STUDY OF THE KINETICS OF STATIC RECRYSTALLIZATION USING AVRAMI EQUATION AND STRESS RELAXATION METHOD

In order to design and use forming processes, one has to cover a body of knowledge including the recovery phenomena. Recovery can be described using the Avrami equation, which yields the proportion of recrystallized structure as a function of various deformation-related variables. However, some constants and exponents of the equation must be known. Their values can be obtained by measuring - either by metallographic or mechanical methods - the fraction of recrystallized structure in specimens deformed under prescribed conditions. A metallographic technique consists in quenching the material and mapping the locations of grains, their sizes and shapes in an optical microscope to find the ratio of recrystallized grains. It is possible to use a more up-to-date technique: electron microscopy combined with electron backscattered diffraction. However, these methods are in fact very complicated and often infeasible. Mechanical methods include intermittent tests in compression or torsion plastometers, which can be evaluated using various techniques. The difficulties of this approach lie in separating recovery from recrystallization, as well as in the vast number of test, which must be performed in order to describe the kinetics of static recrystallization in a single material. Another modern technique uses the stress relaxation phenomenon. By analysing the stress vs. time plot, one can obtain information on the specimen's flow stress during the hold after deformation. A Gleeble simulator was used for an experiment with four temperatures and five strain rates and the resulting strain magnitude of 0.2. After deformation, specimen was held in the grips at constant temperature and stress was recorded. The resulting stress vs. time plot was analysed. Points denoting the start and end of recovery were identified on the curve. Constants of the Avrami equation were determined using these points. The advantages of stress relaxation tests include their simplicity, the opportunity to obtain a full-range curve for the volume fraction of recrystallized structure in a single test run and the effectiveness of mapping softening phenomena.