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Thermal stability of ultrafine-grained aluminum in the presence of Mg and Zr additions
Materials Science and Engineering A265 (1999) 188 – 196
Thermal stability of ultrafine-grained aluminum in the presence of Mg and Zr additions Hideaki Hasegawa a, Shogo Komura a, Atsushi Utsunomiya a, Zenji Horita a, Minoru Furukawa b, Minoru Nemoto a, Terence G. Langdon c,* a
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Uni6ersity, Fukuoka 812 -8581, Japan b Department of Technology, Fukuoka Uni6ersity of Education, Munakata, Fukuoka 811 -4192, Japan c Departments of Materials Science and Mechanical Engineering, Uni6ersity of Southern California, Los Angeles, CA 90089 -1453, USA Received 14 September 1998; received in revised form 11 November 1998
Abstract Superplastic deformation occurs at high temperatures and requires the presence of a very small grain size. Experiments demonstrate that equal-channel angular (ECA) pressing is capable of introducing ultrafine grain sizes in pure Al, Al–Mg and Al –Zr alloys, with grain sizes lying in the sub-micrometer range for the alloys. It is shown by static annealing that these ultrafine grain sizes are not stable in pure Al and the Al–Mg alloys at elevated temperatures but in the Al – Zr alloys the grains remain small up to temperatures of :600 K. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Aluminum alloys; Equal-channel angular pressing; Grain growth; Superplasticity; Ultrafine grain size
1. Introduction Superplastic forming is now an established process for the fabrication of complex shapes from sheet metals [1]. In practice, however, industrial forming is conducted at the strain rates associated with optimum superplastic ductility and these rates tend to be slow ( :10 − 3 –10 − 2 s − 1) so that the forming times are consequently fairly long (typically, :20 – 30 min). There is experimental evidence showing that the optimum strain rate for superplasticity is increased when the grain size is decreased [2] and this led to the suggestion that it may be possible to achieve superplasticity at faster strain rates by making a substantial reduction in the specimen grain size [3]. Equal-channel angular (ECA) pressing is a processing method in which an intense plastic strain is introduced into a material through simple shear [4,5]. This process is capable of producing ultrafine grain sizes in polycrystalline materials [6 – 9] and very recent experiments have confirmed the feasibility of using ECA * Corrresponding author. Tel.: + 1-213-7400491; fax: + 1-2137407797. E-mail address: [email protected] (T.G. Langdon)
pressing to attain high strain rate superplasticity (HSR SP) in various aluminum-based alloys containing Zr and Sc additions [10–13]. Although these experiments have been successful in demonstrating the potential for HSR SP in cast aluminum-based alloys, nevertheless they have also revealed a possible difficulty. It is now well established that superplasticity is a diffusion-controlled process which occurs at high temperatures, typically above :0.5Tm where Tm is the absolute melting temperature of the material [14]. This leads in practice to the requirement that any ultrafine grain size introduced by ECA pressing must remain small and stable at the high temperatures associated with superplastic flow. Experiments on an Al–3%Mg solid solution alloy revealed the possibility of achieving a grain size of :0.2 mm by ECA pressing but there was substantial grain growth when the material was heated above : 500 K corresponding to : 0.5Tm [15,16]. By contrast, an ultrafine grain size was successfully retained in a commercial Al–5.5%Mg–2.2%Li–0.12%Zr alloy up to temperatures as high as : 700 K, equivalent to :0.75 Tm, because of the presence of a fine dispersion of b%-Al3Zr precipitates [17]. This latter alloy was capable of ex-
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hibiting high tensile ductilities in the ECA-pressed condition, including elongations of \1100 and :340% at strain rates of 10 − 2 and 1 s − 1 when testing at a temperature of 623 K [10,13]. The present work was motivated by the lack of any quantitative information on the thermal stability of ultrafine-grained aluminum-based alloys containing either Mg or Zr as additives. Experiments were conducted to evaluate the potential for using these additions in order to maintain an ultrafine grain size up to temperatures within the range required for superplastic deformation. As will be demonstrated, the presence of only very dilute additions of zirconium in a pure aluminum matrix is sufficient to retain the very small grain sizes produced by ECA pressing up to the high temperatures where superplasticity may be anticipated.
2. Experimental materials and procedures
2.1. Materials The experiments were conducted using five different materials. A commercial ingot of aluminum of 99.99%
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purity was rolled into a plate at room temperature, rods were cut having dimensions of 25× 25× 150 mm3, and these rods were swaged to a diameter of 10 mm and then cut to lengths of : 60 mm. Prior to ECA pressing, each piece was annealed for 1 h at 773 K to give an initial grain size of : 1.0 mm. Earlier reports gave details of the microstructural evolution occurring in pure Al during ECA pressing [18,19]. An Al–1.0wt.%Mg alloy (equivalent to 1.1 at.%) was obtained in the form of a cold-rolled billet with a width of 150 mm, a thickness of 25 mm and a length of :1 m. The major impurities in this alloy were 0.003% Si and 0.001% Fe. Rods were cut with dimensions of 25× 25× 150 mm3 and these rods were swaged to give a circular cross-section with a diameter of 10 mm. Samples for ECA pressing were cut with lengths of : 60 mm and then annealed for 1 h at 773 K. In the annealed and unpressed condition, the measured grain size of this material was :500 mm. An Al–3.0wt.%Mg alloy (equivalent to 3.3 at.%) was also obtained as a cold-rolled billet and the major impurities in this alloy were 0.004% Si and 0.001% Fe. Samples were prepared from this alloy for ECA pressing using the same procedure as for the Al–1%Mg alloy and the samples were also annealed at 773 K for 1 h to give an initial grain size of :500 mm.
