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Influence of preliminary extrusion conditions on the superplastic properties of a magnesium alloy processed by ECAP
Acta Materialia 55 (2007) 1083–1091 www.actamat-journals.com
Influence of preliminary extrusion conditions on the superplastic properties of a magnesium alloy processed by ECAP Mitsuaki Furui a, Hiroki Kitamura a, Hiroshi Anada a, Terence G. Langdon b
b,*
a Department of System Engineering of Materials and Life Science, Faculty of Engineering, University of Toyama, Toyama 930-8555, Japan Departments of Aerospace and Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA
Received 28 March 2006; received in revised form 11 June 2006; accepted 22 September 2006 Available online 5 December 2006
Abstract The microstructures and properties of a two-phase Mg–8% Li alloy were evaluated in three different conditions: after casting; after casting followed by extrusion at different temperatures and speeds; and after casting, extrusion and processing by ECAP for four passes at room temperature using a die with a channel angle of 135. The results show extrusion introduces significant grain refinement and there is additional refinement in ECAP. In tensile testing, the elongations to failure increase with decreasing extrusion temperature, but are essentially independent of the extrusion speed. The ductilities are low in the cast condition, intermediate in the extruded condition and high after extrusion and ECAP. For the last condition, an exceptionally high elongation of 1780% was achieved at a testing temperature of 473 K. It is shown that it is advantageous to use a die with a channel angle of 135 because it permits pressing at room temperature where grain growth is limited. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Equal-channel angular pressing; Extrusion; Magnesium alloys; Severe plastic deformation; Superplasticity
1. Introduction Over the last decade, considerable attention has been focused on the processing of metals through the application of severe plastic deformation (SPD) [1,2] using procedures such as equal-channel angular pressing (ECAP) where a billet, in the form of a rod or bar, is pressed through a die constrained within a channel bent through a very abrupt angle [3,4]. When materials are processed by ECAP, the grains are usually refined to exceptionally small sizes that are not attained using conventional thermo-mechanical processing. Thus, experiments on pure aluminum with an initial grain size of 1.0 mm showed that the grain size was reduced to 1.3 lm after pressing through a total of only four passes in ECAP [5,6]. Similar results were reported for other fcc metals including copper and nickel with initial grain sizes of 80 lm and 50 lm *
Corresponding author. Tel.: +1 213 740 0491; fax: +1 213 740 8071. E-mail address: [email protected] (T.G. Langdon).
where processing by ECAP reduced the grain sizes to 300 nm [7] and 270 nm [8], respectively. Despite considerable success in using ECAP for the production of ultrafine grain sizes in fcc metals, problems are encountered in extending the use of ECAP to hexagonal close-packed materials such as magnesium. Early experiments showed that the application of ECAP to pure Mg in a cast and annealed condition was sufficient only to reduce the grain size from 400 lm to 120 lm after pressing at 673 K [9] and for a cast and annealed Mg– 0.9% Al alloy the initial grain size of 100 lm was only reduced to 78 lm after pressing at 673 K, to 48 lm after pressing at 573 K and to 17 lm after pressing at 473 K [9]. These results show that it is difficult to achieve substantial grain refinement in hcp materials where there is a limited number of active slip systems and twinning is an important mode of deformation. It was established in earlier work that, in the absence of processing by ECAP, significant grain refinement and superplastic ductilities may be achieved in commercial
1359-6454/$30.00 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.09.027
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magnesium alloys by using conventional extrusion [10,11], because extrusion produces a texture where a majority of the basal (0 0 0 1) planes lie parallel to the extrusion direction [12–14]. Since this texture means that the basal planes are no longer oriented for easy slip in ECAP, it is reasonable to anticipate that shear deformation in ECAP will be accompanied by the activation of a multiplicity of slip systems and this may favor the development of an ultrafine equiaxed microstructure. To verify this proposal, samples of an Mg–0.6% Zr alloy with an initial as-cast grain size of 70 lm were tested using combinations of conventional extrusion and ECAP and then examined using transmission electron microscopy (TEM) [15]. The results showed that casting and ECAP without extrusion produces a heterogeneous microstructure of elongated grains with lengths of 30 lm, casting and extrusion without ECAP produces a microstructure with a reasonably equiaxed grain size of 11 lm and a combination of casting, extrusion and ECAP produces a uniform and equiaxed microstructure with a measured grain size of 1 lm [15]. This two-step procedure of extrusion and ECAP was subsequently designated the EX-ECAP process [16] and it was used successfully to achieve arrays of ultrafine grains and superplastic ductilities in a number of magnesium alloys [16–22]. The results available to date demonstrate that a preliminary extrusion step prior to ECAP provides a valuable tool for increasing the effectiveness of grain refinement. Nevertheless, there have been no attempts to identify the extrusion conditions that lead to the greatest grain refinement and the optimum superplastic ductilities. The present investigation was initiated to address this deficiency by examining the influence of the extrusion speed and the extrusion temperature on the microstructures and tensile properties of a magnesium-based alloy. The experiments were conducted using a two-phase Mg– 8% Li alloy. This material was selected because it is well known that the addition of lithium to magnesium reduces the density [23], increases the ductility and introduces potential for achieving superplastic elongations at elevated temperatures [24]. Furthermore, Mg–Li alloys are of considerable current interest because of their potential for use in portable electronic devices and consumer products. In practice, the addition of lithium introduces a bcc bphase incorporated within an hcp a-phase [25] and for the Mg–8 wt.% Li alloy these two phase distributions are approximately equal. An earlier report showed that this alloy exhibits superplastic elongations after the EX-ECAP process, with a maximum reported elongation of 970% at 473 K when using an initial strain rate of 1.0 · 10 4 s 1 [19]. However, there was no attempt in the earlier investigation to evaluate or optimize the preliminary extrusion conditions. 2. Experimental material and procedures The experiments were conducted using a Mg–8 wt.% Li two-phase alloy containing a and b phases consisting of
