Recovery Stage of Annealing

11 Key excerpts on "Recovery Stage of Annealing"

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  Recrystallization and Related Annealing Phenomena

  • Anthony Rollett, F J Humphreys, Gregory S. Rohrer, M. Hatherly(Authors)
  • 2004(Publication Date)
  • Pergamon

    (Publisher)

  Whether any or all of these occur during the annealing of a particular specimen will depend on a number of parameters, including the material, purity, strain, deformation temperature and annealing temperature. In many cases some of these stages will have occurred during the deformation as dynamic recovery . Although the recovery stages tend to occur in the order shown, there may be significant overlap between them. Before considering the details of recovery, we should note that recovery and recrystallization are competing processes as both are driven by the stored energy of the deformed state (§2.2). Once recrystallization has occurred and the deformation substructure has been consumed, then clearly no further recovery can occur. The extent of recovery will therefore depend on the ease with which recrystallization occurs. Fig. 6.1. Various stages in the recovery of a plastically deformed material. 170 Recrystallization Conversely, because recovery lowers the driving force for recrystallization, a significant amount of prior recovery may in turn influence the nature and the kinetics of recrystallization. The division between recovery and recrystallization is sometimes difficult to define, because recovery mechanisms play an important role in nucleating recrystallization. Additionally, as will be discussed at the end of this chapter, there are circumstances in which there is no clear distinction between the two phenomena. The extensive early work on the recovery of deformed metals has been reviewed by Beck (1954), Bever (1957) and Titchener and Bever (1958), but despite its obvious importance, recovery attracted little interest in later years. However, the current industrially driven move to produce quantitative physically-based models for annealing processes has resulted in renewed interest in recovery (e.g. Nes 1995a). 6.1.2 Properties affected by recovery During recovery, the microstructural changes in a material are subtle and occur on a small scale.

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  Recrystallization and Related Annealing Phenomena

  • Anthony Rollett, Gregory S. Rohrer, John Humphreys(Authors)
  • 2017(Publication Date)
  • Elsevier

    (Publisher)

  dynamic recovery . Although the recovery stages tend to occur in the order shown, there may be significant overlap between them.

  Before considering the details of recovery, we should note that recovery and recrystallization are competing processes as both are driven by the stored energy of the deformed state (Section 2.2 ). Once recrystallization has occurred and the deformation substructure has been consumed, then clearly no further recovery can occur. The extent of recovery will therefore depend on the ease with which recrystallization occurs.

  Figure 6.1  Various stages in the recovery of a plastically deformed material.

  Conversely, because recovery lowers the driving force for recrystallization, a significant amount of prior recovery may in turn influence the nature and the kinetics of recrystallization. The division between recovery and recrystallization is sometimes difficult to define, because recovery mechanisms play an important role in nucleating recrystallization. Additionally, as will be discussed at the end of this chapter, there are circumstances in which there is no clear distinction between the two phenomena.

  The extensive early work on the recovery of deformed metals has been reviewed by Beck (1954) , Bever (1957) , and Titchener and Bever (1958) , but despite its obvious importance, recovery attracted little interest in later years. However, the current industrially driven move to produce quantitative physically based models for annealing processes has resulted in renewed interest in recovery (e.g., Nes, 1995a ).

  6.1.2. Properties Affected by Recovery

  During recovery, the microstructural changes in a material are subtle and occur on a small scale. The microstructures as observed by optical microscopy do not usually reveal much change and for this reason, recovery is often measured indirectly by some bulk technique—for example, by following the change in some physical or mechanical property.

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  Modern Physical Metallurgy

  • R. E. Smallman, A.H.W. Ngan(Authors)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  The formation of atmospheres by strain ageing is one method whereby the metal reduces its excess lattice energy, but this process is unique in that it usually leads to a further increase in the structure-sensitive properties rather than a reduction to the value characteristic of the annealed condition. In that case, it is necessary to increase the temperature of the deformed metal above the strain ageing temperature before it recovers its original softness and other properties.

