GRAIN BOUNDARY CONTRIBUTION TO THE HIGH-TEMPERATURE BACKGROUND OF INTERNAL FRICTION IN METALS

10.25712/ASTU.1811-1416.2022.03.006

Abstract

The paper considers the contribution to the high-temperature background of internal friction of grain boundaries containing defects that violate the ideally flat configuration of the boundaries. Such defects, for example, include steps at the borders, pores, facets. Under the influence of periodic external stresses, they become sources and sinks of vacancies. Diffusion flows of boundary vacancies between sources lead to mutual displacement of mating grains. This is the reason for the dissipation of oscillation energy. The spectrum of internal friction in coordinates  –  in all these cases is a line having two or three rectilinear sections with different angles of inclination to the axis of the abscissa. This indicates different values of the activation energy in different temperature ranges. The value of the background activation energy takes values from half of the activation energy of migration of grain-boundary vacancies to the activation energy of boundary self-diffusion. The break points correspond to the full development of the diffusion process in the area corresponding to the size of the defect. The location of the break point on the graph makes it possible to find the relationship between the oscillation frequency, the boundary diffusion coefficient and the defect sizes. Taking into account the size distribution of defects leads to the appearance of a transition region between rectilinear sections. Two types of logarithm size distribution are considered: uniform and normal. From the analysis of the location of the transition region, it is possible to estimate the distribution parameter.