Papers by Keyword: Structural Superplasticity

Abstract: The equation σ=Kέ^m, where σ is the applied stress, έ is the strain rate, K and m are material constants that depend on stress / strain rate, temperature and grain size is often used to describe structural superplasticity. The general shape of the logσ-logέ curve is sigmoidal. Based on limited data, it was suggested by us earlier that a universal σ-έ curve could exist in a properly normalized space. έ and m are normalized with respect to έ_opt and m_max, the strain rate at which m is a maximum and the maximum m value respectively. Here a multi-dimensional relationship involving σ/σ_opt-έ/έ_opt-m/m_max-ΔF₀/kT-η/η_opt is developed; σ_opt corresponds to έ_opt, ΔF₀ is the free energy of activation for the rate controlling mechanism, k the Boltzmann constant, T the absolute test temperature, η the (apparent) viscosity of the superplastic alloy and η_opt is the viscosity of the same alloy for m=1 in a dimensionless σ-έ space. Using data concerning many systems, the phenomenology of structural superplasticity in all classes of materials is shown to be unique.

Abstract: The paper deals with three-level model of polycrystal inelasticity based on crystal plasticity. This model allows to regard the most important inelastic deformation mechanisms of polycrystals including grain boundary sliding. The inflow of intragranular dislocations, changing of the boundary structure under realization of grain boundary sliding and diffusion processes are taken into account in equations for grain boundary sliding. Consistency conditions of constitutive relations at the different scale levels are used in constructing model. The results of computational experiments under uniaxial tension of a representative volume are obtained with developed model. The results show that grain boundary sliding is important and must be taking into account.

Abstract: Structural superplasticity is observed in materials of different classes with μm–, sub–μm– or nm– grain size. In all cases mesoscopic grain/interphase boundary sliding (~ grain diameter or more) is suggested to be the rate controlling mechanism. Sub–μm grained metallic and ceramic systems are analyzed here and good agreement with experimental results is established. Compared with earlier works, the numerical procedure is more robust and the free energy of activation for the rate controlling process is matched with the value for the same obtained using Eshelby’s equation.

Abstract: An internal variable theory has been proposed to account for the essential microstructures during inelastic deformation. The framework of the theory is built on the basis of well known dislocation dynamics to provide the concept of an internal strain tensor as the most fundamental deformation state variable. The plastic and inelastic strain rate tensors are then naturally defined and also a kinematics relation among them can further be derived from the time rate of change of this internal strain tensor, which in fact accounts for the evolution of microstructures during inelastic deformation. To complete the theory, the constitutive relations between the various stress variables and their conjugate deformation rate variables are then derived based on the dislocation kinetics. The theory is then further extended to describe the structural superplasticity, taking this slip zone model with dislocation pile-ups as the major accommodation mechanism for grain boundary sliding. The experimental results obtained from the various crystalline materials are then presented and compared with each other in relation to the internal variable theory for inelastic deformation.