Solidification behaviour and microstructure development of Mg-Al-RE (Ce and La) alloys

Wong, C 2019, Solidification behaviour and microstructure development of Mg-Al-RE (Ce and La) alloys, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Solidification behaviour and microstructure development of Mg-Al-RE (Ce and La) alloys
Author(s) Wong, C
Year 2019
Abstract The high specific strength of magnesium alloys makes them attractive for use in transport applications where potential weight savings can be significant. However, the most common magnesium alloys such as AZ91 (Mg-9Al-1Zn) and AM60 (Mg-6Al-0.3Mn) tend to suffer from poor creep resistance, limiting their use in high-temperature environments. The elevated temperature applications, such as powertrain components, offer significant potential for vehicle weight reductions. This limitation has stimulated the development of a number of high creep resistance magnesium alloys for powertrain components. Moreover, rare earth (RE) alloying additions in magnesium-aluminium based alloys are widely accepted to improve the creep resistance. The development of magnesium casting alloys depends significantly on the ability to control the as-cast microstructure. Microstructure in hyper-eutectic Mg-Al-RE (RE = Ce and La) alloys are analogous to those in A390 alloys used for pistons and cylinder liners. Both of these alloy systems form a large amount of hard secondary phase particles and fine eutectic similar to the composite structure. These attribute to improved properties of the alloys.

Previous studies have been performed to understand the effect of REs on the microstructure of Mg-Al-RE (AE) alloys, but different phases were reported. In addition, there is limited information about the effect of individual RE elements on AE alloys, especially on the hyper-eutectic region of these alloy systems. The lack of a comprehensive understanding of the microstructure evolution in AE alloys hinders the accuracy of thermodynamic predictions and hence limit the knowledge in magnesium alloy design and optimisation. Herein, the work described in this thesis aims to gain an improved understanding of the microstructural development of AE alloys.

The first objective in this research is to understand better the solidification sequence and thermodynamic predictions of the investigated alloys, which would be advantageous for the research and development of improved magnesium alloys. The second objective is to investigate the impacts of the individual RE (lanthanum and cerium) elements on the solidification path and microstructural development of AE alloys. Finally, the third objective of this thesis is to investigate the effects of cooling rates on the solidification behaviour and microstructure development of AE alloys. This includes the study of processing-microstructure relationships for the investigated alloys.

The investigations in this research study were carried out on a series of Mg-4Al-xCe and Mg-4Al-xLa alloys with "x" being the Ce and La concentration range from 4 wt.% to 13 wt.%. All alloys were gravity cast into a wedge shape mould comprising two sections: permanent mould and sand mould. The use of a wedge mould with the combination of permanent and sand moulds allows a wide range of cooling rates to be explored within a single casting. The solidification behaviour and microstructure formation of these alloys were studied experimentally using both ex-situ and in-situ techniques. The as-cast microstructures of these alloys were studied by X-ray powder diffraction, scanning and transmission electron microscopy. The microstructure evolution was investigated and observed under a real-time solidification condition using in-situ synchrotron radiography. The effect of the cooling parameters on the microstructure and solidification behaviour was studied through computer-aided cooling curve analysis. The experimental results were compared with thermodynamic predictions calculated using the CALPHAD method.

There has been relatively little research done on the thermodynamic properties and phase equilibria of the Mg-Al-RE systems compared to other systems. In this study, thermodynamic calculations were utilised to understand the solidification sequence of the intermetallic phases through comparison with experimental data, which includes a real-time in-situ observation of solidification using the synchrotron radiation. It was determined that the thermodynamic predictions were contradictory to the experimental data. The discrepancy observed in this comparison, especially in the liquid-solid and solid-solid phase transformations, suggests that the Mg-Al-Ce and Mg-Al-La systems should be remodelled to improve the accuracy of the thermodynamic database.

The effects of alloying compositions on the solidification and microstructure development are also discussed in this thesis. The resultant microstructures from the as-cast alloys were characterised. The low RE content alloys, such as ALa44 and ACe44, exhibited a primary α-Mg phase surrounded by an interdendritic region of Mg and intermetallic(s). On the other hand, the high RE content alloys, such as ALa413 and ACe413, exhibited a primary intermetallic phase before the α-Mg grains developed. The morphology of the eutectic was also very distinct between the investigated alloys. During phase characterisation, there were distinct unknown peaks observed in the XRD measurements indicating a new type of intermetallic phase. After a detailed investigation, a new phase, (Al,Mg)3La, was identified, which was found to be the dominant phase in the ALa44 alloys. The morphology and crystallographic information of this new (Al,Mg)3La phase is also described in this thesis.

Another goal of this research is to study the relationships between the processing condition (i.e. cooling rate) and the resultant microstructure of the as-cast AE alloys. The effects of different cooling rates on the microstructures were analysed. Microstructural features, such as grain size and secondary dendrite arm spacing (SDAS) of the α-Mg phase, were measured and their influence on hardness has also been investigated. It was observed that the microstructural features contribute significantly to the hardness of the alloys. A generalised relationship between freezing time and SDAS using the commonly used empirical equation has been developed. In addition, microstructure-property relationships between yield stress and microstructure features (grain size and SDAS) for these alloys were discussed and established based on the Hall-Petch equation. This linear relationship with SDAS is observed to be similar to that obtained for average grain size, in which smaller SDAS and grain size leads to higher yield stress and hardness. Using these relationships developed in this study, it is possible to predict the hardness and microstructure features at a given cooling rate for the investigated alloys.

The consolidation of these objectives and the results of this research study can improve the knowledge base of AE alloys, especially from the perspective of solidification behaviour and microstructure development. The results should prove useful for alloy design and development that are targeted for specific applications, as well as for alloy optimisation to improve cast magnesium alloys.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Manufacturing Engineering not elsewhere classified
Metals and Alloy Materials
Keyword(s) Casting
As-cast magnesium alloys
Solidification process
Cooling rate
Mg-Al-La system
Mg-Al-Ce system
Thermodynamic prediction
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Created: Wed, 11 Sep 2019, 13:38:41 EST by Adam Rivett
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