Tensile deformation characteristics of a nano-structured 5083 Al alloy

Journal of Alloys and Compounds 386 (2005) 197–201

Tensile deformation characteristics of a nano-structured 5083 Al alloy Si-Young Chang a,∗ , Byong-Du Ahn b , Sung-Kil Hong c , Shigeharu Kamado d , Yo Kojima d , Dong Hyuk Shin b a

Department of Materials Engineering, Hankuk Aviation University, 200-1 Hwajon-dong, Koyang-shi, Kyunggi-do 412-791, South Korea b Department of Metallurgy and Materials Science, Hanyang University, Ansan, Kyunggi-do 425-791, South Korea c Department of Metallurgical Engineering, Chonnam National University, Kwangju 500-757, South Korea d Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-2188, Japan Received 22 September 2003; received in revised form 3 March 2004; accepted 29 March 2004

Abstract The microstructure and tensile deformation characteristics in the commercial 5083 Al alloy after equal channel angular pressing (ECAP) at 373 and 473 K were investigated and compared with those of one reannealed at 473 K for 1 h after ECAP at 373 K. All specimens revealed nano-structure with grains of ∼300 nm and no distinct grain boundaries except the reannealed alloy. The strength in the alloy pressed at 373 K was much higher than that of as-received alloy but the elongation to failure was significantly smaller, while, in case of pressing at 473 K, the improvement in strength was found concurrent with slight work hardening, without sacrificing much of the elongation. The tensile properties of the reannealed alloy were in between those of ones pressed at 373 and 473 K. These tensile deformation characteristics were analyzed based on the observations of deformed microstructure and fracture surface by TEM and FE-SEM, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: 5083 Al alloy; Equal channel angular pressing; Nano-structure; Tensile properties; Fracture surface

1. Introduction A number of research studies have been recently attempted to obtain the nano-structure in metallic materials by imposing severe plastic deformation (SPD). Among SPD techniques, particularly, the equal channel angular pressing (ECAP) has been widely used to produce the bulk nano-crystalline materials without residual porosity and volume change [1,2]. Such nano-structured metallic materials are expected for high performance applications because of their excellent strengths exceeding those of conventional coarse-grained metallic metals. For example, nano-structured low carbon steel has strengths in excess of 900 MPa, which is three times higher than that of coarse-grained low carbon steel [3], and pure nano-structured Cu also shows six times higher yield stress compared to pure coarse-grained Cu [4,5]. However, nano-structured metallic materials often exhibit low tensile ductility at room temperature, which limits their

∗

Corresponding author. Tel.: +82-2-300-0168; fax: +82-2-3158-3770. E-mail address: [email protected] (S.-Y. Chang).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.148

practical application. Therefore, it is important to attain to at least the elongation comparable to that of conventional coarse-grained metallic materials, with satisfying strength, in the nano-structured metallic materials. Recently, Wang et al. [6] reported that a marked improvement in uniform elongation was found concurrent with pronounced strain hardening, without sacrificing much of strength, in pure Cu with the bimodal microstructure consisting of micrometer-sized grains embedded inside a matrix of nano-sized grains. However, there is still a lack of reports on the high ductility concurrent with high strength at room temperature of Al alloys produced by ECAP. Among Al alloys, the 5083 Al alloy is a representative non-age-hardenable Al–Mg based alloy which possesses many interesting characteristics as a structural material, such as low price, moderately high strength, good corrosion resistance, high formability in conjunction with superplasticity, etc. [7,8]. These advantages of the alloy are quite attractive in the automobile industry for producing vehicles with high fuel efficiency by replacing steels with the alloy as a body sheet material. In particular, the superplastic property of the alloy provides cost-effectiveness by minimizing the processing steps such as stamping, machin-

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ing, joining, etc. In our previous investigation, therefore, a nano-sized 5083 Al alloy was processed by ECAP under a certain condition and the feasibility of superplasticity at relatively low temperature was confirmed [9]. Next, our strategy is to efficiently use the moderate conditions of pressing temperature and subsequent annealing to achieve much higher strength, without sacrificing the ductility, than which would be expected in the conventional alloy at room temperature. In the present study, therefore, the aim is to manifest tensile deformation characteristics of the alloy processed by taking advantage of the moderate conditions, based on the observations of deformed microstructure and fracture surface. Fig. 2. TEM micrograph and corresponding SAED pattern of 5083 Al alloy after eight ECAPs at 373 K followed by annealing at 473 K for 1 h.

