On the recovery and recrystallization which attend static softening in hot-deformed copper and...

Acta metall, mater. Vol. 38, No. 1, pp. 41-54, 1990 0956-7151/90 $3.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1990 Pergamon Press plc

ON THE RECOVERY A N D RECRYSTALLIZATION WHICH A T T E N D STATIC S O F T E N I N G IN H O T - D E F O R M E D

COPPER A N D A L U M I N U M

O. KWONI'+ and A. J. D e A R D O 2

tResearch Institute of Industrial Science and Technology, Pohang, 680, Korea and -~Basic Metals Processing Research Institute, Department of Materials Science and Engineering,

University of Pittsburgh, Pittsburgh, PA 15261, U.S.A.

(Received 3 February 1989; in revised form 28 June 1989)

Abstract---The fundamental nature of the static restoration processes which result in static softening after a hot deformation has been studied in copper and aluminum. The kinetics of static softening were determined using the double-hit technique applied to hot compression while the microstructural changes were characterized by the quantitative metallography of quenched specimens. A static softening parameter based on the area under the compression flow curve was used to describe the static softening kinetics. The static softening curves exhibited a simple sigmoidal shape showing no inflection. The relative softening occurring prior to the initiation of recrystallization was found to be small when compared with that occurring after the onset of recrystallization, and was dependent on deformation temperature, amount of deformation, purity and stacking fault energy. The static softening was related to the fractional recrystallization in a nonlinear manner; the degree of nonlinearity was dependent on the occurrence of recovery and dynamic recrystallization. The recrystallization process in A1 was of the classical type with the nucleation stage being either the boundary bulge or subgrain growth mechanism. In Cu, twinning appeared to be the major nucleation mechanism for recrystallization. When the applied prestrain was greater than the critical strain for dynamic recrystallization, recrystallization was observed to be completed before the completion of static softening. In this case, the remaining softening occurred by the operation of multiple recrystatlization where high-order twins formed in the already twinned regions.

R6sam6~La nature fondamentale des m6canismes de restauration statique, qui sont fi l'origine d'un adoucissement statique apr6s une d6formation fi chaud, a 6t6 &udi6e dans le cuivre et dans l'aluminium. La cin&ique de l'adoucissement statique a 6t6 d6termin6e en utilisant la technique du double choc appliqu6e fi la compression fi chaud, tandis que les modifications structurales ont &6 caract6ris6es par m&allographie quantitative des 6chantillons tremp6s. Pour d6crire la cin&ique de l'adoucissement statique, on a utilis6 un param&re d'adoucissement statique bas6 sur l'aire comprise sous la courbe d'6coulement de compression. Les courbes d'adoucissement statique ont une forme sigmo'/dale simple qui ne pr6sente pas de point d'inflexion. L'adoucissement relatif qui a lieu avant le d+but de la recristallisation est faible par rapport fi celui qui a lieu apr6s, et il d6pend de la temperature de d6formation, du taux de d6formation, de la puretb et de l'6nergie de faute d'empilement. L'adoucissement statique est li6 fi la recristallisation partielle de faqon non lin6aire; l'6cart fi la lin6arit6 d6pend de l'existence d'une restauration et d'une recristallisation dynamique. Le processus de recristallisation dans A1 est de type classique, le stade de germination correspondant fi une avancke de joints ou ft. la croissance de sous-grains. Dans Cu, le maclage semble 6tre le m6canisme de germination essentiel pour la recristallisation. Quand la pr6d&ormation appliqu~e est sup6rieure ~i la d&ormation critique pour la recristallisation dynamique, la recristallisation est totale avant l'ach6vement de l'adoucissement statique. Dans ce cas, l'adoucissement final a lieu par une recristallisation multiple ot~ des macles d'ordre 61ev6 se forment dans les r6gions d@i macl6es.

Zusammenfassang--An Kupfer und Aluminium wurde die grundlegende Natur statischer Restau- rationsprozesse, die zu statischer Entfestigung nach Warmverformung fiihren, untersucht. Die Kinetik der statischen Entfestigung wurde mit der Doppelschlag-Technik, angewendet auf Warm-Kompression, bes- timmt; .Anderungen in der Mikrostruktur wurden mittels quantitativer Metallographie an abgeschreckten Proben verfolgt. Mit einem Parameter der statischen Entfestigung, basierend auf der F1/iche unter der Kompressions-FlieBkurve, wurde die Kinetik der statischen Entfestigung beschrieben. Die Kurven der statischen Entfestigung wiesen eine einfache s-f6rmige Form ohne Biegung auf. Die vor dem Beginn der Rekristallisation auftretende relative Entfestigung war klein im Vergleich zu der nach Beginn der Rekristallisation; sie hing yon der Verformungstemperatur, dem Verformungsgrad, der Reinheit und der Stapelfehlerenergie ab. Die statische Entfestigung hing mit dem Bruchteil der Rekristallisation auf nichtlineare Weise zusammen; der Grad der NichtlinearitS.t hing davon ab, ob Erholung und dynamische Rekristallisation auftrat. Der RekristallisationsprozeB in A1 war klassisch, der Mechanismus im Stadium der Keimbildung war entweder Ausbauchen von Korngrenzen oder Wachstum von Subk6rnern. In Cu schien Zwillingsbildung haupts~ichlich vorzuliegen. War die vorher erhaltene Dehnung gr6Ber als die kri- tische Dehnung fiir dynamische Rekristallisation, dann war die dynamische Rekristallisation vor dem Ende der statischen Entfestigung abgeschlossen. In diesem Fall lief die restliche Entfestigung tiber Vielfachre- kristallisation ab, bei der sich Zwillinge h6herer Ordnung in den schon verzwillingten Gebieten bildeten.

