Effects of grain size on room temperature deformation behavior of Zn–22Al alloy under uniaxial and biaxial loading conditions

Materials Science & Engineering A 672 (2016) 78–87

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Effects of grain size on room temperature deformation behavior of Zn– 22Al alloy under uniaxial and biaxial loading conditions M.E. Cetin a, M. Demirtas a,b, H. Sofuoglu a, Ö.N. Cora a, G. Purcek a,n a b

Department of Mechanical Engineering, Karadeniz Technical University, Trabzon 61080, Turkey Department of Mechanical Engineering, Bayburt University, Bayburt 69000, Turkey

art ic l e i nf o

a b s t r a c t

Article history: Received 11 March 2016 Received in revised form 23 June 2016 Accepted 24 June 2016 Available online 25 June 2016

Effects of grain size on room temperature (RT) deformation behavior of superplastic Zn–22Al alloy under uniaxial and biaxial loading conditions were investigated. Two–step equal channel angular pressing (ECAP) and subsequent annealing processes were applied to the alloy in order to obtain microstructures with various grain sizes ranging from submicron to micron sizes. Grain size of 200 nm was achieved after ECAP of the alloy, and it was increased up to
2.60 mm by annealing at 250 °C for different time periods. Changes in deformation behaviors of the alloy with various grain sizes were found to be in good agreement under the uniaxial tensile and biaxial Erichsen test conditions. Increasing grain size decreased both the maximum elongation under uniaxial deformation and limiting dome height (LDH) under biaxial deformation. The high elongation to failure and LDH values for the samples with submicron grain size were attributed to their high strain rate sensitivity. & 2016 Elsevier B.V. All rights reserved.

Keywords: Severe plastic deformation Ultrafine grained structure Formability Superplastic Zn–22Al alloy

1. Introduction Superplastic materials are fine-grained (grain sizes less than 10 mm) polycrystalline solids which show exceptionally high tensile elongation at high temperatures (above 0.5 Tm where Tm is the melting point of the material in Kelvin) and at low strain rates (strain rates in the range of 10  5–10  3 s  1) [1,2]. This unique behavior of some materials brought about the development of superplastic forming (SPF) technology. SPF is a near–net shape forming process which is capable of producing complex–shaped parts in a single forming process, and it has a wide range of applications especially in the fields of aerospace, communication, medical and architecture [3]. Deformation mechanisms and requirements of superplasticy under uniaxial loading conditions have been widely studied so far [1,2,4–13]. It has been noted that grain size is the most important structural parameter affecting the superplastic elongation of a material at a constant temperature [14]. Grain size affects primarily the grain boundary sliding (GBS) which is known as the main superplastic deformation mechanism [8,11,14]. Smaller grain size means more grain boundary area for effective GBS, and it brings about higher superplastic elongations. Grain size has also a considerable effect on the strain rate, which is an experimental parameter affecting superplastic behavior under uniaxial loading n

Corresponding author. E-mail address: [email protected] (G. Purcek).

http://dx.doi.org/10.1016/j.msea.2016.06.072 0921-5093/& 2016 Elsevier B.V. All rights reserved.

conditions. Decreasing grain size leads to an increase in strain rate at which the maximum superplastic elongation is achieved [1], and thus superplasticity is observed at relatively high strain rates. Nonetheless, effects of grain size on superplastic materials under biaxial loading condition which is more appropriate to characterize the SPF have not been studied in detail. In addition, there is no dedicated study which compares the formability of superplastic materials under uniaxial and biaxial loading conditions. Therefore, the purpose of this study is to investigate the effects of grain size on the formability of superplastic materials under uniaxial and biaxial loading conditions, and to compare the results of those. For this purpose, Zn–22Al alloy was chosen as a model superplastic material. To achieve different grain sizes in the ranges of sub– micron to micron sizes, equal channel angular pressing (ECAP) and subsequent post–ECAP annealing were devised and applied to the alloy after casting. Then, uniaxial tensile tests and biaxial Erichsen tests were conducted at different strain rates and punch speeds to reveal the effect of grain size on formability of the alloy.

