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Microstructure and tensile properties of a commercial 5052 aluminum alloy processed by equal channel angular extrusion
Materials Science and Engineering A342 (2003) 144 /151 www.elsevier.com/locate/msea
Microstructure and tensile properties of a commercial 5052 aluminum alloy processed by equal channel angular extrusion T.L. Tsai, P.L. Sun, P.W. Kao *, C.P. Chang Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan Received 25 January 2002; received in revised form 22 April 2002
Abstract A commercial 5052 aluminum alloy was processed by equal channel angular extrusion at 423 K to a strain of /8. The asextruded microstructure is quite uniform, which can be characterized as a mixture of elongated and equiaxed subgrains with submicrometer size and high misorientations. In static annealing, the as-extruded structure is quite stable at 423 K and shows slow coarsening at 473 K. The microstructure resulted from more ECAE passes (higher strain) is more readily to develop a uniform structure of equiaxed grains in static annealing. Very high strength (394 MPa yield stress and 421 MPa ultimate tensile strength) and reasonable ductility (10.5% tensile elongation) can be obtained in this non-heat treatable aluminum alloy. The high strength can be related to the structure of submicron-sized subgrains (grains). To increase the strain from 4 to 8 is beneficial for producing finer structure and higher strength. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Tensile properties; 5052 Aluminum alloy; Equal channel angular extrusion
1. Introduction It has been shown that ultrafine grain structure may be introduced into metals through the use of equal channel angular extrusion (ECAE) [1,2]. In ECAE, a sample is pressed through a die in which two channels, equal in cross section, intersect at an angle of F, with an additional angle of C defining the arc of curvature at the outer point of the intersection of the two channels. By the use of ECAE, it is possible to repeat this process to many cycles to accumulate large plastic strain. ECAE has attracted much attention recently because it offers the potential to produce bulk samples with a very fine structure. Berbon and Langdon [3] summarized the important parameters in ECAE. Among them are the die angle, which determines the strain introduced in the material for each deformation pass, and the number of the passes through the die, which corresponds to the total accu-
* Corresponding author E-mail address: [email protected] (P.W. Kao).
mulated strain applied to the billet. The deformation route, which involves rotating the billet between each pass, is another critical parameter in texture and microstructure development. In addition, the pressing speed, the deformation temperature, and the friction between the die walls and the billet are also important parameters. By the use of ECAE at room temperature, ultrafine grain structure of micrometer size can be obtained in 99.99% pure Al [4] and in commercial pure 1050 Al [5], SMG structure can be achieved. In both cases, the resulted ultrafine grain structure has been shown to have large misorientations. Apparently the achieved grain size is affected by the presence of the impurity atoms. With detailed TEM observations, Chang et al. [6] showed the evolution of boundary structure from dislocation wall to grain boundary with increasing strain in ECAE. In a systematic study of pure Al and Al /Mg alloys, Iwahashi et al. [7] indicates that the addition of magnesium decreases the rate of recovery, which leads both to an increase in the number of passes required to establish a homogeneous microstructure and to a decrease in the ultimate equiaxed grain size. In addition,
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Fig. 1. Microstructure of as-extruded state (B8) revealed by TEM (a) banded structure containing elongated subgrains, (b) mixture of elongated and equiaxed subgrains, and (c) SAD pattern from an area of 6 mm in diameter.
it has been shown by static annealing that the ultrafine grain structure in pure Al and Al /Mg alloys is not stable at elevated temperatures, and a minor addition of Zr to aluminum can improve the thermal stability of the ultrafine grain structure to a temperature approaching 600 K [8]. Yamashita et al. recently have reported the influence of deformation temperature on microstructural development in ECAE [9]. Two important effects are reported. The grain size tends to increase with increasing deformation temperature. Grain refinement is achieved at all temperatures but there is a transition from arrays of high angle boundaries at the lower temperatures to low angle boundaries at high temperatures. The transition temperature depends on the alloy composition. It is
473 K in pure Al and 573 K in an Al /3% Mg alloy, whereas an Al /3% Mg /0.2%Sc alloy exhibits high angle boundaries at all temperatures to 573 K. According to the literature mentioned before, the addition of Mg to Al is beneficial for grain refinement and the presence of small amounts of transition metals is very effective in stabilizing the ultrafine grain structure. The major alloying elements in commercial 5052 aluminum alloy are 2.5 wt.% Mg and 0.25 wt.% Cr with other impurity atoms such as Mn, Fe and Si. It is a potential candidate to achieve submicrometer grain (SMG) structure by the application of ECAE. In the present work, the microstructure and tensile properties of a commercial 5052 aluminum alloy processed by ECAE to achieve SMG structure were investigated.