~Former address: University of Pittsburgh, PA 15261, U.S.A,

41

42 KWON and DeARDO: SOFTENING

1. INTRODUCTION

Hot working is a process of changing the shape and microstructure of metals and alloys at elevated temperatures. The hot working process was initially developed because it lowered the energy required to deform the metals and increased the ability of the metals to flow without forming cracks. However, many recent studies of hot working have focussed principally on the microstructural changes which occur as a result of the hot deformation. Much of this interest is a direct result of the well-recognized improvement in mechanical properties which may be observed in metals and alloys which have undergone proper hot deformation processing. This is particularly true in the case of structural steels where appropriate hot rolling of the parent phase (austenite) can lead to a much finer distribution of the final or product phase (ferrite) with the attendant improvement in both strength and resistance to brittle fracture. Hence, the study of the origins of microstructural refinement present an interesting area of study from both the scientific and technological perspectives.

The important structural changes occurring during hot working are recovery and recrystallization, which are the restoration processes that result from the release of the internal energy stored during deformation. The structural changes can occur either during concurrent straining under stress (dynamic) or during intervals between rolling passes in the absence of applied stress (static). For example, in industrial plate rolling on a reversing mill, the time available for dynamic microstructural change is very short, less than one second [1]. On the other hand, the time available for static microstructural change is much longer, typically in the range 20-30 s [1]. Since the time available for dynamic microstructural change is very short at reasonable high strain rates, certainly much shorter than the time normally required for dynamic events to occur, it has been concluded that the microstructural changes which occur during multi- pass hot rolling are more closely associated with static microstructural changes [1]. The present study was, therefore, conducted to increase the basic under- standing of the nature of static restoration which may accompanying multi-pass hot deformation.

Various techniques based on interrupted mechanical tests have been employed to investigate the progress of static restoration. These include interrupted tension [2-8], compression [9-18], torsion [19-22] tests and hardness measurements [22-24]. Among these studies, the static softening studies performed by Petkovic et al. [10-12, 25] are considered important because the model proposed by these workers has been widely accepted as a general description of the occurrence of the static softening processes. A series of carbon steels and copper were studied under various experimental conditions (temperature, strain rate, prestrain, etc.). It was concluded from these results that static restoration takes place by the operation of three distinct mechanisms; (1) static

IN HOT-DEFORMED Cu AND Al

recovery, (2) classical recrystallization, and (3) metadynamic (post-dynamic) recrystallization.

Attempts to rationalize the three-mechanism static restoration model with both well-documented observations concerning static annealing behavior and widely accepted theories of static recovery and recrystallization have raised several significant questions concerning the amount of energy released during the recovery period, the effect of deformation on recovery and recrystallization, and the nucleation mechanism of recrystallization.

According to both conventional observation and theory of annealing, the amount of energy released during the recovery stage is relatively small and decreases as the deformation is increased [26-29]. But the model described above demonstrated a different behavior [10-12, 25]. Also, twinning has been known as the common feature observed during recrystallization in the low stacking fault energy materials such as copper and austenitic stainless steels [30-35], but their modelling study using copper [10] over looked this feature. The present study was designed and conducted principally to address the inconsistencies which exist between the established static softening model and traditional static annealing behavior, and to elucidate more clearly the nature of the progress of the static restoration processes. An extensive metallographic analysis was performed to determine the interrelationships between the softening measured using mechanical methods and the structural changes observed by metallographic techniques.

2. E X P E R I M E N T A L

The present work was carried out using two face- centered cubic (f.c.c.) metals: 99.9% OFHC Cu and 99.0% commercial purity A1. The metals Cu and A1 were selected based upon two facts: (1) they do not have a phase change occurring when cooled from the deformation temperature to room temperature, and (2) they have different stacking fault energies which has been cited as one of the important factors influencing the hot deformation behavior of F.C.C. materials. Aluminum is a high stacking fault energy metal, and the stacking fault energy of Cu is intermediate and is approximately the same as that of austenite in HSLA steels [35].

Small cylindrical compression specimens were machined from the as-received round bars. The dimensions of the compression specimens were 12.7 mm (0.5in.) diameter X 19.05 mm (0.75in.) height. For detailed analysis of microstructural changes occurring during recrystallization, large grain size specimens are desirable. To this end, specimens were solution-treated at high homologous temperatures. The preliminary experiment to control grain size showed that a certain amount of deformation was required to obtain a homogeneous microstructure in the large grain size condition. The detailed treatments and the resulting grain sizes are

KWON and DeARDO: SOFTENING IN HOT-DEFORMED Cu AND AI 43

listed in Table 1 [37]. The deformation was applied at a constant strain rate 1.0/s. Specimens were heated to a given temperature of hot deformation (550°C for Cu and 450°C for A1) and deformed isothermally using the interrupted compression technique. A combination of proper lubricants and a modified Rastegaev's design [38] was used to keep friction to a minimum. Preliminary experiments proved that teflon sheet was most effective as a lubricant for AI, and a combination of teflon and a water-base graphite (Delta Forge 182, Acheson Colloids Co.) was optimum for Cu. A detailed description of the equipment and procedures is given elsewhere [37].

Static softening was studied using the interrupted compression technique. A softening parameter based on the yield strength (Xy) has been most widely used. In the present study, a different softening parameter, XA, which was based on the area under the flow curve, was used for the evaluation, Fig. 1. The primary reason for using this parameter was based on the fact that the precise measurement of yield strength was extremely difficult at high temperatures because of the lubricants used, the grooves intro- duced on the end surfaces of compression specimens, and the use of the relatively high compliance hydraulically-powered testing machine. The present softening parameter has been applied to describe the static softening behax ior of HSLA steels [39], and was found to correlate better with the degree of microstructural restortion than did the conventional parameter based on the yield strength.

All specimens for optical microscopic examination were quenched using an in situ quenching system [37]. Following the standard metallographic procedures for Cu and A1, the specimens were etched for optical metallography. Etching of the A1 samples was performed electrolytically with Baker's reagent (5ml HBF + 200 ml H20) The Cu specimens were etched

Table 1. Heat treatments used and the resulting grain sizes

Materials Processing Grain size (~m)

AI 800°C, l0 h 750 +10% c.w. + 600~C, 4h

Cu 2% c.w. 450 + 950'~C, 12h

using an alcoholic-ferric solution consisting of 5 g ferric chloride, 100ml ethanol and 15ml hydro- chloric acid. The average grain size was determined using the standard circular intercept method, and the fractional recrystallization was determined using the point-count method. The linear intercept method was applied to determine the density of twins formed during the recrystallization of CU [37].