Fig. 1. Microstructures after ECA pressing for pure Al after four passes, Al – 1%Mg after six passes, Al – 3%Mg after eight passes and Al–0.2%Zr after eight passes.
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Fig. 2. Microstructures of pure Al after ECA pressing and annealing for 1 h at 423, 473, 523 and 573 K.
Ingots of Al–0.12wt.%Zr (equivalent to 0.036 at.%) and Al–0.2wt.%Zr (equivalent to 0.06 at.%) were prepared to the dimensions of 17×55 ×120 mm3 using the same high purity (99.99%) Al and additions of either 0.12 or 0.2% Zr. These ingots were homogenized for 24 h in air at 903 K and then rolled at room temperature to thicknesses of 11.5 mm. Rods were cut from the rolled ingots, machined to a diameter of 10 mm and then cut into pieces with lengths of : 60 mm. Prior to ECA pressing, these pieces were annealed for 1 h at 903 K to give initial grain sizes of :900 mm in both alloys.
2.2. Procedure for equal-channel angular pressing Full details of the procedures for ECA pressing were given earlier [15,18,19]. In the present investigation, the die consisted of a solid block of high strength SKD11 tool steel containing a single channel of circular crosssection with diameters of 10.3 and 10.0 mm at the points of entrance and exit, respectively. The channel formed an L-shaped configuration within the die, with an angle of intersection between the two portions of the channel of F=90° and with the outer arc of curvature at the point of intersection having an angle of C= 90°. For these values of the angles F and C, it can be shown
from first principles [20], and has been confirmed in model experiments [21], that the strain accrued within the sample on a single passage through the die is : 1. Since the cross-section of the sample remains unchanged in a single passage through the die, repetitive pressings were used to attain high strains. In the present experiments, the samples were pressed at room temperature for a total of up to eight passes through the die, equivalent to a total strain of : 8. The ECA pressing was conducted with a pressing speed of : 19 mm s − 1 and with the samples coated in MoS2 as a lubricant. There is experimental evidence showing that the microstructural evolution in ECA pressing depends not only upon the total strain accrued in the sample but also upon the nature of any rotation of the sample between consecutive pressings [18,19,22– 24]. In practice, it has been shown that a homogeneous equiaxed grain structure develops most rapidly when the specimen is rotated in the same direction by 90° between each pressing in the procedure designated route BC [19]: accordingly, all of the present experiments were conducted using this procedure. Except only for the Al–0.12%Zr alloy, samples were pressed until it became apparent that a reasonably homogeneous microstructure of grains with high angle boundaries had been achieved. This required totals of
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four and six passes through the die for the pure Al and the Al–1%Mg alloy, respectively, and eight passes for the Al–3%Mg and the Al – 0.2%Zr alloys. For the Al–0.12%Zr alloy, ECA pressing was discontinued after four passes but careful inspection of the microstructure revealed subsequently some inhomogeneities such that some areas contained grain boundaries having low angles of misorientation. In order to provide a direct comparison between the characteristics of ECA pressing and cold rolling (CR), unstrained samples of the pure Al and the Al –0.12%Zr alloy were rolled at room temperature to a reduction ratio, (ti −tf)/ti, of 0.77, where ti and tf are the initial and final thicknesses of the sample, respectively. This reduction ratio is equal to an equivalent strain of : 1.7.
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2.4. Microstructural examination Specimens were prepared for microstructural examination by transmission electron microscopy (TEM) both immediately after the ECA pressing and after static annealing. All observations were made in the x plane which is defined as the plane perpendicular to the longitudinal axis of the pressed sample [19]. Small discs with diameters of 3 mm and thicknesses of : 0.15 mm were prepared by cutting and mechanical grinding and these discs were then thinned to perforation at a temperature of 278 K using a twin-jet electropolishing unit with a solution of 10% HClO4, 20% C3H8O3 and 70% C2H5OH. Specimens were examined using an Hitachi H-8100 electron microscope operating at 200 kV. Selected area electron diffraction (SAED) patterns were obtained from regions having diameters of 12.3 mm except where noted otherwise.
2.3. Static annealing experiments 2.5. Compression testing Following ECA pressing, samples were cut having dimensions of 3×3 × 5 mm3. Each piece was encapsulated separately in a glass tube under an argon atmosphere and it was then statically annealed for 1 h at a selected temperature within the range from 373 to 773 K.
Compression specimens were prepared after ECA pressing and after static annealing. These specimens had dimensions of 3× 3× 5 mm3 and they were tested in compression at room temperature using an initial strain rate of 3.3 ×10-4 s − 1. The load was recorded
Fig. 3. Microstructures of Al–1%Mg after ECA pressing and annealing for 1 h at 423, 473, 523 and 573 K.
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Fig. 4. Microstructures of Al–3%Mg after ECA pressing and annealing for 1 h at 423, 473, 523 and 573 K.
continuously as a function of time and measurements were taken to determine the 0.2% proof stresses.
