Faculty of Engineering


Exceptionally ductile magnesium alloys

Our research has discovered a new magnesium alloy system with very high ductility.    This project aims to understand the cause of this ductility at a microstructural level, and further optimize the mechanical properties of magnesium alloys for industrial applications.

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Magnesium alloys are light weight, have high strength-to-weight ratio, good electrical/heat conductivity and damping properties. Magnesium alloys are attractive for applications in aerospace, transportation and portable electronics where weight reduction is critical.

Use of magnesium alloy parts in automobiles has experienced rapid growth in recent years as the weight reduction means direct energy saving and less pollution. However, there are problems that limit the applications of magnesium alloys: the ductility of magnesium alloys is intrinsically poor, due to their crystal structure, which severely limits their use.

Our newly developed alloy system contains a small amount of tin and other alloy elements. With this new alloy system it is possible to use various forming processes including cold-rolling, extruding or stamping, and net-shaped parts can be directly produced from a cast plate.

Current major developments


The new alloy system we have developed has tensile elongation of 20-28% at room temperature in the casting state, similar to many cast aluminium alloys, and significantly greater than the 3-6% elongation possible with existing popular commercially available alloys.

At the microstructural level, typical magnesium alloys such as AZ91 have intermetallics distributed along grain boundaries, which results in brittle fracture of the alloy when stress is applied and microcracks form along the boundaries. We have shown that the microstructure of the new alloy tends to have wide and dispersed blurry zones instead of sharp grain boundaries, and shows tiny stars and balls of intermetallic phases dispersed in the alloy.

 

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Key focus areas/issues


This research project aims to answer the following questions:

  1. How can very small amounts of tin and other small amounts of alloy elements change the nature and shape of the intermetallics?
  2. Why have the grain boundaries changed into a ‘blurry’ zone?
  3. What are the mechanisms of plastic deformation in these alloys?
  4. Can this mechanism be used to improve other alloy systems?

Key achievements


Composition, microstructure and mechanical properties of Mg alloys.

The mechanical properties of Mg-Zn-Sn, Mg-Zn-Pb and Mg-Zn-Sn-Pb cast alloys were tested in order to understand the effect of Sn and Pb on the mechanical behaviours of Mg-Zn based alloy. The microstructures of those alloys were characterized. The heat treatments (solid solution and ageing) were also conducted, and the tensile properties of heat treated alloys were tested and the effect of microstructure is investigated. In addition, Sn+Pb added AZ61 (Mg-6Al-Zn) alloys were cast at various solidification conditions which render different grain sizes. The microstructure was characterized and the effect of grain size on the mechanical properties is under close investigation.

The effect of Sn and Pb alloying elements on thermodynamic reactions during solidification of Mg alloys.

The Mg-Al-Zn (AZ series) alloys are the most commonly used Mg alloys. The eutectic solidification is the key thermodynamic reaction determining the microstructure and mechanical properties of these alloys. Using DSC (differential scanning calorimetry), the eutectic reactions of Sn and Pb alloyed AZ61 (Mg-6Al-Zn) and AZ91 (Mg-9Al-Zn) cast alloys were investigated. It was found that addition of Sn and Pb suppressed the eutectic reaction, which retarded the formation of intermetallic compounds in the alloy. This may contribute positively to the ductility of the alloys.

Deformation modes of Mg-4Zn-Pb cast alloy.

Three points bending tests were conducted on a rectangular Mg-4Zn-Pb specimen with a polished surface. The deformation patterns were observed on the surface using optical microscopy, SEM. Both slipping and twining were observed in strained grains. It was also tried using the EBSD (electron backscatter diffraction) technique to indentify the deformation modes (planes and directions of slip and twin). This has not been successful due to the surface oxidation of the Mg specimen and limitations of the EBSD technique. More systematic study on deformation modes will be continued, employing the more advanced techniques of Neutron and Synchrotron diffractions, in order to characterize the deformation modes and textures of bended Mg specimens.

The forging ability of Mg cast alloys at low to medium temperatures.

According to the previous results, two compositions of Mg-4Zn-Pb and Mg-6Al-2Pb-Zn-Sn were selected for a forging ability test. The cast blocks were made as starting materials, and the Mg part was forged at temperatures ranging from 150 to 350ºC which is lower than the normal forging temperature of over 400 ºC. The forged Mg part was examined and tensile tested. The Mg alloys show good forging ability at medium temperature, and incompletion was observed at the tip of the Mg forged part forged at low temperature. The very fine grains formed due to dynamic recrystallization during forging enhanced the mechanical properties of the forged Mg part.

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Key people


Wei Gao
Chemical and Materials Engineering

Balan Zhu
Chemical and Materials Engineering

Michael Hodgson
Chemical and Materials Engineering

Mark Taylor
Light Metals Research Centre

Contact


Wei Gao
Email: w.gao@auckland.ac.nz
Phone: +64 9 373 7599 extn 88175

Related publications


Liu, H., Chen, Y., Zhao, H., Wei, S., Gao, W., 2010. Effects of strontium on microstructure and mechanical properties of as-cast Mg-5 wt.%Sn alloy, Journal of Alloys and Compounds, 504 (2), pp. 345-350.

Zhu, T., Gao W., et al, 2010.  Dissolution of eutectic β-Mg17Al12 in Mg AZ91 cast alloy at temperature close to eutectic temperature, Journal of Materials Engineering and Performance, vol.19, pp. 860-867.

Zhu, T., Gao W., et al, 2010. Effect of cooling conditions during casting on fraction of β-Mg17Al12 in Mg-9Al-1Zn cast alloy, Journal of Alloys and Compounds, vol. 501, pp. 291-296.

Ma, Y.  Feng, J-Y., Ma, Y-Z, Zan, H. and Gao, W., 2010.  Comparative Study on Characterization of Corrosion Resistance of Micro-Arc Oxidation Coatings on Mg Alloys, Journal of Chinese Society for Corrosion and Protection, Vol. 30 (6), pp. 442-448.

Liu, H., and Gao, W., 2010. The influence of Sn and Pb additions on the tensile properties of Mg alloys, Magnesium Technology 2010, pp. 461-465.

Zhu T., Chen Z.W., and Gao W., 2006. Incipient melting in partially melted zone during arc welding of AZ91D magnesium alloy, Materials Science and Engineering A, vol. 416(1-2), pp. 246-252.

Zhu T., Chen Z.W., and Gao W., 2010.  Grain Boundary Wetting and Solidification of Constitutional Liquid in AZ91 Cast Alloy, Inter. J. Modern Physics B, Vol. 24, Nos. 15 & 16, pp. 2249-2255.

Zhu T.P. and Gao, W., 2010.  Formation of Intermetallic Compound Coating on AZ91 Cast Alloy, Editors: Yu Lung Chiu et al, IOP Conference Series, Materials Science and Engineering, Processing, Microstructure and Properties of Materials, ISSN 1757-8981, 012024

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This project has been made possible with the support of the Marsden Fund.