3. RESULTS

3.1. Static restoration behavior of copper

The static softening curves obtained from the interrupted compression test are shown in Fig. 2. The four softening curves in Fig. 2 correspond to the four different prestrains applied before interruption. The overall shape of the four curves was essentially identical, showing a typical sigrnoidal form. The effect of prestrain was to increase the kinetics of static softening. A significant increase in the kinetics was observed when prestrain was raised from 0.065 to 0.13, or from 0.13 to 0.35. However, a small increase in the static softening kinetics was observed when the prestrain was increased from 0.35 to 0.6.

The data shown in Fig. 2 indicate that the kinetics of static softening were extremely fast for the prestrains of 0.35 and 0.6. The amounts of static softening taking place for the delay time of 1 s are close to 80% for both conditions. Rapid kinetics of this

%

0- 2

(c)

E

J

St.ra in S t , r a i n

A 3 - - A 2 0" 3 - - o - 2

X A = - - X y -

A 3 - A I (7 3 - - o - I

Fig. 1. Flow curves of interrupted compression test. (a) After some delay time. (b) After infinitely small holding time. -¥A and Xy indicate softening parameters based on area and yield strength respectively (37).

44 KWON and DeARDO: SOFTENING IN HOT-DEFORMED Cu AND AI

kind have been attributed to the operation of metadynamic recrystallization [9, 11]. Metadynamic recrystallization occurs by the continued growth of nuclei formed as a result of the occurrence of dynamic recrystallization during prestraining. Hence, the operation of metadynamic recrystallization does not require an incubation time. This view is, in fact, in agreement with the rapid kinetics of static softening observed for the prestrains of 0.35 and 0.6, as displayed in Fig. 2. Furthermore, more than 10% of the softening has already taken place for a very short delay time of 0.1 s. It is noted that the initial flattening of the softening curves for the two conditions in Fig. 2 (i.e. pre-strains of 0.065 and 0.13) should not be considered as an incubation period. The flattening is attributed to the nature of the log(time)-scale used for the abscissa.

An overall examination of the microstructures in the early stage of softening revealed that small, new features were formed at the grain boundaries or deformation bands as a sign of the initiation of recrystallization. These new features which occurred during the static holding period in Cu have often been referred to as recrystallized grains. However, in many (if not most) cases, these new features were not simply new grains at all, but were, rather, twinned regions of the initial deformed grains. Some of the new features are shown in Fig. 3. The top microstructure shows a small twin emerging from the grain boundary, the middle one shows an elliptical grain which consists of several new twins, and the bottom one shows a complicated twin reaction occurring along a high angle grain boundary. It has been long disputed whether this type of twinning reaction can be considered a recrystallization process. However, recent studies have shown experimental evidence that twinning is, in fact, a recrystallization reaction, where twins form and grow to consume dislocations [33-35,40~7]. Hence, the stored energy of defor-

mation remains the driving force for this new mechanism of softening by twinning, as it is for traditional or classic static recrystallization. Since the twinned regions a r e the recrystallized regions, and will subsequently be referred to as the recrystallized regions, the volume fraction of twinned regions was determined using the point-counting technique. The re- suits are shown in Fig. 4. The softening curves in Fig. 4 are the reproduction of the curves in Fig. 2, and the numbers in the rectangles positioned along the softening curves represent the percentage of recrystallization at the corresponding points.

When the applied prestrain was 0.065, recrystallization started after a significant amount of softening had taken place, approximately 21% of softening as observed in Fig. 4. The recrystallization was initiated exclusively at the grain boundaries. One of the interesting observation concerns the softening curve for a pre-strain of 0.065 in Fig. 4. After a holding time of 30,000 s, there was complete static softening even though there was only 40% of the microstructure having undergone recrystallization. The remaining 60% of the volume seemed to regain its as-annealed properties by a recovery process alone. This observation indicates that both recovery and recrystallization took place during this delay period: the former occurred within the grain and the latter on the grain boundary area. This concurrent recovery during recrystallization is known to be typical behavior during the annealing process and has been reported in previous investigations [28, 48-50]. No significant progress in recrystallization or softening was observed for delay times longer than 10,000 s.

When the prestrain was 0.13, some deformation bands were present in the as-deformed microstructure. Recrystallization started after 6 0 mechanical softening, which was much smaller than the 21% observed for the prestrain of 0.065. Recrystallization started initially at the grain boundaries, but occurred

Static softening behavior of copper

550°C 1.0/s

2O

\ . \ ~ ~ t ~ o 0.600

g o 60 •

8O

100 I I I I l l l l l I I I])1111 I k ItHhT t I I 1 ~ I ] IIII1[I } I II[IH] J i l l l lHI 0.01 0.1 t 'TO '100 1000 10000 100000

Time (s)

Fig. 2. Static softening curves of Cu obtained from interrupted compression tests. The applied prestrains of 0.065 and 0.13 are lower than the critical strain for dynamic recrystallization, but the prestrains of 0.35

and 0.6 are greater than the critical strain (37).

KWON and DeARDO: SOFTENING IN HOT-DEFORMED Cu AND AI 45

both at grain boundaries and deformation bands in the later stages. Many small twins were observed in the newly formed recrystallization grains. Some of these twins appeared to be formed as a bundle of microtwins as shown in Fig. 5. The twins increase in thickness by the coalescence of neighboring twins of the same orientation. The thickening of twins in- volves the displacement of coherent twin boundaries which are immobile in nature. However, the displacement of coherent twin boundaries can occur by the motion of incoherent twin ledges.

Since the applied prestrain of 0.35 exceeded the critical value for dynamic recrystallization, the as-deformed microstructure was partially recrystallized. The dynamically-formed new grains, or, more precisely, newly twinned regions, were the source of metadynamic recrystallization which occurred immediately after deformation. Non-metadynamic recrystallization, or classical recrystallization as referred to by Djaic et al. [12], occurred by the mechanism presented above in the remaining dynamically-recovered area, initiating at both grain boundaries and deformation bands. The distinction between grains formed in the non-metadynamic and metadynamic patterns was not clear because both types of new grains were similar in size and shape.