solid solutions of Li in hcp Mg and Mg in bcc Li. The alloy was produced from commercial high-purity Mg and Li by melting in a vacuum high-frequency induction furnace in an argon atmosphere without any flux. The molten alloy was cast into steel moulds having diameters of 140 mm and heights of 180 mm and the as-cast material was homogenized in an argon environment at a temperature of 573 K for 86.4 ks. A small portion of the alloy was cut from one of the ingots to test in the homogenized condition and the ends were cut from the remaining ingots to give lengths of 100 mm and average diameters of 50 mm. In order to evaluate the precise significance of the preliminary extrusion condition, these pieces were extruded into rods with diameters of 10 mm using three different extrusion temperatures of 373, 473 and 573 K and three different extrusion speeds of 1, 5 and 10 mm s 1. All extrusions were performed using a reduction ratio of 25:1; this ratio was selected because it is similar to the reduction ratios used in investigations on other Mg alloys [15,16]. Following extrusion, rods were cut into lengths of 60 mm for processing by ECAP. All of the ECAP processing was performed using a solid die containing an internal channel having a diameter of 10 mm bent through an internal angle of U = 135. This channel angle is large by comparison with conventional ECAP, where experiments have shown that an optimum equilibrium microstructure is achieved most readily when using a die with U = 90 [26]. However, it was found in early experiments that the Mg–8% Li alloy is only capable of sustaining a single pass without any cracking at room temperature when the pressing is performed using a die with a high angle between the channels. Thus, recognizing that pressing at room temperature is also more convenient in any future industrial applications, a die was selected having an internal channel angle of U = 135. A preliminary evaluation showed that the Mg–8% Li alloy was easily pressed up to 10 passes at room temperature without any surface cracking when using a die with a channel angle of 135. It is instructive to note that a similar approach was reported in an earlier investigation of the pressing of commercial purity tungsten where it was necessary to increase the die angle to 110 in order to avoid cracking of the tungsten billets when pressing at 1273 K [27]. The ECAP die used in this work also contained an angle of W 20 at the outer arc of curvature where the two parts of the channel intersect. It follows from the conventional relationship for the strain imposed in ECAP [28] that values of U = 135 and W 20 lead to an imposed strain of 0.5 on each separate pass of the billet through the die. In the present experiments the billets were pressed for a total of four passes at room temperature, corresponding to an imposed strain of 2, using a pressing speed of 7.5 mm s 1 and processing route BC in which the billets are rotated by 90 in the same sense between each pass [29]. Tensile specimens were machined from billets prepared in three different conditions. First, for the unpressed material after casting, henceforth designated the ‘‘Cast’’ condi-
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tion. Second, for the unpressed material after casting followed by conventional extrusion, henceforth designated the ‘‘Cast + Extrude’’ condition. Third, for the cast material after conventional extrusion followed by ECAP through four passes at room temperature, henceforth designated the ‘‘Cast + Extrude + ECAP’’ condition. All of the tensile specimens had gauge lengths of 4 mm and cross-sectional areas of 3 · 2 mm2. For the Cast + Extrude and Cast + Extrude + ECAP conditions, the gauge lengths lay parallel to the extrusion and pressing directions. These specimens were pulled to failure at a temperature of 473 K using a testing machine operating at a constant rate of cross-head displacement and with initial strain rates in the range from 1.5 · 10 4 to 1.5 · 10 1 s 1. The microstructures were examined in several different conditions using optical microscopy. For the materials subjected to extrusion or pressing, the structures were examined on planes cut parallel to the working directions. All specimens were prepared by sectioning, mechanically polishing and then etching in a 10% HNO3 solution in ethanol to reveal the interphase boundaries. The microstructures were characterized by measuring the average widths of the a and b phases in directions perpendicular to the working directions using the linear intercept method. 3. Experimental results 3.1. Microstructural characteristics Representative microstructures for the three different processing conditions are shown in Fig. 1, where (a) is the Cast condition, (b) is the Cast + Extrude condition where the sample was extruded at 373 K at a speed of 1 mm s 1 and (c) is the Cast + Extrude + ECAP condition where the sample was extruded under the same conditions as in (b) and then pressed for four passes at room temperature: for Fig. 1b and c, the extrusion and pressing directions were horizontal. In Fig. 1a, the a-phase is the lighter discontinuous phase, the b-phase is the darker continuous phase and thus in the as-cast condition the a-phase is elongated and dispersed within a matrix of the b-phase. The volume fractions of the a and b phases were measured
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as 55% and 45% in this condition, respectively. By contrast, in Fig. 1b and c there is significant microstructural refinement and the a and b phases form banded structures with the bands lying reasonably parallel to the extrusion and pressing directions. There is also a clear difference between the Cast + Extrude and the Cast + Extrude + ECAP conditions because Fig. 1c shows that the a and b phases are finer and generally more uniform after ECAP. Optical microscopy was used to measure average phase widths of 60–70 lm in the Cast condition, 3–5 lm in the Cast + Extrude condition after extrusion at 373 K and 1–3 lm in the Cast + Extrude + ECAP condition. The last result is consistent with an earlier report of the presence of a homogeneous microstructure in a Mg–8% Li–1% Al alloy after extrusion and ECAP through four passes at 403 K [30]. As reported earlier [19], microstructural observations by TEM revealed poorly defined arrays of reasonably equiaxed grains within these separate phases, with average grain sizes in the Cast + Extrude + ECAP condition of the order of 1 lm. To obtain more detailed information on the nature of the microstructural refinement associated with the extrusion and ECAP processes, additional microstructures are shown in Fig. 2 for the Cast + Extrude + ECAP condition: again the extrusion and pressing directions are horizontal. All of these microstructures were obtained after ECAP through four passes at room temperature, but the samples were processed using different extrusion conditions: in Fig. 2a–c, the samples were extruded at a temperature of 473 K at speeds of 1, 5 and 10 mm s 1, respectively, and in Fig. 2d the sample was extruded at a higher temperature of 573 K at a speed of 1 mm s 1. Two conclusions may be drawn by comparing the various microstructures shown in Figs. 1 and 2. First, inspection of Fig. 1c, Fig. 2a and d, relating to the same extrusion speed of 1 mm s 1 at three different temperatures, shows that the a and b phases both tend to coarsen with increasing extrusion temperature. Second, inspection of Fig. 2a– c, relating to three different extrusion speeds at a temperature of 473 K, shows that the extrusion speed has an essentially negligible influence on the microstructure attained after ECAP.
Fig. 1. Optical micrographs showing the alloy in: (a) the Cast condition, (b) the Cast + Extrude condition after extrusion at 373 K at a speed of 1 mm s 1 and (c) the Cast + Extrude + ECAP condition after extrusion at 373 K at a speed of 1 mm s 1 and ECAP for four passes at room temperature. The extrusion and pressing directions are horizontal in (b) and (c).
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Fig. 2. Optical micrographs showing the alloy after ECAP through four passes at room temperature following different extrusion conditions, where the samples in (a), (b) and (c) were extruded at a temperature of 473 K at speeds of 1, 5 and 10 mm s 1, respectively, and the sample in (d) was extruded at 573 K at a speed of 1 mm s 1. The extrusion and pressing directions are horizontal in all micrographs.