2. Experimental procedure Zn–22Al alloy was produced by melting and casting of Zn and Al with 99.9% purity. The ingot was then subjected to first homogenization annealing at 375 °C for 24 h. The billets for ECAP were machined from the ingot with the dimensions of 13 mm  13 mm  120 mm. Later on, a second homogenization step was applied to the billets at 375 °C for 48 h along with

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subsequent quenching in order to obtain equiaxed Zn–rich η– and Al–rich α–phases in the microstructure. The billets were then subjected to two–step ECAP which was especially applied to the alloy in order to increase the effectiveness of ECAP as the grain refinement tool and achieve submicron grain–sized microstructure [4]. For this purpose, four passes of ECAP were applied to the billets at 350 °C followed by four more passes at room temperature (RT) (8 passes in total). The ECAP was carried out at an extrusion speed of 1 mm s  1 following route Bc (after each pass the billets were rotated by 90° in the same direction along the longitudinal axis). In order to increase the grain size achieved by ECAP, post–ECAP annealing processes were applied to the alloy at 250 °C for different durations, namely, 2.5 h, 24 h, 144 h and 192 h. Microstructure of the sample after two-step ECAP process was examined by transmission electron microscope (TEM). TEM examinations were conducted using an FEI Tecnai F20 microscope operated at a nominal voltage of 200 kV. More detailed microstructural examination of two-step ECAPed sample was undertaken using electron backscatter diffraction (EBSD) technique combined with a LEO Supra 35 field emission scanning electron microscope (SEM). The grain size of the ultrafine-grained (UFG) sample processed only by ECAP was determined using EBSD analysis software. In that determination, both the grains with HAGBs and the sub-grains with LAGBs were considered by setting the EBSD system to detect a very low misorientation value for grain boundary setting. Microstructures of post–ECAP annealed samples were examined using SEM. Samples for the SEM analyses were ground, polished and then etched in a solution containing 5 g CrO3, 0.25 g Na2SO4 and 100 ml H2O. Samples perpendicular to extrusion direction were extracted from both ECAPed and post–ECAP annealed samples for all metallographic examinations using wire electro– discharge machining (wire–EDM). RT deformation behavior of processed Zn–22Al alloy under uniaxial loading condition was determined by means of uniaxial tensile tests. For this purpose, samples with the gauge section dimensions of 2 mm  3 mm  5 mm were cut from the billets with their tensile axes aligned parallel to the extrusion direction (Fig. 1). Tests were conducted at the strain rates ranging between 1  10  3 s–1 and 1  10  1 s  1 using Instron 3382 universal tension and compression test frame having non–contact video type extensometer. In general, the tests were repeated at least three times for each condition. But some with large scatters were repeated up to six times to be able to achieve more accurate results, and the mean of them were reported in the present study. Biaxial deformation behavior of Zn–22Al alloy after all applied processes was evaluated using a miniaturized Erichsen test system

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in 1/4 scale described in the ISO 20,482 standard. Erichsen samples with the dimensions of 13 mm  13 mm  0.7 mm were cut from the billets perpendicular to extrusion direction (Fig. 1). Surfaces of the samples were ground and then polished with 0.3 mm alumina solution to eliminate the surface scratches before tests. Erichsen tests were performed at RT and at three different punch speeds of 0.005 mm/s, 0.05 mm/s and 0.5 mm/s, which correspond to the strain rates of 1  10  3 s  1, 1  10  2 s  1 and 1  10  1 s  1, respectively, in the tensile tests. Any lubricant was not used during the Erichsen tests and, as in the case of tensile tests at least three experiments were performed on each condition. Surface features and deformation characteristics of the stretched samples were investigated via SEM. It is known that limited grain growth may occur in severely deformed Zn-22Al alloy by means of some self-annealing during its storage at RT due to its low melting point of 753 K [15,16]. Therefore, the mechanical tests (tensile and Erichsen tests) and microstructural examinations were performed as soon as possible after ECAP and post-ECAP annealing processes in order to minimize the effects of self-annealing.

3. Results 3.1. Microstructure TEM micrograph of Zn–22Al alloy after two–step ECAP is shown in Fig. 2(a). The ECAP resulted in a UFG microstructure having equiaxed grains of both zinc–rich η–phase (dark contrast) and aluminum–rich α–phase (bright contrast). In addition, both phases distributed homogeneously throughout the microstructure [4]. The EBSD map showing the grainy morphology of the ECAPed alloy and the grain size distribution histograms determined from the EBSD data for both α– and η–phases are shown in Fig. 2(b–d) [4]. From the histograms, grain sizes of α– and η–phases were calculated to be 150 nm and 250 nm, respectively. Considering both phases together, the average grain size of the alloy was determined as 200 nm which is the smallest one achieved for Zn– 22Al alloy so far by means of ECAP [4]. SEM micrographs showing the microstructures of post–ECAP annealed Zn–22Al alloy are shown in Fig. 3(a–d). The mean grain sizes of the annealed samples were determined from these micrographs using linear intercept method. Annealing for 2.5 h at 250 °C resulted in growing in grain size of ECAPed sample, and the average grain size was recorded as 750 nm (Fig. 3(a)). Hereafter, this sample and ECAPed one will be named as “submicron grain– sized samples”, because both samples have the grain sizes below

Fig. 1. Schematic illustrations of the tensile test, Erichsen test and metallographic examination samples inside the ECAPed billet.

