Pseudo-Superplastic Characteristics of ZK60 Alloy with Fibrous Microstructure
2018 Volume 59 Issue 4 Pages 674-678
Details
2018 Volume 59 Issue 4 Pages 674-678
The superplasticity of ZK60 alloy was investigated after thermomechanical treatments (combination of rolling at 473 K and 673 K). ZK60 alloy, with an equiaxial grains microstructure (average grain size 2.6 µm), showed a maximum elongation of 865.2% with the strain rate 1 × 10−4 s−1 under high temperature tensile test at 573 K. Surprisingly, hot-rolled ZK60 samples with fibrous microstructure also displayed pseudo-superplastic behavior with an elongation of 535.2% at the same temperature. Microstructures indicated that the hot-rolled samples with elongated grains exhibited dynamic recovery and recrystallization during high temperature deformation, transforming the fibrous to fine-grained crystals, and thus retaining pseudo-superplastic behavior.
Currently, magnesium alloy is one of the lightest structural materials for industrial applications.1,2) With the continuous trend in developing lighter and thinner 3C (Computers/Communications/Consumer) products, wrought magnesium alloys have become even more important for their applications. However, it is difficult to deform magnesium alloys at ambient temperature because of its HCP crystal structure;3) consequently, any efforts to improve the formability of magnesium alloys have received much attention.4,5)
The superplasticity is a well-recognized property in polycrystalline materials that exhibits high ductility in which the formability of metals could be improved.6) Now, it is well-established that superplasticity is anticipated when metals are processed to fine grain size. There have been many studies to develop severe plastic deformation (SPD) techniques, in which conventional coarse grained bulk solids are transformed into metals having small grain sizes.7–9) The two most effective procedures are equal-channel angler pressing (ECAP)10–17) and high-pressure torsion (HPT),18–20) which are effective for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. Figueiredo et al. presented a model for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization.21) Torbati-Sarraf et al. confirmed the successful production of an ultrafine-grained structure with a mean grain size of ∼700 nm by 5 turns of HPT at room temperature.22) The superplasticity of magnesium alloys can be achieved after grain refinement. The ZK60 showed excellent superplastic elongation of 628% at 523 K under strain rate of 1.67 × 10−3 s−1 after four passes of ECAP at 473 K.23) The Mg-5 mass% Sn alloy displayed very fine grain size of 10 µm after equal channel angular extrusion (ECAE). The maximum elongation was 550% at high temperature 623 K with strain rate of 1 × 10−3 s−1.24)
Deformation Mechanism Map (DMM) with insightful information was used to evaluate the formability of magnesium alloys.25) DMM allows us to understand (1) the optimal forming temperature, (2) forming rate required in forging process, (3) plastic deformation mechanisms such as dislocation glide, grain boundary sliding (superplasticity), and (4) dynamic recrystallization. Earlier studies showed that deformation of ZK60 at 0.5Tm resulted in grain boundary sliding, which led to superplasticity.26–31) However, Galiyev et al.31) reported that deformation at 0.5Tm caused dynamic recrystallization that led to massive deformation under the same testing conditions. These two phenomena belong to two different plastic deformation regions in DMM: (1) superplasticity in high temperature and low stress region, with a stress exponent of 2, and (2) dynamic recrystallization in the high temperature and high stress region, with a stress exponent greater than 4.25) In this study, we adjusted the microstructure of the materials by thermomechanical treatments to investigate the effect of microstructures on the plastic deformation mechanism of ZK60 at high temperatures. It is well known that the fine equiaxial grains microstructure led to superplasticity. However, we found fibrous microstructure also showed noticeable elongation. X. Yang et al. found the similar behavior in an unrecrystallized coarse-grained 7075 aluminum alloy.32) To our knowledge, it is probably for the first time shown in magnesium alloy. We named this phenomenon as pseudo-superplastic behavior temporarily.
Cast ingot of ZK60 (Mg-5.29 wt.%Zn-0.59 wt.%Zr) with 8′′ in diameter was extruded with the extrusion ratio of 60:1. The extruded sheets with thickness of 6 mm were homogenized at 673 K for 12 h. The homogenized sheets were subsequently rolled along the extrusion direction (ED) at constant temperature (HR200 and HR400) and different rolling temperatures (DTR), described as follows.
HR200: Samples were rolled at a constant temperature of 473 K with a 5% reduction per rolling, and a total rolling reduction of 80%. The sample was heated at 473 K for 20 minutes between the rolling passes.
HR400: Samples were rolled at a constant temperature of 673 K with a 20% reduction per rolling, and a total rolling reduction of 80%. The sample was heated at 673 K for 20 minutes between the rolling passes.
DTR: This process combines rolling at constant temperatures of 473 K and 673 K, respectively. Samples were first rolled at 473 K as same as HR200 process, with an accumulated rolling reduction of 40%, followed by rolling at 673 K as same as HR400 process, with an accumulated rolling reduction of 40% to reach a final total reduction of 80%.
