Phase-stress partition during uniaxial tensile loading of a TiC-particulate-reinforced Al composite
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INTRODUCTION
INCORPORATING a stiff second phase as a load bearer in a compliant and ductile matrix is a classical idea for composite strengthening. Because of its importance, an analytical formalism for load transfer through interfacial shear, known as the shear-lag theory,[1] was developed in the 1950s and has been in continuous development since then.[2] This analytical framework is widely used, from internal stress predictions[3] to large-scale computer simulation[4] of deformation and failure in composites. More recently, models of internal stress analysis at the mean phase-stress level[5,6] have been developed using the Eshelby equivalent inclusion method,[7] and have successfully addressed a wider range of composite mechanical performance. Modern metal matrix composite (MMC) theories attribute strengthening to a variety of sources.[8] In addition to the simple load-transfer from matrix to reinforcement, matrix microstructural changes[9,10] and constrained plastic flow (by the development of matrix triaxial stresses[11] and Orowan dislocation loops[12]) also contribute to strengthening. However, regardless of the mechanisms, each relates, directly or indirectly, to the internal stresses. In addition, internal stresses prior to and during loading also affect postyield behavior.[13,14] Accordingly, a good understanding of the development of these quantities can lead to enhanced composite properties. Despite their importance, high-fidelity measurements of N. SHI, formerly Technical Staff Member, Los Alamos National Laboratory, is Development Scientist, IBM, San Jose, CA 95193. M.A.M. BOURKE, and J.A. ROBERTS, Technical Staff Members, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. J.E. ALLISON, Staff Scientist, is with the Scientific Research Laboratory, Ford Motor Company, Dearborn, MI 48121. Manuscript submitted November 27, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
internal stresses in MMC systems are usually limited. Model verifications usually rely on the prediction of global mechanical responses such as stiffness and yield strength, etc., which are a net-average from the individual phases. To overcome the limitations of a global-response-based approach, there has been a recent focus on using neutron and X-ray diffraction techniques to measure discrete phase behavior.[15,16,17] For either radiation technique, elastic strains are determined by measuring crystal lattice spacings, but a distinction arises in the volume of material examined, since, in general, neutrons are used as a bulk probe whereas Xrays generally address near-surface properties.[18–21] In this investigation, the evolution of the lattice elastic mean phase (LEMP) strains in a 15 vol pct TiC-particulatereinforced aluminum was evaluated by neutron diffraction as a function of applied tensile loads. Corresponding phase stresses were calculated using Hooke’s law. The composite was modeled using the finite-element method (FEM), and both the measured LEMP strains and stresses calculated from them were compared with the
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