3. Experimental results
3.1. Microstructures after equal-channel angular pressing It was shown earlier that a total of four passes through the die, to a strain of :4, is required with pure Al when using processing route BC in order to achieve a homogeneous array of equiaxed grains separated by high angle grain boundaries [19]. A systematic approach was used in the present investigation and it was found that reasonably similar homogeneous microstructures were achieved after six passes with the Al–1%Mg alloy and after eight passes with the Al– 3%Mg and the Al – 0.2%Zr alloys. Fig. 1 shows the microstructures for these four materials in the homogeneous condition: for pure Al after four passes, for Al – 1%Mg after six passes, for Al– 3%Mg after eight passes and for Al – 0.2%Zr after eight passes, respectively, together with the corresponding SAED patterns taken from regions with a diameter of 6.3 mm. Inspection shows that the SAED patterns
exhibit diffracted beams which are scattered around rings indicative of a microstructure containing grains separated by high angle grain boundaries and the TEM photomicrographs reveal a reasonably equiaxed grain structure in each material. However, it is apparent also that the average grain sizes of these materials decrease significantly with alloying addition and measurements gave average sizes of : 1.3, 0.45, 0.3 and 0.7 mm for pure Al, Al–1%Mg, Al–3%Mg and Al–0.2%Zr, respectively. These average sizes are not influenced by the numbers of passes through the die because it was shown earlier that the grain size of an ECA-pressed material remains essentially unchanged during repetitive pressings and microstructural evolution occurs solely through an increase in the average misorientation angles of the boundaries as they evolve from low angle sub-boundaries to boundaries having high angles of misorientation [18,19]. The differences arise, however, because there is experimental evidence demonstrating a reduction in the equilibrium grain size when the rate of recovery in the material is reduced [25]. The microstructures shown in Fig. 1 confirm that, as in earlier reports [26–28], ECA pressing is a viable procedure for attaining ultrafine grain sizes in Al-based materials.
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3.2. Grain growth during static annealing Each material was annealed for 1 h over a range of temperatures and Figs. 2 – 5 show the microstructures for these four materials at the four different annealing temperatures of 423, 473, 523 and 573 K, respectively. For annealing temperatures up to 423 K, the microstructures of all four materials remained essentially unchanged by comparison with the as-pressed material shown in Fig. 1. However, annealing at 473 K gave a duplex microstructure of large and small grains in pure Al and the two Al – Mg alloys whereas in the Al– 0.2%Zr alloy no large grains were visible at this temperature and the grain size remained similar to the condition after ECA pressing. This duplex structure of small and large grains persisted in pure Al and the Al–Mg alloys at 523 K but there was a corresponding increase in the proportion of the larger grains. After annealing at 573 K, the pure Al and Al – Mg alloys exhibited only the larger grains with no evidence for the original small grains. A comparison of Figs. 2 –5 shows that the behavior of the pure Al and the two Al–Mg alloys is essentially similar except only that the grain sizes are smaller in the two alloys. By contrast, the fine-grained structure remained stable up to an annealing temperature of 573 K in the Al – 0.2%Zr alloy
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although there is evidence in Fig. 5 for a minor but uniform increase in the average grain size with increasing temperature. The rate of grain growth may be documented over the entire temperature range of static annealing up to 773 K by plotting the average measured grain size against the annealing temperature as shown in Fig. 6 for all five materials: in this plot, the values plotted for the duplex structures represent the average size of the dominant structure. Inspection of Fig. 6 shows that, although the as-pressed grain sizes are different for pure Al and the two Al–Mg alloys, all three materials exhibit extensive and rapid grain growth when the annealing temperature reaches : 473 K. By contrast, the microstructures of the Al–0.12%Zr alloy and the Al–0.2%Zr alloy are stable up to a temperature of : 573 K.
3.3. Values of the 0.2% proof stress The values of the measured 0.2% proof stresses are plotted in Fig. 7 as a function of the annealing temperature for all five materials. For pure Al, it is apparent that the proof stress remains constant up to :473 K but there is an abrupt decrease with increasing temperature up to 573 K and thereafter the 0.2% proof stress
Fig. 5. Microstructures of Al–0.2%Zr after ECA pressing and annealing for 1 h at 423, 473, 523 and 573 K.
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Fig. 6. Variation of the average grain size with annealing temperature for the five materials used in this investigation.
remains reasonably constant. This decrease in the values of the proof stress is associated with the development of the larger grains, as documented in Figs. 2 and 6. The temperature dependence of the 0.2% proof stress is similar also for the two Al – Mg alloys except that the proof stress decreases gradually up to : 473 K and thereafter decreases more rapidly. However, the behavior of the two Al – Zr alloys is different: initially, the proof stress appears to increase slightly with annealing
Fig. 7. Variation of the 0.2% proof stresses with annealing temperature for the five materials used in this investigation.
Fig. 8. Demonstration of the validity of the Hall – Petch relationship.
temperature up to :473 K for both alloys, but thereafter there is an abrupt decrease in the proof stress from : 523 to 623 K and at even higher temperatures the proof stresses are similar to those recorded in the other materials. This initial increase in the proof stress is consistent with an earlier report showing a similar effect for aluminum alloys with Zr contents at and above 0.05% when cold rolled to a reduction ratio of 0.90 [29]. The Hall–Petch equation requires an inverse relationship between the measured proof stress and the reciprocal of the square root of the grain size [30,31].
Fig. 9. Variation of the 0.2% proof stresses with annealing temperature for samples subjected to ECA pressing and CR.
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The validity of this relationship was checked as shown in Fig. 8 where the 0.2% proof stress is plotted against d − 1/2, where d is the grain size. It is apparent that these materials exhibit a reasonably linear relationship within the experimental scatter, thereby confirming the validity of the Hall–Petch relationship over the range of grain sizes attained in this investigation through static annealing: the individual Hall – Petch relationships are given in Fig. 8 for each material, where sy is the 0.2% proof stress. It is important to note that deviations from the simple Hall – Petch relationship may occur in more complex alloys where there are concomitant changes in the precipitate morphology with temperature and time [32].
