Contents

## **Preface XI**


Preface

Polycrystalline structures are conglomerates of a large number of crystals irregularly situated, yet bound to each other strongly enough to behave as a whole. As the size and the shape of these crystals are irregular too, the latter are called grains or crystallites. The boundary surfaces connecting the grains have crystal structures that are not identical to those of the adjacent crystallites. They are distortions, which allow a smooth transition between the structures within the grains in contact. The mosaic of these borderline

A mechanical loading of these materials leads to deformations. With small loadings, the deformation is elastic, slip and elastic (Young's) modulus being its characteristics. The Young's modulus is an important parameter of the polycrystal materials

For higher loadings, the deformation becomes plastic. Theoretical studies, experimental data, as well as practical observations show that this type of deformation involves slippage in the material and active participation of its twodimensional imperfections. With lower temperatures (less than 0.4 Tm for metals and 0.6 Tm for alloys, Tm denoting the melting point) slippage does not occur uniformly, but remains confined to smaller regions, which appear successively. At higher temperatures, the critical break tension drops down. Thus, smaller loadings prove sufficient to bring about deformation effects, such as dislocations slip, twinning,

Stress level, stress rate, and temperature are the parameters characterizing the plastic

In these materials, the macroscopic value of their parameters is a mean value, resulting from taking the average over domains comprising a considerable number of grains with usually different orientations. In this way, they turn out isotropic as compared to the monocrystals, in which sharp anisotropy is observed. In special cases, the orientations of the grains show more or less preferred directions, leading to anisotropy. To the best of my knowledge, one of them – polycrystalline tungsten, is dealt with for the first time in specialized literature by Dr. P. V. Galptshyan in the

regions represents an extended block of two dimensional imperfections.

determining its resistance to deformation.

sliding of grain boundaries, etc.

deformation of polycrystalline materials.

chapter: "Strength of Polycrystalline Materials".
