Annealing Behavior of Solution Treated and Aged

Al-0.2wt% Sc Deformed by ARB

Ehsan Borhani1, a, Hamidreza Jafarian1, b, Hiroki Adachi1, c, Daisuke Terada1,d

and Nobuhiro Tsuji1,e 1 Department of Materials Science and Engineering, Kyoto University, Yoshida Hommachi,

Sakyo-Ku, Kyoto 606-8501, Japan.

a ehsan.borhani@ks7.ecs.kyoto-u.ac.jp,b hamidreza.jafarian@ky2.ecs.kyoto-u.ac.jp,

c hiroki.adachi@materials.mbox.media.kyoto-u.ac.jp, d daisuke.terada@ky7.ecs.kyoto-u.ac.jp,

e nobuhiro.tsuji@ky5.ecs.kyoto-u.ac.jp

Keywords: Recrystallization, Al-Sc, precipitation, ARB.

Abstract. The effect of prior and subsequent precipitation on recrystallization behavior during the

isothermal annealing of an Al-0.2%wt.Sc alloy heavily deformed by accumulative roll bonding

(ARB) up to 10 cycles at ambient temperature was investigated. Three kind of different

microstructures, i.e., solution treated one, 300°C pre-aged one and 400°C pre-aged one, were

prepared as the starting structures for the ARB process. It is found that precipitation pinning effect

of Al3Sc suppresses recrystallization and especially the 400°C pre-aging was effective to stabilize

the ultrafine grained structure of the matrix. Dissolution of pre-aging Al3Sc precipitates was

suggested by re-precipitation during annealing of the ARB processed specimens at around 300°C.

Introduction

Microstructural restoration processes of deformed materials, like recovery and recrystallization,

have significant impact on the mechanical properties of the materials. Therefore, these softening

processes are of great scientific and technological interest in particular with regard to thermo-

mechanical processing [1]. In some alloys, a complicated situation may arise in which the processes

of recrystallization and precipitation occur concurrently. If recrystallization of a deformed

supersaturated solid solution is not completed before the start of precipitation, the material may

exhibit complex behaviors due to the mutual interaction of recrystallization and precipitation [2,3].

On the other hand, pre-aged deformed microstructure may affect the kinetics of the recrystallization,

and consequently result in different behavior. Especially, annealing behaviors of such alloys after

severe plastic deformation have been scarcely studied [4,5].

The purpose of the present work is to investigate the interaction between recrystallization and

precipitates in an ARB (accumulative roll bonding) processed binary Al-Sc alloy during annealing.

In Al-Sc alloys, fine and thermally stable Al3Sc precipitates, which significantly inhibits recovery

and recrystallization of the matrix. In this study, three different starting microstructures were

prepared. They were heavily deformed by ARB process, and then the annealing behaviors were

studied.

Experiments

An Al-0.2 wt% Sc alloy was prepared as sheets with thickness of 2mm, width of 60 mm and length

of 200 mm. The sheets were firstly solution treated (ST) at 913 K for 86.4 ks and immediately

water-quenched. The average grain size of the ST sheets was 0.5 mm. Some ST sheets were aged at

573 K and 673 K for 10ks, to have different sizes of Al3Sc precipitates, which are denoted as Aged-

Materials Science Forum Vols. 667-669 (2011) pp 211-216© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.667-669.211

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sheets. The specimen pre-aged at 300°C had fine Al3Sc precipitates with size of 3.62 nm in

diameter, while that pre-aged at 400°C had relatively coarse precipitates with sizes of about 50 nm

in diameter. These three types of sheets were used as the starting materials for ARB. The starting

sheets with thickness of 2 mm were firstly cold-rolled by 50% reduction in thickness. This

procedure is considered as the first ARB cycle. The 50% rolled sheets 1mm thick were cut into two,

degreased and wire-brushed the surfaces, and then roll-bonded by 50% reduction in one pass at

ambient temperature with lubrication. This is considered as the 2nd ARB cycle. The same

procedures were repeated up to 10 cycles including the first cold-rolling, which corresponded to the

total equivalent strain of 8. Hereafter, the ST- and Aged-sheets ARB processed are denoted as ST-