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Fig. 2. (a) TEM micrograph showing the microstructure of Zn–22Al alloy after ECAP and (b–d) EBSD map displaying the evolution of microstructure after ECAP and related grain size distribution histograms of α- and η-phases. The micrographs and the EBSD map given here were also published in previous study [4].

Fig. 3. SEM micrographs representing the microstructures of ECAPed samples after annealing at 250 °C for: (a) 2.5 h, (b) 24 h, (c) 144 h and (d) 192 h.

1 mm. Extending the annealing duration to 24 h and 144 h brought about further grain growth in the microstructure and yielded a microstructure with 1.80 mm and 2.40 mm grain sizes, respectively (Fig. 3(b–c)). Nevertheless, further increase in the annealing duration to 192 h had a slight effect on the grain size, and finally an average grain size of 2.60 mm was achieved (Fig. 3(d)). Because the last three samples have the grain sizes above 1 mm, they will be named as “micron grain–sized samples” in the rest of the study. The grain growth mechanism of Zn–22Al alloy was studied earlier by Senkov and Myshlyaev [17]. They subjected the alloy to annealing at different temperatures for different time periods and proposed an empirical formula describing the kinetics of grain growth during the static annealing as provided below:

d05 –dt5 = Bt exp ( –Q /RT )

(1)

where d0 is the initial grain size, dt is grain size after t seconds of annealing, B ¼3.5  104 (mm)5/s, t is the annealing duration in seconds, Q is the activation energy equal to 83.000 J/mol, R is the gas constant, and T is the annealing temperature in Kelvin [17]. Substituting the temperature and the annealing durations of the present study into Eq. (1) yields the grain sizes of 1.10 mm, 1.72 mm, 2.47 mm, and 2.62 mm after 2.5 h, 24 h, 144 h and 192 h, respectively. These results show that the grain size values measured from real microstructures and calculated using above given formula are in good agreement especially for the annealing time periods of 24 h, 144 h and 192 h. In Ref. [17], grain boundary solute diffusion was suggested as the mechanism controlling the grain growth of Zn–22Al alloy. Regarding the validity of empirical formula given in Ref. [17] to the results of the present study, it can be concluded that grain boundary solute diffusion that controls the grain boundary migration is also responsible from the grain growth of

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Fig. 4. Stress–strain curves of the samples under uniaxial loading condition: (a) ECAPed sample and (b–e) post–ECAP annealed samples for (b) 2.5 h, (c) 24 h, (d) 144 h and (e) 192 h.

Zn-22Al alloy in the present study. It is evident from the grain sizes achieved for different time periods of annealing that the grain growth rate decreases with increasing annealing time. This is due to the change in the surface energy of the grain boundaries with increasing the gran size. Surface energy of grain boundaries increases during the ECAP due to formation of UFG microstructure, since smaller grain size means more grain boundary area which results higher total surface energy [18]. Furthermore, superplastic materials tend to decrease the surface energy of their grain boundaries which provides the driving force for grain growth [14]. At the initial stages of annealing process, a rapid grain growth occurs as a result of high surface energy of grain boundaries. Since increasing the annealing time decreased the surface energy of grain boundaries, the grain growth rate also decreased, and more stable microstructure was observed. 3.2. Uniaxial tension behavior Uniaxial tensile behavior of Zn–22Al alloy after ECAP and post– ECAP annealing processes are shown in Fig. 4(a–e) as a function of grain size and strain rate. A sharp strain softening was observed after the stress reached to the maximum value at
5% strain at all strain rates for the submicron grain–sized samples (Fig. 4(a–b)). Strain softening also occurred at all strain rates for the sample annealed for 24 h. However, increasing strain rate decreased the efficiency of strain softening for that sample (Fig. 4(c)). For the last two samples (the samples annealed for 144 h and 192 h), strain softening behavior was observed only at a low strain rate of 1  10  3 s  1, and apparent strain hardening occurred at relatively high strain rates of 1  10  2 s  1 and 1  10  1 s  1. The effect of strain hardening increased for these two samples with increasing annealing time and strain rate from 1  10  2 to 1  10  1 s  1 (Fig. 4(d–e)). Strain rate dependency of elongation to failure under uniaxial tension loading after ECAP and post–ECAP annealing processes is illustrated in Fig. 5(a), and the mean values with the scatters are given in Table 1. The maximum elongation was recorded as 400%