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Fig. 2. The variation of hardness as a function of annealing temperature indicating the softening behavior of ECAE processed 5052 alloy in static annealing.
2. Experimental procedure A commercial 5052 aluminum alloy produced by China Steel Aluminum Corporation was used. The chemical compositions (in wt.%) of this alloy are 2.57 Mg, 0.22 Cr, 0.26 Fe, 0.08 Si, 0.05 Mn and bal. Al. The material was annealed at 838 K for 12 h to give a grain size of about 200 mm. Specimens of 12 /12 /80 mm were subjected to ECAE for four or eight passes at 423 K. The die for ECAE consists of two channels with square cross-section intersecting at an angle F /908 and with an outer arc of angle C /208. The von Mises equivalent strain on pressing through the die was calculated to be 1.04 [10]. In the ECAE process, the die was heated and maintained at 423 K, the sample was coated with a lubricant containing MoS2, and the strain rate was estimated to be about 10 2 s 1. After each pass, the sample was quenched in water immediately. The approximate time for sample staying in the heated die is within 1000 s. Between each pass, the specimen was rotated 908 clockwise about the long axis, i.e. route Bc [11]. Microhardness tests were used to follow the softening behavior of the as-extruded material after various static annealing treatments. The tests were carried out with 200 g load and 15 s holding time on a Shimadzu HMV2000 microhardness tester. Transmission electron microscope (TEM) was used to examine the microstruc-
Fig. 3. The results of differential scanning calorimetry (heating rate: 10 K min 1), where (a) is for sample B8 and (b) is for sample B4.
ture. Specimens were cut from the flow plane of the extruded material, which is parallel to the extrusion direction. Thin foils prepared by twin-jet electropolishing were examined by TEM (Philips CM200) at 200 kV. In addition, the as-extruded samples were also analyzed by differential scanning calorimetry (DSC) using a
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Fig. 4. The change of hardness during isothermal annealing at 473 K.
SETARAM DSC 131 calorimeter. The DSC analysis was carried out in an argon atmosphere with a heating rate of 10 8C min 1. Tensile specimens with 14 mm gauge length and 3 mm dia. were tested with an Instron 5582 universal testing machine. The tensile tests were carried at room temperature with an initial strain rate of 7 /10 4 s 1.
3. Results and discussion 3.1. As-extruded microstructure The alloy was extruded at 423 K with route Bc for a total of four or eight passes, which will be denoted as B4 or B8, respectively. Generally speaking, the as-extruded samples, B4 or B8, exhibit the typical cold-worked structure after large strain, i.e. subgrains (grains) with high dislocation densities in the interior. Because the boundaries in structures resulted from large strain are often mixtures of low angle and high angle boundaries, the use of ‘subgrains’ or ‘grains’ to describe the structural feature becomes a problem. In the following text, the term ‘grains’ will be reserved only for those ‘dislocation free’ areas with large misorientation, which are of polyhedral shape and completely enclosed by sharp boundaries. The predominant feature in both samples B4 and B8 are elongated subgrains forming banded structure with some equiaxed subgrains. Fig. 1 shows the typical microstructure of the as-extruded state; banded structure of elongated subgrains as shown in Fig. 1(a), and mixture of elongated and equiaxed subgrains as shown in Fig. 1(b). The SAD pattern, which was taken from an area having a diameter of 6 mm, contains well-defined rings demonstrating the sub-
grains with a large spread in orientations. The most noticeable differences between B4 and B8 are that B8 has a finer structure and more equiaxed subgrains. 3.2. Thermal stability of the microstructure The extruded samples were subjected to isochronal annealing, in which the holding time at each temperature is 1 h. Fig. 2 shows the variation of microhardness as a function of annealing temperature, in which each data point is an average of ten measurements. The B4 sample shows a slight softening below 473 K, which may be related to recovery, and a typical sigmoidal curve with significant softening at temperatures higher than 473 K. For sample B8, considerable softening already takes place between 423 and 473 K with more drastic softening above 473 K. However, both samples exhibit similar hardness after being annealed at 673 K. For both samples, annealing at 673 K for 1 h results in a fully recrystallized structure. The results of DSC are shown in Fig. 3, that each sample has single exothermic peak, which is related to recrystallization. The exothermic peak starts at a temperature 25 K lower for the sample B8 than for the sample B4. By comparing with B4, the softening of B8 starts at a lower temperature (423 /473 K in Fig. 2) and may be related to the phenomenon that the recrystallization starts at lower temperature. It is known that the fraction of high angle boundaries in deformed structure increases significantly with increasing strain at low temperature [12,13]. The sample B8 is expected to have more high angle boundaries. It is believed that these high angle boundaries are more ready to migrate during annealing; as a consequence, the recrystallization in B8 may take place at a lower temperature.