In order to obtain a more precise characterization of the microstructures for each condition, the width distributions of the a and b phases were recorded and plotted in the form of histograms as shown in Figs. 3 and 4 for the Cast + Extrude and the Cast + Extrude + ECAP conditions, respectively; the extrusion speed was 1 mm s 1 for all of these plots, the extrusion temperature is denoted by Tex, the upper and lower histograms are for the a and b phases, the average a widths determined from these histograms are denoted by w b for the b phase, and the plots denoted for the a phase and w by (a), (b) and (c) are for extrusion temperatures of 373, 473 and 573 K, respectively. It is immediately apparent from these plots that the histograms are very similar for both the a and w b , are a and the b phases and the average values, w essentially identical to within the accuracy of the measurements. Furthermore, although there is an increase in the average widths of the two phases when the extrusion temperature is increased from 373 to 573 K, this increase is by a factor of <3. Finally, processing by ECAP after the extrusion step gives a further, but relatively minor, reduction in the average width of the phases by a factor of <2. Another striking feature of these histograms is the consistent increase in the widths of the distributions with increasing extrusion temperature in both the Cast + Extrude and the Cast + Extrude + ECAP conditions. Furthermore, at the lowest extrusion temperature of 373 K, the distribution is especially narrow for the Cast + Extrude + ECAP condition shown in Fig. 4a, suggesting the presence of a reasonably uniform microstructure under these conditions. 3.2. Tensile properties after processing Several recent reports have demonstrated the potential for achieving high, and often superplastic, ductilities in
Mg-based alloys through the use of the two-step process of extrusion followed by ECAP [15–17,19–22]. To evaluate the mechanical properties in the present investigation, specimens were pulled to failure over a range of testing conditions. Fig. 5 shows the influence on the measured elongations to failure of (a) the extrusion temperature, Tex, and (b) the extrusion speed, rex, where all samples were pulled to failure at an absolute testing temperature, T, of 473 K using an initial strain rate, e_ , of 1.5 · 10 3 s 1. A first impression from inspection of these plots is that the data are remarkably consistent: thus, the elongations to failure are always higher, by a factor of two or more, in the specimens processed to incorporate the additional step of ECAP. Furthermore, this is consistent with the microstructural observations in Figs. 3 and 4, where the phase widths are smaller in the Cast + Extrude + ECAP samples. The ductilities also increase with decreasing extrusion temperature and thus with decreasing phase width. By contrast, Fig. 5b shows that the extrusion speed has an insignificant effect on the ductilities achieved after ECAP, at least within the limited range from 1 to 10 mm s 1. For the extrusion conditions of Tex = 373 K and rex = 1 mm s 1, Fig. 6 records the measured elongations to failure as a function of the initial testing strain rate for the three conditions of Cast, Cast + Extrude and Cast + Extrude + ECAP. Again, all of the results are very consistent and exceptionally high elongations are attained at the lowest strain rates for the samples subjected to ECAP. In practice, the highest elongation recorded in this work was 1780% when using an initial strain rate of 1.5 · 10 4 s 1. By comparison, the results in Fig. 6 show that the measured elongations to failure at this strain rate are only 200% in the Cast condition and 630% in the
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60
Frequency (%)
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α-phase wα = 3.5 μm
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Mg-8%Li Tex = 473 K Cast + Extrude
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Frequency (%)
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Width of phase (μm) Fig. 3. Width distributions for the a (upper) and b (lower) phases for the Cast + Extrusion condition after extrusion at temperatures of: (a) 373, (b) 473 a and w b denote the measured average widths for the a and b phases. and (c) 573 K with an extrusion speed of 1 mm s 1; w
Cast + Extrude condition. At testing strain rates below 10 2 s 1, the elongations are consistently low in the ascast material, intermediate in the cast material after extrusion and very high when the extruded samples are also processed by ECAP. By contrast, all processing conditions exhibit fairly similar elongations to failure at strain rates above 10 2 s 1.
4. Discussion 4.1. Implications of the different processing procedures The first important conclusion from these experiments is that the overall ductility of the Mg–8% Li alloy may be significantly enhanced by introducing an extrusion step and it
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Mg-8%Li Tex = 373 K Cast + Extrude + ECAP
Frequency (%)
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Mg-8%Li Tex = 473 K Cast + Extrude + ECAP
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Width of phase (μm) Fig. 4. Width distributions for the a (upper) and b (lower) phases for the Cast + Extrusion + ECAP condition after extrusion at temperatures of (a) 373, a and w b denote the measured average (b) 473 and (c) 573 K with an extrusion speed of 1 mm s 1 and ECAP through four passes at room temperature; w widths for the a and b phases.
may be even further enhanced by subjecting the extruded material to processing by ECAP. The elongations recorded in Fig. 6 after this two-step process are exceptionally high for this Mg–Li alloy. For example, there is a report that thermo-mechanical processing may be used to reduce the
grain size in the Mg–8% Li alloy to 8 lm and in this condition the maximum recorded elongation was 640% at a strain rate of 1.7 · 10 4 s 1 and temperature of 573 K [31]. An earlier investigation of the same Mg–8% Li alloy gave a maximum elongation of 970% at an initial strain
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a
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Elongation to failure (%)
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Elongation to failure (%)
T = 473 K . ε = 1.5 x 10-3 s-1
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Cast + Extrude Cast + Extrude + ECAP 0 0
2
4
6
8
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12
Extrusion speed, rex (mm s-1) Fig. 5. Variation of the elongation to failure for the Cast + Extrude and the Cast + Extrude + ECAP conditions with: (a) the extrusion temperature for samples extruded at a speed of 1 mm s 1 and (b) the extrusion speed for samples extruded at a temperature of 473 K; all specimens were pulled to failure at 473 K using an initial strain rate of 1.5 · 10 3 s 1.
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Elongation to failure (%)
T = 473 K
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Cast Cast + Extrude Cast + Extrude + ECAP
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rate of 1.0 · 10 4 s 1 and testing temperature of 473 K when using a material where the extrusion was conducted under conditions of Tex = 573 K and rex = 5 mm s 1 followed by ECAP through two passes at 473 K using a die with a channel angle of U = 110 [19]. The latter result is generally consistent with the present experiments when it is noted that larger grain sizes are introduced at the higher extrusion temperatures and it is well known there is less grain refinement when materials are processed by ECAP at higher temperatures [32]. In addition, the maximum elongation of 1780% attained in these experiments, although less than the maximum elongation of 2040% reported recently for a ZK60 alloy processed by ECAP [33,34], is nevertheless exceptionally high for a magnesium-based alloy. A significant conclusion follows from inspection of these various results. It appears that the selection of a die with a channel angle of 135 gives no significant reduction in the measured elongations to failure but, on the contrary, it provides the capability of performing the ECAP at room temperature and this leads to greater microstructural refinement and hence to higher superplastic ductilities. This result would have been impossible to achieve using an ECAP die with a sharper channel angle (for example, 90 or 110) because of the necessity of conducting the ECAP at a higher pressing temperature and thereby introducing significant grain growth during processing. The second important conclusion is that the extrusion step, which is intermediate between casting and ECAP, may be significantly optimized by extruding at a lower temperature, as demonstrated by the results shown in Fig. 5a. This result is a direct consequence of the greater microstructural refinement demonstrated in Fig. 4a, where the a and b phases have smaller average widths giving an improved microstructural homogeneity. The result is consistent also with earlier data reported for the AZ31 (Mg– 3% Al–1% Zn) alloy which show that a more homogeneous microstructure may be achieved by extruding the alloy at a lower processing temperature [35]. Conversely, it follows from Fig. 5b that the speed of the extrusion has little or no effect on the achievement of high ductilities after ECAP. 4.2. The flow process in the Mg–8% Li alloy
1000
0 10-4
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-1
Strain rate (s ) Fig. 6. Variation of the elongation to failure with the initial strain rate at a testing temperature of 473 K for samples in the Cast, Cast + Extrude and Cast + Extrude + ECAP conditions: the extrusion was conducted at 373 K using an extrusion speed of 1 mm s 1.
In order to obtain information on the flow process in the superplastic region, Fig. 7 shows logarithmic plots of the 0.2% proof stress versus the initial strain rate for (a) the Cast + Extrude condition and (b) the Cast + Extrude + ECAP condition: these points correspond to the experimental conditions shown in Fig. 6, where Tex = 373 K, rex = 1 mm s 1 and the tensile tests were conducted at T = 473 K. Although the datum points in Fig. 7 are very limited, the results suggest a transition from a strain rate sensitivity, m, of 0.3 at the faster strain rates where the elongations to failure are low to 0.5 at the slower strain rates where the alloy exhibits superplastic behavior. These numbers
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0.2 % proof stress (MPa)
a
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superplastic flow but it is consistent with other recent reports of superplasticity in Mg-based alloys processed by ECAP. For example, superplastic flow was reported at a testing temperature of 423 K in an extruded commercial AZ31 alloy after processing by ECAP [18], where this temperature corresponds to 0.46 Tm. Similarly, elongations of up to 1190% were reported in an extruded AZ61 alloy processed by ECAP and tested in tension at 448 K [43]. The present results are consistent with these earlier investigations but they provide also the first demonstration that it is possible to optimize the superplastic properties by adjusting the conditions associated with the preliminary extrusion prior to processing by ECAP.
