No abstract

No abstract

Creep in polycrystalline oxide ceramics at temperatures 1400 -1600°C occurs by diffusion. The creep curves are very sensitive to the presence of nonequilibrium defects and stress concentrators which were "annealed" during cooling after high-temperature kilning of the materials. Apparently, rupture is due to vacancies forming in stretch-zone sections tied to crystal boundaries followed by their coagulation on the boundaries. Cracks form and develop differently in different materials, depending on the crystal-chemical properties of the materials.

No abstract

The results of an investigation of creep in a polycrystalline metal-oxide ceramic at temperatures to 1600°C and under high loads are presented. These temperatures lie near the transition from the brittle to plastic behavior of the ceramic. The loads are close to the ultimate strength of the materials at the given temperature. It is shown that the elementary mechanism of deformation in the entire plasticity range of the samples is diffusion-viscous (vacancy or interstitial) flow in its different forms -diffusion motion of dislocations and crystal boundaries. The data obtained are in agreement with the supposition that for a transition from brittle to plastic fracture of a ceramic a critical vacancy concentration in the ceramic is required. These vacancies are formed as a result of thermal activation and determine the deformation of the materials.The results of systematic studies of creep of metal-oxide polycrystalline ceramics are presented in [1,2]. The experiments were performed in the region of variation of the experimental conditions where the samples show appreciable irreversible flow deformation -plasticity 4 -at comparatively high temperatures (> 1600°C) and under low loads (to 3 MPa). It was shown that under such conditions the main deformation mechanism at the atomic level is the diffusion of atoms (ions) along vacant sites of the crystal lattice or interstices with all its variety -the movement of dislocation or crystal boundaries along vacancies.The works cited also note the main features of the behavior of the samples -practically only stationary creep (i.e., with constant rate) is observed; as stress increases, the rate of the process increases as a power law with exponent n about 1 or slightly higher (to 2 -2.5); as temperature increases the creep rate increases exponentially, and the activation energy of the process corresponds to the activation energy of selfdiffusion. Exceptions from this regular behavior occurred only in measurements of the structure of the materials in the course of the tests or with a change of the deformation mechanism. The studies indicated above were continued for the behavior of the experimental materials at lower temperatures and under higher loads, close to the ultimate strength of the samples [3]. In the opinion of the authors, such experiments are of great interest from the standpoints of applications as well as theory.The aim of the present article is to present experimental results on the determination of creep in an air atmosphere of polycrystalline oxide ceramic in the temperature interval 1450 -1550°C and under loads 5 -60 MPa.The investigations were performed at temperatures and under loads for which the measured rate of deformation (strain) was in the range 10 -1 -10 -5 h -1 . This was convenient for a laboratory study, because the longest duration of an individual determination did not exceed 100 h with motion sensor sensitivity of about 1 ìm. At the lower limit (1400°C) of the experimental temperature range (i.e., for measured strain rates of the order of 10 -4...