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The role of intermetallic phases in the corrosion of magnesium-rare earth alloys
Citation Link: https://doi.org/10.15480/882.1318
Publikationstyp
Doctoral Thesis
Publikationsdatum
2016
Sprache
English
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2016-07-25
The role of intermetallic phases in the corrosion of Mg-RE alloys
A new concept to develop a RE based Mg alloy with improved corrosion resistance was
followed in the current work. Based on subsequent characterisation steps to eliminate less
suitable RE elements the best microstructure for improved corrosion resistance was identified. At first, the corrosion properties of selected RE elements were determined. Based on these results RE elements that have a potential to enhance the corrosion resistance of Mg-RE alloys were selected. Two aspects of RE elements were important for the selection: the electrochemical properties and the solid solubility in Mg. If the solubility limit of RE elements in the Mg matrix is exceeded, they form intermetallic phases with Mg. By performing galvanic coupling measurements the compatibility between Mg matrix and intermetallic phases were estimated.
At that point three binary Mg-RE alloys systems remained (Mg-Ce, Mg-La, and Mg-Gd). To
evaluate the influence of composition (amount of intermetallic phases) on the corrosion
behaviour, four concentrations were cast with 1, 5, 10 and 15 wt. % of RE. Ce and La have a lower solid solubility in Mg matrix generating higher volume fraction of the secondary phases, thus higher dissolution rates in the binary Mg-RE alloys. While Gd with higher solid solubility shows a different behaviour. Additions of up to 10 wt. % Gd resulted in similar behaviour compared to 1 wt. % Gd addition.
The most promising results were obtained for the Mg-Gd system with 10 wt. % Gd. Thus, the microstructure of this alloy was further modified by heat treatments to understand the influence of microstructural changes on corrosion behaviour. A ternary element was used to attempt further optimisation of the corrosion performance. Additions of Al, Zn, Ga and Y did not show any improvement in the corrosion resistance of Mg10Gd. This is due to increasing volume fractions of critical more noble phases and the microstructure dominated by eutectic phase formation. Thus galvanic effects became much stronger due to the increased amount of cathodic phases in the eutectic regions. Mn was the only suitable ternary alloying element as it did not lead to the formation of Mn-rich intermetallics. It was found in solid solution in the intermetallics and to a lesser extent in the matrix without modifying the microstructure but increasing the corrosion resistance. The results of this work allow the design of new corrosion resistant Mg-Gd-Mn alloys by electrochemical evaluation and understanding of the basic corrosion mechanisms and interactions of the different phases. Better performance was predicted for reduced Gd contents and was finally experimentally verified.
A new concept to develop a RE based Mg alloy with improved corrosion resistance was
followed in the current work. Based on subsequent characterisation steps to eliminate less
suitable RE elements the best microstructure for improved corrosion resistance was identified. At first, the corrosion properties of selected RE elements were determined. Based on these results RE elements that have a potential to enhance the corrosion resistance of Mg-RE alloys were selected. Two aspects of RE elements were important for the selection: the electrochemical properties and the solid solubility in Mg. If the solubility limit of RE elements in the Mg matrix is exceeded, they form intermetallic phases with Mg. By performing galvanic coupling measurements the compatibility between Mg matrix and intermetallic phases were estimated.
At that point three binary Mg-RE alloys systems remained (Mg-Ce, Mg-La, and Mg-Gd). To
evaluate the influence of composition (amount of intermetallic phases) on the corrosion
behaviour, four concentrations were cast with 1, 5, 10 and 15 wt. % of RE. Ce and La have a lower solid solubility in Mg matrix generating higher volume fraction of the secondary phases, thus higher dissolution rates in the binary Mg-RE alloys. While Gd with higher solid solubility shows a different behaviour. Additions of up to 10 wt. % Gd resulted in similar behaviour compared to 1 wt. % Gd addition.
The most promising results were obtained for the Mg-Gd system with 10 wt. % Gd. Thus, the microstructure of this alloy was further modified by heat treatments to understand the influence of microstructural changes on corrosion behaviour. A ternary element was used to attempt further optimisation of the corrosion performance. Additions of Al, Zn, Ga and Y did not show any improvement in the corrosion resistance of Mg10Gd. This is due to increasing volume fractions of critical more noble phases and the microstructure dominated by eutectic phase formation. Thus galvanic effects became much stronger due to the increased amount of cathodic phases in the eutectic regions. Mn was the only suitable ternary alloying element as it did not lead to the formation of Mn-rich intermetallics. It was found in solid solution in the intermetallics and to a lesser extent in the matrix without modifying the microstructure but increasing the corrosion resistance. The results of this work allow the design of new corrosion resistant Mg-Gd-Mn alloys by electrochemical evaluation and understanding of the basic corrosion mechanisms and interactions of the different phases. Better performance was predicted for reduced Gd contents and was finally experimentally verified.
Schlagworte
intermetallic phases
corrosion
rare earth alloy
magnesium
Korrosionseigenschaft
intermetallische Phase
Magnesiumlegierung
Korrosionsbeständigkeit
DDC Class
530: Physik
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