**1. Introduction**

Most metals and alloys have become increasingly important in a variety of applications. The most important of these properties are ease of high strength, relatively good ductility, and good corrosion resistance. One of the ways of acquiring the best combination of these prop‐ erties is to select the microstructure, which in turn depends on thermo-mechanical history as well as on chemical composition [1]. An optimization of the thermo-mechanical process can be achieved through an understanding of the entire forming process and the metallurgical variables affecting the micro-structural features occurring during deformation operations carried out during deformation operations carried out at high temperatures. Most applications of these alloys are in the chemical process equipment, petrochemical, aerospace industry, and medical tools. The understanding of metals and alloys behavior at hot deformation condition has a great importance for designers of hot metal forming processes (hot rolling, forging and extrusion) because of its effective role on metal flow pattern, and the constitutive relationships are often used to describe the plastic flow properties of metals and alloys in a form that can be used in computer code to model the forging response of mechanical part members under the prevailing loading conditions [2].

During hot forming process alloy is liable to undergo work hardening (WH), dynamic recovery (DRV) and dynamic recrystallization (DRX), three metallurgical phenomena for controlling microstructure and mechanical properties [3]. The relay of softening mechanism from strainhardening and dynamic recovery to DRX is the reason the term discontinuous has been earned. At a microstructural level DRX begins when strain hardening plus recovery can no longer store more immobile dislocations. When the critical strain is reached, on face centered cubic (fcc) metals of medium to low stacking fault energy, strain-hardening and dynamic recovery cease to be the principle mechanisms responsible of the stress-strain response, DRX accompanies

© 2013 Guo-Zheng; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the process. However, DRX is not a phenomenon restricted to fcc metals, it has been described on ice, some minerals, and even high purity α–Fe (bcc metal) [4-6]. In the deformed material DRX would affect the crystallographic texture and thus, material anisotropy. For example, DRX would eliminate some crystal defects, such as part of dislocations resulting from work hardening, which will improve hot plasticity, refine microstructure, and reduce the deforma‐ tion resistance [7]. High stacking fault energy (SFE) metals, such as aluminium alloys, alpha titanium alloys, and ferritic steels, undergo continuous dynamic recrystallization (CDRX) rather than discontinuous dynamic recrystallization (DDRX) during high temperature deformation. In particular, due to the high efficiency of dynamic recovery, new grains are not formed by a classical nucleation mechanism; the recrystallized microstructure develops instead by the progressive transformation of subgrains into new grains, within the deformed original grains. Dislocations produced by strain hardening accumulate progressively in lowangle (subgrain) boundaries (LABs), leading to the increase of their misorientation angle and the formation of high-angle (grain) boundaries (HABs), when a critical value of the misorien‐ tation angle is reached. The microstructure is thus intermediate between a subgrain and a grain structure: while grains and subgrains are entirely delimited by HABs and LABs, respectively, it will be referred to as an aggregate of crystallites, which are bounded partly by LABs and partly by HABs. On the contrary, low stacking fault energy (SFE) metals, such as magnesium alloys, austenitic steels, and beta titanium alloys, undergo discontinuous dynamic recrystal‐ lization (DDRX) rather than continuous dynamic recrystallization (CDRX) during high temperature deformation [8].

lower than in the surrounding matrix; this is a necessary primary condition for nucleation. During this initial period when the angles between the sub-grains are small and less than one degree, the sub-grains form and grow quite rapidly. However, as the sub-grains grow to such a size that the angles between them become of the order of a few degrees, the growth of any given sub-grain at the expense of the others is very slow. Eventually one of the sub-grains will grow to such a size that the boundary mobility begins to increase with increasing angle. A large angle boundary, *θ* ≈ 30~40°, has a high mobility because of the large lattice irregularities or 'gaps' which exist in the boundary transition layer. The atoms on such a boundary can easily transfer their allegiance from one crystal to the other. This sub-grain is then able to grow at a much faster rate than the other sub-grains which surround it and so acts as the nucleus of a recrystallized grain. The further it grows, the greater will be the difference in orientation between the nucleus and the matrix it meets and consumes, until it finally becomes recogniz‐ able as a new strain-free crystal separated from its surroundings by a large-angle boundary [9].

Characterization for Dynamic Recrystallization Kinetics Based on Stress-Strain Curves

http://dx.doi.org/10.5772/54285

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Fig.1 shows typical flow curves during cold and hot deformation. During hot deformation, the shape of the flow curve can be 'restricted', or work hardening rates counterbalanced, by dynamic recovery or by dynamic recrystallization (i.e. discontinuous dynamic recrystalliza‐ tion). Dynamic recovery is typical of high-SFE metals (e.g. aluminium, low-carbon ferritic steel, etc.), where the flow stress saturates after an initial period of work hardening. This saturation value depends on temperature, strain rate and composition. On the other hand, as shown in Fig.1, a broad peak (or multiple peaks) typically accompany dynamic recrystallization. Fig.2 illustrates schematically the microstructure developments during dynamic recovery and dynamic recrystallization. During dynamic recovery, the original grains get increasingly strained, but the sub-boundaries remain more or less equiaxed. This implies that the substruc‐ ture is 'dynamic' and re-adapts continuously to the increasing strain. In low-SFE metals (e.g. austenitic stainless steel, copper, etc.), the process of recovery is slower and this, in turn, may allow sufficient stored energy build-up. At a critical strain, and correspondingly at a value/ variation in driving force, dynamically recrystallized grains appear at the original grain boundaries – resulting in the so-called 'necklace structure'. With further deformation, more and more potential nuclei are activated and new recrystallized grains appear. At the same time, the grains, which had already recrystallized in a previous stage, are deformed again. After a certain amount of strain, saturation/equilibrium sets in (see Fig.2b). Typically equili‐ brium is reached between the hardening due to dislocation accumulation and the softening due to dynamic recrystallization. At this stage, the flow curve reaches a plateau and the microstructure consist of a dynamic mixture of grains with various dislocation densities. It is important, at this stage, to bring out further the structural developments and structure– property correlation accompanying dynamic recovery and dynamic recrystallization respec‐

The true compressive stress-strain curves for as-extruded 7075 aluminum alloy under different temperatures and strain rates are illustrated in Fig. 3a~d. The flow stress as well as the shape of the flow curves is sensitively dependent on temperature and strain rate. Comparing these curves with one another, it is found that increasing strain rate or decreasing deformation temperature makes the flow stress level increase, in other words, it prevents the occurrence of

tively [10].
