DYNAMIC RECRYSTALLIZATION DURING HOT DEFORMATION OF Mg-Zn-Ca. Monika HRADILOVÁ a, Pavel LEJČEK b

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1 DYNAMIC RECRYSTALLIZATION DURING HOT DEFORMATION OF Mg-Zn-Ca Monika HRADILOVÁ a, Pavel LEJČEK b a Department of Metals and Corrosion Engineering, Institute of Chemical Technology Prague, Technická 5, Prague 6, Czech Republic, hradilom@vscht.cz b Department of Advanced Structural Materials, Institute of Physics, ASCR, Na Slovance 2, Prague 8, Czech Republic Abstract Mechanical and chemical properties of magnesium and its alloys can be significantly improved by decreasing grain size which can be achieved by continuous or discontinuous dynamic recrystallization activated during hot deformation. The present work is focused on the study of the mechanism of dynamic recrystallization modifying the microstructure of Mg 4Zn and Mg 4Zn 0.4Ca alloys (in wt.%) during uniaxial compression at 240 C with constant strain rate of s -1 and three different final strains of 0.09, 0.3 and close to 1. Light microscopic and scanning electron microscopic analyses of the microstructures showed that the grains were gradually fragmented with increasing strain. As a result inhomogeneous structure is generally formed and it is constituted of fine dynamically recrystallized grains of several micrometres and coarse unrecrystallized initial grains. Besides, the deformation condition led to deformation-induced precipitation. To elucidate the main mechanism of dynamic recrystallization contributing to the grain refinement, the electron backscatter diffraction analysis was used. Preliminary results showed that the evolution of the microstructure was favourably influenced by Ca addition that evoked the formation of the second phase particles. Keywords: Magnesium alloys, deformation, microstructure, recrystallization 1. INTRODUCTION Magnesium and its alloys are promising engineering materials for application in transport industry or in medical field. The mechanical and chemical properties can be further modified by the addition of appropriate solutes as well as by reduction of their grain size [1]. Magnesium alloys containing zinc in combination with calcium represent beneficial combination of price and properties such as a relatively high strength, hardness, corrosion resistance. The role of zinc consists predominantly in increasing strength of Mg alloys [2, 3]. On the other hand, additions of calcium induce the grain refinement and usually result in precipitation during solidification and hot processing [4]. In addition, calcium can improve corrosion resistance due to the formation of an oxide layer [3]. Some studies [4, 5] have shown that Ca-doped Mg alloys can exhibit the comparable effects achieved by alloying with rare earth metals. Additional improvement of the properties of magnesium alloys can be attributed to refined grain size as well as to the existence of fine precipitates formed during hot deformation and heat treatment [6]. The grain refinement in various magnesium alloys can be achieved by dynamic recrystallization (DRX) activated at various thermo-mechanical processes such as torsion, uniaxial compression, extrusion or equal channel angular pressing [7]. The mechanism of dynamic recrystallization can be generally distinguished as either continuous or discontinuous. The continuous DRX (CDRX) is partly dynamic recovery process. The CDRX consists in continuous absorption of dislocations in low-angle grain boundaries (LAGBs) subgrains - followed by the formation of new grains separated by the high-angle boundaries [8]. On the other hand, during discontinuous DRX (DDRX) new grains are formed usually by conventional nucleation and nucleus growth [8]. In addition, other mechanisms discussed in connection with deformation of magnesium alloys are twin-aided DRX (TDRX) [7, 9] or particle-stimulated nucleation (PSN) [10]. The mechanism of PSN, which is based on the existence of misorientation gradients around large hard particles during deformation, resulting in the creation of new high-angle grain boundaries (HAGBs), has been studied in more detail in aluminium alloys [10]. Moreover, PSN mechanisms can lead to weakening of the deformation texture that can be also attractive feature for magnesium alloys [11]. Studies [12, 13] have shown that the presence of second-phase particles can more or less favourably influence the deformation behaviour of Mg alloys, depending on their size, spacing, fraction and the conditions of deformation [10].

