The Viability of Mg Alloy with Nano/Sub-micron Structure as a new Material for Practical Applications

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1 The Viability of Mg Alloy with Nano/Sub-micron Structure as a new Material for Practical Applications E. Aghion and A. Arnon Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel 1 Abstract The development of new magnesium alloys with consolidated nano/sub-micron structure may introduce a new generation of super-light alloys that possess significantly higher specific strength. Consolidated nano/sub-micron structure relates to alloys with the combined microstructure of the nano-crystalline phase and the sub-micron phase. The aim of the present study was to evaluate the viability of a consolidated nano/sub-micron magnesium alloy having the composition of AZ31, as a new structural material for practical applications. The viability evaluation was based on metallurgical examination using SEM, HRSEM, TEM, mechanical testing, and electrochemical corrosion measurements. The results show that the nano/sub-micron structured alloy has nearly more than twice the hardness and strength of the conventional reference alloy. However, in terms of ductility and corrosion resistance, the nano/sub-micron structured alloy showed significantly inferior properties which limits its potential use in practical applications. 2 Introduction The large spectrum of new magnesium alloys developed in the last few years highlights the increasing demand of the transportation and electronic industries for advanced structural material with improved properties [1 8]. The requirements become even more particular when new magnesium alloys are developed to address specific and critical applications [9,10]. One of the most important advantages of magnesium based alloys is their specific strength (strength to density ratio). This property can be improved if the microstructure is composed of relatively small grains, as in the case of nano-structured material. The aim of the present paper is to examine the viability of Mg alloys having a consolidated nano/sub-micron structure as a new structural material for practical applications. The magnesium alloy selected for this study was based on the composition of AZ31 ( % Al, % Zn, 0.2 %Mn, max. 0.3 % others, and Mg balance). The alloy was produced by MBN Nanomaterialia using mechanical alloying synthesis technology [11]. This was followed by direct extrusion of the nano-structured powder to obtain consolidated rods for metallurgical assessment. 3 Experimental procedure The chemical composition of the consolidated nano/sub-micron magnesium alloy was within the following range: % Al, % Zn, % Mn, <1 % others, with Magnesi- 7th International Conference on Magnesium Alloys and Their Applications. Edited by K. U. Kainer 2007 WILEY-VCH Verlag GmbH, Weinheim. ISBN:

2 4 um making the balance. This composition basically complies with the composition of AZ31 magnesium alloy which was used in this study as a reference material in the form of regular extruded rods. The microstructure examination was carried out using optical and electron microscopy (SEM with EDS, HRSEM and TEM). TEM analysis was obtained after developing a special method for sample preparation which incorporated an electro-polishing process with nitric acid solution. The mechanical properties were determined using standard round specimens according to E8-95 ASTM spec. Hardness measurements were carried out using the standard range of Rockwell B. Potentiodynamic polarization analysis in 3.5 % NaCl solution was carried out to evaluate the environmental behavior of the tested materials. The scanning rate of the potentiodynamic measurements was 5 mv/sec and the potential range was nearly 2000 mv. The consolidated nano/ sub-micron structured alloy was tested in as-received condition and after annealing at 345 C for 1 hr. The results obtained were introduced in terms of corrosion current which was converted into standard corrosion rate expressions (mpy). 4 Results and Discussion The microstructure of the nano/sub-micron structured alloy obtained by SEM and HRSEM is shown in Figures 1 and 2, respectively. The SEM analysis revealed the presence of pure magnesium stringers along the extrusion direction. In addition, the microstructure contained micro-porosity and Fe-Mn base inclusions. Figure 1: Microstructure of nano/sub-micron structured alloy obtained by SEM and EDS analysis HRSEM revealed a combined microstructure of nano-crystalline and sub-micron phases. The typical crystal size of the nano phase was up to 40 nm, while the crystal size of the sub-mi-

3 5 cron phase was up to 150 nm. It is believed that the sub-micron phase developed from the nanostructured powder during the extruding process at a relatively high temperature. Figure 2: Microstructure of nano/sub-micron structured alloy obtained by HRSEM TEM analysis showing the dislocation morphology of the nano/sub-micron structured alloy, along with its selected area diffraction pattern, is shown in Figure 3. This reveals the presence of high dislocation density which was the result of plastic deformation of the nano-structured powder during the direct extrusion process. The non-cyclic pattern diffraction is indicative of the fact that the micro-structure was not made of pure nano-scale phase. Figure 3: TEM analysis of nano/sub-micron structured alloy (a) Typical microstructure (b) Selected area diffraction pattern In terms of the mechanical properties, it was evident the hardness and tensile strength of the nano/sub-micron structured alloy was 104 Rockwell B and 440 MPa, compared to 44 Rockwell