100
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0.5 1
Mg-8%Li Tex = 373 K rex = 1 mm s-1
T = 473 K Cast + Extrude 1 10-4
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5. Summary and conclusions
Strain rate (s-1)
0.2 % proof stress (MPa)
b
100
0.3 1
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Mg-8%Li T = 473 K Cast + Extrude + ECAP 1 10-4
10-3
10-2
Tex = 373 K rex = 1 mm s-1
10-1
100
-1
Strain rate (s ) Fig. 7. Variation of the 0.2% proof stress with the initial strain rate at a testing temperature of 473 K for: (a) the Cast + Extrude condition and (b) the Cast + Extrude + ECAP condition; the extrusion was conducted at 373 K using an extrusion speed of 1 mm s 1 and the slopes of the lines denote the strain rate sensitivities.
are consistent with a transition in the flow process from a regime controlled by the intragranular movement of dislocations at the faster strain rates [36] to superplastic flow controlled by grain boundary sliding at the slower strain rates [37]. Furthermore, a strain rate sensitivity of m 0.5 is consistent with the flow behavior observed when superplastic ductilities were achieved in an Al–3% Mg–0.2% Sc alloy after processing by ECAP [38,39] and the ductilities recorded in the Cast + Extrude + ECAP condition at the two slowest strain rates in Fig. 6 are generally consistent with the expectations from the standard plot of the strain rate sensitivity vs. elongation to failure for superplastic alloys [40]. The high tensile elongations at the two lowest strain rates in Fig. 6 are also reasonable for an alloy having a grain size of the order of 1 lm [41]. The absolute melting temperature, Tm, for the Mg–8% Li alloy is 877 K [42] so that the testing temperature of 473 K in Figs. 6 and 7 corresponds to an homologous temperature of 0.54 Tm. This temperature is relatively low for
1. A two-phase Mg–8% Li alloy was examined and tested in three different conditions: in the Cast condition, in the Cast + Extrude condition where extrusion was conducted at different temperatures and speeds, and in the Cast + Extrude + ECAP condition where the alloy was extruded under different conditions and then processed by ECAP for four passes at room temperature using a die with a channel angle of 135. 2. Microstructural observations showed that there was a very significant reduction in the average widths of the phases due to extrusion and there was an additional microstructural refinement due to ECAP. Typically, the average phase widths were 60–70 lm in the as-cast condition, 3–10 lm after extrusion and 1–7 lm after extrusion and ECAP. An exceptionally fine and uniform microstructure was achieved after ECAP when the extrusion was conducted at the lowest temperature of 373 K. 3. The elongations to failure in tensile testing increased with decreasing extrusion temperature but they were reasonably independent of the extrusion speed within the range from 1 to 10 mm s 1. 4. For an extrusion temperature of 373 K and an extrusion speed of 1 mm s 1, the elongations to failure at 473 K were high in the Cast + Extrude + ECAP condition, intermediate in the Cast + Extrude condition and low in the Cast condition for testing strain rates lower than 10 2 s 1. The highest elongation achieved in this investigation was 1780% in the Cast + Extrude + ECAP condition using an initial strain rate of 1.5 · 10 4 s 1. 5. The use of a die with a channel angle of 135 proved to be advantageous in this investigation because it permitted processing by ECAP at room temperature and this restricted the extent of grain growth during the pressing operation.
Acknowledgements We are grateful to Professor Shigeharu Kamado (Nagaoka University of Technology) and Professor Zenji Horita
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(Kyushu University) for the provision of experimental facilities and Megumi Kawasaki for assistance with the figures. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan under Grant No. 17560629 and in part by the National Science Foundation of the United States under Grant No. DMR0243331. References [1] Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mater Sci 2000;45:103. [2] Valiev RZ, Estrin Y, Horita Z, Langdon TG, Zehetbauer MJ, Zhu YT. JOM 2006;58(4):33. [3] Segal VM. Mater Sci Eng 1995;A197:157. [4] Segal VM. Mater Sci Eng 2004;A386:269. [5] Iwahashi Y, Horita Z, Nemoto M, Langdon TG. Acta Mater 1997;45:4733. [6] Iwahashi Y, Horita Z, Nemoto M, Langdon TG. Acta Mater 1998;46:3317. [7] Neishi K, Horita Z, Langdon TG. Mater Sci Eng 2002;A325:54. [8] Komura S, Horita Z, Nemoto M, Langdon TG. J Mater Res 1999;14:4044. [9] Yamashita A, Horita Z, Langdon TG. Mater Sci Eng 2001;A300:142. [10] Kainer KU. In: Lorimer GE, editor. Proceedings of the third international magnesium conference. London: The Institute of Materials; 1997. p. 533. [11] Mukai T, Watanabe H, Higashi K. Mater Sci Technol 2000;16:1314. [12] Wilson DV. J Inst Met 1970;98:133. [13] Hilpert M, Styczynski A, Kiese J, Wagner L. In: Mordike BL, Kainer KU, editors. Magnesium alloys and their applications. Weinheim: Wiley–VCH; 1998. p. 319. [14] Mukai T, Yamanoi M, Watanabe H, Higashi K. Scripta Mater 2001;45:89. [15] Horita Z, Matsubara K, Makii K, Langdon TG. Scripta Mater 2002;47:255. [16] Matsubara K, Miyahara Y, Horita Z, Langdon TG. Acta Mater 2003;51:3073. [17] Matsubara K, Miyahara Y, Horita Z, Langdon TG. Metall Mater Trans 2004;35A:1735. [18] Lin HK, Huang JC, Langdon TG. Mater Sci Eng 2005;A402:250.