2 As briefly outlined above, various mechanisms can be connected with the grain refinement during deformation of magnesium alloys. Thus, further research is necessary to better understand of DRX phenomena. The present paper describes refinement of microstructures of magnesium alloys during deformation at intermediate temperature with the aim to reveal the mechanism of DRX that takes place in the Mg Zn Ca alloy. 2. EXPERIMENTAL DETAILS The alloys were prepared from pure Mg (~ %) and Zn (~ 99,96 %), and a Mg10Ca (wt. %) master alloy by induction melting (Balzers VSG-02 vacuum furnace) in a carbon crucible under argon atmosphere and followed by casting into a steel mould. The composition of the Mg 4Zn and Mg 4Zn 0.4Ca alloys (in wt. %) was verified by XRF analysis. The ingots were annealed 24 h at 340 C, quenched into water and machined into cube (10 mm 10 mm 10 mm) for compression tests (SCHENCK servohydraulic compression machine). The uniaxial compression (UC) tests were realized at 240 C with three different final equivalent Von Mises strain about 0.09, 0.3 and close to 1 at constant strain rates s -1. Before tests, samples were held at the process temperature for 10 min. After UC tests, they were quenched into water to freeze microstructure for subsequent observation. The microstructures were observed by light microscope (Olympus GX51) and scanning electron microscope (Jeol JSM-6500F) equipped with an EBSD detector and evaluated by the HKL Channel 5 EBSD software. The samples were mechanically ground, polished and finally etched. For the electron back-scatter diffraction (EBSD) analysis, the samples were further electro-polished in a solution of orthophosphoric acid and ethanol (3:5) at a voltage of 2 4 V and etched for a few seconds in a solution of 10 ml of nitric acid, 30 ml of acetic acid, 40 ml of distilled water and 120 ml of ethanol. The linear intercept method was used for measurement of the grain size. 3. RESULTS AND DISCUSSION 3.1 Microstructure evolution To describe the evolution of the microstructure of magnesium alloys, the compression tests carried out at 240 C were terminated at a strain of about 0.09 (i.e., before achieving the maximum stress), at a strain of 0.3, after passing this maximum and at strain close to 1 (see the σ ε curve in Fig. 1). Before deformation, the as-cast alloys were annealed 24h at 340 C to homogenize their structures. Besides, large precipitates (with various shapes and the size from units to tens of µm) with almost uniform dispersion were present in the case of the ternary alloy Mg4Zn0.4Ca. The average grain size of these annealed alloys was of about 100 µm. The small deformation (ε = 0.09) led to significant grain fragmentation via twins occurred in both alloys Fig. 1 (a). These twins were heterogeneously distributed within the samples and appeared inside 30 40% of the Fig. 1 Measured stress strain curves of the two magnesium alloys at 240 C and strain rate s -1. grains. The multiple twinning described in [7] also contributed to further grain fragmentation, as well as grain boundary serrations (Fig. 2(a)).

3 Fig. 2 Microstructures of Mg4Zn (a) and Mg4Zn0.4Ca (b, c) after UC at 240 C with the strain rate of s -1 to ε = 0.09 and 0.3, respectively (LM, SEM). After reaching the maximum stress at a strain of 0.3, the microstructure was still heterogeneous, but new fine grains were present in addition to twins (Fig. 2 (b)). Let us to note that few fine grains were found in the vicinity of large precipitates (Fig. 2 (c)) in the ternary alloy. Moreover, in both alloys new fine precipitates were observed (Fig. 2 (c)) as well as a dense distribution of kinks at grain and twin boundaries. Highly heterogeneous structures were also reached in the case of a strain close to 1. A mix of fine and coarse grains formed bimodal structures where the recrystallized grains were concentrated along the original boundaries. Furthermore the deformation led to the strain localization predominantly in the case of binary alloy. In contrast, the imposed strain was more effectively distributed probably due to the presence of the large precipitates in the ternary alloy. 3.2 Mechanism of dynamic recrystallization The mechanism of DRX was deduced on basis of analyses of inverse pole figure (IPF) and local misorientation (LM) maps provided by EBSD. Selected maps are shown in Fig. 3. In the present case the IPF map is related to compression direction (CD) and thus the normal axis of the basal plane is parallel to the compression loading direction in the case of the red colour 1 (Fig. 3 (a)). In contrast, LM mapping can identify small orientation changes within the sample and highlight the regions of higher deformation. Nonetheless, this LM map enables us to relate local orientation changes to dislocation substructures such as dislocation walls or subgrain boundaries (indicated by the green colour in Fig. 3 (b)). An example of the formation of substructure is highlighted by the red arrow in Fig. 3 (b). Fig. 3 (a) IPF map and (b) LM map of Mg4Zn after UC at 240 C with the strain rate of s -1 and a final strain close to 1. 1 White colour indicates non-indexed points.