  11.6.2 Three stages of annealing

  The removal of the cold-worked condition occurs by a combination of three processes, namely, (i) recovery, (ii) recrystallization and (iii) grain growth. These stages have been successfully studied using light microscopy, transmission electron microscopy and X-ray diffraction. Mechanical properties, e.g. hardness, and physical properties, e.g. density, electrical resistivity and stored energy, have also been measured. Figure 11.8 shows the change in some of these properties on annealing. During the recovery stage the decrease in stored energy and electrical resistivity is accompanied by only a slight lowering of hardness, and the greatest simultaneous change in properties occurs during the primary recrystallization stage. However, while these measurements are no doubt striking and extremely useful, it is necessary to understand them to correlate such studies with the structural changes by which they are accompanied.

  Figure 11.8 Rate of release of stored energy (ΔP ), increment in electrical resistivity (Δρ ) and hardness (VPN) for specimens of nickel deformed in torsion and heated at 6 K/min (Clareborough et al., 1955).

  11.6.3 Recovery

  This process describes the changes in the distribution and density of defects with associated changes in physical and mechanical properties which take place in worked crystals before recrystallization or alteration of grain orientation occurs. From the previous section the structure of a cold-worked metal consists of dense dislocation networks, formed by the glide and interaction of dislocations, and, consequently, the Recovery Stage of Annealing is chiefly concerned with the annihilation and rearrangement of these dislocations to reduce the lattice energy and does not involve the migration of large-angle boundaries. This rearrangement of the dislocations is assisted by thermal activation. Mutual annihilation of dislocations is one process.

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  Physical Metallurgy

  • William F. Hosford(Author)
  • 2010(Publication Date)
  • CRC Press

    (Publisher)

  115 9 Annealing 9.1 GENERAL Annealing is heating of metal after it has been cold worked to soften it. Most of the energy expended in cold work is released as heat during the deformation. However, a small percent is stored by dislocations and vacancies (Figure 9.1). The stored energy is the driving force for the changes during annealing. There are three stages of annealing. In order of increasing time and temperature, they are 1. Recovery—often a small drop in hardness, rearrangement of dislocations to form subgrains. Otherwise overall grain shape and orientation remain unchanged. 2. Recrystallization—replacement of cold-worked grains by new ones. There are new orientations, a new grain size, and a new grain shape (but not nec-essarily equiaxed). Recrystallization causes the major hardness decrease. 3. Grain growth—growth of recrystallized grains at the expense of other recrystallized grains. 9.2 RECOVERY The energy release during recovery is largely due to annealing out of point defects and rearrangement of dislocations. Most of the increase of electrical resistivity dur-ing cold work is attributable to vacancies. These anneal out during recovery, so the electrical resistivity drops (Figure 9.2) before any major hardness changes occur. During recovery, residual stresses are relieved and this decreases the energy stored as elastic strains. The changes during recovery cause no changes in microstruc-ture that would be observable under a light microscope. Figure 9.3 shows the energy release and the changes of resistivity and hardness with increasing annealing temperatures. There is also a rearrangement of dislocations into lower energy configurations such as low-angle tilt boundaries (Figure 8.10). In single crystals, these boundaries lead to a condition called polygonization , in which a bent crystal becomes facetted (Figure 9.4). In polycrystals, these low-angle boundaries form subgrains that differ in orienta-tion by only a degree or two.

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  Physical Metallurgy

  Principles and Design

  • Gregory N. Haidemenopoulos(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  Deformation temperature. The stored energy increases when the deformation takes place at low temperatures. In this case the dislocations are not aided by thermal activation to overcome obstacles. As a result there are more dislocation interactions leading to an increase of dislocation density.

  7.7.3 Property Changes During Annealing

  The main change during annealing is softening, which is attributed to the reduction of dislocation density. However other physical properties change as well, such as the density and the electrical resistivity of the metal. These property changes are shown as a function of annealing temperature and are correlated to the reduction of stored energy in Figure 7.20 . The hardness drops drastically in the recrystallization stage, where there is a large decrease in dislocation density, coming from the formation of new grains. The drop in electrical resistivity is due to the reduction of the density of point defects, vacancies and interstitials, which normally impede electron migration. During plastic deformation, the density of a metal decreases due to the generation of new vacancies and dislocations. During annealing, the population of these defects decreases and the density of the metal increases. As shown in Figure 7.20 , the largest part of the stored energy is released during the recrystallization stage.

