EFFECT OF MICROSTRUCTURAL PARAMETERS ON TWINNING ACTIVITY OF MAGNESIUM ALLOYS

Transcription

1 EFFECT OF MICROSTRUCTURAL PARAMETERS ON TWINNING ACTIVITY OF MAGNESIUM ALLOYS VLIV MIKROSTRUKTURNÍCH PARAMETRŮ NA AKTIVITU DVOJČATĚNÍ V SLITINÁCH HOŘČÍKU Kristian MÁTHIS, Jan ČAPEK, Zuzana ZDRAŽILOVÁ FACULTY OF MATHEMATICS AND PHYSICS, CHARLES UNIVERISTY, Ke Karlovu 3, Praha 2, Czech Republic, mathis@met.mff.cuni.cz Abstract Magnesium-based alloys are in the focus of researchers and engineers interest for many years for their high specific strength and specific modulus. Nevertheless, limited ductility and formability of magnesium alloys especially at moderate temperatures (up to 200 C) hinder their wider applications. It is due to hexagonal close packed structure of most magnesium alloys with the limited number of the crystallographic equivalent glide systems. The work is aimed to explanation of the influence of microstructural parameters on the twinning activity that is one of the main deformation modes in magnesium alloys at moderate temperatures. The results explain the role of twins in the deformation process of continuous casted AZ31 magnesium alloy. Keywords: Magnesium alloys; Twinning; Dislocations; Acoustic Emission 1. INTRODUCTION Magnesium-based alloys, as one of the lightest structural materials, are in the focus of researchers and engineers interest for many years. Their unique properties as high specific strength (the ratio of the yield stress to density) and superior damping capacity predetermine these materials for using in many structural applications, e.g. in automotive and aerospace industry. The main restriction in a wider use of magnesium alloys is their limited ductility and poor formability at ambient temperature, poor creep and corrosion resistance and degradation of their mechanical properties at higher (above 200 C) temperatures [1]. Matter of poor ductility of magnesium is in its hexagonal close packed structure. Despite of similar true stress-true strain curves of hexagonal (hcp) and face-centered cubic (fcc) polycrystals there are differences in the activity of slip systems. Contrary of the fcc metals, in hcp polycrystals the von Mises criterion [2] requiring the activity of five independent slip systems for plastic deformation is not fulfilled [3]. Basal slip of a dislocations is the easy glide system in Mg [4]. a dislocations can operate also in prismatic and first order pyramidal systems. All of slip systems mentioned provides only a total four independent slip systems [5]. None of them can produce strain parallel to the c axis. It is clear that either second order pyramidal slip system must be activated or deformation twinning has to occur. Aside from the slip in the second order pyramidal system { 1122} 1/ with c + a Burgers vectors, which is recognizably a very hard slip direction and it has been observed mainly at higher temperatures (above 200 C) [6], twinning is the only active deformation mode that can provide straining along the c-axis at room temperature [7]. Generally, those hcp metals that exhibit substantial twinning (e.g. Ti, Zr) possess higher ductility [8]. The acoustic emission (AE) that originates from transient elastic waves generated within a material owing to sudden localized irreversible structure changes yields information on the dynamic processes involved in plastic deformation of alloys [9]. It was found [10] that deformation twinning and dislocation glide are the