It is interesting to note that approximately 96% of the total volume was observed to be recrystallized although the mechanical softening reaches only 63% after 0.5 s of delay time. The remaining 37% of the stored energy was observed to be released without any significant change in fractional recrystallization. This observation leads to the conclusion that new grains were formed in the region which had already been recrystallized. Since the recrystallization in Cu has been shown to be a twinning process, the new "grains" formed during the later stage of softening were simply the second generation of twins. This repeated recrystallization can be understood by the concept of multiple twinning as reported recently by many investigators [43-47]. Hence, the recrystallization process in Cu deformed in the range of prestrain greater than the critical strain for dynamic recrystallization can be characterized by the concurrent operation of metadynamic and classical recrystallization which themselves occur by a twinning process. Additional softening can then occur by repeated or multiple twinning of the previously twinned volumes.

3.2. Static restoration behavior of aluminum

In order to determine the softening behavior of AI, a series of interrupted compression tests was performed with increasing delay times. The true strains of 0.2 and 0.8 were employed as the prestrains in the experiments. An examination of specimens strained to 0.2 and 0.8 revealed that the microstructures were totally different in the as-deformed condition: few deformation bands were observed when specimens

J

Fig. 3. Microstructures showing the formation of twins in the initial stage of recrystallization in Cu.

were deformed to 0.2, but an abundance of deformation bands were observed on straining to 0.8. The results obtained from the compression tests are presented in Fig. 6. In Fig. 6, the softening curve of high purity (99.99%) AI (dotted line) is also included for comparison.

The softening curves in Fig. 6 display the typical sigmoidal shape. The rate of softening was very slow initially, becoming faster in the intermediate stage, and finally decreasing as the delay time was increased. The effect of increasing prestrain was to increase the kinetics, which is similar to the observed effect observed in Cu. Approximately an order of magnitude difference in kinetics was observed when the applied prestrain was quadrupled from 0.2 to 0.8. However, the effect of prestrain in AI did not seem to be as great

46 KWON and DeARDO: SOFTENING IN HOT-DEFORMED Cu AND Al

as that in Cu although the change in prestrain was larger and homologous temperature used for the experiments was higher in Al. This may be attributed to active dynamic recovery occurring during the prestraining period and, hence, to the ineffective stored energy buildup as the driving force in A1.

The as-deformed grain structure observed under polarized light showed nonuniform colors within the grains and somewhat unclear grain boundaries. The nonuniform colors represented the disturbance of crystal registry created during deformation. The newly-formed recrystallized grains exhibited a uniform color within the grains and sharp grain boundaries, and were readily distinguished. When the applied prestrain was 0.2, recrystallization appeared to start at a holding time of approximately 1000 s. During this period (1000 s), recovery was the only operating restoration process, and the mechanical softening observed during the period was about 20%

as seen in Fig. 6. Recrystallization was observed to be initiated primarily at grain boundaries. In addition, some new grains were also formed within the original grains. This observation was in good agreement with earlier work [24, 51-54], indicating that both strain- induced grain boundary migration or bulging and subgrain coalescence are the nucleation mechanisms responsible for recrystallization in A1. When the delay time exceeded 2000 s, the small new grains gradually increased in size. But the recrystallization did not seem to spread throughout the whole structure. In- stead, some areas appeared to have their properties restored to their original as-annealed states entirely by the recovery process. This type of recrystallization behavior appeared to be similar to that observed in Cu when the applied prestrain was small. Even after a long delay of 10,000 s, the structure returned to its as-annealed condition without the appreciable motion of high angle grain boundaries.

5 5 0 ° 6 , 1.0 / s

~ \ ~ ~ P res t ra i n

zO , - ~ ' , ~ ' Z i \ , ~ 0.065 1751 ~ ~, ~ ~ ~ 0.130

','\ ~ ~ 0.350 -- '\ '\ \ ~ 0.600

.~ \

100 I I I I I I I I I I I I I l l l l l I I l i l . . I I IIIIIII [ I l l l l l l l I I I I l l l l l I I l l l l l l l

0.01 0.1 1 10 100 1 0 0 0 1 0 0 0 0 1 0 0 0 0 0

T i m e (s)

Fig. 4. Relationship between softening and fractional recrystallization in Cu. Numbers in boxes represent the volume percentage of recrystallization measured by the point-count method (37).

Fig. 5. Microtwins observed during recrystallization in Cu.

KWON and DeARDO: SOFTENING IN HOT-DEFORMED Cu AND A1 47

When the prestrain was 0.8, recrystallization started at approximately 100 s holding time. The amount of mechanical softening which occurred before the start of recrystallization was about 20%. This is shown in Fig. 7, where the volume fractions recrystallized are indicated at the corresponding points on the softening curve. The kinetics of softening were increased substantially as recrystallization began. Recrystallization initiated both at grain boundaries and deformation bands within the grains. Both mechanical softening and recrystallization were essentially completed upon holding for 10,000 s.

4. DISCUSSION

4.1. Stat ic softening curves

Numerous investigations have been made on the static restoration behavior of materials using the

interrupted technique [2, 5, 9-12, 14, 16, 17]. Since the major static restoration processes are known to be recovery and recrystallization, most of the previous studies have attempted to correlate the observed softening behavior to the occurrence of recovery and recrystallization. An analysis of the reported softening curves of single-phase f.c.c, materials showed that most of them exhibited the typical sigmoidal shape without any inflections, while some of them [5, 1(~12] showed one or two arrests in the softening behavior when experiments were carried out on single-phase materials. When the softening curves show the smooth sigmoidal behavior, the initial slow increase in fractional softening may indicate the presence of a recovery stage and the subsequent major increase reflects recrystallization. This static softening behavior agrees well with the softening behavior of Cu and AI in the present study, and the occurrence of recovery and recrystallization has been confirmed metallographically.

