Warm deformation mechanism of hot-rolled Mg alloy
LIU Jun-wei(刘俊伟), CHEN Ding(陈 鼎), CHEN Zhen-hua(陈振华)
College of Materials Science and Engineering, Hunan University, Changsha 410082, China
Received 12 June 2008; accepted 5 September 2008
Abstract: Tension attachment of high temperature microscopy was proposed to research the microstructure evolution and plastic behavior of AZ31 magnesium alloy in a temperature range of 473-523 K and a load range of 80-160 N. Transmission electron microscopy(TEM) was utilized to observe the morphology of twins after deformation process. The results show that as Zener- Hollomon parameter Z increases (temperature falls, strain rate rises), the peak stress obviously increases, while the ductility tends to become worse. A great amount of twins can be found at moderate temperatures. Therefore, basal slip, a+c non-basal slipping and twinning are considered the dominant mechanisms at moderate temperatures. Some DRXed grains can be observed in the twinned regions and grain boundaries, suggesting both twinning-induced DRX and continuous DRX occurs in the deformation process.
Key words: magnesium alloy; twinning; high temperature microscopy; microstructure
1 Introduction
Due to the high specific strength, superior damping capacity, excellent machinability and good electromagnetic shielding characteristics, Mg alloys have attracted great attention with regard to their application in transportation, electronics and aerospace industries [1-3]. However, casting defects, such as porosity and inclusions, as well as the rather low ductility of Mg alloys at room temperature greatly restrict the wide use of Mg alloy. Moreover, magnesium alloys with hcp structure show a strong crystallographic anisotropy of their mechanical properties, especially at low temperatures[4-6]. By these reasons, further researches of the microstructure evolution and mechanical properties at moderate temperatures have been required to extend the application of Mg alloys.
YIN et al[7] has carefully investigated the relationship between dynamic recrystallization (DRX) and twinning, and suggested a twinning induced DRX model in a temperature range of 323-473 K. MYSHLYAEV[8] described a thermomechanical processing of hot worked AZ31 Mg alloy at moderate temperatures and showed that the enhanced dynamic recovery (DRV) and DRX play a significant role in reducing flow stress and raising ductility to improve industrial processing at moderate temperatures. They found that since twinning and other features described at low temperatures were also found at high temperatures, with decreasing frequency, the microstructures showed severe heterogeneity which accounted for the limited ductility. A lot of research showed that three types of twins were frequently reported in Mg alloys: extension twins, contraction twins and double twins. LAN et al[9] pointed out that the softening induced by both contraction twinning [10-11] and tension twinning[10-12] overrides the dislocation hardening and twinning-induced hardening at moderate temperatures. Nevertheless, the research on the microstructure evolution and mechanical properties for the samples deformed at moderate temperatures has still not been further systematically studied.
For this purpose, the current study is undertaken to investigate these issues further. By using high temperature microscopy(HTM), X-ray diffraction(XRD) and high-resolution transmission electron microscopy (HRTEM), the heterogeneous combination of slip, twinning, DRV and DRX during deformation processing is analyzed.
2 Experimental
2.1 Materials
The chemical composition of the used AZ31 Mg alloy is presented in Table 1. The material used in this study was a commercial hot-rolled AZ31 Mg alloy with a thickness of 1.2 mm and average grain size of 17.5 μm (Fig.1). It consisted of fine and equiaxed grains with a mean grain size of about 20 μm. It can be clearly seen that some small recrystallized grains with low density of dislocations were found to surround some large recrystallized grains.
Table1 Chemical composition of hot-rolled AZ31 alloy (mass fraction, %)
Fig.1 Microstructure of hot-rolled AZ31 Mg alloy
Specimens with a gauge of 27 mm in length, 2 mm in width and 1.2 mm in thickness were electro- discharge machined with the tensile axis parallel to final rolling direction.
2.2 Experimental procedure
Tension tests were carried out using a tensile attachment of HTM at a temperature rage of 473-523 K and a load rage of 8-16 kg with a high vacuum circumstance (≤1.33×10-4 Pa) in HTM. Before tensile, specimens for HTM were sectioned and cold mounted. A solution of phosphoric acid in absolute ethyl alcohol at room temperature was used at potential 4 V. The mean grain size was determined using linear intercept method (d=1.74 L, where L is the linear interception size). Then tension tests were carried out using a computer servo controlled Gleeble-1500 machine at strain rates of 10-3-10-1 s-1 and deformation temperatures of 373-523 K.