4 One of the evidence of the continuous DRX mechanism is that the microstructure consists of crystallites only partly separated by HAGBs [14]. As can be seen in the circle, there are grains with similar orientations (IPF map Fig. 3 (a)), but they are only partly separated by HAGBs and LAGBs. Moreover, the LM map in Fig. 3 (b) indicates that a majority of the new fine grains are not strain-free, which again support the idea of continuous dynamic recrystallization (CDRX) [15, 16]. In addition, the cumulative misorientation measured within the remaining grains is again connected with CDRX mechanism [9]. In the ternary alloy, crystallites were not fully separated by HAGBs and grains contained residual strains, as in Mg4Zn at the end of deformation. In addition, the presence of Ca did not change the final texture (Fig. 4) as could be expected from the presence of large precipitates and possible PSN mechanism. Consequently, the addition of 0.4 wt.% Ca to Mg4Zn did not change the main DRX mechanism as in [17]. Fig. 4 Pole figures of Mg4Zn ( a; max = 8.16) and Mg4Zn0.4Ca (b; max = 5.5) alloys after uniaxial compression at 240 C with the strain rate of s -1 and a final strain close to 1 (compression direction: CD). formation of fine precipitates also supports the process of continuous DRX. Finally, fine precipitates formed during deformation could effectively contribute to transformation of LAGBs into HAGBs due to the pinning effect. As a consequence, the dislocations were continuously absorbed and the subgrains transformed into the grains without their growth by migration of HAGBs [14]. Thus the The obtained data suggest that the first grain fragmentation is connected with the formation of twins at beginning of deformation. Extensive initial twinning can be expected as a result of coarse initial grain size [18] and relatively low temperature of the deformation process [6]. Moreover the grains and twins boundaries are serrated due to the differences in dislocation densities [16]. The newly-formed kinks become attractive for further absorption of dislocations produced by increased imposed strain [19]. The rearrangement of dislocations by climb and their enhanced density will result in increasing misorientation of newly formed LAGBs and, consequently, in the formation of new recrystallized grains separated by HAGBs. Further imposed strain lead to progress of the recovery and recrystallization processes. At the end of the deformation, the twin boundaries are fragmented and the initial coarse grains are partly replaced by new DRX grains. The volume fraction of DRX was slightly higher in the ternary alloy (50 60%). 4. CONCLUSION The evolution of microstructures during uniaxial compression of Mg 4Zn and Mg 4Zn 0.4Ca (in wt.%) magnesium alloys showed that coarse primary grains are gradually replaced by new fine recrystallized grains as a result of dynamic recrystallization. The process of DRX was accompanied by extensive twinning, serration of grain and twin boundaries and fine precipitation. The microstructural analyses revealed that primarily formed twins are continuously fragmented and new fine recrystallized grains are formed. The presence of large precipitates in the ternary alloy contributes to increasing number of sites for formation of new DRX grains. Because new grains are rarely found in the vicinity of large precipitates, the PSN mechanism can contribute to increasing the DRX volume fraction in the ternary alloy, although it plays a minor role in the evolution of microstructures. In addition, the volume fraction of DRX is increased in comparison with the binary alloy which is free of large precipitates. The microstructure evolution observed during compression of magnesium alloys at 240 C was predominantly influenced by continuous DRX. The occurrence of this mechanism is further supported by the formation of fine precipitates during deformation of both alloys. However, neither the large precipitates nor the addition of Ca contribute to significant changes of in the main DRX mechanism and final texture. As the more advantageous material properties can be attributed to the refined grain size as well as to the presence of fine precipitates formed during hot deformation, these magnesium alloys represent interesting and prospective materials for further applications.