2 major sources of AE in magnesium alloys. Therefore, AE is an appropriate method to study dynamic processes involved in plastic deformation of pure magnesium. 2. EXPERIMENTAL Magnesium alloy AZ31 investigated in this work was prepared by continuous casting. Tensile testing was carried out using specimen with a rectangular cross section of 5 x 5 mm 2 and a gauge length of 30 mm. The rectangular shaped compression specimens had a cross section of 15 x 15 mm 2 and height of 30 mm. Both types of tests were performed at room temperature and at an initial strain rate of 10-3 s -1. Since the specimens exhibit a strong texture, the tensile tests were carried out parallel (longitudinal direction - LD) and perpendicular (transversal direction - TD) to the casting direction. Acoustic emission (AE) was monitored directly using a computer-controlled DAKEL-XEDO-3 facility (DAKEL-ZD Rpety, Czech Republic). The facility incorporated a high temperature S9215 transducer (Physical Acoustic Corporation) with a flat response between 50 and 650 khz and a built-in preamplifier giving a gain of ~30 db. The total gain was about 94 db. The AE analyzer detects the AE signals at two settable threshold levels, which corresponded to voltages of 730 mv for the total AE count 1 and 1450 mv for the burst AE count 2 (total range of the A/D converter is from 0 to 2400 mv). The specimens for light microscopy were mechanically polished and finally etched (90 s) in 3% nital solution. 3. RESULTS The initial microstructure in both directions (Fig. 1) specimens exhibits a dendritic structure. The grain size was estimated as 620 µm [11]. Fig. 1 Initial microstructure of AZ31 specimen in a) longitudinal b) transversal direction Comparison of tensile and compression true stress true strain curves is shown in Fig. 2. The loading mode influences the shape of the deformation curves. In the compression an S-shaped curves are observed, i.e. the curve exhibit a moderated and a significant hardening part. On the other hand the tension curves are convex. Similar behavior was observed by several authors [12, 13] and it is connected with mechanical twinning [14].

3 Fig. 2 Dependence of deformation curves on loading mode in transversal direction Fig. 3 Loading mode dependence of count rate 1 in tension. The main twinning system in magnesium is { 10 12} which causes a misorientation of 86.3 between he twinned and untwinned regions. A pole mechanism for twin formation was proposed by Thompson and Millard [15], who suggested the following dislocation reaction to form a pole source of twinning: [0001] α[10 11] + [10 10] (1) where α is a small fraction, ranging from 1/12 up to 1/4. It should be noted that a dislocation with Burgers vector [0001] is sessile and therefore these dislocations are responsible for strain hardening in hexagonal close-packed materials [3]. Once pyramidal slip systems are activated, the dislocation of this type is formed by the reaction 1/ 3[1120] + 1/ 3[1 123] [0001] (2) where the first dislocation moves in a basal plane and the second one in the pyramidal plane. Twinning dislocations in Eq.(1) could be formed also by mutual interaction of basal dislocations [16]. As it has been reported in numerous works [17, 18], the AE response is very sensitive on twinning formation, since it is always accompanied by strong bursts. In our AE experiments two threshold levels were set. The first level was used for the detection of the global signal (count rate 1). By setting the second level, we recorded only strong signals caused largely by twins (count rate 2). Fig. 3 shows the loading mode dependence of count rate 1 in tension. The signal is the stronger for the tensile mode. The maximum of both curves is connected with { } twinning. Their activation in the compression experiment is easy [19] that causes moderated strengthening part. Subsequently, the new twins start to growth and the interaction of twin boundaries with dislocations [20] causes significant strengthening. Since the AE method is sensitive only on the twin nucleation, not on the twin growth [21], the AE signal decreases to nearly zero. During the tensile test, despite of low volume fraction of favorable oriented grains, tensile twinning takes place as well. Nevertheless, it needs higher stresses and their growth is limited [19]. Therefore further twin formation is necessary for continuing the plastic deformation. As a consequence of this mechanism, the observed signal

4 strength is higher. Fig. 4 shows the true stress-true strain curves for both orientations at the room Fig. 4 Orientation dependence of deformation curves in compession Fig. 5 Orientation dependence of count rate 2 in compression temperature in compression. It is seen that the TD sample exhibit a higher strength. This difference could be explained in terms of different initial texture of the LD and TD orientation, respectively. In the initial state, the basal plane (0001) is nearly parallel to the plane of the sheet [11]. Consequently, slip of basal <a> dislocations is the dominant mechanism for samples with the LD orientation. Since this mechanism itself Fig. 6 Microstructure after deformation in a) longitudinal b) transversal direction does not fulfill the von Mises criterion for plastic deformation [2], { 10 12} twinning takes place as well [20]. As it has been shown by Yoo [8], the twin boundaries are impenetrable for basal dislocations. Therefore twinning on planes { 10 12} lowers the spacing between the non-dislocation type obstacles, which causes hardening. In TD direction, where basal slip is not favorable, the twinning activity is more pronounced. Fig. 5 demonstrated the strain dependence of count rates 2 for both orientations of the samples in compression. The value of the count rate 2 is significantly higher for TD direction, which indicates more intensive twinning. Consequently, TD samples have a higher amount of twin boundaries that could cause the observed higher hardening rate. Dislocation pile-ups are formed at the twin boundaries. The stress concentrations at the head