O -

20

A 40

#

~ 6 o

80

Static softening behavior of aluminum T=4500C ~ = l . 0 / s

I I I I I l l ~ t -4 I I I I I I I I I I I I I I I I I I I I I I h = ~ I I I I I I I I I I l I I l ( I IOO lo lOO lOOO 10000 100000 1000000

Time (s)

Fig. 6. Static softening curves of A1 obtained from interrupted compression tests. The two solid curves represent commercial-purity while the dashed curve represents high-purity A1. The high-purity A1 was

prestained 0.8.

O

20 ~ Aluminium

o~ 40

L .F 6o

Prestrain = 0.8 L_~ Temperature = 450°C

90 Strain rote : 1.0Is

100 ~ I I I I I I I I I I I IH I~ -~ , ' - , - , ' I I I I I I I0 100 1000 I0000 I00000

Time (s)

Fig 7. Relationship between softening and fractional recrystallization. Numbers in boxes represent the percentage of recrystallization measured by the point-count method (37).

48 KWON and DeARDO: SOFTENING IN HOT-DEFORMED Cu AND A1

The static softening behavior which showed an inflection in the softening curve was reported by Capeletti et al. [5] using a 304 stainless steel, and the softening behavior which showed one or two inflections was reported by Petkovic and coworkers [1 0-12] using Cu and plain carbon steels. The results obtained by Capeletti et al. [5] demonstrated that the arrest in the softening curve was attributed to the sequential operation of recovery and recrystallization. The presence of one or two arrests in softening curves observed by Petkovic and co-workers [10-12] was explained by the operation of three distinct softening processes: static recovery, classical recrystallization and metadynamic recrystallization. The important difference between these two groups is that the former observed the inflection only in a low temperature range (less than 1039°C), but the latter suggested that the presence of inflections is a uni- versal feature of the static restoration process as a result of the sequential operation of three proposed softening processes.

The sequential operation of recovery and recrystallization has been widely observed when a cold worked metal was heated slowly and the energy released during heating was measured by a calor- imeter. But numerous studies [48, 50, 55-60] have shown that recovery and recrystallization can also occur concurrently. It is now believed that some overlap between recovery and recrystallization occurs during the isothermal annealing of deformed metals [48]. The degree of overlap depends on various factors including temperature, deformation and purity.

One of the important sources responsible for the concurrent behavior is the heterogeneity of deformation. When the level of deformation is high, the heterogeneity of deformation is evidenced by the formation of microbands, transition bands and shear bands [60]. For deformations less than 5% macro- scopic strain, a considerably higher local strain was observed in the grain boundary regions as compared to the grain interiors [61] and, hence, recrystallization starts on the grain boundaries. The observed microstructures in the present study were seen to conform well with the general behavior of strain distribution and recrystallization. Therefore, the concurrent behavior of recovery and recrystallization was shown to be obvious at particularly low levels of deformation in Cu and A1 where recovery took place in the grain interiors while recrystallization occurred at the grain boundaries. The consequence of concurrent recovery during recrystallization is that the softening curves exhibit no arrest when the static restoration behavior of a single phase metal or alloy is studied under the isothermal condition. The absence of inflections in the static softening curve is consistent with the results observed in the present study and in most other investigations, but does not agree with the softening behavior displayed by Capeletti et aL [5] or by Petkovic and co-workers [10-12].

4.2. Energy release during static restoration It has been generally accepted that the relative

energy released during the recovery stage (Er), or, more strictly, the relative energy released before the start of recrystallization, is small compared with that released after the onset of recrystallization (ER) [26-28]. This behavior was confirmed in the present study. In Cu, the relative amount of static softening occurring during the recovery stage was 21% for the prestrain of 0.065, and 6% for the prestrain of 0.13. In AI, the applied prestrain does not seem to affect the relative softening which occurs by recovery significantly, about 20% softening was observed for both prestrains of 0.2 and 0.8. In general, the Er/ER-ratio is a sensitive function of purity, temperature, deformation, and material properties such as stacking fault energy [26, 28, 29].

Impurities are known to raise the temperature of recrystallization during anisothermal heating experiments, and are responsible for a rise in the Er/ER-ratio because the temperature interval of recovery is extended. For example, in Cu, the Er/ER- ratio is equal to 0.05 for 99.988% pure copper, 0.3 for 99.967% copper, and 0.6 for Cu with 0.35 As [29]. In the present study of the static softening behavior of high purity OFHC Cu, the fractional softening observed before the start of recrystallization was found to be less than 20%. This contrasts with softening as great as 50% in the previous studies of low purity tough pitch Cu conducted by Petkovic and co-workers [10]. The greater Er/ER-ratio observed in tough pitch copper may be attributed to the impurity effect on the occurrence of recovery and recrystallization. A similar impurity effect was also observed in AI as shown in Fig. 6, where the softening curves of high (dotted line) and commercial purity (solid line) AI are compared. The fractional softening observed before the initiation of recrystallization was approximately 20% for commercial purity A1, but was only about 5% for high purity A1. The sudden increase in the softening kinetics observed after a softening of 5% in high purity AI was attributed to the initiation of recrystallization, as was also confirmed metallographically.

The effect of temperature on the Er/ER-ratio can be visualized by the concept of the thermally-activated process. Both recovery and recrystallization are known as thermally-activated processes, and, hence, the occurrence of each process is sensitive to temperature. At high temperatures, thermal fluctuations assist both the initiation and progress of recrystallization. The earlier start of the recrystallization process will reduce the relative amount of energy released before the initiation of recrystallization, or during recovery, and result in the reduction of Er/ER-ratio [26]. This analysis is generally in good agreement with the softening behavior reported earlier, where the arrest in the softening curve occurred at smaller fractional softening values as the temperature was increased [5, 11].