5 ACKNOLEDGEMENT This work was done as part of the internship of M.H. at EMSE, Saint Etienne, France. The authors would like to thank the Czech Science Foundation (grant P108/12/G043) and the Ministry of Education, Youth and Sports of the CR (grant LM and MSMT No. 20/2013 of Specific University Research). REFERENCES [1] HONO, K., MENDIS, C.L., et al. Towards the development of heat-treatable high-strength wrought Mg alloys. Scripta Materialia, 2010,vol. 63, nr. 7, pp [2] GAO, X., ZHU, S.M., et al. Precipitation-hardened Mg Ca Zn alloys with superior creep resistance. Scripta Materialia, 2005,vol. 53, nr. 12, pp [3] YANG, M., CHENG, L., et al. Comparison about effects of Ce, Sn and Gd additions on as-cast microstructure and mechanical properties of Mg 3.8Zn 2.2Ca (wt%) magnesium alloy. J Mater Sci, 2009,vol. 44, nr. pp [4] STANFORD, N. The effect of calcium on the texture, microstructure and mechanical properties of extruded Mg Mn Ca alloys. Materials Science and Engineering: A, 2010,vol. 528, nr. 1, pp [5] LARIONOVA, T.V., PARK, W.-W., et al. A ternary phase observed in rapidly solidified Mg Ca Zn alloys. Scripta Materialia, 2001,vol. 45, nr. 1, pp [6] OH-ISHI, K., MENDIS, C.L., et al. Bimodally grained microstructure development during hot extrusion of Mg 2.4 Zn 0.1 Ag 0.1 Ca 0.16 Zr (at.%) alloys. Acta Materialia, 2009,vol. 57, nr. 18, pp [7] XU, S.W., KAMADO, S., et al. Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation. Materials Science and Engineering: A, 2009,vol. 527, nr. 1 2, pp [8] GALIYEV, A., KAIBYSHEV, R., et al. Correlation of plastic deformation and dynamic recrystallization in magnesium alloy ZK60. Acta Materialia, 2001,vol. 49, nr. 7, pp [9] XU, S.W., OH-ISHI, K., et al. Twins, recrystallization and texture evolution of a Mg 5.99Zn 1.76Ca 0.35Mn (wt.%) alloy during indirect extrusion process. Scripta Materialia, 2011,vol. 65, nr. 10, pp [10] HUMPHREYS, F.J., HATHERLY, M., Recrystallization and Related Annealing Phenomena, Elsevier, Amsterdam, the Netherlands, [11] ZHANG, B., WANG, Y., et al. Effects of calcium on texture and mechanical properties of hot-extruded Mg Zn Ca alloys. Materials Science and Engineering: A, 2012,vol. 539, nr. 0, pp [12] ZHANG, J., FANG, C., et al. A comparative analysis of constitutive behaviors of Mg Mn alloys with different heat-treatment parameters. Materials & Design, 2011,vol. 32, nr. 4, pp [13] STANFORD, N., BARNETT, M.R. Effect of particles on the formation of deformation twins in a magnesium-based alloy. Materials Science and Engineering: A, 2009,vol. 516, nr. 1 2, pp [14] GOURDET, S., MONTHEILLET, F. An experimental study of the recrystallization mechanism during hot deformation of aluminium. Materials Science and Engineering: A, 2000,vol. 283, nr. 1 2, pp [15] YI, S., BROKMEIER, H.-G., et al. Microstructural evolution during the annealing of an extruded AZ31 magnesium alloy. Journal of Alloys and Compounds, 2010,vol. 506, nr. 1, pp [16] TAN, J.C., TAN, M.J. Dynamic continuous recrystallization characteristics in two stage deformation of Mg 3Al 1Zn alloy sheet. Materials Science and Engineering: A, 2003,vol. 339, nr. 1 2, pp [17] XU, S.W., KAMADO, S., et al. Recrystallization mechanism and the relationship between grain size and Zener Hollomon parameter of Mg Al Zn Ca alloys during hot compression. Scripta Materialia, 2010,vol. 63, nr. 3, pp [18] DOBROŇ, P., CHMELÍK, F., et al. Grain size effects on deformation twinning in an extruded magnesium alloy tested in compression. Scripta Materialia, 2011,vol. 65, nr. 5, pp [19] MIURA, H., ITO, M., et al. Mechanisms of grain refinement in Mg 6Al 1Zn alloy during hot deformation. Materials Science and Engineering: A, 2012,vol. 538, nr. 0, pp