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[19] Furui M, Xu C, Aida T, Inoue M, Anada H, Langdon TG. Mater Sci Eng 2005;A410–411:439. [20] Miyahara Y, Matsubara K, Horita Z, Langdon TG. Metall Mater Trans 2005;36A:1705. [21] Miyahara Y, Horita Z, Langdon TG. Mater Sci Eng 2006;A420:240. [22] Figueiredo RB, Langdon TG. Mater Sci Eng 2006;A430:151. [23] Haferkamp H, Boehm R, Holzkamp U, Jaschik C, Kaese V, Niemeyer M. Mater Trans 2001;42:201. [24] Inoue M, Yamaguchi T, Kamado S, Kojima Y. J Jpn Inst Light Met 1998;48:174. [25] Hori S, Fujitani W. J Jpn Inst Light Met 1990;40:285. [26] Nakashima K, Horita Z, Nemoto M, Langdon TG. Acta Mater 1998;46:1589. [27] Alexandrov IV, Raab GI, Shestakova LO, Valiev RZ, Dowding RJ. In: Greenfield MS, Oakes JJ, editors. Tungsten, hard metals and refractory alloys, vol. 5. Princeton (NJ): Metal Powder Industries Federation; 2000. p. 27. [28] Iwahashi Y, Wang J, Horita Z, Nemoto M, Langdon TG. Scripta Mater 1996;35:143. [29] Furukawa M, Iwahashi Y, Horita Z, Nemoto M, Langdon TG. Mater Sci Eng 1998;A257:328. [30] Liu T, Zhang W, Wu SD, Jiang CB, Li SX, Xu YB. Mater Sci Eng 2003;A260:345. [31] Fujitani W, Furushiro N, Hori S, Kumeyama K. J Jpn Inst Light Met 1992;42:125. [32] Yamashita A, Yamaguchi D, Horita Z, Langdon TG. Mater Sci Eng 2000;A287:100. [33] Lapovok R, Cottam R, Thomson PF, Estrin Y. J Mater Res 2005;20:1375. [34] Lapovok R, Thomson PF, Cottam R, Estrin Y. Mater Sci Eng 2005;A410–411:390. [35] Murai T. J Jpn Inst Light Met 2004;54:472. [36] Langdon TG. Z Metallkd 2005;96:522. [37] Langdon TG. Acta Metall Mater 1994;42:2437. [38] Komura S, Horita Z, Furukawa M, Nemoto M, Langdon TG. Metall Mater Trans 2001;32A:707. [39] Balasubramanian N, Langdon TG. Mater Sci Eng 2005;A410– 411:476. [40] Woodford DA. Trans Am Soc Met 1969;62:291. [41] Langdon TG. Metall Trans 1982;13A:689. [42] Massalski TB, editor. Binary alloy phase diagrams, vol. 2. Metals Park (OH): American Society for Metals; 1986. p. 1483. [43] Yoshida Y, Arai K, Itoh S, Kamado S, Kojima Y. Mater Trans 2004;45:2537.
Influence of preliminary extrusion conditions on the superplastic properties of a magnesium alloy processed by ECAP Mitsuaki Furui a, Hiroki Kitamura a, Hiroshi Anada a, Terence G. Langdon b
b,*
a Department of System Engineering of Materials and Life Science, Faculty of Engineering, University of Toyama, Toyama 930-8555, Japan Departments of Aerospace and Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA
Received 28 March 2006; received in revised form 11 June 2006; accepted 22 September 2006 Available online 5 December 2006
Abstract The microstructures and properties of a two-phase Mg–8% Li alloy were evaluated in three different conditions: after casting; after casting followed by extrusion at different temperatures and speeds; and after casting, extrusion and processing by ECAP for four passes at room temperature using a die with a channel angle of 135. The results show extrusion introduces significant grain refinement and there is additional refinement in ECAP. In tensile testing, the elongations to failure increase with decreasing extrusion temperature, but are essentially independent of the extrusion speed. The ductilities are low in the cast condition, intermediate in the extruded condition and high after extrusion and ECAP. For the last condition, an exceptionally high elongation of 1780% was achieved at a testing temperature of 473 K. It is shown that it is advantageous to use a die with a channel angle of 135 because it permits pressing at room temperature where grain growth is limited. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Equal-channel angular pressing; Extrusion; Magnesium alloys; Severe plastic deformation; Superplasticity
1. Introduction Over the last decade, considerable attention has been focused on the processing of metals through the application of severe plastic deformation (SPD) [1,2] using procedures such as equal-channel angular pressing (ECAP) where a billet, in the form of a rod or bar, is pressed through a die constrained within a channel bent through a very abrupt angle [3,4]. When materials are processed by ECAP, the grains are usually refined to exceptionally small sizes that are not attained using conventional thermo-mechanical processing. Thus, experiments on pure aluminum with an initial grain size of 1.0 mm showed that the grain size was reduced to 1.3 lm after pressing through a total of only four passes in ECAP [5,6]. Similar results were reported for other fcc metals including copper and nickel with initial grain sizes of 80 lm and 50 lm *
Corresponding author. Tel.: +1 213 740 0491; fax: +1 213 740 8071. E-mail address: [email protected] (T.G. Langdon).
where processing by ECAP reduced the grain sizes to 300 nm [7] and 270 nm [8], respectively. Despite considerable success in using ECAP for the production of ultrafine grain sizes in fcc metals, problems are encountered in extending the use of ECAP to hexagonal close-packed materials such as magnesium. Early experiments showed that the application of ECAP to pure Mg in a cast and annealed condition was sufficient only to reduce the grain size from 400 lm to 120 lm after pressing at 673 K [9] and for a cast and annealed Mg– 0.9% Al alloy the initial grain size of 100 lm was only reduced to 78 lm after pressing at 673 K, to 48 lm after pressing at 573 K and to 17 lm after pressing at 473 K [9]. These results show that it is difficult to achieve substantial grain refinement in hcp materials where there is a limited number of active slip systems and twinning is an important mode of deformation. It was established in earlier work that, in the absence of processing by ECAP, significant grain refinement and superplastic ductilities may be achieved in commercial
1359-6454/$30.00 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.09.027
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magnesium alloys by using conventional extrusion [10,11], because extrusion produces a texture where a majority of the basal (0 0 0 1) planes lie parallel to the extrusion direction [12–14]. Since this texture means that the basal planes are no longer oriented for easy slip in ECAP, it is reasonable to anticipate that shear deformation in ECAP will be accompanied by the activation of a multiplicity of slip systems and this may favor the development of an ultrafine equiaxed microstructure. To verify this proposal, samples of an Mg–0.6% Zr alloy with an initial as-cast grain size of 70 lm were tested using combinations of conventional extrusion and ECAP and then examined using transmission electron microscopy (TEM) [15]. The results showed that casting and ECAP without extrusion produces a heterogeneous microstructure of elongated grains with lengths of 30 lm, casting and extrusion without ECAP produces a microstructure with a reasonably equiaxed grain size of 11 lm and a combination of casting, extrusion and ECAP produces a uniform and equiaxed microstructure with a measured grain size of 1 lm [15]. This two-step procedure of extrusion and ECAP was subsequently designated the EX-ECAP process [16] and it was used successfully to achieve arrays of ultrafine grains and superplastic ductilities in a number of magnesium alloys [16–22]. The results available to date demonstrate that a preliminary extrusion step prior to ECAP provides a valuable tool for increasing the effectiveness of grain refinement. Nevertheless, there have been no attempts to identify the extrusion conditions that lead to the greatest grain refinement and the optimum superplastic ductilities. The present investigation was initiated to address this deficiency by examining the influence of the extrusion speed and the extrusion temperature on the microstructures and tensile properties of a magnesium-based alloy. The experiments were conducted using a two-phase Mg– 8% Li alloy. This material was selected because it is well known that the addition of lithium to magnesium reduces the density [23], increases the ductility and introduces potential for achieving superplastic elongations at elevated temperatures [24]. Furthermore, Mg–Li alloys are of considerable current interest because of their potential for use in portable electronic devices and consumer products. In practice, the addition of lithium introduces a bcc bphase incorporated within an hcp a-phase [25] and for the Mg–8 wt.% Li alloy these two phase distributions are approximately equal. An earlier report showed that this alloy exhibits superplastic elongations after the EX-ECAP process, with a maximum reported elongation of 970% at 473 K when using an initial strain rate of 1.0 · 10 4 s 1 [19]. However, there was no attempt in the earlier investigation to evaluate or optimize the preliminary extrusion conditions. 2. Experimental material and procedures The experiments were conducted using a Mg–8 wt.% Li two-phase alloy containing a and b phases consisting of
