Elastoplastic interaction of grains in polycrystalline materials

A new method is developed for solving boundary value problems of elastoplastic deformation of polycrystalline materials based on the field-theoretical approach. The boundary value problem for inhomogeneous global strain fields in differential form is transformed into a system of integral equations for mesostrain tensors in grains. In this approach, strain at any point of any grain represented as a superposition of homogeneous macrostrain and contributions of interactions with strain in given grain and all another grains of polycrystalline body . It is shown that the effects of the interaction of strains in polycrystal grains can be described using tensors of the fourth rank. This tensor has 36 independent components. The interactions are additive in nature, that drastically simplifies the solution of some problems, for example, search for extreme microstructures of a polycrystal in which critical localized phenomena arise, such as nucleation of the first plastic slips occur. Constitutive equations of deformation type are used for whole body and separate grains. A model of elastoplastic deformation of single crystals of grains is constructed. The physical mechanisms of plastic deformation are shifts in slip systems of crystals. General expressions are obtained for calculating the secant modules of single crystals for any multiaxial deformation. To solve systems of integral equations for mesostrains in grains the perturbation theory upon intergrains interaction used. Nonlinear systems of equations for plastic strains are solved by the iteration method. The features of the elastoplastic interaction of grains are theoretically investigated. The intensity of the elastoplastic interaction depends on the deformed state of the grains. For two identical grains, the elastoplastic interaction of the pair is several times more intense than the elastic one. In this case, the effect of plastically deformed grain on elastically deformed grain is much higher than the inverse effect. An increase in the intensity of interaction with the development of plastic deformations leads to the effect of homogenization of mesodeformations. Computational experiments were performed using polycrystalline titanium as an example.

Crystal orientation effects in scratch testing with a spherical indenter

Spherical scratch tests were conducted in individual grains of a randomly oriented polycrystalline body-centered-cubic (bcc) Ti–Nb alloy. For each grain, scratch tests were conducted at four different levels of normal load, which resulted in varying amounts of plastic strain during indentation. The results show a dependence of the horizontal load component on the crystallographic orientation and on the amount of plastic strain. The component of the horizontal force that resulted from plastic deformation was found to correlate with the active slip systems for the particular grain orientation.

Thermodynamics of processes of consolidation of an assembly of dispersed particles and deconsolidation of a polycrystalline body

Thermodynamic functions that describe the processes both of consolidation of an assembly of dispersed particles and deconsolidation of a polycrystalline body are considered. Expressions have been derived for the shrinkage pressure, which arises from the consolidation of particles, and the migration pressure, which arises from deconsolidation of a polycrystalline body. The values of the shrinkage and migration pressures are described in terms of structure parameters of a composite body (particle size and shape, phase composition, values of surface tension at the solid-solid and solid-moving phase interfaces).

Application of Analytical Electron Microscopy to glass-bonded aluminas

Liquid phase sintering represents one of the most common methods of producing aluminas and other ceramic materials. Regardless of the specific processing details, this sintering procedure invariably results in a polycrystalline body containing a certain volume fraction of an intergranular binder phase. This phase is typically a glass, which can control many important properties of the sintered material. Clearly, microstructural characterization - describing not only the physical distribution of the binder phase but also its chemical composition - represents its an important part in the study of these materials.

Plasticity of grain boundaries

Plastic deformation of a polycrystalline body requires the propagation of slip bands from one grain to another. This process may occur by dislocations being forced through boundaries, new dislocations being nucleated in or adjacent to the grain boundary, or pinned dislocations being released. Whichever of these processes dominates, it is necessary for the grain boundary structure to be reshaped, and in previous work (Pond & Smith, 1977; Pond et al . 1977) we have described experimental observations of grain boundary dislocation processes involved in such activity.