  Figure 7.20: Property changes in a metal with annealing temperature.

  7.7.4 Recovery

  Recovery is the first stage of annealing. The mechanisms activated during recovery depend on annealing temperature since each mechanism exhibits a different activation energy. After cold working and due to the dislocation interactions, the microstructure is characterized by a large density of sessile dislocations in the grain interiors (Figure 7.21a ). At relatively low annealing temperatures, the main mechanism activated is vacancy diffusion. The fraction of vacancies is reduced since they are absorbed at grain boundaries or at dislocation sites. At intermediate annealing temperatures the dislocations become mobile. They rearrange and form cells . The interior of the cells is relatively free of dislocations, which now concentrate at the cell walls (Figure7.21b). This is accompanied by a slight decrease of dislocation density, which is attributed to a mutual annihilation of dislocations of opposite sign. At high annealing temperatures dislocation climb is activated, aided by diffusion. The dislocations are released from obstacles and with combined glide and climb transform the cell walls into low-angle grain boundaries with lower energy (Figure 7.21c and Figure 7.22 ). The mechanism has been termed polygonization and is observed in single crystals, which have been deformed by bending with the activation of a single slip system (Figure 7.23a ). During annealing the dislocations rearrange into low-angle grain boundaries, separating the crystal into polygon segments (Figure 7.23b

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  Handbook of Thermal Process Modeling Steels

  • Cemil Hakan Gur, Jiansheng Pan, Cemil Hakan Gur, Jiansheng Pan(Authors)
  • 2008(Publication Date)
  • CRC Press

    (Publisher)

  This process also removes the residual stresses formed due to cold working signi fi cant [1,5]. The recovering of physical and mechanical properties varies with the temperature and time. Recovery is a relaxation process with the following charac-teristics: 1. There is no incubation period 2. Recovery rate is large at the beginning, then it slows down till it is near zero 3. Recovery has a limit value varying with temperature; the higher the temperature, the greater is the limit value and the shorter is the time needed to reach the limit value 4. The greater the deformation, the greater is the initial recovery rate, and decrease in grain size helps to accelerate the recovery process The characteristic of recovery can be expressed as the following equation [5]: d x d t ¼ cx ( 3 : 2 ) where t is the time of heating under constant temperature x is the fraction of property increase caused by cold work after heating c is a constant related with material and temperature The value of constant parameter c can be described with the Arrhenius equation: c ¼ c 0 exp Q RT ( 3 : 3 ) Flow stress s s e c e max Work hardening Work hardening + dynamic recovery Plastic strain Work hardening + dynamic recovery + dynamic recrystallization FIGURE 3.10 Variation in fl ow stress due to hot forming. (From Yanagida, A. and Yanagimoto, J., J. Mater. Process. Technol ., 151, 33, 2004.) 96 Handbook of Thermal Process Modeling of Steels Mechanisms of recovery are different in different temperature ranges. At low temperatures recovery is mainly caused by dislocation translation whereas at medium temperatures, it is caused by dislocation movement and redistribution. Dislocations on the same slip surface attract and then are eliminated. Recovery at higher temperatures is caused by climbing of edge dislocations which is activated at about 0.3 T m . 3.2.4 K INETICS OF R ECRYSTALLIZATION The process of formation of new grains by heat treating cold-worked steels is known as recrystal-lization.

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  Structure

  • Gengxiang Hu, Xun Cai, Yonghua Rong(Authors)
  • 2021(Publication Date)
  • De Gruyter

    (Publisher)

  During cooling, after hot working and removal of external force, recovery and recrystallization occur since their driving force is derived from strain energy by hot working. Such recovery and recrystallization are called static recovery and static recrystallization since no external are involved, which is similar to recovery and recrystallization of cold deformed metal during annealing. Therefore, only dynamic recovery and dynamic recrystallization will be described.

  1. Dynamic recovery

  Usually, extended dislocation in metals with high stacking fault energy (such as Al, α-Fe, Zr, Mo, and W) is very narrow in distance between partial dislocations. Both cross-slip of screw dislocations and climb of edge dislocations can be carried out easily. These dislocations tend to leave from nodes and dislocation nets and then counteract with dislocations of opposite signs. Therefore, the dislocation density is very low so that the storage energy is not enough to trigger the dynamic recrystallization. Dynamic recovery is a dominant softening mechanism in hot working for this kind of metals.