3 Results and discussion
3.1 Mechanical behaviors
A summary of mechanical properties of specimens deformed in HTM is shown in Table 2. From Table 2, it is evident that the elongation value is dependent upon the temperature and load. At 473 K, the elongation value is not high. With temperature increasing to 523 K, the elongation value shows an obvious, improvement which indicates the activation of additional deformation mechanisms. This implies that the screw dislocations stocked in grain boundaries are activated with the increase of temperature and move to the next slip plane. The relaxation of stocked dislocations in the grain boundary leads to softening. In this way, the improvement of elongation can be obtained. As shown in Table 2, the elongation values also rise significantly with the decrease of strain rate and load. This is because that DRX in the deformation process is suppressed with the increasing strain rate. The stress concentration is difficult to be relaxed at high strain rates due to increased severity of dislocation pile-up and depressed DRX, thus deteriorating the ductility.
Table 2 Experimental conditions and results in constant loading tension
A typical tensile stress—strain curve at various strain rates and temperatures is shown in Fig.2. It is obvious that the ductility of AZ31 Mg alloy rises sharply with increasing temperatures, while the strain hardening rate before peak stress declines significantly. Otherwise, the plasticity in tension decreases with increasing strain rate, showing obvious strain rate sensitivity. The strain hardening rate before peak stress increases apparently with increasing strain rate, and the softening stage after peak stress is markedly reduced, thus, shortening the steady flow process and deteriorating the ductility of the alloy.
Fig.2 Typical engineering tensile stress—strain curves of hot- rolled AZ31 samples deformed at 523 K and various strain rates
In magnesium alloys, basal slip, a+c non-basal slip, twinning and DRX are the four major mechanisms involved in the release of stress. At temperatures below those at which individual atoms are mobile, slip and twinning are the major deformation modes which enable a solid to change shape under the action of an applied stress[10]. However, the activation of pyramidal and prismatic slip systems in magnesium alloy aggregates occurs primarily due to the large stresses generated in grain-boundary regions because of misorientation between neighboring grains. So the critical resolved shear stress(CRSS) of pyramidal and prismatic slip is difficult to be surpassed and there are few slip systems activated during deformation processing[11].
As reported, CRSS for a basal slip system is much lower than those of non-basal slip systems on pyramidal planes, as well as the twinning modes. For the present hot-rolled samples which have a strong basal plane texture, stress direction is nearly parallel to the basal plane. Because of the small orientation factor, which denotes an extremely low shear stress on the basal slip system, the basal slip is limited in the tensile loading conditions. However, the CRSS for basal slip is only 2 MPa in AZ31 Mg alloy[12]. Thereby, a slight misalignment (about 100) of basal plane can induce the operation of basal slip. By these reasons, softening mechanisms of Mg alloys often exhibit by twinning and basal slip, when the temperature is below recrystallization temperature.
3.2 Microstructure evolution
To analyze the deformation mechanism of AZ31 Mg alloy from the viewpoint of microstructure, micro-image of HTM were used to observe the microstructure evolution in deformed specimens. Fig.3 shows the different stage (60, 240, 480 and 600 s) microstructures of unfractured specimens deformed at 523 K and 120 N. Since some of twins are immediately investigated in the grains at the beginning of the deformation (Fig.3 (a)), it is evident that twins are extremely sensitive to applied stress. It is known that there are two types of twins frequently reported in alloys: delayed twinning (common in fcc metals) which usually has a rather small effect on the actual stress vs strain curve and immediate twinning (common in hcp metals) which is often characterized by very rapid formation of twinned regions[10]. Twinning of the latter type is very sensitive to temperature of deformation and to applied stress. Since twinning and other features described at low temperature are also found at high ones, with decreasing frequency, the microstructures show severe heterogeneity which accounted for the limited ductility. Across the entire range, twinning occurs in grains poorly oriented for slip to produce orientations suitable for basal slip thus creating stress concentrations enhancing multiple slip in line with reported behavior.
Fig.3 Microstructures of specimens deformed at 523 K and 120 N: (a) 120 s; (b) 240 s; (c) 480 s; (d) 660 s
The strongest evidence for this in our opinion is the formation of subgrains that is aided by additional slip system operation as a result of both higher temperature and higher, more complex stresses near twin intersections. The clear progression in microstructure development with both rising T and decreasing 
is fully consistent with the decrease in stress and rise in ductility. The effects of temperature and strain rate on forming can be integrated with employment of the Zener- Hollomon parameter Z:
(1)
It is obvious that Z parameter decreases with increasing T or decreasing . GUO et al[13] described that Z parameter plays a significant role on average DRX grain size Drex. They found that the relationship between Z and Drex during hot compression can be expressed by the formula via regression
ln Drex= -0.105 ln Z+4.695 (2)
Fig.3(b) illustrates that with the deformation pursuing, a few slip lines (as indicated in region A in Fig.3(b)) can be found near the grain boundaries. Due to the hindrance of grain boundaries, the stress concentration is generated. To relax the stress concentration, the basal slip gradually plays the dominant effect. As shown in Fig.3(c), it is clear that the nucleation and growth of DRXed grains is improbable unless there is a combination of extremely high stress and energies. Due to the high stress concentration, some fine DRXed grains appear in the grain boundaries (indicated in Region A and B). At the end of deformation process, nucleation of cavities can be observed in Fig.3(d). Due to the improvement of load, the nucleated cavities tend to coalesce with the neighboring cavities and the interlinkage of cavities occurs easily. At the end of deformation process, the cavities interlinked seriously and lead to fracture at last. As shown in Fig.3(d), some of the large grains are very sensitive to DRX and thus are
Fig.4 Microstructure of alloy deformed at constant strain rate of 10-2 s-1 and 523 K
replaced by DRXed grains, while fine grains can not be replaced by fine grains.