3.4. Comparison between ECA pressing and cold rolling Fig. 9 plots the 0.2% proof stress against the annealing temperature for pure Al and for the Al –0.12%Zr alloy after either equal-channel angular pressing to a total of four passes, equivalent to a strain of : 4, or CR to an equivalent strain of : 1.7. Inspection shows that, for both materials, the proof stresses after ECA pressing are higher than after CR because of the higher strain. In both materials, the stress level after ECA pressing decreases abruptly at a critical temperature within the range : 450 – 650 K because of the rapid dissipation of the strain. By contrast, the strain dissipates less rapidly after CR so that the decrease in stress with increasing temperature is more gradual. It is interesting to note that, for each material, the transition temperatures associated with the decrease in stress are fairly similar after both ECA pressing and CR. This similarity may be fortuitous because there is experimental evidence from the CR of dilute Al–Zr alloys showing that these transition temperatures are reduced when the alloys are rolled to higher strains [29]. On the other hand, it is reasonable that the transition temperature is higher in the Al – Zr alloy by comparison with pure Al because it is well established that the addition of Zr leads to the presence of Al3Zr particles which inhibit grain growth and thereby increase the recrystallization temperature [29].
4. Discussion Two fundamental requirements must be fulfilled in order to achieve superplasticity in metals: (1) there must be a very small grain size, typically B10 mm, and (2) testing must be conducted at a temperature above : 0.5 Tm [14]. The present experiments show that ECA pressing leads to a very substantial grain refinement in pure Al and in dilute Al – Mg and Al – Zr alloys so that the as-pressed grain sizes are within the superplastic range for all materials, but these ultrafine grain sizes
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are not retained at elevated temperatures unless, as in the Al–Zr alloys, precipitates are present to pin the grain boundaries and minimize grain growth. The present observations on the significance of minor additions of Zr are generally consistent with earlier observations showing the influence of Zr additions on the recrystallization behavior of aluminum after CR [29]. Specifically, it was shown earlier that: (1) the transition temperature is higher in Al alloys containing Zr by comparison with pure Al and (2) there is evidence for an initial increase in stress with temperature in dilute Al–Zr alloys. Both of these trends are apparent also in Fig. 9. However, a possible discrepancy arises because the earlier work showed that the transition temperature decreased with increasing strain in CR whereas the present results, documented in Fig. 9, show fairly similar transition temperatures for each material after both CR to an equivalent strain of :1.7 and ECA pressing to an equivalent strain of :4. These observations suggest, therefore, that there may be significant differences between the rolling and ECA pressing procedures. It is now well established that dilute additions of Mg to aluminum lead to enhanced ductility in coarsegrained materials at elevated temperatures [33–36]. However, this behavior is not associated with superplastic flow but rather it arises because the strain rate sensitivity is high in these alloys at high temperatures when deformation occurs by a viscous glide process in which dislocations drag solute atom atmospheres [37]. The present results reveal, in Figs. 2–4 and in Fig. 6, the retention of smaller grains in the two Al–Mg alloys by comparison with pure Al but, nevertheless, the difference is lost through grain growth at elevated temperatures and it provides no opportunity to achieve superplastic flow in the Al–Mg alloys. By contrast, it is apparent that the ultrafine grain sizes are more stable in the Al–Zr alloys and the grain sizes remain small up to : 600 K. It should be noted that recent experiments have demonstrated a potential for achieving HSR SP in Al–Mg alloys by introducing minor additions of Sc [11]. Specifically, the addition of 0.2% Sc to an Al– 3%Mg alloy led to a grain size on ECA pressing of : 0.2 mm, which is comparable to the present experiments, but an average grain size of B 1 mm was retained to annealing temperatures as high as 673 K because of the presence of a very fine dispersion of coherent Al3Sc precipitates. Under these conditions, HSR SP was achieved in this alloy at 673 K with an elongation of \ 1000% at a strain rate of 3.3 ×10 − 2 s − 1. The present results suggest, however, that elongations of this order are not feasible in Al–Mg alloys in the absence of any precipitates to restrict the grain boundary mobility. It is concluded instead that the simultaneous addition of both Mg and Zr may be
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especially effective in promoting subsequent superplastic ductilities after ECA pressing because the addition of Mg is effective in reducing the grain size through ECA pressing and Zr additions are effective in retaining the ultrafine grain sizes to relatively high testing temperatures: these two trends are evident in Fig. 6.
5. Summary and conclusions 1. Samples of pure Al, Al – 1%Mg, Al – 3%Mg and Al–0.2%Zr were subjected to ECA pressing at room temperature to strains where the microstructures were reasonably homogeneous. The grain sizes in these conditions were : 1.3, 0.45, 0.3 and 0.7 mm for these four materials, respectively. Additional ECA pressing was also conducted on an Al– 0.12%Zr alloy to introduce a less homogeneous ultrafine-grained structure. 2. The ultrafine grain sizes introduced by ECA pressing were not stable at elevated temperatures in the pure Al and Al – Mg alloys but a reasonably finegrained structure was retained in the Al – Zr alloys up to temperatures as high as :600 K. 3. The lack of thermal stability at elevated temperatures shows that pure Al and dilute Al – Mg alloys are not suitable candidate materials for attaining superplasticity through ECA pressing.
Acknowledgements We thank Takayoshi Fujinami, Hiroaki Kawahara and Yoshinori Iwahashi for experimental assistance. This work was supported in part by the Light Metals Educational Foundation of Japan, in part by a Grantin-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan, in part by the Japan Society for the Promotion of Science, and in part by the National Science Foundation of the United States under Grants No. DMR-9625969 and INT-9602919.