The samples from each rolling treatment were mounted, ground, polished with alumina powders of 1 µm, 0.3 µm and 0.05 µm sizes in order to a mirror finish and etched with picric acid for 2–4 s. The microstructures were observed with an optical microscope, followed by imaging with Charge-Coupled Device, and the grain sizes were calculated using linear intercept method. The grain size is given by the relation of d = 1.74 L, where L is the mean intercept length and 1.74 is the geometrical factor between the mean length in the plane with the spatial size of the grain in 3 dimensions.33)
Samples rolled at different temperatures were punched into tensile specimens with a gauge length of 10 mm, thickness of 1.2 mm, and corner of 2.0 mm. The tensile direction is along the rolling direction (RD). High temperature tensile tests were performed at 573 K after holding 30 min at constant strain rates of 1 × 10−2 s−1, 1 × 10−3 s−1, and 1 × 10−4 s−1, respectively.
The microstructure of the as-extruded ZK60 samples shown in Fig. 1(a) exhibited uneven, chaotic microstructures and large variation in grain sizes. These severe grain distortions observed in large grains were caused by the uneven deformation of the materials during extrusion process.
Microstructures of (a) as-extruded ZK60, (b) extruded ZK60 after homogenization treatment at 673 K for 12 h, (c) homogenized ZK60 sheet after constant temperature rolling at 673 K with 80% rolling reduction, (d) homogenized ZK60 sheet after constant temperature rolling at 473 K with 80% rolling reduction, (e) homogenized ZK60 sheet after different temperature rolling with 80% rolling reduction.
Figure 1(b) shows the microstructure of specimens after homogenization treatment for 12 h at 673 K. It can be seen that, the fibrous microstructure observed in as-extruded samples was no longer present. The grains became equiaxial, but the grain size distribution was still not uniform. The observed uneven grain size distribution is attributed to the different amount of plastic deformation in the material during extrusion process. Based on the linear-intercept calculation, the average grain size was about 13.3 ± 0.6 µm.
Figure 1(c) shows the microstructure of HR400. The micrograph shows that the rolling process is effective in refining the grain size of the ZK60 alloys. Rolling at 673 K resulted in an equiaxial microstructure and an extensive grain size refinement to an average size of 2.6 ± 0.3 µm. Obviously, high temperature (673 K) rolling combined with larger deformation reduction (20%) per rolling could drastically refine the grain size. The grain refinement is due to the fact that the rolling temperature at 673 K is higher than the recrystallization temperature of the alloy, and the effect of recovery and dynamic recrystallization allowed the microstructure to retain its fine equiaxial grain.
For HR200 sample, the samples still remained a fibrous microstructure, as shown in Fig. 1(d). The reason for this behavior is that the recrystallization temperature of ZK60 is very close to 473 K; as a result, complete dynamic recrystallization could not be achieved at the rolling temperature of 473 K. Moreover, the severe deformation caused by the rolling process made the slip bands at the interior of the grains very narrow that led to retained fibrous microstructure. On the other hand, some finer grains observed in certain areas may appear, because the rolling temperature was close to the recrystallization temperature, and the large amount of deformations in some local areas may contribute to the local dynamic recrystallization. Since the grains of a fibrous microstructure were elongated along the RD, it was difficult to measure the exact grain size.
Figure 1(e) shows the micrograph of samples that were rolled following the DTR procedures. It can be seen that the last rolling processes at 673 K served to eliminate the fibrous microstructure developed during rolling at 473 K. In this process, the fibrous microstructure developed at 473 K rolling could be recrystallized by the subsequent hot rolling at 673 K. Furthermore, the two hot rolling at 673 K, each with a rolling reduction of 20%, led to an additional dynamic recrystallization. Combination of these two effects accentuated the effect of grain refinement, reducing the average grain size to 2.1 ± 0.2 µm, which is smaller than the 2.6 ± 0.3 µm observed in the samples treated with HR400 process.
3.2 High temperature tensile testSpecimens prepared by the three different rolling procedures were subjected to high temperature tensile tests at 573 K, along the RD, under different strain rates of 1 × 10−2 s−1, 1 × 10−3 s−1, and 1 × 10−4 s−1, respectively. The results are shown in Fig. 2. The HR400 specimen, which possesses an equiaxial grains with average size of 2.6 ± 0.3 µm, showed a maximum elongation of 865.2% with the strain rate 1 × 10−4 s−1 under high temperature tensile test at 573 K. On the other hand, the hot-rolled ZK60 samples with fibrous microstructure (HR200) also showed superplasticity with an elongation of 535.2% at the same temperature.
True stress-true strain curves of ZK60 samples (a) after HR200 process at 573 K, (b) after HR400 process at 573 K, (c) after DTR process at 573 K under different strain rates.
Figure 2 shows that the flow stress of the materials decreased with decreasing strain rates during the tensile tests at 573 K. Equation (1) allows calculation of strain rate sensitivity for materials rolled by different processes.34)
| \begin{equation} \sigma=k(\dot{\varepsilon})^{m} \end{equation} | (1) |
In the equation σ is the flow stress taken at a plastic strain of 0.1; k is a constant; $\dot{\varepsilon }$ is strain rate; m is strain rate sensitivity.