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Fig. 5. TEM micrographs of the sample B4 annealed at 473 K for (a) 1 h and (b) 6 h.
In addition, the thermal stability of the as-extruded structure was also tested isothermally at 423 and 473 K. It was followed by microhardness tests after different holding time at the temperatures. Both samples have similar behavior. At 423 K, the hardness shows little change for a period of 8 h. However, at 473 K, the hardness drops slightly in the first 2 h and shows little change over the period of 2 /8 h as shown in Fig. 4. This may imply that the as-extruded structure is quite stable at 423 K and coarsens slowly at 473 K. It is also of interest to examine the microstructure and tensile properties for samples annealed to a state just before the onset of significant softening. The samples annealed at 473 K for various times were examined by the use of TEM. The TEM specimens were cut from the flow plane, which is parallel to the extrusion direction. In general, as the annealing time increases, the fraction of equiaxed subgrains increases with the expense of the
Fig. 6. TEM micrographs of the sample B8 after annealing: (a) annealed at 473 K for 1 h showing mixture of elongated and equiaxed subgrains, and (b) annealed at 473 K for 6 h showing equiaxed grains free of dislocations.
elongated subgrains and the dimension of subgrains grows. For the same annealing treatment, the sample B8 has smaller subgrain size and higher fraction of equiaxed subgrains than the sample B4 does. Typical TEM micrographs are shown in Figs. 5 and 6. The characteristics of the microstructures for both as-extruded and annealed states are summarized in Table 1. It is quite interesting that after annealing at 473 K (0.51 Tm) for 6 h, the sample B8 has developed a structure of equiaxed ‘grains’ free of dislocations, which are delineated by sharp boundaries as indicated in Fig. 6, and the average grain size is about 0.35 mm. The present observations can be supported by the observations of Oscarsson et al. [14]. In a study of a strip cast aluminum alloy, they observed a transition from
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Table 1 Summary of the microstructural features of the as-extruded state and some annealed states B4
B8
As-extruded
Mainly elongated subgrains with high dislocation density; very small number of equiaxed subgrains. Size of elongated subgrains: length, 1 /2 mm; width, 0.2 /0.4 mm
Mainly elongated subgrains with high dislocation density; some noticeable amount of equiaxed subgrains. Size of elongated subgrains: length, 1 /2 mm; width, 0.1 /0.2 mm Size of equiaxed subgrains: 0.1 /0.2 mm
473 K, 1 h annealing
Mixture of elongated and equiaxed subgrains.
Mixture of elongated and equiaxed subgrains.
Size of elongated subgrains: length, 1 mm; width, 0.2 mm Size of equiaxed subgrains: 0.2 /0.4 mm
Size of elongated subgrains: length, 1 /2 mm; width, 0.1 /0.2 mm Size of equiaxed subgrains: 0.1 /0.2 mm
Mainly equiaxed grains and some elongated subgrains.
Equiaxed grains nearly free of dislocations.
