J. Cent. South Univ. Technol. (2010) 17: 197-201
DOI: 10.1007/s11771-010-0030-6 
Effect of twinning on flow stress during initial stage of hot compression of twin roll cast Mg-5.51Zn-0.49Zr alloy
LIU Zhi-yi(刘志义)1, 2, XU Jing(徐静)1, 2, HOU Yan-hui(侯延辉)1, 2, KANG S. B.3
1. Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education,
Central South University, Changsha 410083, China;
2. School of Materials Science and Engineering, Central South University, Changsha 410083, China;
3. Korea Institute of Materials Science, Changwon, Korea
? Central South University Press and Springer-Verlag Berlin Heidelberg 2010
Abstract:
A Thermecmastor-Z hot deformation simulator, optical microscopy, XRD and TEM were employed to characterize the flow stress behavior and microstructure of twin roll cast ZK60 magnesium alloy during initial stage of hot compression at elevated temperature of 300 ℃ and 400 ℃ and a given strain rate of 10-2 s-1. The results suggest that flow stress drop during initial stage of hot compression at 300℃, generally led by dynamic recrystallization, is attributed to twinning, correspondingly to dynamic recrystallization as deformation temperature is raised to 400 ℃.
Key words:
magnesium alloy; basal slip; non-basal slip; twinning; hot deformation; recrystallization；
1 Introduction
In spite of lack of sufficient independent slip system magnesium alloy is of good formability at elevated temperature due to activation of pyramidal plane slip〈c+a〉, which, together with twinning, provides plastic deformation along c axis besides basal plane slip, as well as prismatic plane slip, allows a plastic contribution along 〈a〉direction. Accordingly, investigations have been mainly focused on microstructural evolution and deformation mechanism during plastic deformation at elevated temperature in recent years [1-3]. Twinning is an important deformation mechanism during hot deformation. A lot of attention has recently been paid to the effect of twinning on flow behavior [4-6] and correlation of twinning with recrystallization [7] during hot deformation of magnesium alloy. For instance, JIANG et al [4], and WANG and HUANG [5] demonstrated that all contraction twinning, extension twinning and double twinning could result in a flow softening behavior and decrease the yield stress due to their contribution to plastic deformation along the applied stress direction. BEER and BARNETT [6], and PROUST et al [8] suggested that twinning could result in a flow hardening behavior and enhance the peak stress due to its grain refinement effect and Hall-Petch mechanism. All of the previous work demonstrated twinning behavior and its effect are related with the texture of as received samples of magnesium alloys.
Twin-roll strip casting is not only a cost-effective manufacturing process but also has grain-refinement effect due to a solidification rate as high as 103 s-1 [9]. Therefore, it received more and more attention recently [10-11]. Most of them [9-10, 12], however, were mainly focused on microstructure and properties. Little work was reported on twinning behavior in twin-roll cast magnesium alloy during hot deformation. Consequently in this work, twin-roll cast strip of ZK60 alloy was taken as experimental materials to identify the twinning behavior during hot deformation.
2 Experimental
The material investigated in this work is commercial magnesium alloy ZK60, with its chemical composition of Mg-5.51Zn-0.49Zr (mass fraction, %). The alloy was obtained in the form of as twin-roll cast strip with thickness from 2.90 to 3.50 mm. Plate-form specimens with a length of 50 mm and a width of 20 mm and a thickness of 3 mm were machined from the strip for uniaxial compression tests. Uniaxial compression tests were performed on a Thermecmastor-Z hot deformation simulator, at the temperature of 300 ℃ and 400 ℃, and a strain rate of 1.0×10-2 s-1 with final true strain of 0.511. The stress was employed in the direction of thickness during hot compression for simulating subsequent hot rolling of as received twin-roll cast strip. The specimens were quenched by nitrogen gas immediately after compression in order to keep the microstructure after deformation.
Texture measurements were carried out by the Schulz reflection method using nickel filtered Cu Kα radiation. The measurements were conducted on the mid-plane sections. (0002) pole figure for as twin-roll cast strip was constructed using diffracted X-ray intensities measured on (0002) reflection. Specimens for microstructural observation were cut from the strip along the plane formed by the rolling and normal direction and mechanically polished using emery papers up to #1200 followed by final polishing on a Struers OP-Chem polishing cloth using polishing suspension OP-S (0.24 μm sized diamond particles). Both deformed and as received samples for optical microscopy were etched in a solution of ethanol (100 mL), picric acid (5 g), acetic acid (5 mL) and water (10 mL). The optical microstructure of both surface layer and the center of the deformed samples was examined and compared for considering strain gradient in stress direction. The TEM microstructure examination of the deformed samples was performed on a JEM-2000FXⅡ instrument. Two-beam technique was employed to distinguish basal slip and non-basal slip during hot compression. TEM samples were prepared by cutting from the deformed samples compressed to a strain of 0.51, along the plane perpendicular to applied stress direction.