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Thermal stability of ultrafine-grained aluminum in the presence of Mg and Zr additions Hideaki Hasegawa a, Shogo Komura a, Atsushi Utsunomiya a, Zenji Horita a, Minoru Furukawa b, Minoru Nemoto a, Terence G. Langdon c,* a
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Uni6ersity, Fukuoka 812 -8581, Japan b Department of Technology, Fukuoka Uni6ersity of Education, Munakata, Fukuoka 811 -4192, Japan c Departments of Materials Science and Mechanical Engineering, Uni6ersity of Southern California, Los Angeles, CA 90089 -1453, USA Received 14 September 1998; received in revised form 11 November 1998
Abstract Superplastic deformation occurs at high temperatures and requires the presence of a very small grain size. Experiments demonstrate that equal-channel angular (ECA) pressing is capable of introducing ultrafine grain sizes in pure Al, Al–Mg and Al –Zr alloys, with grain sizes lying in the sub-micrometer range for the alloys. It is shown by static annealing that these ultrafine grain sizes are not stable in pure Al and the Al–Mg alloys at elevated temperatures but in the Al – Zr alloys the grains remain small up to temperatures of :600 K. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Aluminum alloys; Equal-channel angular pressing; Grain growth; Superplasticity; Ultrafine grain size
1. Introduction Superplastic forming is now an established process for the fabrication of complex shapes from sheet metals [1]. In practice, however, industrial forming is conducted at the strain rates associated with optimum superplastic ductility and these rates tend to be slow ( :10 − 3 –10 − 2 s − 1) so that the forming times are consequently fairly long (typically, :20 – 30 min). There is experimental evidence showing that the optimum strain rate for superplasticity is increased when the grain size is decreased [2] and this led to the suggestion that it may be possible to achieve superplasticity at faster strain rates by making a substantial reduction in the specimen grain size [3]. Equal-channel angular (ECA) pressing is a processing method in which an intense plastic strain is introduced into a material through simple shear [4,5]. This process is capable of producing ultrafine grain sizes in polycrystalline materials [6 – 9] and very recent experiments have confirmed the feasibility of using ECA * Corrresponding author. Tel.: + 1-213-7400491; fax: + 1-2137407797. E-mail address: [email protected] (T.G. Langdon)
pressing to attain high strain rate superplasticity (HSR SP) in various aluminum-based alloys containing Zr and Sc additions [10–13]. Although these experiments have been successful in demonstrating the potential for HSR SP in cast aluminum-based alloys, nevertheless they have also revealed a possible difficulty. It is now well established that superplasticity is a diffusion-controlled process which occurs at high temperatures, typically above :0.5Tm where Tm is the absolute melting temperature of the material [14]. This leads in practice to the requirement that any ultrafine grain size introduced by ECA pressing must remain small and stable at the high temperatures associated with superplastic flow. Experiments on an Al–3%Mg solid solution alloy revealed the possibility of achieving a grain size of :0.2 mm by ECA pressing but there was substantial grain growth when the material was heated above : 500 K corresponding to : 0.5Tm [15,16]. By contrast, an ultrafine grain size was successfully retained in a commercial Al–5.5%Mg–2.2%Li–0.12%Zr alloy up to temperatures as high as : 700 K, equivalent to :0.75 Tm, because of the presence of a fine dispersion of b%-Al3Zr precipitates [17]. This latter alloy was capable of ex-
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hibiting high tensile ductilities in the ECA-pressed condition, including elongations of \1100 and :340% at strain rates of 10 − 2 and 1 s − 1 when testing at a temperature of 623 K [10,13]. The present work was motivated by the lack of any quantitative information on the thermal stability of ultrafine-grained aluminum-based alloys containing either Mg or Zr as additives. Experiments were conducted to evaluate the potential for using these additions in order to maintain an ultrafine grain size up to temperatures within the range required for superplastic deformation. As will be demonstrated, the presence of only very dilute additions of zirconium in a pure aluminum matrix is sufficient to retain the very small grain sizes produced by ECA pressing up to the high temperatures where superplasticity may be anticipated.
2. Experimental materials and procedures
2.1. Materials The experiments were conducted using five different materials. A commercial ingot of aluminum of 99.99%
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purity was rolled into a plate at room temperature, rods were cut having dimensions of 25× 25× 150 mm3, and these rods were swaged to a diameter of 10 mm and then cut to lengths of : 60 mm. Prior to ECA pressing, each piece was annealed for 1 h at 773 K to give an initial grain size of : 1.0 mm. Earlier reports gave details of the microstructural evolution occurring in pure Al during ECA pressing [18,19]. An Al–1.0wt.%Mg alloy (equivalent to 1.1 at.%) was obtained in the form of a cold-rolled billet with a width of 150 mm, a thickness of 25 mm and a length of :1 m. The major impurities in this alloy were 0.003% Si and 0.001% Fe. Rods were cut with dimensions of 25× 25× 150 mm3 and these rods were swaged to give a circular cross-section with a diameter of 10 mm. Samples for ECA pressing were cut with lengths of : 60 mm and then annealed for 1 h at 773 K. In the annealed and unpressed condition, the measured grain size of this material was :500 mm. An Al–3.0wt.%Mg alloy (equivalent to 3.3 at.%) was also obtained as a cold-rolled billet and the major impurities in this alloy were 0.004% Si and 0.001% Fe. Samples were prepared from this alloy for ECA pressing using the same procedure as for the Al–1%Mg alloy and the samples were also annealed at 773 K for 1 h to give an initial grain size of :500 mm.