Size of equiaxed grains: 0.4 /0.5 mm
Size: 0.35 mm
473 K, 6 h annealing
discontinuous recrystallization to a continuous recrystallization during annealing for cold reductions exceeding about 95%. They have attributed this phenomenon to the increasing proportion of high angle boundaries in the cold rolled structure with increasing rolling reduction. Since the low and high angle boundaries may exhibit quite different growth rates, for a mixed structure containing a small fraction of high angle boundaries the growth process becomes highly abnormal, displaying the characteristics of discontinuous recrystallization. As the proportion of high angle boundaries increases with increasing strain, the growth process becomes more normal with more rapid initial rates of coarsening but slower rates later on. Such behavior seems in good agreement with the softening behavior of B8 as indicated by the results of microhardness (Fig. 2) and DSC analysis (Fig. 3). It is of interest to compare the annealing behavior of the 5052 aluminum alloy with that of copper deformed to large strain. It has been documented that similar to aluminum alloys, large fraction of high angle boundaries (HABs) can be generated in pure copper by the use of ECAE to large strains [12]. However, it was reported that pure copper, after deformed to a strain of /8 by ECAE route Bc at 373 K (0.27 Tm), showed discontinuous recrystallization at an annealing temperature as low as 393 K (0.29 Tm) [15]. By comparing the results of the 5052 Al /Mg alloy and pure copper, it seems that in
addition to high fraction of HABs, the stored energy and the mobility of the boundaries must play important roles in the growth process during annealing. Compared with copper, a lower stored energy is expected in the 5052 Al /Mg alloy because of its higher stacking fault energy and homologous temperature, which may enhance the recovery process. Thus, the migration of HABs in the 5052 Al /Mg alloy may experience a relatively lower driving force. On the other hand, as compared with pure copper, the boundary mobility in the 5052 Al/Mg alloy may be reduced significantly due to the presence of alloying elements. Therefore, the development of uniform submicron-grained structure in the 5052 Al /Mg alloy by the use of ECAE and annealing may be resulted from the combined effects of the presence of high fraction of HABs, low stored energy, and reduced boundary mobility. 3.3. Tensile properties It is of interest to determine the tensile properties of the samples with a microstructure of submicron-sized subgrains (grains). The results of tensile tests are listed in Table 2, in which the properties of 5052 aluminum alloy reported in the handbook [16] are also included for comparison. The four selected samples processed by ECAE, which have submicron-sized structure, exhibit better ductility and much higher strength than 5052-H38
Table 2 Summary of the tensile properties
B4 B4, 473 K, 1 h B8 B8, 473 K, 6 h 5052-O [15] 5052-H38 [15]
0.2% yield stress (MPa)
Ultimate tensile strength (MPa)
Elongation to failure (%)
336 292 394 350 90 255
365 330 421 370 195 285
9 10 9 10.5 25 7
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Fig. 7. Tensile stress /strain curves obtained at room temperature with an initial strain rate of 7 /10 4 s 1.
does. Fig. 7 shows the tensile stress /strain curves, which have similar characteristics, i.e. high yield stress followed by rapid strain hardening and reaching the ultimate strength after small strain. The failure occurs by localized necking along a plane, which is inclined 458 from the tensile axis. The mechanisms to account for the deformation and failure of submicron-grained alloys are not quite clear at the present time. By correlating the tensile properties (Table 2) to the microstructure (Table 1), one may find that the strength strongly depends on the dimension of subgrains (grains), and the low temperature annealing only slightly improves the ductility of the as-extruded state.
2) The as-extruded microstructure can be characterized as a mixture of elongated and equiaxed subgrains with high density of dislocations distributed, in which high misorientations are associated with the subgrain structure. A higher strain (more ECAE passes) results in a finer structure. 3) In general, the as-extruded structure is quite stable at 423 K and shows slow coarsening at 473 K. 4) It seems that the microstructure resulted from more ECAE passes (higher strain) is more readily to develop a uniform equiaxed grain structure after static annealing. For material fabricated by eight ECAE passes, a uniform structure of equiaxed grains with an average size of 0.35 mm could be obtained by annealing at 473 K for 6 h.
4. Summary Acknowledgements A commercial 5052 aluminum alloy was processed by equal channel angular extrusion to an equivalent strain about 8. After extrusion, static annealing was also applied to the alloy. The important results of this study can be summarized as follows. 1) Very high strength (394 MPa yield stress and 421 MPa ultimate tensile strength) and reasonable ductility (10.5% tensile elongation) can be obtained in this non-heat treatable aluminum alloy by the application of ECAE to a strain of /8. The high strength may be attributed to the submicron-sized structure.
This work was supported by the National Science Council of R.O.C. under contract NSC89-2216-E110039.
References [1] V.M. Segal, Mater. Sci. Eng. A197 (1995) 157. [2] S. Ferrasse, V.M. Segal, K.T. Hartwig, R.E. Gorforth, Met. Mater. Trans. 28A (1997) 1047. [3] P.B. Berbon, T.G. Langdon, in: R.S. Mishra, et al. (Ed.), Ultrafine Grained Materials, TMS, 2000, p. 381.
T.L. Tsai et al. / Materials Science and Engineering A342 (2003) 144 /151 [4] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 46 (1998) 3317. [5] P.L. Sun, P.W. Kao, C.P. Chang, Mater. Sci. Eng. A283 (2000) 82. [6] C.P. Chang, P.L. Sun, P.W. Kao, Acta Mater. 48 (2000) 3377. [7] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Met. Mater. Trans. 29A (1998) 2503. [8] H. Hasegawa, S. Komura, A. Utsunomiya, Z. Horita, M. Furukawa, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A265 (1999) 188. [9] A. Yamashita, D. Yamaguchi, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A287 (2000) 100.