Fig. 5. Strain rate dependency of: (a) elongation to failure and (b) flow stress after the applied processes.

at a high strain rate of 5  10  2 s  1 for the ECAPed sample [4]. Also, relatively high elongation values after ECAP, namely 340%, 250% and 183%, were obtained at strain rates of 1  10  1, 1  10  2

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Table 1 Elongation to failure and flow stress values obtained after all applied processes. Sample ECAPed Annealed Annealed Annealed Annealed

for for for for

2.5 h 24 h 144 h 192 h

Elongation to Failure (%) 1  10  3 2  10  3 1837 15 – 328 7 20 390 7 9 1857 15 – 1257 5 – 1007 8 –

3  10  3 – 325 7 8 – – –

2.5 h 24 h 144 h 192 h

Flow Stress (MPa) 1  10  3 2  10  3 967 5 – 170 71 1907 5 300 73 – 2737 2 – 242 74 –

3  10  3 – 215 78 – – –

Sample ECAPed Annealed Annealed Annealed Annealed

for for for for

5  10  3 – 265 7 5 – – –

and 1  10  3 s–1, respectively [4]. While nearly 400% elongation was also achieved with the sample annealed for 2.5 h, it was obtained at a relatively low strain rate of 2  10  3 s  1. Below and above this strain rate elongation to failure decreased for that sample. The maximum elongation values were achieved at the lowest strain rate of 1  10  3 s  1 for all other annealed samples and increasing strain rates resulted in decrease in elongation. The maximum elongations for the samples annealed for 24 h, 144 h and 192 h were recorded as 185%, 125% and 100%, respectively. Fig. 5(b) shows the changes in flow stresses with initial strain rates, and the mean values of the flow stresses are given in Table 1. It was seen that the flow stresses were very sensitive to the strain rates for the submicron grain–sized samples, and increasing strain rate resulted in increase in the flow stresses. Nevertheless, the nearly strain independent flow stresses were obtained for micron grain–sized samples, especially at high strain rates. Another inspection from Fig. 5(b) is that grain growth up to 1.80 mm increased the flow stress. However, further increase of grain size to 2.40 mm and 2.60 mm resulted in a considerable decrease in flow stress. It can, therefore, be concluded that there is a critical grain size value below which decreasing grain size also decreases the flow stress. Similar observations were also recorded with some

1  10  2 250 716 1627 5 92 75 82 75 75 78

5  10  3 – 245 74 – – –

1  10  2 1407 7 304 75 363 7 3 3107 10 2757 7

3  10  2 305 730 – – – –

5  10  2 400 715 – – – –

1  10  1 340 720 60 75 63 72 63 72 65 73

3  10  2 166 710 – – – –

5  10  2 1887 12 – – – –

1  10  1 2357 10 4007 6 408 73 340 710 2787 3

dilute Zn–Al alloys showing superplastic behavior at RT [6,19]. It was stated that below a critical grain size, transition from non– superplastic region to superplastic region occurs. Thus, decreasing grain size also decreases the flow stress as a result of easier GBS which occurs at more grain boundaries [6,19]. Similarly, in the present study, further grain refinement below the critical grain size (approximately 1.8 mm) resulted in a decrease in flow stress due to the presence of GBS as a dominant deformation mechanism. Strain rate sensitivity parameters (m–value) in the optimum superplastic region were determined as the slope of flow stress– strain rate curves for the submicron grain–sized samples since these samples showed superplastic behavior (Fig. 5(b)). The m–value for the regions where the maximum elongations were achieved were determined as 0.30. Nonetheless, increasing grain size from 200 nm to 750 nm shifted the optimum superplastic region having the highest m–value along with the highest superplastic elongation to relatively low strain rates as expected [1]. 3.3. Biaxial Tension Behavior Load (F)–displacement (X) curves of Zn–22Al alloy obtained

Fig. 6. Load–displacement curves of the samples under biaxial loading via Erichsen tests: (a) ECAPed; and (b–e) post–ECAP annealed samples for (b) 2.5 h, (c) 24 h, (d) 144 h and (e) 192 h.

