Grain refinement of magnesium

Lee, Young (2002). Grain refinement of magnesium PhD Thesis, School of Engineering, The University of Queensland.

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Author Lee, Young
Thesis Title Grain refinement of magnesium
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2002
Thesis type PhD Thesis
Supervisor Dr Ame Dahle
Prof. David St John
Total pages 198
Collection year 2002
Language eng
Subjects 09 Engineering
Formatted abstract

The mechanisms involved in the grain refinement of magnesium alloys were investigated. Based on the findings, several attempts were made to develop a new, reliable grain refinement method, or maximise on existing method, for various magnesium alloys. The magnitude of grain growth during the solution heat treatment was investigated first so that more accurate grain size measurement can be made. The importance of solute in grain refinement is taken into account so that the effect of various solute additions on grain size can be verified. Finally, the additions of particles considered to be potent nucleants for magnesium crystals were made and their effect on grain size of different magnesium alloys was investigated. The combined effect of solute and particle additions was also investigated to maximise the effect of each constituent.

The grain growth of binary Mg-3 wt% Al (high purity) and Mg-9 wt% Al (commercial purity) alloys under common solution heat treatment conditions was investigated using Electron Back Scatter Diffraction (EBSD) as well as with conventional optical microscopy. The results indicate that more significant grain growth occurs in Mg-3 wt% Al alloy than in Mg-9 wt% Al alloy, presumably due to its low alloy and impurity contents. Grain growth was not observed to obscure the grain refining effect, neither has abnormal grain growth been observed under the experimental conditions described in the work.

GRF of various elements in magnesium has been identified and separate solute additions have been made to investigate their effect on grain size of pure magnesium. The results show that small additions of Zr, Si, Ca, and Sr effectively refine pure magnesium. Their relatively high GRF is responsible for the significant grain size reduction. Zr produces more significant grain refinement than other additives investigated in this study, and its high GRF and undissolved Zr particles are considered to contribute to the significant grain size reduction. Impurities, particularly Fe and Mn, tend to increase the grain size of Mg-Al alloys by poisoning existing nucleants, and the tendency of such poisoning is more significant with increasing Fe content. Addition of Sb into Al-containing Mg alloys produced a significant grain size reduction and the degree of grain refinement increased with increasing Sb content. The formation of Mg3Sb2 particles is believed to be responsible for the grain refinement through retarding the advancing solid/liquid interface.

Superheating of Al-containing Mg alloys to 900°C produced significant grain refinement. The cooling rate from the superheating temperature to casting temperature played a significant role. Carbon containing intermetallics are believed to be the favoured nucleant and Al, Fe, Mn and Si may interact with the carbon constituents. Upon fast cooling from the superheating temperature, those Al-Fe-Mn-(C, Si) particles tended to be small in size with irregular shape. On the other hand, larger polyhedral particles were commonly observed with slow cooling.

Various particles with relatively small lattice disregistry have been directly added into magnesium alloy melts, both in powder and master alloy form. Particles with smaller disregistry tend to produce more significant grain refinement, but some particles have no significant influence on grain size, despite their small lattice disregistry. The less than 1% lattice disregistry of ZrB2 suggests that it is potentially a good nucleant for magnesium. The results showed that ZrB2 produced significant grain refinement in alloys containing less than 3 wt% Al. However, less significant grain refining effect is observed with increasing Al content. Improvement in grain refinement upon ZrB2 particle addition was observed in Zn containing Mg alloys.

Aluminium boride particles produced a significant grain refining effect in Al containing Mg alloys under certain casting conditions. The elevated operating temperatures as well as Mn content for successful grain refinement indicate that the formation of intermetallics, presumably MnB2, is responsible for the grain refinement.

Keyword Magnesium alloys

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
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Created: Tue, 06 Dec 2011, 13:14:05 EST by Mrs Puvanarachaki Ravichandran on behalf of Scholarly Communication and Digitisation Service