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[10] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143. [11] M. Furukawa, Y. Iwahashi, Z. Horia, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A257 (1998) 328. [12] D.A. Hughes, N. Hansen, in: R.S. Mishra, et al. (Ed.), Ultrafine Grained Materials, TMS, 2000, p. 195. [13] F.J. Humphreys, P.B. Prangnell, J.R. Bowen, A. Gholinia, C. Harris, Phil. Trans. R. Soc. Lond. A 357 (1999) 1663. [14] A. Oscarsson, B. Hutchinson, B. Nicol, P. Bate, H.-E. Ekstrom, Mater. Sci. Forum 157 /162 (1994) 1271. [15] Y.J. Wang, M.S. Thesis, National Sun Yat-Sen University, 2001. [16] Metals Handbook, vol. 2, Properties and Selection: Nonferrous Alloys and Pure Metals, 9th ed., ASM, 1979, p. 101.
Microstructure and tensile properties of a commercial 5052 aluminum alloy processed by equal channel angular extrusion T.L. Tsai, P.L. Sun, P.W. Kao *, C.P. Chang Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan Received 25 January 2002; received in revised form 22 April 2002
Abstract A commercial 5052 aluminum alloy was processed by equal channel angular extrusion at 423 K to a strain of /8. The asextruded microstructure is quite uniform, which can be characterized as a mixture of elongated and equiaxed subgrains with submicrometer size and high misorientations. In static annealing, the as-extruded structure is quite stable at 423 K and shows slow coarsening at 473 K. The microstructure resulted from more ECAE passes (higher strain) is more readily to develop a uniform structure of equiaxed grains in static annealing. Very high strength (394 MPa yield stress and 421 MPa ultimate tensile strength) and reasonable ductility (10.5% tensile elongation) can be obtained in this non-heat treatable aluminum alloy. The high strength can be related to the structure of submicron-sized subgrains (grains). To increase the strain from 4 to 8 is beneficial for producing finer structure and higher strength. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Tensile properties; 5052 Aluminum alloy; Equal channel angular extrusion
1. Introduction It has been shown that ultrafine grain structure may be introduced into metals through the use of equal channel angular extrusion (ECAE) [1,2]. In ECAE, a sample is pressed through a die in which two channels, equal in cross section, intersect at an angle of F, with an additional angle of C defining the arc of curvature at the outer point of the intersection of the two channels. By the use of ECAE, it is possible to repeat this process to many cycles to accumulate large plastic strain. ECAE has attracted much attention recently because it offers the potential to produce bulk samples with a very fine structure. Berbon and Langdon [3] summarized the important parameters in ECAE. Among them are the die angle, which determines the strain introduced in the material for each deformation pass, and the number of the passes through the die, which corresponds to the total accu-
* Corresponding author E-mail address: [email protected] (P.W. Kao).
mulated strain applied to the billet. The deformation route, which involves rotating the billet between each pass, is another critical parameter in texture and microstructure development. In addition, the pressing speed, the deformation temperature, and the friction between the die walls and the billet are also important parameters. By the use of ECAE at room temperature, ultrafine grain structure of micrometer size can be obtained in 99.99% pure Al [4] and in commercial pure 1050 Al [5], SMG structure can be achieved. In both cases, the resulted ultrafine grain structure has been shown to have large misorientations. Apparently the achieved grain size is affected by the presence of the impurity atoms. With detailed TEM observations, Chang et al. [6] showed the evolution of boundary structure from dislocation wall to grain boundary with increasing strain in ECAE. In a systematic study of pure Al and Al /Mg alloys, Iwahashi et al. [7] indicates that the addition of magnesium decreases the rate of recovery, which leads both to an increase in the number of passes required to establish a homogeneous microstructure and to a decrease in the ultimate equiaxed grain size. In addition,
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 2 8 3 - 6
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Fig. 1. Microstructure of as-extruded state (B8) revealed by TEM (a) banded structure containing elongated subgrains, (b) mixture of elongated and equiaxed subgrains, and (c) SAD pattern from an area of 6 mm in diameter.