  Fig. 4.67: The true stress–true strain curve of the dynamic recovery.

  a. Stress–strain curves of dynamic recovery

  Figure. 4. 67 shows the true stress–true strain curve of the dynamic recovery. Dynamic recovery can be divided into three different stages:

  Stage I – microstrain stage. Stress increases rapidly, and work hardening appears with the total strain less than 1%.

  Stage II – homogeneous strain stage. The slope of the stress-strain gradually declines, and uniform plastic deformation, as well as dynamic recovery, occur at the same time in metals, the “work-hardening” effect is counteracted by the “softening” effect caused by the dynamic recovery.

  Stage III – a steady flow stage. The work hardening and dynamic recovery almost balance, and the work-hardening rate tends to be zero accompanying the formation of a steady state where stress does not increase with the strain. The steady flow stress is evidently affected by the temperature and strain rate.

  b. Mechanism of dynamic recovery

  With the increase of strain, the density of dislocations increases by proliferation (multiplication) accompanying the appearance of dislocation tangles as well as cellular structures. But high hot deformation temperature provides thermal activation energy for recovery process, during which the climbing of edge dislocations, cross-slip of screw dislocations, and the counteracting of dislocations with opposite signs will occur, and thus the dislocation density will decrease continuously. Also when proliferation rate and elimination rate of dislocations reach a balance, work hardening cannot appear, and the stress–strain curve turns to steady flow stress stage.

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  Essentials of Materials Science and Engineering, SI Edition

  • Donald Askeland, Wendelin Wright(Authors)
  • 2018(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  The dislocation density decreases dramatically during recrystallization as new grains nucleate and grow. ● Grain growth, which typically should be avoided, occurs at still higher temperatures. In cold-worked metallic materials, grain growth follows recovery and recrystallization. In ceramic materials, grain growth can occur due to high temperatures or the presence of a liquid phase during sintering. ● Hot working combines plastic deformation and annealing in a single step, permitting large amounts of plastic deformation without embrittling the material. ● Annealing of glasses leads to the removal of stresses developed during cooling. Thermal tempering of glasses is a heat treatment in which deliberate rapid cooling of the glass surface leads to a compressive stress at the surface. We use tempered or laminated glass in applications where safety is important. ● In metallic materials, compressive residual stresses can be introduced using shot peening. This treatment will lead to an increase in the fatigue life. Glossary Annealed glass Glass that has been treated by heating above the annealing point temperature (where the viscosity of glass becomes 10 12 Pa · s) and then cooled slowly to minimize or eliminate residual stresses. Annealing In the context of metals, annealing is a heat treatment used to eliminate part or all of the effects of cold working. For glasses, annealing is a heat treatment that removes thermally induced stresses. Bauschinger effect A material previously plastically deformed under tension shows decreased flow stress under compression. Cold working Deformation of a metal below the recrystallization temperature. During cold working, the number of dislocations increases, causing the metal to be strengthened as its shape is changed. Deformation processing Techniques for the manufacturing of metallic and other materials using such processes as rolling, extrusion, drawing, etc. Copyright 2019 Cengage Learning. All Rights Reserved.

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  The Science and Engineering of Materials, Enhanced, SI Edition

  • Donald Askeland, Wendelin Wright, Donald Askeland(Authors)
  • 2020(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  The dislocation density decreases dramatically during recrystallization as new grains nucleate and grow. ● Grain growth, which typically should be avoided, occurs at still higher temperatures. In cold-worked metallic materials, grain growth follows recovery and recrystallization. In ceramic materials, grain growth can occur due to high temperatures or the presence of a liquid phase during sintering. ● Hot working combines plastic deformation and annealing in a single step, permitting large amounts of plastic deformation without embrittling the material. ● Annealing of glasses leads to the removal of stresses developed during cooling. Thermal tempering of glasses is a heat treatment in which deliberate rapid cooling of the glass surface leads to a compressive stress at the surface. We use tempered or laminated glass in applications where safety is important. ● In metallic materials, compressive residual stresses can be introduced using shot peening. This treatment will lead to an increase in the fatigue life. Glossary Annealed glass Glass that has been treated by heating above the annealing point temperature (where the viscosity of glass becomes 10 12 Pa · s) and then cooled slowly to minimize or eliminate residual stresses. Annealing In the context of metals, annealing is a heat treatment used to eliminate part or all of the effects of cold working. For glasses, annealing is a heat treatment that removes thermally induced stresses. Bauschinger effect A material previously plastically deformed under tension shows decreased flow stress under compression. Cold working Deformation of a metal below the recrystallization temperature. During cold working, the number of dislocations increases, causing the metal to be strengthened as its shape is changed. Deformation processing Techniques for the manufacturing of metallic and other materials using such processes as rolling, extrusion, drawing, etc. Copyright 2022 Cengage Learning. All Rights Reserved.