2. Experimental procedure A commercial 5083 Al alloy (Al–4.4%Mg–0.7%Mn– 0.15%Cr (in mass %)) was supplied in the form of an extruded bar. Cylindrical samples of φ 18 mm × 130 mm were prepared from the extruded bar, as starting materials for ECAP, followed by annealing at 723 K for 1 h. The starting materials showed the average grain size of approximately 200 ␮m. The ECAP of eight passes was carried out using route C and a press speed of 2 mm s−1 at 373 and 473 K that selected for the restraint of recrystallization and much formation of high angle grain boundaries during ECAP [9], respectively. In addition, some samples were annealed at 473 K for 1 h after ECAP at 373 K. Much information for ECAP adopted in this study has been listed in previous reports [9–13]. Tensile tests were carried out using the tensile specimens with 25 mm in gauge length at the initial strain rate of 1.00 × 10−3 s−1 at room temperature. The initial microstructures and tensile deformed microstructures of as ECA pressed samples were examined with a transmission electron microscope (TEM) utilizing a JEOL 2010 operated at 200 kV and the fractured surfaces after tensile test were also observed with a field emission scanning electron microscope (FE-SEM).

3. Results 3.1. Microstructures of as-pressed and as-reannealed 5083 Al alloys Fig. 1 represents TEM micrographs showing the microstructure evolution of the as-pressed 5083 Al alloy, together with their corresponding SAED patterns. Near equiaxed nano-sized grains of ∼300 nm were introduced by eight ECAPs at 373 and 473 K, although the grain size at 373 K was slightly smaller than that at 473 K. TEM micrograph and its corresponding SAED pattern of the alloy ECA pressed at 373 K when compared with those of one ECA pressed at 473 K showed that only a few grain boundaries were well-defined and less diffused spots were observed, which indicates less formation of a high angle boundary. After annealing at 473 K for 1 h, recovery has occurred, and the dislocation density was much reduced as shown in Fig. 2. And also, the recrystallized grains had well-defined, high angle boundaries. It has been previously known that the unclear grain boundaries in severely plastic-deformed metallic materials include many facets and steps of regular

Fig. 1. TEM micrographs and corresponding SAED patterns of 5083 Al alloy after eight ECAPs at 373 K (a) and 473 K (b).

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Fig. 3. Tensile stress–strain curves for 5083 Al alloy pressed at 373 K (a) and 473 K (b).

or irregular alignment, and the lattices near grain boundaries are severely distorted [14]. In addition, the extrinsic dislocation density is high in the grain boundaries, leading to the drastic increase of grain boundary energy. Such grains with non-equilibrium grain boundaries could have pronounced experience of not only the dynamic recovery and recrystallization during ECAP at higher temperature but also the static recovery and recrystallization during subsequent annealing as shown in Figs. 1 and 2, respectively. 3.2. Tensile behavior of as-pressed and as-reannealed 5083 Al alloys Typical true stress–strain curves obtained for as-pressed 5083 Al alloys are shown in Fig. 3. It was found that much improvement in strength was obtained at lower pressing temperature 373 K, than 473 K. This results from that the use of lower temperature suppresses dynamic recovery, allowing the density of the accumulated dislocations to reach a higher stable state than that achievable at 473 K [15,16]. The strength in the alloy pressed at 373 K was much higher than that of as-received alloy but the elongation to failure was significantly smaller, with losing the work hardening. Such a trend of strengthening accompanied by a loss of ductility is generally true for low carbon steel [3,17] and other metals [4,5,18] processed in various SPD methods. In addition, it has been recently known [6] that the ultra-fine grains tend to lose the work hardening on deformation owing to their very low dislocation storage efficiency inside the tiny grains. Such a material is therefore prone to unstably plastic-deform, limiting the desirable uniform elongation. However, when in case of pressing at 473 K, the improvement in strength was found without sacrificing much of elongation. In addition, the alloy pressed at 473 K exhibited small strain hardening with satisfying the elongation unlike that pressed at 373 K. These results are quite different from general phenomena that occur in the ultra-fine grained low carbon steel and other

metals [2–6,17,18]. The tensile properties of the reannealed alloy, as observed for tensile stress–strain curves shown in Fig. 4, were in between those of ones pressed at 373 and 473 K. However, the stress–strain curves represented similar shape to those shown at 373 K rather than those shown at 473 K. 3.3. Tensile deformed microstructures TEM micrographs of the deformed microstructure in the vicinity of fracture surface in the alloy pressed at 373 and 473 K are shown in Fig. 5. In addition, FE-SEM images showing fracture surfaces are exhibited in Fig. 6. There were still less-defined grain boundaries in the alloy pressed at 373 K. The elongated grains were observed along the tensile direction, resulting from less formation of high angle grain boundary. The fracture surface exhibited relatively brittle fracture. These results indicate that the alloy experiences the