ARB, and Aged-ARB specimens, respectively. Deformed specimens were annealed in a salt bath

set at different temperatures from 200°C up to 600°C for 1.8ks. All samples were water quenched

after annealing. Electron backscattering diffraction (EBSD) analysis was carried out in a scanning

electron microscope with field emission type gun (FE-SEM; Philips XL30) operated at 15 KV. The

specimens were mechanically and then electro-polished before the measurements. Shimadzu HMW

micro hardness tester was used for Vickers hardness measurement on the planes perpendicular to

the normal direction (ND) of the Aged-ARB and ST-ARB sheets using a load of 0.098 kgf and a

time period of 10sec at room temperature.

Result and discussion

The effects of precipitation on recrystallization were investigated by annealing the ST- and Aged-

ARB materials. The ST-ARB and Aged-ARB alloys revealed well-defined lamellar boundary

structures after ARB process. The average interval of the lamellar boundaries was 290 nm. These

microstructures were typical in ARB processed aluminum alloys [6]. There remained many

coherent precipitates for 400°C Aged-ARB specimen, while most of precipitates lost their

coherency in 300°C pre-aged specimens during ARB process[7]. This affected the change in

hardness in the 10-cycle ARB process (Fig. 1) [7]. To investigate the effect of annealing

temperature, the Vickers microhardness was measured and plotted in Fig. 1. Three kinds of

specimens showed similar changes in hardness during annealing, though the 400°C Aged-ARB

specimens always showed higher hardness than others. It should be noted that all specimens

exhibited the increase in hardness by low temperature annealing and showed the peaks at 300°C.

Fig. 1 Vickers hardness of the ST-ARB and Aged-ARB specimens as a function of annealing

temperature.

The microstructural change during annealing of the three different specimens alloys were

observed using EBSD. All the specimens annealed at low temperatures (200-300°C) did not show

any traces of recrystallization and kept the same microstructures as those in the as-ARB processed

specimens. On the other hand, all specimens showed more equiaxed microstructures after annealing

212 Nanomaterials by Severe Plastic Deformation: NanoSPD5

400°C (Fig. 2). However, some grains are still elongated and there are still many low-angle

boundaries.

Fig. 2 EBSD microstructures of (a) ST-ARB (b) 400°C Aged-ARB and 300°C Aged-ARB

specimens annealed at 400°C for 1.8ks.

At temperature range of 450–550 °C, the recrystallized microstructures were found to be very

heterogeneous, as shown in Fig. 3 and Fig. 4. It was found that the ST-ARB and 300°C Aged-ARB

materials showed abnormal grain growth at 450°C annealing temperature, while the 400°C Aged-

ARB specimen did not commence abnormal grain growth until 550 °C.

Fig. 3 EBSD microstructures of (a) ST-ARB (b) 400°C Aged-ARB (c) 300°C Aged-ARB

specimens annealed at 450°C for 1.8ks.

Fig. 4 EBSD microstructure of the 400°C Aged-ARB specimen annealed at 550°C for 1.8ks.

Compared with the pre-aged alloy, recrystallization in the ST- ARB alloy was less inhibited by

precipitation. It can be seen that the ST- ARB specimen has bigger grain size after annealing at

450°C, probably due to less presence of precipitates than the Aged-ARB materials. The 400°C

Aged-ARB specimen still displays a fine structure even after annealing at 500°C, though two other

starting materials are mostly recrystallized at this stage, as shown in Fig. 5.

(c)

(a) (c) (b)

(b) (a)

10µm 10µm

20µm 20µm 20µm

50µm

LABs 2° ≤θ<15° HABs 15°≤θ

RD

ND 10µm

Materials Science Forum Vols. 667-669 213

Fig. 5 EBSD microstructures of (a) ST-ARB (b) 400°C Aged-ARB (c) 300°C Aged-ARB

specimens annealed at 500°C for 1.8ks.