4 6 B and 240 MPa for the regular extruded AZ31 alloy. This result was supported by the fact that the average elongation of the nano/sub-micron structured alloy was only 1 % compared to more than 20 % for the regular reference alloy. The results obtained by the hardness and elongation measurements are clearly indicative of the brittle nature of the nano/sub-micron structured alloy. The electrochemical behavior of the nano/sub-micron structured alloy and conventional AZ31 reference alloy in 3.5 % NaCl solution is shown in Figure 4 in as-received condition and after annealing. The results obtained show that the corrosion currents of the nano/sub-micron alloy in as-received condition and after annealing were A/cm 2 (443 mpy) and A/cm 2 (142 mpy), respectively. This clearly reveals that the annealing process has no significant effect on the magnitude of the corrosion rate. The corrosion current of the conventional AZ31 reference alloy was significantly lower, A/cm 2 (14 mpy). a) b) Figure 4: Electrochemical behavior of nano/sub-micron structured alloy and conventional AZ31 reference alloy obtained by potentiodynamic polarization in 3.5 % NaCl solution (a) In as-received condition (b) After annealing at 345 C for 1 hr Comparing the corrosion resistance of the nano/sub-micron alloy with the conventional alloy, it is evident that the corrosion rate of the nano/sub-micron alloy is one order of magnitude higher, indicative of extremely low corrosion resistance. The significantly reduced corrosion resistance of the nano/sub-micron structured alloy is mainly attributed to presence of residual pure magnesium stringers and cathodic phases such as Fe base inclusions. The interaction between the two phases in a corrosive environment creates a significant galvanic effect and, consequently, detrimental corrosion degradation. This degradation mechanism is amplified by the presence of micro-porosity. 5 Conclusions The results obtained by the present study clearly indicate that the newly developed nano/submicron structured magnesium alloy in its present form and quality can not be used as a structural material for practical applications. This is mainly attributed to the inherent defects in terms of pure magnesium residue, heavy element inclusions, and micro-porosity that have a significant detrimental effect on ductility and corrosion performance. The inherent defects are genera-

5 7 ted during the mechanical alloying synthesis process. Hence, it is believed that the status of the nano/sub-micron magnesium alloy as a new structural material can be improved only if the inherent defects are adequately addressed in terms of the mechanical alloying process. 6 References [1] K. Jereza, R. Brindle, S. Robison, J. N. Hryn, D. J. Weiss, B. M. Cox, The road to 2020: Overview of the Magnesium casting industry technology roadmap, Magnesium technology, TMS San Antonio, Texas, USA, March 2006, pp [2] B. Bronfin, E. Aghion, V. Von Buch, S. Schumann, H. Friedrich, M. Katzer, High temperature resistant magnesium alloys, Patent numbers: EP A1 & EP B1 (European patent office), DE C0 (Germany), AT E (Austria), CA AA (Canada), US (USA) [3] E. Aghion, B. Bronfin, F. von Buch, S. Schumman and H. Friedrich, Newly developed Magnesium alloys for power train applications, JOM, Nov. 2003, pp [4] M.O Pekguleryuz, A. A. Kaya, Magnesium die-casting alloys for high temperature applications, Magnesium technology, TMS Charlotte, N. Carolina, USA, March 2004, pp [5] M. Kettner, F. Pravdic, W. Fragner, K.U. Kainer, Vertical Direct Chill (VDC) Casting of a Novel Magnesium Wrought Alloy with Zr and Re Additions (ZK10): Alloying Issues, TMS San Antonio, Texas, USA, March 2006, pp [6] A. Luo, Research and development challenges for Magnesium applications in Automotive structures, Second International Conference on Magnesium, Beijing, China, June 2006, paper No, A 038. [7] S. Kamado, Y. Kojima, Development of Magnesium alloys with high performance, Second International Conference on Magnesium, Beijing, China, June 2006, paper No. A 308. [8] D. St. John, Overview of CAST and Australian Magnesium research, Second International Conference on Magnesium, Beijing, China, June 2006, paper No. A 302. [9] H. Westengen, P. Bakke, J. I. Skar, H. Gjestland, New Magnesium die casting alloys: Driving developement of critical automotive applications, 63 rd annual World Magnesium Conference IMA, Beijing, China, May [10] T. Abbot, M. Murray, G. Dunlop, AM-lite: A new Magnesium diecasting alloy for decorative applications, 63 rd annual World Magnesium Conference IMA, Beijing, China, May [11] P Matteazzi, G. Le Caer, A. Mocallin, Synthesis of Nanostructured materials by mechanical alloying, Ceramics International 1997, pp