Carbon inoculation of AZ91 magnesium alloy and effects of grain morphology on the microstructure of Mg17Al12
File(s)
Author(s)
Lin, Cheng-Jung
Type
Thesis
Abstract
This thesis investigates the factors affecting the development of microstructure in Mg-9Al-0.7Zn-0.2Mn (AZ91, wt%) during casting and solution heat treatment, with a focus on the effects of carbon inoculation. Four questions are addressed: (i) how carbides form after carbon inoculation of the melt, (ii) how different types of carbon additives affect carbide formation and grain refinement of α-Mg during casting, (iii) how the α-Mg morphology affects the partially divorced growth of the α-Mg+Mg17Al12 eutectic, and (iv) how the different microstructural features determine the dissolution of Mg17Al12 during solution heat treatment.
Water-free sample preparation methods were developed so that carbides were not hydrolysed before characterisation. Three carbides formed when AZ91 inoculated with graphite, diamond or α-SiC was remelted to 700-900 ˚C and slow cooled: T1-Al2MgC2, T2-Al2MgC2 and Al4C3. All three carbides grew with the {0001} facet and Al4C3 grew with two types of growth twin. Graphite, diamond and α-SiC additions each gave strong grain refinement in gravity castings. Multiple examples were measured of the T2-Al2MgC2 carbide with a simple basal-to-basal orientation relationship (OR) with α-Mg. Based on this and the relatively low atomic disregistry between T2-Al2MgC2 and α-Mg, T2-Al2MgC2 is considered as the nucleant in the carbon inoculation method.
During partially divorced eutectic growth, Mg17Al12 grew in orientation domains crossing multiple α-Mg grains. The degree of partially divorced growth is shown to depends on a simple geometrical measure for the space available for eutectic growth. Fully coupled eutectic microstructures were generated by manipulating the α-Mg dendrite solidification time and the eutectic cooling rate. During heat treatment, the Mg17Al12 dissolution kinetics were dominated by the α-Mg secondary dendrite arm spacing (SDAS) Dissolution data from samples with a wide range of α-Mg grain size and SDAS could be approximated as a single curve when all data were normalised by a lengthscale-corrected time.
Water-free sample preparation methods were developed so that carbides were not hydrolysed before characterisation. Three carbides formed when AZ91 inoculated with graphite, diamond or α-SiC was remelted to 700-900 ˚C and slow cooled: T1-Al2MgC2, T2-Al2MgC2 and Al4C3. All three carbides grew with the {0001} facet and Al4C3 grew with two types of growth twin. Graphite, diamond and α-SiC additions each gave strong grain refinement in gravity castings. Multiple examples were measured of the T2-Al2MgC2 carbide with a simple basal-to-basal orientation relationship (OR) with α-Mg. Based on this and the relatively low atomic disregistry between T2-Al2MgC2 and α-Mg, T2-Al2MgC2 is considered as the nucleant in the carbon inoculation method.
During partially divorced eutectic growth, Mg17Al12 grew in orientation domains crossing multiple α-Mg grains. The degree of partially divorced growth is shown to depends on a simple geometrical measure for the space available for eutectic growth. Fully coupled eutectic microstructures were generated by manipulating the α-Mg dendrite solidification time and the eutectic cooling rate. During heat treatment, the Mg17Al12 dissolution kinetics were dominated by the α-Mg secondary dendrite arm spacing (SDAS) Dissolution data from samples with a wide range of α-Mg grain size and SDAS could be approximated as a single curve when all data were normalised by a lengthscale-corrected time.
Version
Open Access
Date Issued
2023-01-12
Date Awarded
01/08/2023
License URL
Advisor
Gourlay, Christopher
Li, Qianqian
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)