solid solutions of Li in hcp Mg and Mg in bcc Li. The alloy was produced from commercial high-purity Mg and Li by melting in a vacuum high-frequency induction furnace in an argon atmosphere without any flux. The molten alloy was cast into steel moulds having diameters of 140 mm and heights of 180 mm and the as-cast material was homogenized in an argon environment at a temperature of 573 K for 86.4 ks. A small portion of the alloy was cut from one of the ingots to test in the homogenized condition and the ends were cut from the remaining ingots to give lengths of 100 mm and average diameters of 50 mm. In order to evaluate the precise significance of the preliminary extrusion condition, these pieces were extruded into rods with diameters of 10 mm using three different extrusion temperatures of 373, 473 and 573 K and three different extrusion speeds of 1, 5 and 10 mm s 1. All extrusions were performed using a reduction ratio of 25:1; this ratio was selected because it is similar to the reduction ratios used in investigations on other Mg alloys [15,16]. Following extrusion, rods were cut into lengths of 60 mm for processing by ECAP. All of the ECAP processing was performed using a solid die containing an internal channel having a diameter of 10 mm bent through an internal angle of U = 135. This channel angle is large by comparison with conventional ECAP, where experiments have shown that an optimum equilibrium microstructure is achieved most readily when using a die with U = 90 [26]. However, it was found in early experiments that the Mg–8% Li alloy is only capable of sustaining a single pass without any cracking at room temperature when the pressing is performed using a die with a high angle between the channels. Thus, recognizing that pressing at room temperature is also more convenient in any future industrial applications, a die was selected having an internal channel angle of U = 135. A preliminary evaluation showed that the Mg–8% Li alloy was easily pressed up to 10 passes at room temperature without any surface cracking when using a die with a channel angle of 135. It is instructive to note that a similar approach was reported in an earlier investigation of the pressing of commercial purity tungsten where it was necessary to increase the die angle to 110 in order to avoid cracking of the tungsten billets when pressing at 1273 K [27]. The ECAP die used in this work also contained an angle of W 20 at the outer arc of curvature where the two parts of the channel intersect. It follows from the conventional relationship for the strain imposed in ECAP [28] that values of U = 135 and W 20 lead to an imposed strain of 0.5 on each separate pass of the billet through the die. In the present experiments the billets were pressed for a total of four passes at room temperature, corresponding to an imposed strain of 2, using a pressing speed of 7.5 mm s 1 and processing route BC in which the billets are rotated by 90 in the same sense between each pass [29]. Tensile specimens were machined from billets prepared in three different conditions. First, for the unpressed material after casting, henceforth designated the ‘‘Cast’’ condi-
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tion. Second, for the unpressed material after casting followed by conventional extrusion, henceforth designated the ‘‘Cast + Extrude’’ condition. Third, for the cast material after conventional extrusion followed by ECAP through four passes at room temperature, henceforth designated the ‘‘Cast + Extrude + ECAP’’ condition. All of the tensile specimens had gauge lengths of 4 mm and cross-sectional areas of 3 · 2 mm2. For the Cast + Extrude and Cast + Extrude + ECAP conditions, the gauge lengths lay parallel to the extrusion and pressing directions. These specimens were pulled to failure at a temperature of 473 K using a testing machine operating at a constant rate of cross-head displacement and with initial strain rates in the range from 1.5 · 10 4 to 1.5 · 10 1 s 1. The microstructures were examined in several different conditions using optical microscopy. For the materials subjected to extrusion or pressing, the structures were examined on planes cut parallel to the working directions. All specimens were prepared by sectioning, mechanically polishing and then etching in a 10% HNO3 solution in ethanol to reveal the interphase boundaries. The microstructures were characterized by measuring the average widths of the a and b phases in directions perpendicular to the working directions using the linear intercept method. 3. Experimental results 3.1. Microstructural characteristics Representative microstructures for the three different processing conditions are shown in Fig. 1, where (a) is the Cast condition, (b) is the Cast + Extrude condition where the sample was extruded at 373 K at a speed of 1 mm s 1 and (c) is the Cast + Extrude + ECAP condition where the sample was extruded under the same conditions as in (b) and then pressed for four passes at room temperature: for Fig. 1b and c, the extrusion and pressing directions were horizontal. In Fig. 1a, the a-phase is the lighter discontinuous phase, the b-phase is the darker continuous phase and thus in the as-cast condition the a-phase is elongated and dispersed within a matrix of the b-phase. The volume fractions of the a and b phases were measured
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as 55% and 45% in this condition, respectively. By contrast, in Fig. 1b and c there is significant microstructural refinement and the a and b phases form banded structures with the bands lying reasonably parallel to the extrusion and pressing directions. There is also a clear difference between the Cast + Extrude and the Cast + Extrude + ECAP conditions because Fig. 1c shows that the a and b phases are finer and generally more uniform after ECAP. Optical microscopy was used to measure average phase widths of 60–70 lm in the Cast condition, 3–5 lm in the Cast + Extrude condition after extrusion at 373 K and 1–3 lm in the Cast + Extrude + ECAP condition. The last result is consistent with an earlier report of the presence of a homogeneous microstructure in a Mg–8% Li–1% Al alloy after extrusion and ECAP through four passes at 403 K [30]. As reported earlier [19], microstructural observations by TEM revealed poorly defined arrays of reasonably equiaxed grains within these separate phases, with average grain sizes in the Cast + Extrude + ECAP condition of the order of 1 lm. To obtain more detailed information on the nature of the microstructural refinement associated with the extrusion and ECAP processes, additional microstructures are shown in Fig. 2 for the Cast + Extrude + ECAP condition: again the extrusion and pressing directions are horizontal. All of these microstructures were obtained after ECAP through four passes at room temperature, but the samples were processed using different extrusion conditions: in Fig. 2a–c, the samples were extruded at a temperature of 473 K at speeds of 1, 5 and 10 mm s 1, respectively, and in Fig. 2d the sample was extruded at a higher temperature of 573 K at a speed of 1 mm s 1. Two conclusions may be drawn by comparing the various microstructures shown in Figs. 1 and 2. First, inspection of Fig. 1c, Fig. 2a and d, relating to the same extrusion speed of 1 mm s 1 at three different temperatures, shows that the a and b phases both tend to coarsen with increasing extrusion temperature. Second, inspection of Fig. 2a– c, relating to three different extrusion speeds at a temperature of 473 K, shows that the extrusion speed has an essentially negligible influence on the microstructure attained after ECAP.
Fig. 1. Optical micrographs showing the alloy in: (a) the Cast condition, (b) the Cast + Extrude condition after extrusion at 373 K at a speed of 1 mm s 1 and (c) the Cast + Extrude + ECAP condition after extrusion at 373 K at a speed of 1 mm s 1 and ECAP for four passes at room temperature. The extrusion and pressing directions are horizontal in (b) and (c).
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Fig. 2. Optical micrographs showing the alloy after ECAP through four passes at room temperature following different extrusion conditions, where the samples in (a), (b) and (c) were extruded at a temperature of 473 K at speeds of 1, 5 and 10 mm s 1, respectively, and the sample in (d) was extruded at 573 K at a speed of 1 mm s 1. The extrusion and pressing directions are horizontal in all micrographs.
