manuscript submitted March 12, 2009. Article published online September 30, 2009 3138—VOLUME 40A, DECEMBER 2009
For ductile materials, the engineering properties are determined by the interaction of stress and strain fields with the microstructure of a material. The contribution of deformation processes in the development of voids has been well established. While the growth mechanisms can vary, say, with the temperature of testing,[19] their nucleation invariably occurs where inhomogeneous deformation takes place. In Table I, an overview of research efforts in correlating fracture features (mainly related to dimple geometry and extent) with microstructural constituents (mainly particle size and distribution) and testing variables is given. The list in Table I is not exhaustive, but serves to highlight the connectivity between microstructural constituents and mechanical properties during ductile fracture. The seminal work of Hahn et al.[20] demonstrated the influence of inclusion volume fraction on the fracture toughness of ultrahighstrength steels of comparable yield strengths. Puttick,[21] Crussard,[22] and Rogers[23] were among the first to consider inclusions or intermetallic particles as the initiation sites of voids. Various models
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