To further analyze the relationship between DRX and twinning, a typical microstructure of the hot-rolled AZ31 alloy deformed at constant strain rate of 10-2 s-1 and 523 K is shown in Fig.4. From Fig.4, it is clear that tiny grains of 1-2 μm begin to nucleate and entire microstructure become extremely inhomogeneous. It can be concluded that some of the grains are very sensitive to DRX, and thus are quickly replaced by DRXed grains, while some grains are present which are insensitive to DRX. As shown in Fig.4, there is a large uncrystallized grain, while the surrounding area of this large grain has already been replaced by fine grains (as indicated in region A in Fig.4). The uneven rate of recrystallization at different grains is considered to be the main reason for the inhomogeneous microstructure. Moreover, DRXed grains can be occasionally observed at the twinned regions (as indicated in the region B in Fig.4). It is often observed that the size of DRXed grains can be estimated by the width of the twins, suggesting that the nucleation of DRXed grains is closely related to the specific mechanism associated with the twinning.
In fact, the most commonly observed DRX mechanisms in Mg alloy are twinning-induced DRX and continuous DRX. Continuous DRX which is characterized by direct transformation of subgrains into high angle grains is believed to be the predominant mechanism for the nucleation of new grains in Mg alloys. For twinning-induced DRX, basal dislocations accumulate near twin boundaries in the initial stage of twinning. With the continuous deformation, the dislocation pile-up and stress concentration become serious, which increases the internal stress up to the CRSS for the activation of a+c non-basal slip. Therefore, non-basal slip is activated near grain boundary and three-dimension DRX nuclei are formed by the interaction between a dislocations and a+c dislocations. Otherwise, the intersection of different types of twins also leads to the occurrence of DRXed grains, as shown in Fig.5[14]. By transferring to low-angle boundaries, primary twin lamellas are spitted. Under the control of grain boundary migration(GBM), the DRX nuclei grow to recrystallized small grains.
Fig.6 presents a typical TEM micrograph of the deformation twins. As shown in Fig.6, another feature of the microstructure is the dislocation arrays that can be observed in the top and bottom of twin plates. Some of dislocation arrays also exist in the twin plate. As mentioned before, dislocation slip contributes to the plastic deformation, besides twinning. Dislocations in the top and bottom of twin plates form dislocation network. Normally, a high density of dislocations may cause changes to crystal orientations and eventually become grain boundaries. Previous work pointed out that a sufficient number of dislocations could induce the formations of grains with different orientations[15]. However, due to the limitation of recrystallization temperature and strain energy, the dislocation arrays in twin plate only subdivide the twin platelets into smaller parts that have less different orientations.
Fig.5 Schematic representation of TDRX mechanism: (a) Mutual intersection of primary twins 1 and 2; (b) Subdivision of coarse primary twin lamellas 1 by fine secondary twins 2; (c) Subdivision of primary twin lamellas by transverse low-angle boundaries; (d) Scheme of formation of orientation misfit dislocations (with Burger vectors b3 and b4) in twin boundaries providing a change in misorientation of twin boundaries
Fig.6 TEM image of alloy elongated at 523 K and 10-2 s-1
4 Conclusions
1) The ductility of hot-rolled AZ31 Mg alloy increases with deformation temperature increasing and decreases with increasing load and strain rate.
2) Since twinning and other features described at low temperatures are also found at high ones, with decreasing frequency, the microstructures show severe heterogeneity which accounted for the limited ductility.
3) The main DRX mechanism of AZ31 is conventional continuous DRX. However, some of DRXed grains are continuously nucleated in the twinned regions, suggesting the nucleation of these DRXed grains is closely related to TDRX.
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(Edited by CHEN Ai-hua)
Corresponding author: CHEN Ding; Tel: +86-731-8820648; E-mail: liujw1981@yahoo.com.cn