5 of pile-ups contribute to initiations of the activity of the non-basal slip systems [22]. Interactions between nonbasal and basal dislocations results in sessile dislocations that increase the forest dislocation density [3]. The increased number of dislocation-type obstacles contributes to the hardening. The higher activity of twinning in TD direction is supported also by results of light optical microscopy. As it can be seen in Fig. 6, where the micrographs of as-deformed LD and TD samples are depicted, the amount of twins are much higher for TD direction. CONCLUSIONS The deformation behavior of continuous casted AZ31 magnesium alloy was investigated as a function of sample orientation and loading mode. A strong tension-compression asymmetry was observed on both deformation and AE curves. The reason could be explained in terms of different twinning mechanisms in tension and compression, respectively. The measurement of acoustic emission signal revealed a higher twinning activity in TD samples due to their unfavorable orientation for basal slip. ACKNOWLEDGEMENT This work is a part of the research program MSM that is financed by the Ministry of Education of the Czech Republic. The authors are grateful for the financial support of the Czech Grant Agency under the contract 106/09/0712. LITERATURA [1] AVEDESIAN, M. M. and BAKER, H. Magnesium and Magnesium Alloys (ASM Specialty Handbook). ASM International, [2] VON MISES, R. Mechanics of the ductile form changes of crystals. Zeitschrift Fur Angewandte Mathematik Und Mechanik, 1928, 8, pp [3] LUKAC, P. Hardening and softening during plastic-deformation of hexagonal metals. Czechoslovak Journal of Physics, 1985, 3, 35, pp [4] ROBERTS, C. S. Magnesium and Its Alloys. New York: John Wiley & Sons, Inc., [5] KNEZEVIC, M., LEVINSON, A., HARRIS, R., et al. Deformation twinning in AZ31: Influence on strain hardening and texture evolution. Acta Materialia, 2010, 19, 58, pp [6] MATHIS, K., NYILAS, K., AXT, A., et al. The evolution of non-basal dislocations as a function of deformation temperature in pure magnesium determined by X-ray diffraction. Acta Materialia, 2004, 10, 52, pp [7] AGNEW, S. R. and DUYGULU, O. Plastic anisotropy and the role of non-basal slip in magnesium alloy AZ31B. International Journal of Plasticity, 2005, 6, 21, pp [8] YOO, M. H. Slip, twinning and fracture in hexagonal closed packed metals. Metallurgical Transactions a-physical Metallurgy and Materials Science, 1981, 3, 12, pp [9] HEIPLE, C. R., CARPENTER, S. H. and CARR, M. J. Acoustic emission from dislocation-motion in precipitationstrengthened alloys. Metal Science, 1981, 11-1, 15, pp [10] FRIESEL, M. and CARPENTER, S. H. Determination of the Source of Acoustic Emission Generated during the Deformation of Magnesium. Journal of Acoustic Emission, 1984, 6, pp [11] VIDRICH, G. Grain refinement and dispersion-strengthening with finest ceramic particles. Clausthal-Zellerfeld: Technisches Universität Clausthal, [12] BARNETT, M. R. Twinning and the ductility of magnesium alloys Part I: "Tension" twins. Materials Science and Engineering a-structural Materials Properties Microstructure and Processing, 2007, 1-2, 464, pp. 1-7.

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