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sample. Decreasing punch speed also decreased the LDH values as in the case of uniaxial tension tests, and LDH values of 4.86 mm and 4.71 mm were obtained at the punch speeds of 0.05 and 0.005 mm/s, respectively. Increase in grain size, on the other hand, resulted in a decrease in LDH as in the case of maximum elongation in tension test. The maximum LDH was noted as 4.56 mm at the lowest punch speeds of 0.005 mm/s for the sample annealed for 2.5 h. Further increase in grain size brought about a significant decrease in the LDH, and it took values less than 3.0 mm for 24 h, 144 h and 192 h annealed samples. Dependency of the maximum load to the punch speed (Fig. 7 (b) and Table 2) in Erichsen tests is also consistent with the tensile tests. For the submicron grain–sized samples, the maximum load was found to be sensitive to the punch speed. Nonetheless, load change in micron grain–sized samples with the punch speed showed similar trends to stress change with the strain rates in the tensile tests, and insignificant change in the maximum load was recorded with respect to punch speed. In order to examine the effect of grain size on deformation homogeneity under biaxial loading via Erichsen tests, two samples were chosen: submicron grain–sized samples obtained after ECAP and micron grain–sized samples achieved after post–ECAP annealing for 192 h. The Erichsen tests were performed at the punch speed of 0.05 mm/s and the tests were stopped at certain points as numbered on the load–displacement curves in Fig. 8(a–b). As seen, tests were stopped at two points (points 1 and 2) before the load reached to the maximum value. For ECAPed sample, the tests were also stopped at the peak point of the load–displacement curve (point 3) and at the punch travel of 3 mm (point 4) which is just after the peak point. Sectioned photographs of the domes showing the thickness variation at marked points (where the tests were stopped) are given in Fig. 8(c). In general, reduction in the thickness of both ECAPed and post–ECAP annealed samples occurred in a similar manner up to the load reached to the peak point (points (1) and (2) of both samples (Fig. 8(c)). Following the elastic deformation region, initial stage of plastic deformation occurred by means of the biaxial bending without a considerable thinning of the blanks of both samples. Although, poles are slightly thinner than the rest of the samples due to low friction between the sample and the punch, both samples got thinned quite homogeneously at the end of this stage (at 1 mm dome height). When the dome height reached to 2 mm, transition regions from fixed points to flanges of the blanks were observed, and the thicknesses of the samples did not change much (0.67 mm) at these regions (points (2) for both samples (Fig. 8(c)). Apart from the transition regions, the poles and the flanges of the samples deformed in a homogeneous manner at the second stage, and the thicknesses were measured to be 0.52 mm and 0.56 mm for the pole and flange of the ECAPed sample (point (2) (Fig. 8(c)), respectively. When the load reached the maximum value, the pole of the ECAPed sample was slightly thicker (0.49 mm) than the flange region (0.40 mm). The thicker part at the pole of the sample was formed due to the less biaxial tensile stresses at that region [20] and more adverse effect of the friction [21]. After the peak point,

Fig. 7. Punch speed dependency of: (a) LDH and (b) the maximum load after ECAP and post–ECAP annealing processes.

during the Erichsen tests after ECAP and post–ECAP annealing processes are presented in Fig. 6(a–e). The end point of all curves represents the initiation of the crack. In general, all samples showed the similar load-displacement behavior until the load reached the maximum value (Fig. 6(a-e)). Beyond the peak points of the F-X curves, the load decreased slightly for the submicron grain–sized samples (Fig. 6(a–b)), and crack initiated after the significant load drop. On the other hand, a sudden drop in loads was observed for the micron grain–sized ones, and crack initiation occurred immediately after the peak point of the F-X curves (Fig. 6 (c–e)). It was concluded that the smaller grain size retarded the failure of sample. The changes in the limiting dome height (LDH) (dome height at which failure occurred) and the maximum load as a function of the punch speeds are given in Fig. 7(a–b), respectively, and the mean values with the scatters are given in Table 2. Considering the deformation behavior of the samples under uniaxial and biaxial loading conditions (Fig. 5(a) and Fig. 7(a), respectively), it was clearly seen that the results for both conditions were in perfect agreement. The maximum LDH value was measured as 5.26 mm under biaxial loading at the 0.5 mm/s punch speed for ECAPed Table 2 Limiting dome height and maximum load values obtained after all applied processes. Sample

ECAPed Annealed Annealed Annealed Annealed

Limiting Dome Height (mm)

for for for for

2.5 h 24 h 144 h 192 h

Maximum Load (N)

