Analytic Description of the Effect of Duration of the Procedure of Annealing Performed after Deformation on the Steady-S
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ANALYTIC DESCRIPTION OF THE EFFECT OF DURATION OF THE PROCEDURE OF ANNEALING PERFORMED AFTER DEFORMATION ON THE STEADY-STATE CREEP OF METALS A. K. Rusynko
UDC 539.376
The thermomechanical treatment of metals (including plastic deformation with subsequent stabilizing annealing) is one of the directions of improvement of their refractory properties. In the course of thermomechanical treatment, ordered dislocation structures are formed in the material. They are capable of inhibiting the motion of defects of the crystal lattice (first of all, linear). In this case, the physicochemical properties of the metal of subgrains remain unchanged. At the same time, the mechanical properties and the degree of distortion of the crystal lattice in subvolumes suffer significant changes as a result of the increase in the density of defects, changes in their specific distribution, and the interaction with point defects. This leads to sharp changes in the structural properties of the material, which is, in fact, the aim of all procedures of hardening. In what follows, we propose a mathematical model for the description of the influence of duration of stabilizing annealing in the course of thermomechanical treatment on the steady-state creep of metals (the procedure of thermomechanical treatment is described in [1]). Since the classical theories of creep do not take into account the influence of preliminary treatment, we develop our model within the framework of the synthetic theory of plasticity and creep [2]. 1. The dependence of the rate of steady-state creep ε˙ M on the duration of annealing t in the course of thermomechanical treatment (Fig. 1) [3, 4] possesses an extremum for fixed plastic strains and temperature of annealing. Without stabilizing annealing ( t = 0), the creep rate does not change. For large values of t, the rate ε˙ M returns to its initial value ε˙ (without preliminary treatment). The ε˙ M – t dependence is explained by using the dislocation theory. In the course of stabilizing annealing of a material, a part of dislocations generated by cold hardening annihilates and the remaining dislocations rearrange into a more favorable energy configuration (polygonized substructure). In this case, the temperature of annealing must exceed a certain minimum value typical of any material (required to realize nonconservative motion of dislocations) but, at the same time, must be lower than the temperature of recrystallization. The substructure created in the process of thermomechanical treatment resists the processes running under the conditions of creep and, hence, decreases their rates. The greater the duration of annealing, the larger the number of dislocations entering the boundaries of polygonal subgrains. At the same time, the boundary of a subgrain with large number of dislocations can turn into a center of recrystallization. Moreover, recrystallization can be absent in the process of annealing but occur in the course of creep tests under the favorable influence of high temperatures and force loading [5]. The structur
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