3 Results and analysis
3.1 (0002) pole figure and optical microstructure of as received strip, and stress strain curves
(0002) pole figure of as twin-roll casting ZK60 strip is shown in Fig.1(a), which illustrates the main component of (0001) crystal plane deviating 35? from the normal direction. This suggests that the angle between (0001) crystal plane and rolling plane is 35?, and furthermore the crystal direction perpendicular to (0001) plane is inclined towards rolling direction. The optical micrograph taken along the plane formed by rolling and normal direction of the strip is shown in Fig.1(b), illustrating fine dentrite of 10-25 μm in size and small amount of twins, marked by white arrow in Fig.1(b).
The stress strain curves under a constant strain rate of 1×10-2 s-1 and elevated temperatures of 300 ℃ and 400 ℃ are plotted in Fig.2, in which a stress drop after peak stress appeared when deformed at both temperatures. Nevertheless the stress drop at 300 ℃ to compression is smaller than that at 400 ℃. It is worthy to note that a small stress rebound appeared after flow stress dropped, as compared valley point A with rebound point B in the stress strain curve as deformed at 300 ℃, which was followed by a steady flow. Compared to compression at 300 ℃, however, a steady flow stress rather than stress rebound directly followed the stress drop during hot deformation at 400 ℃.
Fig.1 (0002) pole figure (a) and micrograph (b) of as twin-roll cast strip of ZK60 alloy
Fig.2 Stress strain curves at constant strain rate of 1×10-2 s-1 and different temperatures of 300 ℃ and 400 ℃
3.2 Optical microstructure of specimens compressed at 300 ℃
Figs.3(a) and (b) show the optical microstructures in the surface layer and those in center of samples deformed to a true strain of 0.105, respectively. Figs.3(c) and (d) respectively present microstructures corresponding to the same position as Figs.3(a) and (b) as sample was strained to 0.223. Little distorted twin was found in the surface layer of both samples compressed to true strains of 0.105 (Fig.3(a)) and 0.223 (Fig.3(c)). This is because plastic deformation parallel to the sample surface is limited due to friction between squeeze head and the samples. Nevertheless, a lot of twins as well as some grains in center of the sample were distorted and elongated due to shear deformation. Compared to Fig.1(b) it can be seen that the amount of twins either formed in surface layer or in center of the deformed samples increased with increasing strains, especially the twins formed in the center of samples were distorted more severely as strained from 0.105 to 0.223, as shown in Fig.3. It is suggested that new twins propagated and previously formed twins were distorted when true strain increased from 0.105 to 0.223. The former true strain nearly corresponds to peak stress and the latter to the lowest stress (valley point marked as A) in the stress strain curve compressed at 300 ℃ as shown in Fig.2. A great stress drop appeared during this deformation stage. Metallographic examination showed that no recrystallization occurred during this stage of hot compression. Consequently, it was shear deformation and twinning that predominated the hot compression at this stage (see Fig.3). This suggests that, only twinning can be responsible for this stress drop because shear deformation enhances dislocation density and contributes to strain hardening. Furthermore, the occurrence of twinning results in grain refinement and subsequent stress rebound after stress drop due to Hall-Petch mechanism. The distorted twins as shown in Figs.3(b) and (d) are evidences for this effect, which has already been confirmed by previous work [7-8].
Fig.3 Optical microstructures of specimens compressed with variation of true strain at constant strain rate of 1×10-2 s-1 and deformation temperature of 300 ℃ (Compression stress was applied along horizontal direction): (a) Surface layer (ε=0.105);(b) Center (ε=0.105); (c) Surface layer (ε=0.223); (d) Center (ε=0.223)
3.3 Optical microstructure of specimens compressed at 400 ℃
Figs.4(a) and (b) show the optical microstructures of surface layer and center of samples compressed to a true strain of 0.105 at a deformation temperature of 400 ℃, respectively. Both of them show dynamic recrystallization occurred at grain boundary and twin boundary. This suggests that dynamic recrystallization predominated hot compression when deformed at 400 ℃ although a few twins were present in the center of the deformed sample (see Fig.4(b)). Comparing Fig.4 with Fig.2 it can be seen that dynamic recrystallization was responsible for stress drop during this stage of deformation.
Fig.4 Optical microstructures of specimens compressed with variation of true strain at constant strain rate of 1×10-2 s-1, and deformation temperature of 400 ℃ (Compression stress was applied along horizontal direction): (a) Surface layer (ε=0.105); (b) Center (ε=0.105)
Consequently, it could be summarized that twinning was responsible for the stress drop during early stage of hot compression at 300 ℃, and dynamic recrystallization for that at 400 ℃.
