Grain Boundary Curvature in a Model Ni-Based Superalloy

The local grain boundary (GB) curvature in a model Ni-based superalloy was measured experimentally using Dehoff’s tangent count method. The results show that, in materials containing significant amounts of second-phase particles, the curvature parameter, κ, which relates the mean local curvature to the grain size, can adopt far lower values than have been reported previously. It is also shown that the value of κ is not a constant, as is usually assumed, but instead varies both with the volume fraction of second-phase particles and with the holding time during high-temperature annealing. The lowest values for κ were obtained for high particle volume fractions and long annealing times. Because the local boundary curvature constitutes the driving force for grain growth, these observations could help to explain grain growth phenomena in heavily pinned systems.

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A New Constitutive Model for the Finite Element Simulation of Local Hot Forming of Aluminum 6xxx Alloys

A new internal variable constitutive model for the use in finite element (FE) simulation of local hot forming of 6xxx aluminum alloys is presented. The model relates the flow stress to the temperature, total strain rate, and internal variables, which represent the dislocation density and the contributions to the hardening stress from elements in solid solution and precipitates. The time evolutions of the internal variables are modeled by an equation representing the accumulation/annihilation of dislocations and by a precipitate model developed elsewhere, taking into account a size distribution of precipitates. The parameters of the constitutive model have been fitted to tensile tests at different temperatures, strain rates, and precipitate states. Manuscipt submitted March 22, 2007.

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