The amount of deformation also influences the Er/ER-ratio, and its effect can be understood in a manner similar to that of the effect of temperature. When the applied deformation is great, recrystallization starts earlier and the Er/ER-ratio decreases. However, the results obtained by Petkovic (Djaic) et aL [10, 12] demonstrated a completely reversed behavior, i.e. the relative amount of softening which occurred during the recovery stage was increased with prestrain. In the present study, a decrease from 0.21 to 0.06 in the fractional softening was observed when the applied prestrain was increased from 0.065 to 0.13 for Cu. This agrees well with the general effect of deformation on the Er/ER-ratio; the Er/ER-ratio decreases as the amount of deformation increases. This behavior has been demonstrated in earlier work [62, 63] which showed that the Er/ER-ratio for copper deformed in tension was 0.12 when the elongation was 10.8%, whereas the ratio was as small as 0.03 when the elongation was 39.5%. On the other hand, for A1, little change in fractional softening occurred prior to the start of recrystallization when the applied prestrain was quadrupled from 0.2 to 0.8. The different behavior exhibited by Cu and Al may be attributed to their different flow or work hardening behavior, and hence, in different driving forces for recrystallization. A comparison of the flow curves of Cu and Al indicated that Cu showed a large difference in the flow stresses at strains of 0.065 and 0.13, whereas Al showed little difference at strains of 0.2 and 0.8. This is because the Cu exhibits a flow curve indicative of intense work hardening, under the prevailing deformation conditions, whereas the Al flow curve is indicative of dynamic recovery with very little work hardening. Hence, in A1, an increase in the applied strain greater than 0.2 did not significantly increase the effective driving force for recrystallization and resulted in little change in Er/ER-ratio.

The effect of stacking fault energy on the relative energy released during recovery is displayed in Fig. 8, where the energy release curves for deformed Al and Cu are shown together with the changes in hardness and electrical resistivity with temperature. The absence of an energy peak during the recovery stage appears to be typical for Cu, whereas the high stacking fault energy materials like Al and Ni exhibit one or two peaks [26]. With regard to the relative energy release, it is believed that Al has a higher EjER-ratio than Cu, which agrees well with the observations made on Al and Cu in the present study.

It has been shown in Fig. 8 that the energy released and the changes in mechanical and physical properties are small during the recovery stage. It has been long recognized in the rolling industry that the static recovery occurring between rolling passes has only a small influence on the mill load in the next pass, whereas static recrystallization has a much larger effect. However, the results obtained by Petkovic and co-workers [10-12] demonstrated that as much as

0.4

O.Z

% 200 --

6. <~

1oo E

<3 0

0.4

(a) l Ap I , \

\

HV

15O

100

5O

H V

300

2O0

100

1 5 o

1oo (b) ~o ~Q.

0 / f I I 200 400 600

t (°C)

Fig. 8. Changes in electrical resistivity, hardness and energy released during heating of various metals initially deformed

in torsion (29). (a) Cu. (b) A1.

50% of total softening took place during the recovery stage. This inconsistency was explained by Luton et al. [64] using a relationship between the flow curve and the mobile and immobile dislocation density. They postulated that upon reloading, before completion of recovery, there existed a transient during which the flow stress rapidly increased to the level of the continuous curve, A similar transient flow behavior was reported in AI [7, 65, 66]. Sandstrom and Lindgren [66] analyzed the flow curve of com- mercially pure A1 and concluded that the sudden drop in stress with holding time (usually referred to as ortho-recovery) was attributed to a reduction in the dislocation arrangement of dislocations. Accord- ing to Luton et al. [64], an integration of the flow curve showing a transient period during recovery would give little difference when compared with the integration of the continuous curve. Hence, little change in the mill load is expected during rolling of a workpiece exhibiting the transient behavior in the flow curve although the amount of the static softening may be as much as 50% during recovery stage. Thus, the inconsistency between the required mill load and the amount of static softening is considered to be primarily attributed to the transient flow behavior during recovery, and to the use of the softening parameter based on the yield strength. It is not clear in the present study whether the transient flow behavior exists during recovery because the application of high temperature lubricants

A.MM 3~,1 D

50 KWON and DeARDO: SOFTENING IN HOT-DEFORMED Cu AND Al

to the grooved specimens and the higher compliance of the testing machine prevented a detailed analysis of the initial stage of flow.

4.3. Relationship between static softening and recrystallization

As stated earlier, one of the primary goals in the present study was to investigate the interrelationships which may exist between the static softening obtained from the mechanical testing method and the structural changes observed with metallographic techniques. The fractional recrystallization was measured using the point count method in the present study, and the results have already been presented in Fig. 4 for Cu and Fig. 7 for A1. In order to examine the relationship, the fractional recrystallization (X) was plotted against softening (XA) as shown in Fig. 9 for Cu and in Fig. 10 for AI. The data in Figs 9 and 10 indicate that the fractional recrystallization was non- linearly related to the softening, and the nonlinear behavior was seen to be essentially dependent on the occurrence of recovery and dynamic recrystallization. Under the conditions where no dynamic recrystallization took place during prior deformation, the fractional recrystallization was related to the softening in a nonlinear manner with positive deviation. The degree of deviation appeared to increase as the amount of prestrain was lowered. This follows from the fact that the Er/ER-ratio increased with strain at small applied strains. This behavior was exhibited by Cu at relatively low prestrains (0.065 and 0.13), and A1 at all prestrains. On the other hand, when dynamic recrystallization took place during the prestraining

Copper

T= 550°C ~= 1 . 0 / s

t00 I~

Prestrain • O. 065

20 [] 0.130 • 0.350 o O. 650 / I / /

I I I - - I I 0 20 40 60 80 100

X (%)

Fig. 9. Relationship between recrystallization (X) and static softening (XA) of Cu deformed at 550 C and 1.0/s. Note that the curve shows a positive deviation when the prestrain (0.065 or 0.13) is smaller than the critical strain for dynamic recrystallization (0.23), but shows a negative deviation when the prestrain (0.35 or 0.6) is greater than the critical

strain (37).

A

X

100

80

60

/ 40 [] /

/ /

/ /

201 / /

/

0 ~ / / I 0 20

A l u m i n u m

T = 4 5 0 ° C ~ = 1 .0 /S~ ~pre=0.8

I I I I 40 60 80 t00

X (%)

Fig. 10. Relationship between recrystallization (X) and static softening (XA) of AI deformed at 450 C and 1.0/s (37).

stage, the relationship demonstrated a nonlinearity with negative deviation. In this case, the degree of deviation appeared to increase with the amount of dynamic recrystallization. This behavior was exhibited by Cu at prestrains greater than the critical strain for dynamic recrystallization, but was not observed in Al. In both cases, the degree of nonlinearity was observed to increase as the difference between the applied prestrain and the critical strain for dynamic recrystallization increased.