At higher annealing temperatures like 600°C, more equiaxed microstructures evolved as shown

in Fig. 6, which this is consistent with recrystallization being completed before the occurrence of

significant precipitation [8]. Half hour annealing at 600°C resulted in complete recrystallization in

all these specimens.

Fig. 6 EBSD microstructures of (a) ST-ARB (b) 400°C Aged-ARB (c) 300°C Aged-ARB

specimens annealing at 600°C for 1.8ks.

The data of average grain size (dt) were finally plotted as a function of annealing temperature in

Fig. 7, in order to investigate how the dt was affected by dispersoids during annealing. The grain

size of all specimens greatly increased above 400°C, though the changes are different depending on

the starting microstructures. At 500°C, the 400°C Aged-ARB specimen showed the smallest grain

size, while the 300°C Aged- ARB specimen showed the smallest grain size at 600°C. At 600°C,

existing Al3Sc would dissolve during annealing, because the temperature is above the solubility

limit of Al-0.2%wt.Sc. As a result, the different in grain size in the 600°C specimens is not so large.

Below 600°C (400°C ~500°C), pre-existing precipitates would affect the recrystallization behaviors,

and it is noteworthy that the pre-aged specimens tend to inhibit grain growth compared with the ST-

ARB specimen.

Fig.7 Grain size of the ST-ARB and Aged-ARB specimens as a function of annealing temperature.

(a) (b) (c)

(a) (b) (c)

100µm 100µm 100µm

50µm 50µm 50µm

214 Nanomaterials by Severe Plastic Deformation: NanoSPD5

As was shown in Fig. 1, all specimens showed an increase in hardness around 300°C annealing

temperature. This is probably caused by precipitation of new Sc-containing dispersoids upon heat

treatment at these temperatures (~300°C). It is reasonable that the ST-ARB specimen showed new

precipitation at around 300°C, but it seems strange in the Aged-ARB specimen, since pre-aging

treatment was already carried out before ARB in specimens. This suggests that the Al3Sc

precipitates in the Age-ARB specimen partly dissolve during the ARB and re-precipitate during

subsequent annealing at 300°C. It is also noteworthy, anyway, that the fine Al3Sc precipitates

greatly inhibit recovery and recrystallization and keep the grain sizes very fine up to 400°C ~

450°C ( Fig. 2 and 3). Above 300°C, the decrease in hardness value observed corresponds with

recrystallization or abnormal grain growth (Fig. 3 and 4).

Fig. 8 is a TEM micrograph of the 400°C Aged-ARB specimen after annealing at 500°C. Fine

precipitates actually pin the grain boundary migration, and some dislocations are still pinned to

remain within the grains. It is interesting that the 400°C pre-aging was more affective to stabilize

the matrix that 300°C pre-aging, though the detailed mechanism is unclear at this moment.

Fig.8 TEM micrograph of the 400°C Aged-ARB specimen annealed at 500°C for 1.8ks.

Conclusions

The annealing behaviors of an Al-0.2%wt.Sc alloy heavily deformed by ARB process have been

studied using three kinds of different starting microstructures. Following conclusions can be made:

- During low temperature annealing (< 300°C) of the ARB processed samples, alloys displayed

elongated structures without any signs of recrystallization.

- The microstructure and consequently the hardness of the specimens are controlled by various

factors including recrystallization, grain growth and re-precipitation at around 300°C in a

complicated manner.

- 400°C Aged-ARB specimen displayed a remarkable recrystallization resistance and ultrafine

grains were maintained even after 400°C annealing for 1.8ks.

Acknowledgment

This study was financially supported by the Grant-in-Aid for Scientific Research on Innovative

Area, “Bulk Nanostructured Metals”, through MEXT, Japan, and the support is deeply appreciated.

Materials Science Forum Vols. 667-669 215

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216 Nanomaterials by Severe Plastic Deformation: NanoSPD5