Fig. 1. Microstructures after ECA pressing for pure Al after four passes, Al – 1%Mg after six passes, Al – 3%Mg after eight passes and Al–0.2%Zr after eight passes.
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Fig. 2. Microstructures of pure Al after ECA pressing and annealing for 1 h at 423, 473, 523 and 573 K.
Ingots of Al–0.12wt.%Zr (equivalent to 0.036 at.%) and Al–0.2wt.%Zr (equivalent to 0.06 at.%) were prepared to the dimensions of 17×55 ×120 mm3 using the same high purity (99.99%) Al and additions of either 0.12 or 0.2% Zr. These ingots were homogenized for 24 h in air at 903 K and then rolled at room temperature to thicknesses of 11.5 mm. Rods were cut from the rolled ingots, machined to a diameter of 10 mm and then cut into pieces with lengths of : 60 mm. Prior to ECA pressing, these pieces were annealed for 1 h at 903 K to give initial grain sizes of :900 mm in both alloys.
2.2. Procedure for equal-channel angular pressing Full details of the procedures for ECA pressing were given earlier [15,18,19]. In the present investigation, the die consisted of a solid block of high strength SKD11 tool steel containing a single channel of circular crosssection with diameters of 10.3 and 10.0 mm at the points of entrance and exit, respectively. The channel formed an L-shaped configuration within the die, with an angle of intersection between the two portions of the channel of F=90° and with the outer arc of curvature at the point of intersection having an angle of C= 90°. For these values of the angles F and C, it can be shown
from first principles [20], and has been confirmed in model experiments [21], that the strain accrued within the sample on a single passage through the die is : 1. Since the cross-section of the sample remains unchanged in a single passage through the die, repetitive pressings were used to attain high strains. In the present experiments, the samples were pressed at room temperature for a total of up to eight passes through the die, equivalent to a total strain of : 8. The ECA pressing was conducted with a pressing speed of : 19 mm s − 1 and with the samples coated in MoS2 as a lubricant. There is experimental evidence showing that the microstructural evolution in ECA pressing depends not only upon the total strain accrued in the sample but also upon the nature of any rotation of the sample between consecutive pressings [18,19,22– 24]. In practice, it has been shown that a homogeneous equiaxed grain structure develops most rapidly when the specimen is rotated in the same direction by 90° between each pressing in the procedure designated route BC [19]: accordingly, all of the present experiments were conducted using this procedure. Except only for the Al–0.12%Zr alloy, samples were pressed until it became apparent that a reasonably homogeneous microstructure of grains with high angle boundaries had been achieved. This required totals of
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four and six passes through the die for the pure Al and the Al–1%Mg alloy, respectively, and eight passes for the Al–3%Mg and the Al – 0.2%Zr alloys. For the Al–0.12%Zr alloy, ECA pressing was discontinued after four passes but careful inspection of the microstructure revealed subsequently some inhomogeneities such that some areas contained grain boundaries having low angles of misorientation. In order to provide a direct comparison between the characteristics of ECA pressing and cold rolling (CR), unstrained samples of the pure Al and the Al –0.12%Zr alloy were rolled at room temperature to a reduction ratio, (ti −tf)/ti, of 0.77, where ti and tf are the initial and final thicknesses of the sample, respectively. This reduction ratio is equal to an equivalent strain of : 1.7.
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2.4. Microstructural examination Specimens were prepared for microstructural examination by transmission electron microscopy (TEM) both immediately after the ECA pressing and after static annealing. All observations were made in the x plane which is defined as the plane perpendicular to the longitudinal axis of the pressed sample [19]. Small discs with diameters of 3 mm and thicknesses of : 0.15 mm were prepared by cutting and mechanical grinding and these discs were then thinned to perforation at a temperature of 278 K using a twin-jet electropolishing unit with a solution of 10% HClO4, 20% C3H8O3 and 70% C2H5OH. Specimens were examined using an Hitachi H-8100 electron microscope operating at 200 kV. Selected area electron diffraction (SAED) patterns were obtained from regions having diameters of 12.3 mm except where noted otherwise.
2.3. Static annealing experiments 2.5. Compression testing Following ECA pressing, samples were cut having dimensions of 3×3 × 5 mm3. Each piece was encapsulated separately in a glass tube under an argon atmosphere and it was then statically annealed for 1 h at a selected temperature within the range from 373 to 773 K.
Compression specimens were prepared after ECA pressing and after static annealing. These specimens had dimensions of 3× 3× 5 mm3 and they were tested in compression at room temperature using an initial strain rate of 3.3 ×10-4 s − 1. The load was recorded
Fig. 3. Microstructures of Al–1%Mg after ECA pressing and annealing for 1 h at 423, 473, 523 and 573 K.
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Fig. 4. Microstructures of Al–3%Mg after ECA pressing and annealing for 1 h at 423, 473, 523 and 573 K.
continuously as a function of time and measurements were taken to determine the 0.2% proof stresses.