In order to obtain a more precise characterization of the microstructures for each condition, the width distributions of the a and b phases were recorded and plotted in the form of histograms as shown in Figs. 3 and 4 for the Cast + Extrude and the Cast + Extrude + ECAP conditions, respectively; the extrusion speed was 1 mm s 1 for all of these plots, the extrusion temperature is denoted by Tex, the upper and lower histograms are for the a and b phases, the average a widths determined from these histograms are denoted by w b for the b phase, and the plots denoted for the a phase and w by (a), (b) and (c) are for extrusion temperatures of 373, 473 and 573 K, respectively. It is immediately apparent from these plots that the histograms are very similar for both the a and w b , are a and the b phases and the average values, w essentially identical to within the accuracy of the measurements. Furthermore, although there is an increase in the average widths of the two phases when the extrusion temperature is increased from 373 to 573 K, this increase is by a factor of <3. Finally, processing by ECAP after the extrusion step gives a further, but relatively minor, reduction in the average width of the phases by a factor of <2. Another striking feature of these histograms is the consistent increase in the widths of the distributions with increasing extrusion temperature in both the Cast + Extrude and the Cast + Extrude + ECAP conditions. Furthermore, at the lowest extrusion temperature of 373 K, the distribution is especially narrow for the Cast + Extrude + ECAP condition shown in Fig. 4a, suggesting the presence of a reasonably uniform microstructure under these conditions. 3.2. Tensile properties after processing Several recent reports have demonstrated the potential for achieving high, and often superplastic, ductilities in
Mg-based alloys through the use of the two-step process of extrusion followed by ECAP [15–17,19–22]. To evaluate the mechanical properties in the present investigation, specimens were pulled to failure over a range of testing conditions. Fig. 5 shows the influence on the measured elongations to failure of (a) the extrusion temperature, Tex, and (b) the extrusion speed, rex, where all samples were pulled to failure at an absolute testing temperature, T, of 473 K using an initial strain rate, e_ , of 1.5 · 10 3 s 1. A first impression from inspection of these plots is that the data are remarkably consistent: thus, the elongations to failure are always higher, by a factor of two or more, in the specimens processed to incorporate the additional step of ECAP. Furthermore, this is consistent with the microstructural observations in Figs. 3 and 4, where the phase widths are smaller in the Cast + Extrude + ECAP samples. The ductilities also increase with decreasing extrusion temperature and thus with decreasing phase width. By contrast, Fig. 5b shows that the extrusion speed has an insignificant effect on the ductilities achieved after ECAP, at least within the limited range from 1 to 10 mm s 1. For the extrusion conditions of Tex = 373 K and rex = 1 mm s 1, Fig. 6 records the measured elongations to failure as a function of the initial testing strain rate for the three conditions of Cast, Cast + Extrude and Cast + Extrude + ECAP. Again, all of the results are very consistent and exceptionally high elongations are attained at the lowest strain rates for the samples subjected to ECAP. In practice, the highest elongation recorded in this work was 1780% when using an initial strain rate of 1.5 · 10 4 s 1. By comparison, the results in Fig. 6 show that the measured elongations to failure at this strain rate are only 200% in the Cast condition and 630% in the
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60
Frequency (%)
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α-phase wα = 3.5 μm
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Mg-8%Li Tex = 473 K Cast + Extrude
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40
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20 10
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Frequency (%)
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5
10
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Width of phase (μm) Fig. 3. Width distributions for the a (upper) and b (lower) phases for the Cast + Extrusion condition after extrusion at temperatures of: (a) 373, (b) 473 a and w b denote the measured average widths for the a and b phases. and (c) 573 K with an extrusion speed of 1 mm s 1; w
Cast + Extrude condition. At testing strain rates below 10 2 s 1, the elongations are consistently low in the ascast material, intermediate in the cast material after extrusion and very high when the extruded samples are also processed by ECAP. By contrast, all processing conditions exhibit fairly similar elongations to failure at strain rates above 10 2 s 1.
4. Discussion 4.1. Implications of the different processing procedures The first important conclusion from these experiments is that the overall ductility of the Mg–8% Li alloy may be significantly enhanced by introducing an extrusion step and it
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Mg-8%Li Tex = 373 K Cast + Extrude + ECAP
Frequency (%)
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30 20 10 0 60 0 60
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50 40 30 20 10 0 0
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10
15
20
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Width of phase (μm) Fig. 4. Width distributions for the a (upper) and b (lower) phases for the Cast + Extrusion + ECAP condition after extrusion at temperatures of (a) 373, a and w b denote the measured average (b) 473 and (c) 573 K with an extrusion speed of 1 mm s 1 and ECAP through four passes at room temperature; w widths for the a and b phases.
may be even further enhanced by subjecting the extruded material to processing by ECAP. The elongations recorded in Fig. 6 after this two-step process are exceptionally high for this Mg–Li alloy. For example, there is a report that thermo-mechanical processing may be used to reduce the
grain size in the Mg–8% Li alloy to 8 lm and in this condition the maximum recorded elongation was 640% at a strain rate of 1.7 · 10 4 s 1 and temperature of 573 K [31]. An earlier investigation of the same Mg–8% Li alloy gave a maximum elongation of 970% at an initial strain
M. Furui et al. / Acta Materialia 55 (2007) 1083–1091
a
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T = 473 K . ε = 1.5 x 10-3 s-1
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Mg-8%Li 1000
Elongation to failure (%)
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2
4
6
8
10
12
Extrusion speed, rex (mm s-1) Fig. 5. Variation of the elongation to failure for the Cast + Extrude and the Cast + Extrude + ECAP conditions with: (a) the extrusion temperature for samples extruded at a speed of 1 mm s 1 and (b) the extrusion speed for samples extruded at a temperature of 473 K; all specimens were pulled to failure at 473 K using an initial strain rate of 1.5 · 10 3 s 1.
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Elongation to failure (%)
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Cast Cast + Extrude Cast + Extrude + ECAP
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10-3
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rate of 1.0 · 10 4 s 1 and testing temperature of 473 K when using a material where the extrusion was conducted under conditions of Tex = 573 K and rex = 5 mm s 1 followed by ECAP through two passes at 473 K using a die with a channel angle of U = 110 [19]. The latter result is generally consistent with the present experiments when it is noted that larger grain sizes are introduced at the higher extrusion temperatures and it is well known there is less grain refinement when materials are processed by ECAP at higher temperatures [32]. In addition, the maximum elongation of 1780% attained in these experiments, although less than the maximum elongation of 2040% reported recently for a ZK60 alloy processed by ECAP [33,34], is nevertheless exceptionally high for a magnesium-based alloy. A significant conclusion follows from inspection of these various results. It appears that the selection of a die with a channel angle of 135 gives no significant reduction in the measured elongations to failure but, on the contrary, it provides the capability of performing the ECAP at room temperature and this leads to greater microstructural refinement and hence to higher superplastic ductilities. This result would have been impossible to achieve using an ECAP die with a sharper channel angle (for example, 90 or 110) because of the necessity of conducting the ECAP at a higher pressing temperature and thereby introducing significant grain growth during processing. The second important conclusion is that the extrusion step, which is intermediate between casting and ECAP, may be significantly optimized by extruding at a lower temperature, as demonstrated by the results shown in Fig. 5a. This result is a direct consequence of the greater microstructural refinement demonstrated in Fig. 4a, where the a and b phases have smaller average widths giving an improved microstructural homogeneity. The result is consistent also with earlier data reported for the AZ31 (Mg– 3% Al–1% Zn) alloy which show that a more homogeneous microstructure may be achieved by extruding the alloy at a lower processing temperature [35]. Conversely, it follows from Fig. 5b that the speed of the extrusion has little or no effect on the achievement of high ductilities after ECAP. 4.2. The flow process in the Mg–8% Li alloy
1000
0 10-4
1089
100
-1
Strain rate (s ) Fig. 6. Variation of the elongation to failure with the initial strain rate at a testing temperature of 473 K for samples in the Cast, Cast + Extrude and Cast + Extrude + ECAP conditions: the extrusion was conducted at 373 K using an extrusion speed of 1 mm s 1.