Figure 3 shows the relationship between σ and $\dot{\varepsilon }$ based on the experimental data. It can be seen that varying the rolling conditions changes the strain-rate sensitivity, distributed in the range of 0.4–0.6, and behavior is fitted to the requirement of superplasticity at 573 K.35) It is well known that elongation is increased with m values.35) In this study, the m values of HR400 and DTR samples were 0.6 and 0.5 respectively, which were relatively high. Both samples showed equiaxial grains, with a grain size of 2.6 ± 0.3 µm and 2.1 ± 0.2 µm, respectively. These results suggest that grain boundary sliding among equiaxial grains contribute significantly to superplasticity during high temperature deformation. HR200 samples, on the other hand, showed fibrous microstructure and a relatively lower m value of 0.4. It is inferred that the high temperature deformation mechanism is dominated by dynamic recrystallization during the high temperature tensile test, which is different from those of HR400 and DTR samples.
Strain rate sensitivity of ZK60 samples treated with different processes at 573 K.
The plastic deformation mechanisms34) can be derived by using the constitutive eq. (2).
| \begin{equation} \dot{\varepsilon}=A(D_{s}Gb/kT)(b/d)^{p}(\sigma/G)^{n} \end{equation} | (2) |
| \begin{equation} \dot{\varepsilon}= A'\sigma^{n} \end{equation} | (3) |
Figure 4 shows the experimental results calculated using the above equation, which allows derivation of the plastic deformation mechanism for ZK60 tested with different testing parameters. The slope (stress index n) is approximately 2, which is consistent with the result of Fig. 3 and indicates deformation mechanism of superplasticity at 573 K for ZK60 alloys prepared under different rolling conditions.36)
Stress index of ZK60 at 573 K.
According to the interpretation described in the previous section, all ZK60 samples, prepared under different rolling process conditions, exhibit similar superplasticity properties.
The experimental data indicates the highest elongation for ZK60 with equiaxial grains. This type of microstructure is in agreement with the criteria for superplasticity: equiaxial grains with average size less than 10 µm. The ZK60 with fibrous structure still retains pseudo-superplastic behavior even though its tensile elongation is lower than that of ZK60 with equiaxial grains at room temperature.
How does fibrous microstructure exhibit superplasticity? To explain this unusual phenomenon, we offer the following hypothesis:
(1) Fibrous microstructures, which possess as many grain boundaries as equiaxial microstructures, facilitate direct grain boundary sliding for fibrous grains.
(2) During high temperature tensile testing, equiaxial grains formed due to recovery and continuous dynamic recrystallization occurred first in the matrix of the material, followed by grain boundary sliding.
In order to find out which mechanism is responsible for the high temperature superplasticity in ZK60 with a fibrous structure, we examined along the RD the micrograph of HR200 samples, with tensile tested at 573 K under a strain rate of 1 × 10−3 s−1. Figure 5 shows that the initial fibrous microstructure produced by rolling was no longer present, and was replaced by equiaxial grains with an average grain size of 4.2 µm. Comparing Fig. 5(a) with 5(b), we can see that the grains closer to the fractured surface are smaller than those in the interior of the sample because of the larger plastic deformation in the fractured area. The result indicates that recovery and dynamic recrystallization occur before grain boundary sliding during high temperature tensile deformation.
Micrographs of the fibrous microstructure of ZK60 after tensile test at 573 K, under a strain rate of 1 × 10−3 s−1. (a) Fracture surface, (b) Deformation area that produced dynamic recrystallization.
The reasons for superplasticity behavior observed in fibrous microstructure can be explained as follows:
Fibrous microstructure may produce uniform equiaxial grains through recovery and continuous dynamic recrystallization under high temperature and tensile stress conditions. Therefore, grain boundary sliding only occurs after the formation of equiaxial grains. Thus, the superplasticity observed in fibrous microstructure occurs.
Is it possible to calculate the theoretical grain size required for superplasticity in fibrous microstructure from the empirical high temperature mechanical properties of fibrous microstructure?
According to early researchers, the superplasticity of ZK60 alloy occurred at a stress exponent of 214) and a grain size exponent of 3.7) At constant temperatures and under constant strain rates, using the relationship between grain size and flow stress, eq. (2) can be simplified into eq. (4).
| \begin{equation} d^{3}=B\sigma^{2} \end{equation} | (4) |
Using the experimental data measured at 573 K and under a strain rate of 1 × 10−3 s−1, d and σ can be calculated as shown in Fig. 6. Applying linear regression, and plugging in the tensile stress used for the fibrous microstructure, we found the theoretical grain size to be 3.57 µm, which is close to the experimentally measured grain size of 4.2 µm shown in the Fig. 5(b) after high temperature tensile test. These results show that the grain size conducive to pseudo-superplastic behavior in fibrous microstructure can be calculated theoretically.
The equivalent grain size of fibrous microstructure exhibiting pseudo-superplastic behavior calculated using linear regression.
The authors gratefully acknowledge the financial support of this study from Ministry of Science and Technology (MOST), Taiwan, Republic of China, under the Grants MOST 106-2221-E-259-011 and 104-2221-E-259-021-MY2. The authors also thank Dr. D. C. Jeng’s useful discussions.