0.005 mm/s

0.05 mm/s

0.5 mm/s

0.005 mm/s

0.05 mm/s

0.5 mm/s

4.71 70.2 4.56 7 0.05 2.75 7 0.05 2.6 7 0.06 2.417 0.05

4.86 70.1 3.86 70.05 2.86 70.06 2.357 0.1 2.22 70.15

5.26 7 0.05 2.26 7 0.05 2.66 70.06 2.46 7 0.05 2.2 7 0.1

620 769 1065 7 20 14757 3 1395 7 2 1398 7 60

822 7 34 16447 37 18007 3 1590 750 16007 70

1340 7 34 1981 7 21 20357 3 1839 7 10 16977 80

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Fig. 8. Load–displacement curves showing the points from where the sectioned photographs were taken for: (a) ECAPed sample and (b) post–ECAP annealed sample for 192 h. (c) sectioned photographs of the stretched sample under biaxial loading and (d) enlarged view of sectioned photograph of point 5 of ECAPed sample. (e–f) Optical photomicrographs showing the cavity formation at: (e) pole and (f) flange regions of point 5.

load started to decrease due to the deformation localization at the flange of the samples, and a rapid decrease in thickness down to 0.20 mm was observed (point (4) of ECAPed sample (Fig. 8(c)). For the post–ECAP annealed sample, brittle fracture was observed without further deformation localization (point (3) of post–ECAP annealed sample in Fig. 8(c)). On the contrary, ECAPed sample exhibited ductile fracture, and relatively high LDH values were recorded. Considering these results, it can be concluded that the effect of grain size on the deformation homogeneity is not pronounced until the load reaches the peak value. Nevertheless, beyond this stage, the grain size has a considerable effect on the characteristics of deformation localization. Finer grain size changes the fracture mode from brittle fracture to ductile fracture and delays the necking growth which brings about a relatively high LDH. For the ECAPed Zn–22Al alloy, fracture location shifted from the pole of the dome towards to its equator (Fig. 8(c)). Similar observation was reported for the superplastic 7075 Al alloy in literature [20]. This formation may be due to the cavity or pore formation during stretching of the ECAPed samples. Because, it has been well established that extensive cavity formation occurs during the superplastic deformation [22,23] and fracture is caused by the coalescence of these cavities during high superplastic elongation. At the pole of the dome, additional compressive stress which acts in direction of the dome thickness may prevents the cavity formation and its growth [20]. For clarifying this argument, optical micrographs were taken from the stretched samples (Fig. 8 (e–f)). Fig. 8(e) show that limited cavity formation was observed at the pole of the ECAPed sample, as expected. Thus, further reduction in thickness at the pole of the dome was restricted by less cavity formation. On the other hand, extensive number of cavities

occurred especially at the outer surface of the equator of the sample (Fig. 8(f)) due to the higher stretching force levels. Therefore, the fracture took place at the flange region of the sample as a result of the coalescence of these cavities [20].

4. Discussion The submicron grain-sized samples showed strain softening behavior at all strain rates, and both strain hardening and strain softening behaviors were observed for the micron grain-sized samples based on the strain rates. Considering the previous studies in the literature, strain hardening behavior was attributed to the grain growth [25] and dislocation stocking process [26] during the superplastic deformation. As it is known, dynamic recovery/recrystallization was accepted as the main factor causing strain softening in superplastic materials [25,27–29]. Also, decrease in the true strain rate during the biaxial tension test was reported as another factor causing the strain softening if the material is strain rate sensitive [29]. Regarding these conclusions, strain softening behavior for submicron grain–sized samples can be attributed to the dynamic recovery/recrystallization in the present study. Strain softening was more pronounced for submicron grain–sized samples at the strain rates at which relatively high m–values are obtained (3  10–2  1  10  1 s  1 for ECAPed sample (Figs. 4(a)); and 1  10  3 –  10–3(s)  1 for the sample annealed for 2.5 h (Fig. 4 (b)). Thus, change in the strain rate during the superplastic deformation was considered another factor causing strain softening. It is known that dislocations contribute to the GBS as the dominant deformation mechanism, and they disappear at grain boundaries which act as dislocation sinks during superplastic