3.4 TEM microstructure
Fig.5(a) shows a TEM image of twins formed when sample was compressed to a true strain of 0.105 at 300 ℃. Fig.5(b) shows a selected area diffraction pattern (SADP) corresponding to twin boundary shown in Fig.5(a). Identification of the SADP shown in Fig.5(b) suggested that the twin presented in Fig.5(a) is a 
contraction twin. No 
extension twin was found during TEM examination. This at least suggests that the twins shown in Fig.3 mainly consist of 
contraction twins although extension twin cannot be excluded. Two-beam technique was employed during TEM microstructure examination, in which diffraction parameter of 
and 
was employed to distinguish basal segment of dislocations, as shown in Fig.6(a). Correspondingly and g[0002] were employed to identify non-basal segment of dislocations, as shown in Fig.6(b). Comparing Fig.6(a) with Fig.6(b) basal slip was found to predominate hot compression, and non-basal slip only played an accommodating role in hot deformation. It is sure that either basal slip or non-basal slip contributes to strain hardening. The stress drop during initial stage of hot compression at 300 ℃ is, thus exclusively attributed to twinning.
Fig.5 TEM image showing twins formed when specimen was compressed to true strain of 0.105 at constant strain rate of 1×10-2 s-1 and deformation temperature of 300 ℃ (a), and corresponding selected area diffraction pattern(b) (electron beam B parallel to 
zone axis )
4 Discussion
Fig.1(a) suggests that the (0001) crystal plane of most grains in as received strip deviates 35? from rolling plane. Furthermore, the normal direction of (0001) plane is inclined towards rolling direction. Based on this geometric relation calculation of the angle of rolling plane with six extension twin planes and six contraction twin planes yields 8?, 38.36?, 77.98?, 78.16?, 77.98? and 38.36? for 
and 26.93?, 45.90?, 91.98?, 96.93?, 91.98? and 45.90? for 
respectively. Among them, only 
and contraction twinning planes are coincident with the direction of maximum shear stress in the case of applied stress normal to the strip as illustrated in Figs.3 and 4. This suggests that less twinning can occur at the beginning of hot compression. Compared to extension twin contraction twin is more readily formed, which consists with the result presented in Fig.5. Fig.6 suggests that basal slip predominated plastic deformation, in contrast non-basal slip only played an accommodating role in the beginning of hot compression, for which subsequent rotation of grains started and (0001) crystal plane rotated towards the rolling plane. A lot of previous work [3, 13-15] demonstrated this grain rotation during hot deformation of magnesium alloy. When (0001) crystal plane became to be parallel to or nearly parallel to rolling plane by rotating the angle between (0001) plane and six extension twinning planes to 43?, all the six twinning planes are almost coincident with the direction of maximum shear stress. Compared to the beginning of hot compression more twinning readily occur when sample is compressed to a given strain. This is in agreement with the results presented in Fig.3. Consequently, a great deal of plastic deformation along externally applied stress direction could occur to release and reduce the employed stress. This induced a great resultant stress drop in stress strain curve when sample was compressed at 300 ℃.
Fig.6 TEM images under two-beam diffraction patterns of 
(a) and g[0002] (b) when specimens were compressed at strain rate of 1×10-2 s-1 and elevated deformation temperature of 300 ℃ (Electron beam B parallel to 
zone axis )
5 Conclusions
(1) The stress drop appeared in stress strain curve during initial stage of hot compression at 300 ℃, generally led by dynamic recrystallization, is attributed to twinning.
(2) The stress rebound after stress drops when the sample is deformed at 300 ℃ is due to grain refinement resulting from twinning and Hall-Petch mechanism.
(3) The stress drop when the sample is compressed at 400 ℃ is attributed to dynamic recrystallization.
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Foundation item: Project(10020072) supported by Brain Pool Program of Korea Government and Core Technology R & D Program for the Development of High Performance Eco-friendly Structural Materials of the Korean Ministry of Commerce, Industry and Energy
Received date: 2009-04-14; Accepted date: 2009-07-02
Corresponding author: LIU Zhi-yi, PhD, Professor; Tel: +86-731-88836927; E-mail: liuzhiyi@mail.csu.edu.cn
(Edited by YANG You-ping)
Abstract: A Thermecmastor-Z hot deformation simulator, optical microscopy, XRD and TEM were employed to characterize the flow stress behavior and microstructure of twin roll cast ZK60 magnesium alloy during initial stage of hot compression at elevated temperature of 300 ℃ and 400 ℃ and a given strain rate of 10-2 s-1. The results suggest that flow stress drop during initial stage of hot compression at 300℃, generally led by dynamic recrystallization, is attributed to twinning, correspondingly to dynamic recrystallization as deformation temperature is raised to 400 ℃.