The nonlinear relationship between the fractional recrystallization and softening was previously reported by Sah and Sellars [9] and Barraclough and Sellars [22]. The nonlinear behavior observed by these authors is similar to the one with positive deviation discussed above. Barraclough and Sellars [22] considered two possibilities as the cause of the nonlinearity; continuing effects of static recovery during recrystallization and heterogeneity of deformation between recrystallized and unrecrystallized regions, and concluded that the latter was the major influencing factor. In addition, the results in these studies implied that the effect of ongoing recovery becomes more significant as the applied prestrain is reduced. This follows from the fact that the nonlinearity becomes greater and the total fractional recrystallization is small at the lower prestrain.

The negatively deviating nonlinear behavior has not been reported previously, but is expected to occur whenever dynamic recrystallization takes place during the prestraining stage. Another factor which contributed to this nonlinear behavior was the completion of apparent recrystallization before a saturation of mechanical softening. This has been observed previously and has been attributed to the operation of multiple recrystallization [67], where recrystallization apparently took place in the previously recrystailized regions. The earlier completion


of apparent recrystallization was observed in Cu for the applied prestrains of 0.35 and 0.6. In the present case, the remaining softening took place by the multiple formation of new twins in the already recrystallized or twinned regions.

4.4. Nucleation mechanisms for recrystallization

4.4.1. Overall behatior. The application of fluctu- ation theory [67,68] to explain the nucleation mechanism for recrystallization was found to be inadequate because the driving force (essentially the stored energy of deformation) was too low and the interfacial energy barrier was too high to cause a classical type of nucleation [27, 28, 49, 69, 70]. Hence, modern models are based on the concept that new grains are formed by the growth of pre-existing nuclei formed during deformation and recovery. Recrystal- lization nucleation mechanisms based on this concept are the sub-boundary migration model [71], subgrain coalescence model [72] and strain-induced grain boundary migration model [73].

It is well-known that recrystallization nuclei form preferentially in regions of high local strain. At relatively low deformations, higher local strain was observed in the pre-existing high-angle grain boundary regions [61], providing the preferential nucleation sites for recrystallization. This observation is generally in good agreement with the recrystallization behavior of Cu and A1 studied in the present investigation. An implication of this observation is that the angle grain boundaries play a vital role in the nucleation of new grains.

Formation of new grains at high angle grain boundaries is a central feature of the recrystallization which results from the operation of the strain- induced grain boundary migration model for recrystallization nucleation [51, 73, 74]. The driving force for this process is the local difference in volume strain energies on the two sides of the boundary, which is produced by non-uniform deformation. This type of recrystallization was first reported by Beck and Bailey [51,73] and subsequently observed in Cu [29], AI [75], and steel [76] by many researchers.

The role of the strain induced grain boundary migration model as a nucleation mechanism for recrystallization is known to decrease with increasing deformation [24, 61]. This follows from the fact that some local highly strained regions appear within the grains. At moderate deformations, clusters of long, thin, sheet-like features called "transition bands" develop parallel to the shear strain axis, and at high deformations shear bands develop across grain boundaries. The materials containing these deformation bands show preferred nucleation at transition bands or shear bands as well as grain boundaries. This type of recrystallization behavior was observed in Cu deformed at true strains of 0.35 and 0.6 and in AI at 0.8 in the present investigation. In this case, new grains may be formed from the subgrains formed during deformation and recovery either by

sub-boundary migration or by subgrain coalescence. It has been argued for a long time that the rates of subgrain growth [77, 78] were orders of mgnitude faster than the theoretically calculated values based on coalescence between two neighbouring subgrains [79]. Therefore, the migration of subgrain boundaries was considered to play a predominant part in recrystallization. This process was observed directly in studies of recrystallization in AI and Ni [80] and Cr-Ni alloys [81]. However, recent studies [82, 83] using electron microscopy have shown sufficient experimental evidence that coalescence of subgrains was the responsible nucleation mechanism in AI. These studies also showed that the coalescence of subgrains occurred not only in deformation bands but also at the grain boundaries prior to grain boundary migration.

4.4.2. Recrystallization twinning in Cu. As shown in the microstructures of Figs 3 and 5, an abundance of twins was found in Cu during the course of recrystallization. These twins have been known to be a prominent feature of fully annealed f.c.c, metals with low stacking fault energy. Numerous investigations have been conducted concerning the formation of annealing twins, and various models have been developed to correlate experimental observations with the nature and origin of twins. However, twinning has only recently been identified and accepted as a recrystallization process which is driven by the stored energy of deformation. The growth of twins can take place by the motion of incoherent twin boundaries into the deformed matrix reducing the overall dislocation density. However, in the classical concept of recrystallization, new grains grow by the motion of high angle grain boundaries. There is an additional difference between softening by the motion of incoherent twin boundary segments and high angle grain boundaries. Once a high angle boundary has swept through a deformed grain, the dislocation density within that grain falls to such a low level that there is insufficient driving force for any subsequent boundary motion through that volume. The softening associated with the motion of twin segments is not so efficient in reducing the dislocation density; once a twin segment has passed through a volume, there remain behind enough dislocations to support the motion of additional waves of twins. Evidence of the high dislocation density, existing in material which has undergone a twin-dominated recrystallization has been shown in austenitic stainless steel [84].

One of the earliest models relating twinning and recrystallization, proposed by Mathewson [85], demonstrated that annealing twins could be formed by a lateral growth of extremely fine deformation twins [86] or twin faults [87] formed during prior deformation. This model suggested a relationship between twinning and deformation and indicated that twinning was a recrystallization process. Fullman and Fisher [32] observed the formation of annealing twins during the grain growth stage and proposed that

52 KWON and DeARDO: SOFTENING IN HOT-DEFORMED Cu AND A1

these twins were formed as a result of a decrease in the interfacial energy of grain boundaries that could not be achieved in the absence of twinning. This type of twin reaction is known as the growth accident model, which has been supported by Burke [30] and Carpenter et al. [31].