3. Experimental results
3.1. Microstructures after equal-channel angular pressing It was shown earlier that a total of four passes through the die, to a strain of :4, is required with pure Al when using processing route BC in order to achieve a homogeneous array of equiaxed grains separated by high angle grain boundaries [19]. A systematic approach was used in the present investigation and it was found that reasonably similar homogeneous microstructures were achieved after six passes with the Al–1%Mg alloy and after eight passes with the Al– 3%Mg and the Al – 0.2%Zr alloys. Fig. 1 shows the microstructures for these four materials in the homogeneous condition: for pure Al after four passes, for Al – 1%Mg after six passes, for Al– 3%Mg after eight passes and for Al – 0.2%Zr after eight passes, respectively, together with the corresponding SAED patterns taken from regions with a diameter of 6.3 mm. Inspection shows that the SAED patterns
exhibit diffracted beams which are scattered around rings indicative of a microstructure containing grains separated by high angle grain boundaries and the TEM photomicrographs reveal a reasonably equiaxed grain structure in each material. However, it is apparent also that the average grain sizes of these materials decrease significantly with alloying addition and measurements gave average sizes of : 1.3, 0.45, 0.3 and 0.7 mm for pure Al, Al–1%Mg, Al–3%Mg and Al–0.2%Zr, respectively. These average sizes are not influenced by the numbers of passes through the die because it was shown earlier that the grain size of an ECA-pressed material remains essentially unchanged during repetitive pressings and microstructural evolution occurs solely through an increase in the average misorientation angles of the boundaries as they evolve from low angle sub-boundaries to boundaries having high angles of misorientation [18,19]. The differences arise, however, because there is experimental evidence demonstrating a reduction in the equilibrium grain size when the rate of recovery in the material is reduced [25]. The microstructures shown in Fig. 1 confirm that, as in earlier reports [26–28], ECA pressing is a viable procedure for attaining ultrafine grain sizes in Al-based materials.
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3.2. Grain growth during static annealing Each material was annealed for 1 h over a range of temperatures and Figs. 2 – 5 show the microstructures for these four materials at the four different annealing temperatures of 423, 473, 523 and 573 K, respectively. For annealing temperatures up to 423 K, the microstructures of all four materials remained essentially unchanged by comparison with the as-pressed material shown in Fig. 1. However, annealing at 473 K gave a duplex microstructure of large and small grains in pure Al and the two Al – Mg alloys whereas in the Al– 0.2%Zr alloy no large grains were visible at this temperature and the grain size remained similar to the condition after ECA pressing. This duplex structure of small and large grains persisted in pure Al and the Al–Mg alloys at 523 K but there was a corresponding increase in the proportion of the larger grains. After annealing at 573 K, the pure Al and Al – Mg alloys exhibited only the larger grains with no evidence for the original small grains. A comparison of Figs. 2 –5 shows that the behavior of the pure Al and the two Al–Mg alloys is essentially similar except only that the grain sizes are smaller in the two alloys. By contrast, the fine-grained structure remained stable up to an annealing temperature of 573 K in the Al – 0.2%Zr alloy
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although there is evidence in Fig. 5 for a minor but uniform increase in the average grain size with increasing temperature. The rate of grain growth may be documented over the entire temperature range of static annealing up to 773 K by plotting the average measured grain size against the annealing temperature as shown in Fig. 6 for all five materials: in this plot, the values plotted for the duplex structures represent the average size of the dominant structure. Inspection of Fig. 6 shows that, although the as-pressed grain sizes are different for pure Al and the two Al–Mg alloys, all three materials exhibit extensive and rapid grain growth when the annealing temperature reaches : 473 K. By contrast, the microstructures of the Al–0.12%Zr alloy and the Al–0.2%Zr alloy are stable up to a temperature of : 573 K.
3.3. Values of the 0.2% proof stress The values of the measured 0.2% proof stresses are plotted in Fig. 7 as a function of the annealing temperature for all five materials. For pure Al, it is apparent that the proof stress remains constant up to :473 K but there is an abrupt decrease with increasing temperature up to 573 K and thereafter the 0.2% proof stress
Fig. 5. Microstructures of Al–0.2%Zr after ECA pressing and annealing for 1 h at 423, 473, 523 and 573 K.
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Fig. 6. Variation of the average grain size with annealing temperature for the five materials used in this investigation.
remains reasonably constant. This decrease in the values of the proof stress is associated with the development of the larger grains, as documented in Figs. 2 and 6. The temperature dependence of the 0.2% proof stress is similar also for the two Al – Mg alloys except that the proof stress decreases gradually up to : 473 K and thereafter decreases more rapidly. However, the behavior of the two Al – Zr alloys is different: initially, the proof stress appears to increase slightly with annealing
Fig. 7. Variation of the 0.2% proof stresses with annealing temperature for the five materials used in this investigation.
Fig. 8. Demonstration of the validity of the Hall – Petch relationship.
temperature up to :473 K for both alloys, but thereafter there is an abrupt decrease in the proof stress from : 523 to 623 K and at even higher temperatures the proof stresses are similar to those recorded in the other materials. This initial increase in the proof stress is consistent with an earlier report showing a similar effect for aluminum alloys with Zr contents at and above 0.05% when cold rolled to a reduction ratio of 0.90 [29]. The Hall–Petch equation requires an inverse relationship between the measured proof stress and the reciprocal of the square root of the grain size [30,31].
Fig. 9. Variation of the 0.2% proof stresses with annealing temperature for samples subjected to ECA pressing and CR.
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The validity of this relationship was checked as shown in Fig. 8 where the 0.2% proof stress is plotted against d − 1/2, where d is the grain size. It is apparent that these materials exhibit a reasonably linear relationship within the experimental scatter, thereby confirming the validity of the Hall–Petch relationship over the range of grain sizes attained in this investigation through static annealing: the individual Hall – Petch relationships are given in Fig. 8 for each material, where sy is the 0.2% proof stress. It is important to note that deviations from the simple Hall – Petch relationship may occur in more complex alloys where there are concomitant changes in the precipitate morphology with temperature and time [32].
