Dynamic recrystallization behavior of commercial pure aluminum

LI Hui-zhong(李慧中), ZHANG Xin-ming(张新明), CHEN Ming-an(陈明安), LIU Zi-juan(刘子娟)

School of Materials Science and Engineering, Central South University, Changsha 410083, China

Received 20 April 2006; accepted 30 June 2006

Abstract: The flow stress feature and microstructure evolvement of a commercial pure aluminum were investigated by compression on Gleeble-1500 dynamic materials test machine. Optical microscopy (OM) and transmission electron microscopy (TEM) were applied to analyze the deformation microstructure of the commercial pure aluminum.The results show that the flow stress tends to be constant after a peak value and the dynamic recovery occurs when the deformation temperatures is 220 ℃ with the strain rate of 0.01 s^?1; while the dynamic recrystallization occurs when the deformation temperature is higher than 380 ℃, and the flow stress exhibits a single peak at 460 ℃ with different strain rates from 0.001 s^?1 to 1 s^?1, and continuous dynamic recrystallization and geometric dynamic recrystallization occur during the hot compression of the commercial pure aluminum.

Key words: commercial pure aluminum; hot compression; dynamic recovery; dynamic recrystallization

1 Introduction

It’s commonly believed that the mechanism of dynamic restoration in pure aluminum during hot deformation is essentially dynamic recovery (DRV). This is because the high stacking fault energy of aluminum makes dislocation climb and cross slip easy, therefore the stored energy is too low to cause dynamic recrystallization(DRX). However, many recent results show that DRX may take place in high pure Al under certain conditions. The stacking fault energe is not the only decided element of dynamic restoration. As for high pure aluminum and aluminum alloy, three types of DRX are likely to occur during hot deformation:

1) Discontinuous dynamic recrystallization (DDRX), which is operated by repeating nucleation and limited grain growth. The dynamically recrystallized grains are coarse and distribute heterogemeously throughout the deformation matrix. Ture stress—strain curves of DDRX usually exhibit strong oscillation. DDRX usually occurs in metals with low stacking fault energy or high pure aluminum [4].

2) Continuous dynamic recrystallization(CDRX), which is proceeded by the progressive accumulation of dislocations in low angle boundaries, leading to an increase of their misorientation and the formation of high angle boundaries. The microstructure of CDRX consists of small grains, which distributes uniformly in materials. The stress—strain curve of CDRX shows a single peak[5].

3) Geometric dynamic recrystallization (GDRX), which is performed by the removal of initial grain boundaries, leading to fracture and the appearance of fine grains with high angle boundaries[6].

There are many reports on the hot deformation behavior of high pure aluminum and aluminum alloys home and abroad, but little on commercial pure aluminum’s dynamic recrystallization. Here, the flow stress feature as well as the microstructure evolvement of a commercial pure aluminum was studied by a hot compression test and the mechanism of the DRX was discussed in this work.

2 Experimental

The commercial pure aluminum used for this study was Al-0.11Fe-0.06Si (mass fraction). Compressive specimens with a diameter of 10 mm and a height of 15 mm were used . The hot compression tests were carried on Gleeble-1500 dynamic materials test machine. A graphite type lubricant was utilized to reduce the friction between the specimen and the loading plate. Hot com- pression tests were performed at various strain rates of 0.001, 0.01, 0.1 and 1 s^?1, and the temperature was selected as 220, 300, 380, 460 and 540 ℃. All the specimens were compressed to a true strain of 1.0. The true stress—strain curves were drawn immediately by computer with singals received by a strain-sensor during hot compression. For microscopic observation, the specimens were rapidly cooled to room temperature by water within 1 s after determination of the compression tests. The specimens were electro-polished and sub-sequently anodically oxidized to observe the microstructure under the POLYVER-MET polarized optical microscope. For TEM examination, the thin foils were prepared with standard twin-jet polishing technique.

3 Results and discussion

3.1 Results and analysis of true stress―strain curve

Figs.1(a) and (b) are the true stress―strain curves of the commercial pure aluminum with hot compression deformation.

Fig.1 True stress―strain curves of specimens compressed at various temperatures and strain rates: (a) 0.01 s^?1; (b) 460 ℃

Fig.1(a) shows the true stress―strain curves with the same strain rate. When amount of deformation is small, the true stress increases with the accumulation of dislocations produced by plastic deformation. When the deformation temperature is 220 ℃, the stress becomes smooth with the increase of the strain and goes up eventually. This can be explained as the temperature comes up, DRV occurs. As the temperature rises to   300 ℃, the true stress—strain curve shows steady flow feature. The hardening rate and softening rate caused by DRV which proceeds sufficiently are almost balanced at this time, so the true stress remains fixed. The true stress increases to peak quickly at first when specimens are deformed at 380, 460 and 540 ℃, and then descents gradually. This shows DRX has occurred when the temperature is high under the experimental strain rate scope.

From the true stress―strain curve of Fig.1(b), DRX appears in all specimens at different strain rates with the same temperature(460 ℃). Figs.1(a) and (b) show the true stress decreases more obviously after a peak value with the higher temperature and lower strain rate, that’s to say, DRX is more likely to occur when the temperature raises and the strain rate decreases. Deformation conditions, especially for strain rate and temperature, are the critical factor of the flow stress behavior and microstructure evolvement. The deformation condition can be summed up by the Zenner-Hollomon parameter (Z parameter)[8]:

where  is the strain rate; T is the deformation temperature; Q is the activation energy and R is the gas constant.

The physic meaning of Z parameter is the strain rate modified by temperature. The movement ability of atoms enhances with the temperature increasing and strain rate decreasing (Z parameter is smaller). The dislocation pinches off or regroups with each other under the same change, which leads to an increase of the dislocation density and the subgrains size. So DRX completes sufficiently and the flow stress descends.

