Theoretical progress in non-equilibrium grain-boundary segregation (II): Micro-mechanism of grain boundary anelastic relaxation and its analytical formula

Abstract

Finding the internal-friction peak of grain boundary anelastic relaxation was one of the important breakthroughs in the study of internal friction in the last century. But the micro-mechanism of grain boundary anelastic relaxations is still obscure. Based on the observations of the grain boundary segregation or depletion of solute induced by an applied stress, the following micro-mechanism was suggested: grain-boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted, inducing grainboundary depletion or segregation of solute, respectively. The equations of vacancy and solute concentrations at grain boundaries were established under the equilibrium of grain-boundary anelastic relaxation. With these the kinetic equations were established for grain boundary segregation and depletion during the grain boundary relaxation progress. grain boundary, segregation, anelastic relaxation, kineticsThe variation of micro-structure in materials induced by an applied stress has been an important study field in materials science and engineering. The slip, climb and interaction of dislocations under applied stresses induce the plastic deformation of materials, which are the main content in the research of mechanical properties. A great deal of engineering practice has confirmed that the degradation of properties, embrittlement, creep, fatigue and brittle fracture without any sign will occur in metallic materials during service to produce engineering accidents. Metal usually serves under an applied stress which is lower than its yield limit and at the same time the degradation of its mechanical properties occurs. Therefore, what kind of micro-structure variations will appear for polycrystalline materials under an elastic stress? How will these variations influence the mechanical properties and the performance in service? These have been the most urgent and vexing challenges to the present materials scientists and engineers. In the 1940s, Chinese scientist Ke found the internal friction peak of grain boundary anelastic relaxation, which was one of the important achievements in the field of elastic stress to induce the variations of micro-structure in the last century. But the micro-mechanism of grain boundary anelastic relaxation was still obscure [1] , and the situation gave rise to the argument whether or not the internal friction peak found by Ke was relative to the grain boundary anelastic relaxation [2] . Recently, we found that the main micro-structure variation of polycrystalline under an elastic stress was absorbing or emitting vacancies to produce grain boundary segregation or depletion of solutes during grain boundary anelastic re

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