it has been shown by static annealing that the ultrafine grain structure in pure Al and Al /Mg alloys is not stable at elevated temperatures, and a minor addition of Zr to aluminum can improve the thermal stability of the ultrafine grain structure to a temperature approaching 600 K [8]. Yamashita et al. recently have reported the influence of deformation temperature on microstructural development in ECAE [9]. Two important effects are reported. The grain size tends to increase with increasing deformation temperature. Grain refinement is achieved at all temperatures but there is a transition from arrays of high angle boundaries at the lower temperatures to low angle boundaries at high temperatures. The transition temperature depends on the alloy composition. It is
473 K in pure Al and 573 K in an Al /3% Mg alloy, whereas an Al /3% Mg /0.2%Sc alloy exhibits high angle boundaries at all temperatures to 573 K. According to the literature mentioned before, the addition of Mg to Al is beneficial for grain refinement and the presence of small amounts of transition metals is very effective in stabilizing the ultrafine grain structure. The major alloying elements in commercial 5052 aluminum alloy are 2.5 wt.% Mg and 0.25 wt.% Cr with other impurity atoms such as Mn, Fe and Si. It is a potential candidate to achieve submicrometer grain (SMG) structure by the application of ECAE. In the present work, the microstructure and tensile properties of a commercial 5052 aluminum alloy processed by ECAE to achieve SMG structure were investigated.
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Fig. 2. The variation of hardness as a function of annealing temperature indicating the softening behavior of ECAE processed 5052 alloy in static annealing.
2. Experimental procedure A commercial 5052 aluminum alloy produced by China Steel Aluminum Corporation was used. The chemical compositions (in wt.%) of this alloy are 2.57 Mg, 0.22 Cr, 0.26 Fe, 0.08 Si, 0.05 Mn and bal. Al. The material was annealed at 838 K for 12 h to give a grain size of about 200 mm. Specimens of 12 /12 /80 mm were subjected to ECAE for four or eight passes at 423 K. The die for ECAE consists of two channels with square cross-section intersecting at an angle F /908 and with an outer arc of angle C /208. The von Mises equivalent strain on pressing through the die was calculated to be 1.04 [10]. In the ECAE process, the die was heated and maintained at 423 K, the sample was coated with a lubricant containing MoS2, and the strain rate was estimated to be about 10 2 s 1. After each pass, the sample was quenched in water immediately. The approximate time for sample staying in the heated die is within 1000 s. Between each pass, the specimen was rotated 908 clockwise about the long axis, i.e. route Bc [11]. Microhardness tests were used to follow the softening behavior of the as-extruded material after various static annealing treatments. The tests were carried out with 200 g load and 15 s holding time on a Shimadzu HMV2000 microhardness tester. Transmission electron microscope (TEM) was used to examine the microstruc-
Fig. 3. The results of differential scanning calorimetry (heating rate: 10 K min 1), where (a) is for sample B8 and (b) is for sample B4.
ture. Specimens were cut from the flow plane of the extruded material, which is parallel to the extrusion direction. Thin foils prepared by twin-jet electropolishing were examined by TEM (Philips CM200) at 200 kV. In addition, the as-extruded samples were also analyzed by differential scanning calorimetry (DSC) using a
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Fig. 4. The change of hardness during isothermal annealing at 473 K.
SETARAM DSC 131 calorimeter. The DSC analysis was carried out in an argon atmosphere with a heating rate of 10 8C min 1. Tensile specimens with 14 mm gauge length and 3 mm dia. were tested with an Instron 5582 universal testing machine. The tensile tests were carried at room temperature with an initial strain rate of 7 /10 4 s 1.