3.2 Results and analysis of microstructure

Fig.2 shows the microstructure on different conditions. The grains vertical to the compressive axis elongate at 220℃ from Fig.2(a). The grain boundaries which have a low angle with compressive dimension appear to be dentate at 300℃ from Fig.2(b). There are no new grains on this two conditions, and therefore only DRV occurs. When the deformation temperature rises to 380 ℃, the grain boundaries become irregularly dentate and an obvious deformation zone turns up next to the dentate grain boundaries. It can be supposed that there may be some new fine grains along the deformation zone. The deformation microstructures containing new equiaxed grains shown in Figs.2(d), (e), (g), and (h) confirm the occurrence of DRX. Except those next to the initial grain boundaries, most new grains are blurry. It’s obvious that with the increase of temperature (540 ℃) the initial grains are filled with new fine grains and the

Fig.2 Microstructures of specimens after being compressed on different conditions: (a) 220 ℃, 0.01 s^?1; (b) 300 ℃, 0.01 s^?1; (c) 380 ℃, 0.01 s^?1; (d) 460 ℃, 0.01 s^?1; (e) 540 ℃, 0.01 s^?1; (f) 460 ℃, 1 s^?1; (g) 460 ℃, 0.1 s^?1; (h) 460 ℃, 0.001 s^?1

original boundaries almost can’t be seen. So it’s easy to get the idea that DRX occurs under the latter three conditions.

Grains extend along the direction vertical to the compressive axis at 460 ℃ with a strain rate of 1 s^?1 and the recrystallized grains are not clear. The reason may be the increase of the dislocation density which leads to the subgrains untimely growth. The original grains become flat with a strain rate of 0.1 s^?1. The grain boundaries go dentate and fine equiaxed grains begin to precipitate at this condition. DRX is completed in most areas with a strain rate of  0.01 s^?1.

3.2.1 Dynamic dynamic recovery

From Fig.1(a), it can be seen in the specimens compressed to a true strain of 1.0 at 220 ℃ with a strain rate of 0.01 s^?1 only gradual work hardening is observed, showing a typical DRV. During this process, cross slip of the screw dislocation and climb of the edge dislocation appear, as a result of the formation of substructures. A recovered structure of  polygonization at 220 ℃ with a strain rate of 0.01 s^?1 is seen in Fig.3(a). It can be seen messy dislocation net and thick grain wall made up of dislocation will counteract with opposite dislocation through cross slip and climb. The offset of dislocation makes the grain wall irregular and become subgrain eventually.

3.2.2 Dynamic recrystallization

The flow stress of material hot compressed at high temperature (≥380 ℃) with a low strain rate (≤1 s^?1) enhances quickly at the start of deformation and then decreases gradually with the increase of strain, therefore the softening rate is bigger than hardening rate in the deformation process. This phenomenon is coincident with the flow stress curve of other continuous dynamic recrystallized material.

Figs.2(c)?(h) show the obvious new grains in the extend area after deformation. Figs.3(b) and (c) are TEM microstructures of specimens compressed at 460 ℃ with strain rate of 0.01 s^?1. There are clear dislocation nets in the subgrains in Fig.3(b), which demonstrates the dislocation tangle changes into subgrains. However, Fig.3(c) shows that straight and clear subgrain boundaries has formed. Subgrains vanish and grow to new grains by combination of subgrains or removal of subgrain boundaries. The process which is proceeded by the progressive accumulation and recombination of dislocations and the formation of high angle boundaries is called CDRX.

Figs.2(e) and (h) show some fine grains with coarse boundaries formed a round the initial grain boundaries. The boundaries of these grains whose size is almost the same as that of the grains surrounded by low angle boun-

Fig.3 TEM microstructures of specimens after compressed at the strain rate of 0.01 s^?1: (a) 220 ℃; (b), (c) 460 ℃

daries evidently are from the bending or pinching off the original serration. From the phenomenon it can be imagined that a mechanism which is different from that CDRX occurs. And this mechanism which just takes place around some initial grain boundaries is inchoate. GDRX is firstly introduced by Mcqueen [9, 10] in pure Al. GDRX explains that, during deformation, the original grains are thinned, and their boundaries become serration while subgrains form. Then the surface of grain boundaries per volume increases quickly. Ultimately, when the original grain thickness is reduced to about two subgrain sizes, GDRX will occur by recombination of opposite boundaries of the thinned grain or by pinching off the serration and form fine grains with high-angle grain boundaries. GDRX usually happens when Z parameter is low (high temperature and low strain rate). However, the most helpful condition for it is enough strain under uneven deformation. The grain sizes of material used in this work are heterogeneous, but average size is large. With an inequable compression, new grains are formed at the port of original grains elongated severely by GDRX.

4 Conclusions

1) Under the same strain rate of 0.01 s^?1, the true stress―strain curve tends to be constant after a peak value and the dynamic recovery occurs when the deformation temperatures are lower (220 ℃), while the deformation dynamic recrystallization occurs when the temperatures are higher (≥380 ℃).

2) At the same deformation temperature of 460 ℃, DRX occurs in commercial pure Al with different strain rates and the true stress―strain curve exhibits a single peak.

3) Both CDRX and GDRX can take place in commercial pure Al, but CDRX is primary. CDRX occurs when dynamic restoration level is not high. GDRX occurs when the original high-angel grain boundaries become serration at first, and then pinch off the initial grains. The new fine grains of GDRX have the same microstructure and orientation with the original grains.

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(Edited by CHEN Can-hua)

Foundation item: Project(2005CB623706) supported by the State Key Fundamental Research Program of China

Corresponding author: LI Hui-zhong; Tel: +86-731-8877949; E-mail: lhz606@mail.csu.edu.cn