Models based on a reduction in grain boundary energy had been widely accepted as the general basis of twinning for some time, and were later advanced by Dash and Brown [33], Gleiter [34], and Goodhew [35] with some modifications. According to Dash and Brown [33], twins nucleated in the form of a small packet of stacking faults at migrating grain boundaries during primary recrystallization. The significance of this model was that twinning was a dislocation reaction where the partial dislocations responsible for the stacking faults were generated from the grain boundary at the expense of grain boundary energy. Supporting this model, Gleiter [33] proposed an atomistic model where twins were formed by the growth of { 111 } plane ledges. Meyers and Murr [42], realizing the importance of the dislocation reactions involved in twin formation, concluded that the driving force for annealing twin formation was the overall reduction in dislocation density and interfacial energy. These studies clearly showed that twinning was a recrystallization process which occurred not simply to satisfy certain interfacial energy requirements but to provide an easier reaction path for a rapid reduction in dislocation density as indicated by Form et al. [41].

In order to obtain better insight, in the present study, into the relationship between twinning and recrystallization, the number of twin boundaries was measured during the course of recrystallization.

450

Copper

T : 550"C ~ : 1 .0 / s

o ,o

E 400 _ / o

f, P restre i n

350 • 0.065 o.130

3 0 0 0.350 0.600 =_

250 ( ./o

200 /

150

~00 .~e ....~...-~ / j.•-

-5 5O

0 1 I I I I I 0 20 40 60 80 100

XA (*/ . )

Fig. 1 t. Relationship between static softening (XA) and twinning observed in Cu (37).

45O

E o 4 0 0

350 ,} ~ 3oo ._~

2 5 o -

g 2 0 0 -

~50

.J 5O

Copper

T = 5 5 0 " 0 , ~ = t . 0 / s

Prest.roin

• 0 .065 a 0 .150 • O. 350 o 0 .600

~ " o ~ e ~

I I I I I 0 2 0 4 0 6 0 8 0 1 0 0

X ( % )

Fig. 12. Relationship between fractional recrystallization (X) and twinning observed in Cu(37).

These data were then plotted against the fractional softening (as measured by compression tests) and recrystallization (as measured metallographically) as shown in Figs 11 and 12, respectively. The results in Figs 11 and 12 clearly demonstrated that twinning was the recrystallization process, where twins formed and grew at the expense of the stored energy of deformation, increasing their number as recrystallization progressed. This twinning process is possible in low stacking fault energy materials because a dissociation of grain boundaries into twin components is considered to be energetically more favorable as compared to the dissociation into random orientations [30-35]. Once twin segments nucleate, twins can grow by the migration of the incoherent twin segments into the deformed matrix by strain-induced grain boundary migration as suggested by Cahn [49] and Sample [88]. In this case, the driving force for such migration is the difference in the volume free energies across the boundary.

The data for the prestrain of 0.6 shown in Fig. 11 demonstrate that the number of twin boundaries reached a steady state value before softening was completed. This behavior confirmed the operation of multiple twinning when the remaining softening took place upon further holding. The multiple twinning process has been previously reported by many workers [43-47], and was considered to be a major mechanism for the recrystallization of Cu. According to Wilbrandt and Haasen [45], the main components of static recrystallization texture in Cu could be explained by the formation of first to fifth order twins to the orientation of the deformed state. Multiple twinning was also observed during dynamic recrystallization of Cu by Gottstein et al. [44]. According to them, the orientations of the recrystallized grains


emerged from high-order twin chains, which origin- ated in the orientation of the deformed material. This multiple twinning process accounts for the continuing release of the stored energy or the progress of softening after completion of apparent recrystallization.

5. CONCLUSIONS

1. The static softening curves of Cu and A1 exhibited a simple sigmoidal shape showing no inflections. The absence of inflections in the softening curves can be rationalized by the concurrent operation of recovery and recrystallization during high- temperature isothermal annealing of deformed metals and alloys.

2. The relative amount of fractional softening (or energy release) occurring prior to the initiation of recrystallization was small when compared with that occurring after the onset of recrystallization. The relative softening was also seen to be dependent on temperature, deformation, purity, and materials. Generally, the softening which occurred prior to recrystallization ~as smaller in purer materials with lower stacking fault energy deformed more severely at higher temperature.

3. The mechanical softening was correlated with the fractional recrystallization in a nonlinear fashion, the degree of which was dependent on the occurrence of recovery and dynamic recrystallization.

4. Recrystallization occurred preferentially in regions of high local strain. In AI, the pre-existing high angle grain boundary regions were the most favorable nucleation sites for recrystallization at relatively low deformation, and the nucleation of recrystallization appeared to take place by the strain induced migration of high angle grain boundaries. At high deformations, new grains were also formed on deformation bands by recrystallization which was nucleated by the subgrain growth mechanism.

5. Twinning was the major nucleation mechanism for recrystallization in Cu. Twins can form in low stacking fault energy materials by a dissociation of grain boundaries into twin components, which was considered to be energetically more favorable compared to the dissociation into random orientation. The growth of twins occurred by the migration of the incoherent twin segments into the deformed matrix at the expense of the stored energy of deformation.

6. The rate of static restoration increased with increasing applied prestrain. When the prestrain was greater than the critical strain for dynamic recrystallization, the increase was smaller because of the operation of metadynamic recrystallization immediately after deformation. It was also observed in this prestrain region that recrystallization was completed before the completion of static softening. In this case, the remaining softening occurred by multiple recrystallization where new grains formed on the already recrystallized grains.

7. Analysis of the results has shown that static restoration took place by the operation of four distinct mechanisms: recovery, classical recrystallization, metadynamic recrystallization and multiple recrystallization. A similar conclusion was reached by the present authors in a similar but separate study [89].

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On the recovery and recrystallization which attend static softening in hot-deformed copper and...

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