Analysis of steady-state dynamic recrystallization
Introduction
If recrystallization (RX) occurs during high-temperature deformation it is referred to as dynamic recrystallization (DRX). Owing to its importance for thermomechanical processing of metallic materials, DRX has attracted much attention and has been frequently addressed in the past both experimentally and theoretically [4], [7], [9]. Since the first attempts by Stuewe and Ortner [14] or Luton and Sellars [8] DRX has been considered as a superposition of static recrystallization on the dynamics of the deformation process. Particular attention has been paid to the prediction of the flow curve and the dynamically recrystallized grain size [12]. Typically, the flows curves reveal one maximum or a damped oscillation before they attain a constant value. The strain regime of constant flow stress at constant strain rate is referred to as steady-state deformation [7], [8]. In addition to early empirical descriptions of the flow curve in terms of strain as a state parameter [8], more recent models are based on the evolution of the dislocation density during deformation and typically define an energy criterion for the initiation of dynamic recrystallization [3], [6], [10], [13], [14]. In a recent publication [5] we have contended that DRX is a true dynamic phenomenon which is caused by the evolution of the dislocation arrangement (rather than its density), and we presented a theoretical approach which considers the kinetics of grain boundary motion as an essential character of DRX. In this model the evolution of the DRX structure is closely related to the development of the subgrain structure during deformation. This allowed the critical conditions for initiation of DRX to be defined.
In this contribution we will focus on DRX during steady-state deformation. Typically, the DRX grain size is related to the steady-state flow stress by a power law [8]. However, usually is some real number with no particular physical meaning and changes from material to material. Moreover, the power law is only a convenient way to express monotonic functional behavior but does not necessarily reflect an underlying physical principle. We have proposed recently that the grain size dependence of the flow stress can be interpreted in terms of the grain-size sensitivity of the flow stress [5]. In this study we will corroborate the hypothesis by expanding the experimental results significantly. For this, the database is extended to a second material, commercially pure copper, and to contain data for an advanced experimental setup to perform not only strain-rate but also temperature-jump tests.
Section snippets
Experimental procedure
A commercially pure Cu and the austenitic steel X10NiCrAlTi3220, also referred to as alloy 800H, have been used for the investigations. Their exact chemical compositions are given in Table 1, Table 2.
Cylindrical samples of 5 mm diameter and 7.7 mm height were machined from statically recrystallized material with Rastegaev geometry [11]. The uniaxial compression tests were performed on a servohydraulic Schenck Hydropuls testing machine with true strain rates of at
Constant deformation conditions
Flow curves at different strain rates and temperatures of both materials were recorded and are displayed in Fig. 1. The characteristic shapes of these curves with either single- or multiple-peak behavior depending on the Zener–Hollomon parameter [8] and the occurring steady-state regimes are clearly visible. Increasing deformation temperature or decreasing strain rate lead to a multiple peak behavior and lower flow stresses, whereas single-peak flow curves and higher stresses occur
Grain-size sensitivity
Whereas traditional models of DRX typically assume a critical energy criterion to define the initiation of DRX, Frommert and Gottstein [5] have recently proposed a model based on a mobility criterion for the initiation of DRX. This model is based on the observation that during high-temperature deformation the dislocation structure undergoes a change from a cell structure to a subgrain structure when approaching steady-state deformation. Subgrain boundaries are essentially low-angle grain
Conclusions
The flow stress behavior during strain-rate and temperature-jump tests and the grain-size distribution during steady-state DRX were investigated on commercially pure copper and the austenitic steel alloy 800H. The following results were obtained:
(a) The jump tests confirmed that the DRX grain size is related to the steady-state flow stress via . This confirms the predicted connection of steady-state DRX grain size and deformation-induced subgrain size.
(b) The grain size distribution
Acknowledgments
Financial support by the Deutsche Forschungsgemeinschaft (DFG; Go 335/42-1, BL 512 4/1) is gratefully acknowledged. The authors appreciate valuable discussions with Prof. Luckhaus and PD Dr. Blesgen during the joint project.
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