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deformation [14]. Therefore, no apparent strain hardening effect caused by dislocations was observed for the submicron grain–sized samples. For the micron grain–sized samples, on the other hand, significant strain softening was valid only at the strain rate of 1  10  3 s  1. These samples lost their superplastic behaviors, and they became strain rate insensitive. Thus, strain softening took place only by dynamic recovery/recrystallization in these samples due to the sufficient time at low strain rates, and change in the strain rate during the deformation did not play an effective role. The strain hardening for the micron grain–sized samples was attributed to the increase in dislocation density during deformation [26]. Since, dynamic recrystallization lost its efficiency due to the insufficient time, and the dislocation stocking process occurred at high strain rates of 1  10  2 and 1  10  1 s  1 for these samples. As stated above, strain hardening becomes more significant in micron grain–sized samples with increasing both annealing time and strain rates especially from 1  10  2 to 1  10  1 s  1. This is because of the change in the level of internal energy with increasing annealing time. High internal energy is imposed during UFG formation by ECAP [30], and it provides the driving force for the dynamic recovery/recrystallization. Increasing annealing time resulted in decrease in internal energy. Therefore, strain hardening behavior was not found to be effective at high strain rates for the sample annealed for 24 h after ECAP (Fig. 4(c)) while it was effective for other micron grain-sized samples (Fig. 4(d–e)). Because, some dynamic recovery/recrystallization still took place at the sample annealed for 24 h due to the relatively high internal energy compared to the samples annealed for 144 h and 192 h after ECAP. Thus, increase in dislocation density was restricted and hardening behavior was not observed at the sample annealed for 24 h. The results of the present study show that the elongation to failure is strongly related to the grain size of the Zn-22Al alloy. It is well known that decreasing grain size increases the strain rate at which the maximum superplastic elongation is achieved [1]. Furthermore, smaller grain size results in higher superplastic elongations at high strain rates due to the insufficient time for growth of internal cavities which may lead to premature failure [1]. The elongation values obtained in the present study for all processed samples are in good agreement with the existing literature. While maximum elongation was achieved at 5  10  2 s  1 for the sample having 200 nm grain size; increasing grain size was shifted the strain rate to the lower values at which the maximum elongations were achieved. The samples having grain sizes ranging between 750 nm and 2.60 mm showed the maximum elongations at relatively low strain rates. Furthermore, any increase in grain size led to decrease in elongation to failure, and maximum elongation decreased from 400% for ECAPed sample to about 390% for the post–ECAP annealed sample for 2.5 h. The other post-ECAP annealed samples having grain sizes in the micron levels lost the superplastic behavior and showed the maximum elongations ranging between 100% and 185% (Fig. 5(a) and Table 1). Dependency of elongation to grain size was attributed to the effectiveness of GBS. Since lower grain size means increased grain boundary area, GBS occurs effectively at more boundaries which brings about higher superplastic elongation [24]. In the present study, two-step ECAP process resulted in a RT superplastic elongation of 400% at high strain rate of 5  10  2 s  1. However, higher RT superplastic elongation was reported in an earlier study even for the Zn-22Al alloy sample with larger grains than the present one [31]. In that study, about 500% elongation was achieved with a sample having 600 nm grain size. On the other hand, 570 nm grain sized sample of Zn-22Al alloy showed lower superplastic elongation of about 300% in the same study. These differences in elongations were attributed to the different Si impurity contents of these two samples. While the fist sample had an impurity content of o 10 ppm, the Si content of the second one

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was 1500 ppm. It was stated that Si impurity did not affect the deformation mechanism, but it altered the fracture behavior, and higher Si impurity content resulted in relatively low superplastic elongation [31]. Similar behavior was also reported for Fe impurity in Zn-22Al alloy [32]. Any increase in the level of Fe impurity resulted in a significant decrease in the superplastic elongation. In the present study, the contents of the impurity elements of the Zn22Al alloy were not controlled during the manufacturing process, which roughly indicates that the as-cast Zn-22Al alloy has high amount of impurity atoms. Therefore, it can be speculated that relatively low superplastic elongation of the ECAPed alloy despite it has the smallest grain size in the present study can be attributed to the high level of impurities coming from the un-controlled casting process. In the Erichsen tests high LDH values were achieved for the submicron grain-sized samples while the micron grain-sized samples showed low LDH. The high LDH values for the submicron grainsized samples were attributed to the high m–value (since their maximum loads were found to be sensitive to the punch speed). After the peak value of the punch load was reached, strain rate of the deformation localization zone of the dome became higher than that of the rest of the samples. This difference in strain rates resulted in an increase in the flow stress of the necking zone due to the high m–value, and deformation stopped at this region and started at the other parts of the dome, simultaneously. Hence, an abrupt necking growth was delayed, and thus quasi–stable plastic flow occurred until the thickness of the sample decreased significantly at flange region which led to a slight decrease in load and to high LDH. In order to examine the effects of grain size on the surface quality of the samples after the Erichsen tests, dome surfaces of the ECAPed sample (having the smallest grain size) and the sample annealed for 192 h (having the largest grain size) were examined using SEM, and surface appearances of domes are presented in Fig. 9. In general, ECAPed samples have smoother dome surfaces than those of the annealed ones at all punch speeds. This difference in both states was attributed to the different deformation mechanisms observed for two samples having different grain sizes. From micrographs of dome surfaces of ECAPed samples (given in Fig. 9(b), (d) and (f)), the evidence of GBS can clearly be seen at all punch speeds since grainy morphology on the surface was achieved, and many grains appear without certain orientation. Thus, surfaces of samples are relatively smooth due to absence of surface level differences of neighboring grains, and occurrence of GBS nearly at all grain boundaries [21]. On the other hand, the dome surfaces of the post–ECAP annealed samples were roughened after Erichsen tests (Fig. 9(g–l)). This was attributed to the absence of GBS due to the relatively large grain size of 2.60 mm. It was stated above that the grain size has not a significant effect on deformation homogeneity until the load reaches the peak value. Considering Fig. 6(a–e), it can also be noted that grain size has no considerable effect on the dome height values at which maximum load are reached (the uniform deformation region). The maximum load values were achieved at dome heights of
2.5–3 mm at 1  10  3 s  1,
2.3–2.8 mm at 1  10  2 s  1 and
2.1–2.3 mm at 1  10  1 s  1 for all submicron and micron grain–sized samples. Oppositely, morphological features of dome surfaces suggest that grain size affects the deformation mechanism and surface quality during Erichsen tests under biaxial stretching. Therefore, it is beneficial to decrease grain size of the superplastic materials to achieve high quality surfaces. It is known that impurities are critical factor for the fracture of the superplastic materials as in the case of superplastic elongation. They affect the formation of the dimples and cavities during deformation and thus surface quality of the samples [31]. In case of the high level of impurities, many micro-voids nucleate around the