3. Results and discussion 3.1. As-extruded microstructure The alloy was extruded at 423 K with route Bc for a total of four or eight passes, which will be denoted as B4 or B8, respectively. Generally speaking, the as-extruded samples, B4 or B8, exhibit the typical cold-worked structure after large strain, i.e. subgrains (grains) with high dislocation densities in the interior. Because the boundaries in structures resulted from large strain are often mixtures of low angle and high angle boundaries, the use of ‘subgrains’ or ‘grains’ to describe the structural feature becomes a problem. In the following text, the term ‘grains’ will be reserved only for those ‘dislocation free’ areas with large misorientation, which are of polyhedral shape and completely enclosed by sharp boundaries. The predominant feature in both samples B4 and B8 are elongated subgrains forming banded structure with some equiaxed subgrains. Fig. 1 shows the typical microstructure of the as-extruded state; banded structure of elongated subgrains as shown in Fig. 1(a), and mixture of elongated and equiaxed subgrains as shown in Fig. 1(b). The SAD pattern, which was taken from an area having a diameter of 6 mm, contains well-defined rings demonstrating the sub-
grains with a large spread in orientations. The most noticeable differences between B4 and B8 are that B8 has a finer structure and more equiaxed subgrains. 3.2. Thermal stability of the microstructure The extruded samples were subjected to isochronal annealing, in which the holding time at each temperature is 1 h. Fig. 2 shows the variation of microhardness as a function of annealing temperature, in which each data point is an average of ten measurements. The B4 sample shows a slight softening below 473 K, which may be related to recovery, and a typical sigmoidal curve with significant softening at temperatures higher than 473 K. For sample B8, considerable softening already takes place between 423 and 473 K with more drastic softening above 473 K. However, both samples exhibit similar hardness after being annealed at 673 K. For both samples, annealing at 673 K for 1 h results in a fully recrystallized structure. The results of DSC are shown in Fig. 3, that each sample has single exothermic peak, which is related to recrystallization. The exothermic peak starts at a temperature 25 K lower for the sample B8 than for the sample B4. By comparing with B4, the softening of B8 starts at a lower temperature (423 /473 K in Fig. 2) and may be related to the phenomenon that the recrystallization starts at lower temperature. It is known that the fraction of high angle boundaries in deformed structure increases significantly with increasing strain at low temperature [12,13]. The sample B8 is expected to have more high angle boundaries. It is believed that these high angle boundaries are more ready to migrate during annealing; as a consequence, the recrystallization in B8 may take place at a lower temperature.
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Fig. 5. TEM micrographs of the sample B4 annealed at 473 K for (a) 1 h and (b) 6 h.
In addition, the thermal stability of the as-extruded structure was also tested isothermally at 423 and 473 K. It was followed by microhardness tests after different holding time at the temperatures. Both samples have similar behavior. At 423 K, the hardness shows little change for a period of 8 h. However, at 473 K, the hardness drops slightly in the first 2 h and shows little change over the period of 2 /8 h as shown in Fig. 4. This may imply that the as-extruded structure is quite stable at 423 K and coarsens slowly at 473 K. It is also of interest to examine the microstructure and tensile properties for samples annealed to a state just before the onset of significant softening. The samples annealed at 473 K for various times were examined by the use of TEM. The TEM specimens were cut from the flow plane, which is parallel to the extrusion direction. In general, as the annealing time increases, the fraction of equiaxed subgrains increases with the expense of the
Fig. 6. TEM micrographs of the sample B8 after annealing: (a) annealed at 473 K for 1 h showing mixture of elongated and equiaxed subgrains, and (b) annealed at 473 K for 6 h showing equiaxed grains free of dislocations.
elongated subgrains and the dimension of subgrains grows. For the same annealing treatment, the sample B8 has smaller subgrain size and higher fraction of equiaxed subgrains than the sample B4 does. Typical TEM micrographs are shown in Figs. 5 and 6. The characteristics of the microstructures for both as-extruded and annealed states are summarized in Table 1. It is quite interesting that after annealing at 473 K (0.51 Tm) for 6 h, the sample B8 has developed a structure of equiaxed ‘grains’ free of dislocations, which are delineated by sharp boundaries as indicated in Fig. 6, and the average grain size is about 0.35 mm. The present observations can be supported by the observations of Oscarsson et al. [14]. In a study of a strip cast aluminum alloy, they observed a transition from
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149
Table 1 Summary of the microstructural features of the as-extruded state and some annealed states B4
B8
As-extruded
Mainly elongated subgrains with high dislocation density; very small number of equiaxed subgrains. Size of elongated subgrains: length, 1 /2 mm; width, 0.2 /0.4 mm
Mainly elongated subgrains with high dislocation density; some noticeable amount of equiaxed subgrains. Size of elongated subgrains: length, 1 /2 mm; width, 0.1 /0.2 mm Size of equiaxed subgrains: 0.1 /0.2 mm
473 K, 1 h annealing
Mixture of elongated and equiaxed subgrains.
Mixture of elongated and equiaxed subgrains.
Size of elongated subgrains: length, 1 mm; width, 0.2 mm Size of equiaxed subgrains: 0.2 /0.4 mm
Size of elongated subgrains: length, 1 /2 mm; width, 0.1 /0.2 mm Size of equiaxed subgrains: 0.1 /0.2 mm
Mainly equiaxed grains and some elongated subgrains.
Equiaxed grains nearly free of dislocations.