In order to obtain information on the flow process in the superplastic region, Fig. 7 shows logarithmic plots of the 0.2% proof stress versus the initial strain rate for (a) the Cast + Extrude condition and (b) the Cast + Extrude + ECAP condition: these points correspond to the experimental conditions shown in Fig. 6, where Tex = 373 K, rex = 1 mm s 1 and the tensile tests were conducted at T = 473 K. Although the datum points in Fig. 7 are very limited, the results suggest a transition from a strain rate sensitivity, m, of 0.3 at the faster strain rates where the elongations to failure are low to 0.5 at the slower strain rates where the alloy exhibits superplastic behavior. These numbers
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0.2 % proof stress (MPa)
a
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superplastic flow but it is consistent with other recent reports of superplasticity in Mg-based alloys processed by ECAP. For example, superplastic flow was reported at a testing temperature of 423 K in an extruded commercial AZ31 alloy after processing by ECAP [18], where this temperature corresponds to 0.46 Tm. Similarly, elongations of up to 1190% were reported in an extruded AZ61 alloy processed by ECAP and tested in tension at 448 K [43]. The present results are consistent with these earlier investigations but they provide also the first demonstration that it is possible to optimize the superplastic properties by adjusting the conditions associated with the preliminary extrusion prior to processing by ECAP.
100
0.3 1 10
0.5 1
Mg-8%Li Tex = 373 K rex = 1 mm s-1
T = 473 K Cast + Extrude 1 10-4
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5. Summary and conclusions
Strain rate (s-1)
0.2 % proof stress (MPa)
b
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Mg-8%Li T = 473 K Cast + Extrude + ECAP 1 10-4
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100
-1
Strain rate (s ) Fig. 7. Variation of the 0.2% proof stress with the initial strain rate at a testing temperature of 473 K for: (a) the Cast + Extrude condition and (b) the Cast + Extrude + ECAP condition; the extrusion was conducted at 373 K using an extrusion speed of 1 mm s 1 and the slopes of the lines denote the strain rate sensitivities.
are consistent with a transition in the flow process from a regime controlled by the intragranular movement of dislocations at the faster strain rates [36] to superplastic flow controlled by grain boundary sliding at the slower strain rates [37]. Furthermore, a strain rate sensitivity of m 0.5 is consistent with the flow behavior observed when superplastic ductilities were achieved in an Al–3% Mg–0.2% Sc alloy after processing by ECAP [38,39] and the ductilities recorded in the Cast + Extrude + ECAP condition at the two slowest strain rates in Fig. 6 are generally consistent with the expectations from the standard plot of the strain rate sensitivity vs. elongation to failure for superplastic alloys [40]. The high tensile elongations at the two lowest strain rates in Fig. 6 are also reasonable for an alloy having a grain size of the order of 1 lm [41]. The absolute melting temperature, Tm, for the Mg–8% Li alloy is 877 K [42] so that the testing temperature of 473 K in Figs. 6 and 7 corresponds to an homologous temperature of 0.54 Tm. This temperature is relatively low for
1. A two-phase Mg–8% Li alloy was examined and tested in three different conditions: in the Cast condition, in the Cast + Extrude condition where extrusion was conducted at different temperatures and speeds, and in the Cast + Extrude + ECAP condition where the alloy was extruded under different conditions and then processed by ECAP for four passes at room temperature using a die with a channel angle of 135. 2. Microstructural observations showed that there was a very significant reduction in the average widths of the phases due to extrusion and there was an additional microstructural refinement due to ECAP. Typically, the average phase widths were 60–70 lm in the as-cast condition, 3–10 lm after extrusion and 1–7 lm after extrusion and ECAP. An exceptionally fine and uniform microstructure was achieved after ECAP when the extrusion was conducted at the lowest temperature of 373 K. 3. The elongations to failure in tensile testing increased with decreasing extrusion temperature but they were reasonably independent of the extrusion speed within the range from 1 to 10 mm s 1. 4. For an extrusion temperature of 373 K and an extrusion speed of 1 mm s 1, the elongations to failure at 473 K were high in the Cast + Extrude + ECAP condition, intermediate in the Cast + Extrude condition and low in the Cast condition for testing strain rates lower than 10 2 s 1. The highest elongation achieved in this investigation was 1780% in the Cast + Extrude + ECAP condition using an initial strain rate of 1.5 · 10 4 s 1. 5. The use of a die with a channel angle of 135 proved to be advantageous in this investigation because it permitted processing by ECAP at room temperature and this restricted the extent of grain growth during the pressing operation.
Acknowledgements We are grateful to Professor Shigeharu Kamado (Nagaoka University of Technology) and Professor Zenji Horita
M. Furui et al. / Acta Materialia 55 (2007) 1083–1091
(Kyushu University) for the provision of experimental facilities and Megumi Kawasaki for assistance with the figures. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan under Grant No. 17560629 and in part by the National Science Foundation of the United States under Grant No. DMR0243331. References [1] Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mater Sci 2000;45:103. [2] Valiev RZ, Estrin Y, Horita Z, Langdon TG, Zehetbauer MJ, Zhu YT. JOM 2006;58(4):33. [3] Segal VM. Mater Sci Eng 1995;A197:157. [4] Segal VM. Mater Sci Eng 2004;A386:269. [5] Iwahashi Y, Horita Z, Nemoto M, Langdon TG. Acta Mater 1997;45:4733. [6] Iwahashi Y, Horita Z, Nemoto M, Langdon TG. Acta Mater 1998;46:3317. [7] Neishi K, Horita Z, Langdon TG. Mater Sci Eng 2002;A325:54. [8] Komura S, Horita Z, Nemoto M, Langdon TG. J Mater Res 1999;14:4044. [9] Yamashita A, Horita Z, Langdon TG. Mater Sci Eng 2001;A300:142. [10] Kainer KU. In: Lorimer GE, editor. Proceedings of the third international magnesium conference. London: The Institute of Materials; 1997. p. 533. [11] Mukai T, Watanabe H, Higashi K. Mater Sci Technol 2000;16:1314. [12] Wilson DV. J Inst Met 1970;98:133. [13] Hilpert M, Styczynski A, Kiese J, Wagner L. In: Mordike BL, Kainer KU, editors. Magnesium alloys and their applications. Weinheim: Wiley–VCH; 1998. p. 319. [14] Mukai T, Yamanoi M, Watanabe H, Higashi K. Scripta Mater 2001;45:89. [15] Horita Z, Matsubara K, Makii K, Langdon TG. Scripta Mater 2002;47:255. [16] Matsubara K, Miyahara Y, Horita Z, Langdon TG. Acta Mater 2003;51:3073. [17] Matsubara K, Miyahara Y, Horita Z, Langdon TG. Metall Mater Trans 2004;35A:1735. [18] Lin HK, Huang JC, Langdon TG. Mater Sci Eng 2005;A402:250.
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