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impurities and they interlinkage to form microcracks causing fracture [31]. In the present study, the alloy contains high level of impurities (because impurity level was not controlled during the manufacturing process), and it is not known that how these impurities distributed after ECAP and post-ECAP annealing processes. Any difference in the distribution of these impurities (accordingly chance in the distribution of the micro-voids) can cause different surface qualities achieved after ECAP and annealing process for 192 h. Thus, different surface qualities after ECAP and post-ECAP annealing processes can also be attributed to the presence of high impurity level of the alloy and their distribution after the applied processes.

5. Conclusion Effects of the grain size and strain rate/punch speed on the uniaxial and biaxial tensile deformation of superplastic Zn–22Al alloy were investigated. Main findings and conclusions are summarized below: 1. Equal channel angular processing (ECAP) decreased the grain size of Zn–22Al alloy, and produced ultrafine–grained microstructure with an average grain size of 200 nm. Post–ECAP annealing increased the grain sizes as expected, and grain sizes of 750 nm, 1.80 mm, 2.40 mm and 2.60 mm were obtained after annealing at 250 °C for 2.5 h, 24 h, 144 h and 192 h, respectively. 2. Changes in the deformation behavior and mechanical properties of the alloy with grain size and strain rate/punch speed were found to be in good agreement considering the uniaxial tensile and biaxial Erichsen tests results. Increasing grain size decreased both the maximum elongation under uniaxial deformation and limiting dome height under biaxial deformation. Increasing grain size also shifted the strain rates/punch speeds to the lower values at which the maximum elongation and limiting dome height values were attained. The maximum elongation to failure was decreased from 400% for the ECAPed sample to about 100% for the post–ECAP annealed sample for 192 h, while the limiting dome height decreased from 5.26 mm to 2.20 mm for the same samples, respectively. 3. It was found that grain size has an insignificant influence on the deformation homogeneity or uniformity until the load reaches the peak value in Erichsen test. Beyond this point, grain size has a considerable effect on the characteristics of deformation localization. In addition, finer grain size changes the fracture mode from brittle fracture to ductile fracture and delays growth in necking which brings it about higher limiting dome height. 4. High elongation to failure under uniaxial loading and high limiting dome height under biaxial loading for submicron grain– sized samples were attributed to delay in necking formation due to their relatively high m–values. 5. The coalescence of the extensive number of cavities occurring especially at the outer surface of the domes due to the high stretching forces caused fracture at the equator region of ECAPed samples. Brittle fracture without considerable cavity formation was observed for the sample annealed for 192 h.

Acknowledgements

Fig. 9. Dome surface appearances of ECAPed samples tested at punch speeds of: (a– b) 0.5, (c–d) 0.05, (e–f) 0.005 mm/s. Dome surface appearances of the post–ECAP annealed samples for 192 h tested at punch speeds of: (g–h) 0.5, (i–j) 0.05, (k–l) 0.005 mm/s.

This research was partly supported by Scientific Research Projects of Karadeniz Technical University, under Grant No: 10501. Dr. G. Purcek was also supported by The World Academy of Sciences (TWAS) under the Visiting Researchers program of TWASUNESCO Associateship Scheme.

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