Size of equiaxed grains: 0.4 /0.5 mm
Size: 0.35 mm
473 K, 6 h annealing
discontinuous recrystallization to a continuous recrystallization during annealing for cold reductions exceeding about 95%. They have attributed this phenomenon to the increasing proportion of high angle boundaries in the cold rolled structure with increasing rolling reduction. Since the low and high angle boundaries may exhibit quite different growth rates, for a mixed structure containing a small fraction of high angle boundaries the growth process becomes highly abnormal, displaying the characteristics of discontinuous recrystallization. As the proportion of high angle boundaries increases with increasing strain, the growth process becomes more normal with more rapid initial rates of coarsening but slower rates later on. Such behavior seems in good agreement with the softening behavior of B8 as indicated by the results of microhardness (Fig. 2) and DSC analysis (Fig. 3). It is of interest to compare the annealing behavior of the 5052 aluminum alloy with that of copper deformed to large strain. It has been documented that similar to aluminum alloys, large fraction of high angle boundaries (HABs) can be generated in pure copper by the use of ECAE to large strains [12]. However, it was reported that pure copper, after deformed to a strain of /8 by ECAE route Bc at 373 K (0.27 Tm), showed discontinuous recrystallization at an annealing temperature as low as 393 K (0.29 Tm) [15]. By comparing the results of the 5052 Al /Mg alloy and pure copper, it seems that in
addition to high fraction of HABs, the stored energy and the mobility of the boundaries must play important roles in the growth process during annealing. Compared with copper, a lower stored energy is expected in the 5052 Al /Mg alloy because of its higher stacking fault energy and homologous temperature, which may enhance the recovery process. Thus, the migration of HABs in the 5052 Al /Mg alloy may experience a relatively lower driving force. On the other hand, as compared with pure copper, the boundary mobility in the 5052 Al/Mg alloy may be reduced significantly due to the presence of alloying elements. Therefore, the development of uniform submicron-grained structure in the 5052 Al /Mg alloy by the use of ECAE and annealing may be resulted from the combined effects of the presence of high fraction of HABs, low stored energy, and reduced boundary mobility. 3.3. Tensile properties It is of interest to determine the tensile properties of the samples with a microstructure of submicron-sized subgrains (grains). The results of tensile tests are listed in Table 2, in which the properties of 5052 aluminum alloy reported in the handbook [16] are also included for comparison. The four selected samples processed by ECAE, which have submicron-sized structure, exhibit better ductility and much higher strength than 5052-H38
Table 2 Summary of the tensile properties
B4 B4, 473 K, 1 h B8 B8, 473 K, 6 h 5052-O [15] 5052-H38 [15]
0.2% yield stress (MPa)
Ultimate tensile strength (MPa)
Elongation to failure (%)
336 292 394 350 90 255
365 330 421 370 195 285
9 10 9 10.5 25 7
150
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Fig. 7. Tensile stress /strain curves obtained at room temperature with an initial strain rate of 7 /10 4 s 1.
does. Fig. 7 shows the tensile stress /strain curves, which have similar characteristics, i.e. high yield stress followed by rapid strain hardening and reaching the ultimate strength after small strain. The failure occurs by localized necking along a plane, which is inclined 458 from the tensile axis. The mechanisms to account for the deformation and failure of submicron-grained alloys are not quite clear at the present time. By correlating the tensile properties (Table 2) to the microstructure (Table 1), one may find that the strength strongly depends on the dimension of subgrains (grains), and the low temperature annealing only slightly improves the ductility of the as-extruded state.
2) The as-extruded microstructure can be characterized as a mixture of elongated and equiaxed subgrains with high density of dislocations distributed, in which high misorientations are associated with the subgrain structure. A higher strain (more ECAE passes) results in a finer structure. 3) In general, the as-extruded structure is quite stable at 423 K and shows slow coarsening at 473 K. 4) It seems that the microstructure resulted from more ECAE passes (higher strain) is more readily to develop a uniform equiaxed grain structure after static annealing. For material fabricated by eight ECAE passes, a uniform structure of equiaxed grains with an average size of 0.35 mm could be obtained by annealing at 473 K for 6 h.
4. Summary Acknowledgements A commercial 5052 aluminum alloy was processed by equal channel angular extrusion to an equivalent strain about 8. After extrusion, static annealing was also applied to the alloy. The important results of this study can be summarized as follows. 1) Very high strength (394 MPa yield stress and 421 MPa ultimate tensile strength) and reasonable ductility (10.5% tensile elongation) can be obtained in this non-heat treatable aluminum alloy by the application of ECAE to a strain of /8. The high strength may be attributed to the submicron-sized structure.
This work was supported by the National Science Council of R.O.C. under contract NSC89-2216-E110039.
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