3.4. Comparison between ECA pressing and cold rolling Fig. 9 plots the 0.2% proof stress against the annealing temperature for pure Al and for the Al –0.12%Zr alloy after either equal-channel angular pressing to a total of four passes, equivalent to a strain of : 4, or CR to an equivalent strain of : 1.7. Inspection shows that, for both materials, the proof stresses after ECA pressing are higher than after CR because of the higher strain. In both materials, the stress level after ECA pressing decreases abruptly at a critical temperature within the range : 450 – 650 K because of the rapid dissipation of the strain. By contrast, the strain dissipates less rapidly after CR so that the decrease in stress with increasing temperature is more gradual. It is interesting to note that, for each material, the transition temperatures associated with the decrease in stress are fairly similar after both ECA pressing and CR. This similarity may be fortuitous because there is experimental evidence from the CR of dilute Al–Zr alloys showing that these transition temperatures are reduced when the alloys are rolled to higher strains [29]. On the other hand, it is reasonable that the transition temperature is higher in the Al – Zr alloy by comparison with pure Al because it is well established that the addition of Zr leads to the presence of Al3Zr particles which inhibit grain growth and thereby increase the recrystallization temperature [29].
4. Discussion Two fundamental requirements must be fulfilled in order to achieve superplasticity in metals: (1) there must be a very small grain size, typically B10 mm, and (2) testing must be conducted at a temperature above : 0.5 Tm [14]. The present experiments show that ECA pressing leads to a very substantial grain refinement in pure Al and in dilute Al – Mg and Al – Zr alloys so that the as-pressed grain sizes are within the superplastic range for all materials, but these ultrafine grain sizes
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are not retained at elevated temperatures unless, as in the Al–Zr alloys, precipitates are present to pin the grain boundaries and minimize grain growth. The present observations on the significance of minor additions of Zr are generally consistent with earlier observations showing the influence of Zr additions on the recrystallization behavior of aluminum after CR [29]. Specifically, it was shown earlier that: (1) the transition temperature is higher in Al alloys containing Zr by comparison with pure Al and (2) there is evidence for an initial increase in stress with temperature in dilute Al–Zr alloys. Both of these trends are apparent also in Fig. 9. However, a possible discrepancy arises because the earlier work showed that the transition temperature decreased with increasing strain in CR whereas the present results, documented in Fig. 9, show fairly similar transition temperatures for each material after both CR to an equivalent strain of :1.7 and ECA pressing to an equivalent strain of :4. These observations suggest, therefore, that there may be significant differences between the rolling and ECA pressing procedures. It is now well established that dilute additions of Mg to aluminum lead to enhanced ductility in coarsegrained materials at elevated temperatures [33–36]. However, this behavior is not associated with superplastic flow but rather it arises because the strain rate sensitivity is high in these alloys at high temperatures when deformation occurs by a viscous glide process in which dislocations drag solute atom atmospheres [37]. The present results reveal, in Figs. 2–4 and in Fig. 6, the retention of smaller grains in the two Al–Mg alloys by comparison with pure Al but, nevertheless, the difference is lost through grain growth at elevated temperatures and it provides no opportunity to achieve superplastic flow in the Al–Mg alloys. By contrast, it is apparent that the ultrafine grain sizes are more stable in the Al–Zr alloys and the grain sizes remain small up to : 600 K. It should be noted that recent experiments have demonstrated a potential for achieving HSR SP in Al–Mg alloys by introducing minor additions of Sc [11]. Specifically, the addition of 0.2% Sc to an Al– 3%Mg alloy led to a grain size on ECA pressing of : 0.2 mm, which is comparable to the present experiments, but an average grain size of B 1 mm was retained to annealing temperatures as high as 673 K because of the presence of a very fine dispersion of coherent Al3Sc precipitates. Under these conditions, HSR SP was achieved in this alloy at 673 K with an elongation of \ 1000% at a strain rate of 3.3 ×10 − 2 s − 1. The present results suggest, however, that elongations of this order are not feasible in Al–Mg alloys in the absence of any precipitates to restrict the grain boundary mobility. It is concluded instead that the simultaneous addition of both Mg and Zr may be
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especially effective in promoting subsequent superplastic ductilities after ECA pressing because the addition of Mg is effective in reducing the grain size through ECA pressing and Zr additions are effective in retaining the ultrafine grain sizes to relatively high testing temperatures: these two trends are evident in Fig. 6.
5. Summary and conclusions 1. Samples of pure Al, Al – 1%Mg, Al – 3%Mg and Al–0.2%Zr were subjected to ECA pressing at room temperature to strains where the microstructures were reasonably homogeneous. The grain sizes in these conditions were : 1.3, 0.45, 0.3 and 0.7 mm for these four materials, respectively. Additional ECA pressing was also conducted on an Al– 0.12%Zr alloy to introduce a less homogeneous ultrafine-grained structure. 2. The ultrafine grain sizes introduced by ECA pressing were not stable at elevated temperatures in the pure Al and Al – Mg alloys but a reasonably finegrained structure was retained in the Al – Zr alloys up to temperatures as high as :600 K. 3. The lack of thermal stability at elevated temperatures shows that pure Al and dilute Al – Mg alloys are not suitable candidate materials for attaining superplasticity through ECA pressing.
Acknowledgements We thank Takayoshi Fujinami, Hiroaki Kawahara and Yoshinori Iwahashi for experimental assistance. This work was supported in part by the Light Metals Educational Foundation of Japan, in part by a Grantin-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan, in part by the Japan Society for the Promotion of Science, and in part by the National Science Foundation of the United States under Grants No. DMR-9625969 and INT-9602919.
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