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  Macromolecular Physics V2

  • Bernhard Wunderlich(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  7.1.1 Definitions and Processes of Annealing 349 certain property by heat treatment without large-scale melting. In a broader sense other treatments, such as chemical reaction and mechanical deforma-tion, can also achieve annealing. An extensive knowledge has been collected during the last 100 years about the annealing of metals, and in particualr steel. Macroscopically annealing has metallurgically the object to impart ductility, extensibility, and a certain grade of softness. The opposite properties are achieved by hardening. Hardening in the shop sense signifies, for example, the making of a piece of steel as hard as possible. Intermediate stages between the hardened and an-nealed state are reached by tempering. Microscopically annealing, harden-ing, and tempering can involve changing of crystal perfection, grain sizes and structure, chemical composition, and state of stress. In analogy to the metallurgical terminology, annealing is used in poly-mer science to describe the improvement of crystallization by heating to temperatures below the melting point which should lead to the growing of crystalline areas, perfection of crystals, and a change to more stable crystal structures (Stuart, 1955).f In the plastics industry, annealing is used to improve the heat resistance and dimensional stability of amorphous and crystalline polymers when ex-posed to elevated temperatures. Furthermore, annealing frequently improves the impact strength and prevents crazing and cracking of excessively stressed items. Amorphous, rigid polymers are mainly annealed for stress relief, while crystalline polymers show, in addition, changes in the nature of their crystal-line state (Jastrzebski, 1965). Heat treatment of separate grains (powder) of a material may lead to agglomeration and compaction to a dense body, a process which is called sintering.

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  Treatise on Materials Science and Technology

  Ultrarapid Quenching of Liquid Alloys

  • Herbert Herman(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  TREATISE ON MATERIALS SCIENCE AND TECHNOLOGY, VOL. 20 6 Annealing Effects in Metallic Glasses^ MARIA LASOCKA and HENRYK MATYJA Institute of Materials Science and Engineering Warsaw Technical University Warsaw, Poland I. Introduction 261 II. Thermodynamic Aspects of Annealing 262 A. Transformation Region 262 B. Internal Parameters 263 C. Analogy between Glass Transition and Melting 265 D. Stabilization 266 E. Relaxation Phenomena 267 F. Typical Experiments in Relaxation Studies 271 III. Basic Effects of Annealing in Metallic Glasses 274 A. Experimental Evidence of Structural Relaxation 275 B. General Trends of Property Changes during Annealing 277 IV. Closing Remarks 286 References 286 I. Introduction The properties of glasses can be changed by a variety of thermal and mechanical treatments. Direct structural evidence is difficult to obtain and current views are based on a variety of sometimes contradictory results. Since the glassy state is metastable, even below T g —in the region of high viscosity—structural relaxation occurs in order to approach a lower energy state. The glassy state is generally not well defined thermodynamically, but under particular circumstances pseudoequilibrium conditions can be as-sumed and thermodynamic arguments employed. In general, the proper-ties of glasses may be considered to be a function of the excess enthalpy t Financial support by N S F Grant N o . INT73-02279 A02 is greatly appreciated. 261 Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN-0-12-341820-8 262 MARIA LASOCKA A N D H E N R Y K MATYJA during isothermal annealing over the temperature range below their re-spective glass transition temperatures. The changes in properties are as-sumed to be parallel to the extent of enthalpy relaxation that occurs during the annealing period as a result of the nonequilibrium state of the glassy phase.

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