Fig. 4. Tensile stress–strain curves for 5083 Al alloy pressed at 373 K followed by annealing at 473 K for 1 h, compared with those for 5083 Al alloy pressed at 373 K.

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Fig. 5. TEM micrographs showing the tensile deformed microstructure in the vicinity of fracture surface of 5083 Al alloy after eight ECAPs at 373 K (a) and 473 K (b).

Fig. 6. FE-SEM images showing the tensile fractured surfaces of 5083 Al alloy after eight ECAPs at 373 K (a) and 473 K (b).

undesirable uniform deformation, leading to low elongation. In contrast, as observed for the alloy pressed at 473 K, the nano-sized grains maintained their initial shape to failure and more dimpled ductile structure was observed as shown in Figs. 5b and 6b, respectively, indicating that the overall deformation occurs uniformly. Consequently, this leads to a high tensile ductility—over 20%—with satisfying strength through strain hardening, which is comparable to that of as-received 5083 Al alloy.

Fig. 7a and b represent TEM micrograph showing the deformed microstructure and fracture surface in the reannealed alloy after ECAP at 373 K, respectively. The size and shape of grains maintained to failure and the tensile deformed microstructure was similar to that of one pressed 473 K. In the observation of fracture surface, the cracks were observed in the site that might be grain boundaries. The deformation of nano-sized alloy with high angle boundaries generally depends on grain boundary deformation mechanisms such as

Fig. 7. (a) TEM micrograph showing the tensile deformed microstructure and (b) FE-SEM image showing the tensile fractured surface of 5083 Al alloy after eight ECAPs at 373 K followed by annealing at 473 K for 1 h.

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grain boundary sliding. According to recent reports [19–21], the occurrence of grain boundary sliding at room temperature was confirmed in materials with nano-crystalline grains and even Mg alloys with micrometer-sized grains. However, the grain boundary sliding is insufficient to compensate for the loss of ductility shown in high strength nano-sized materials, because the stress concentration can occur in such high angle grain boundaries acting as the obstacle to disturb the movement of dislocation during deformation, leading to the occurrence of cracks as shown in Fig. 7b. Accordingly, this results in the lower elongation in the reannealed alloy than one pressed at 473 K.

4. Summary The nano-structure of ∼300 nm in grain size was introduced in the commercial 5083 Al alloy by eight ECAPs at 373 and 473 K. The alloy pressed at 373 K showed less distinct grain boundaries than one pressed at 473 K. However, the grain boundaries were clearly defined by reannealing at 473 K for 1 h. It was found that much improvement in strength was obtained at lower pressing temperature 373 K, than 473 K. The strength in the alloy pressed at 373 K was much higher than that of as-received alloy but the elongation to failure was significantly smaller, while, in case of pressing at 473 K, the improvement in strength was found without sacrificing much of the elongation. In addition, the work hardening was found in the alloy pressed at 473 K unlike the ultra-fine grained low carbon steel and Al alloys. These were considered to be due to the deformed microstructure and fracture surface in the alloy pressed at 373 K that revealed the elongated grains along the tensile direction, leading to low elongation, while, at 473 K, the nano-sized grains maintained their initial shape to failure, indicating that the overall deformation occurs uniformly. The reannealed alloy revealed different tensile behavior. The tensile properties were in between those of ones pressed at 373 and 473 K. The tensile deformed microstructure was similar to that of one pressed at 473 K rather than one pressed at 373 K. However, the fracture surface showed cracks occurring in the grain boundaries, leading to lower elongation than one pressed at 473 K.

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Acknowledgements This work was supported by Korea Ministry of Science and Technology through ‘the 21st Century New Frontier Research and Development Program’, and by Korea Institute of Industrial Technology and Kwangju Metropolitan City through ‘the Advanced Elements and Materials Industry Development Program’.

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