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Title: Solidification behaviour of Fe-rich intermetallic compounds in aluminium alloys
Author: Zhou, Yipeng
ISNI:       0000 0004 7658 5105
Awarding Body: Brunel University London
Current Institution: Brunel University
Date of Award: 2018
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The industrial use of recycled aluminium is greatly limited by the degraded mechanical properties due to the increased impurities. Fe, one of the common impurity content in Al alloys, is difficult to eliminate once introduced into aluminium during primary production or recycling processes. Due to the low solid solubility of Fe in Al, the formation of Fe-rich intermetallic compounds (Fe-IMCs) is inevitable, which is one of the main causes for the deterioration of mechanical properties in various cast Al alloys. In order to obtain desirable mechanical properties of recycled Al alloys, modification and refinement of the Fe-IMCs are urgently required as the compact and refined morphologies of such intermetallics are generally non detrimental to Al alloy's performance. However, manipulating the solidification behaviour of the Fe-IMCs phases, including nucleation and growth, is very challenging because of the inherently more difficult heterogeneous nucleation of the Fe-IMCs compared with that of a pure metal or a solid solution; and the strong growth anisotropy. Limited understanding on mechanisms of nucleation and growth of the multicomponent Fe-IMCs is available in the literature. The aim of this study is to gain a deeper understanding on the heterogeneous nucleation and growth behaviour of Fe-IMCs in various Al alloys. The nucleation and growth of both primary and eutectic Fe-IMCs have been investigated during various solidification conditions including a number of different cooling rates and casting temperatures. Based on the experimental results of the solidification of several ternary and quaternary alloys, effect of Mg on the solidification behaviour of Fe-IMCs was investigated. Further the surface modified TiB2 particles were used to enhance the heterogeneous nucleation of Fe-IMCs in order to refine the Fe-IMCs particles. The dominant Fe-IMC in Al-5Mg-2Si-1.2Fe-0.7Mn alloy is identified, using transmission electron microscopy (TEM), as α-AlFeMnSi with a body centred cubic (BCC) lattice structure and lattice parameter of 1.256nm. In the current alloy system, the nucleation of primary α-AlFeMnSi occur at lower cooling rate (≤0.8K/s) when required nucleation undercooling is reached, as the slower cooling rate allows longer diffusion time for the solute to form a stable nucleation embryo. When casting with 20K superheat, the size of primary α-AlFeMnSi increases gradually from 24.5±3.1μm (870K/s) to 251.3±75.3μm (0.02K/s) and the size of α-AlFeMnSi eutectic increased gradually from 102.0μm (870K/s) to 623.3μm (0.02K/s). The Fe and Mn concentration in α-AlFeMnSi appears to reduce with the increased cooling rate due to the relatively insufficient solute supply when solute concentration is low (1.2wt.% Fe and 0.7wt.% Mn). Microstructure observation reveals that the {011} plane, especially on <111> orientation, is the preferred growth orientation of BCC primary α-AlFeMnSi, resulting in rhombic dodecahedral in 3D. The eutectic α-AlFeMnSi, prefers to initiate on the primary α-AlFeMnSi. In addition to the substantial nucleation undercooling, the research revealed that the nucleation of primary α-AlFeMnSi also rely on the local solute concentration and the solute diffusion. Compared with α-Al, the growth of α-AlFeMnSi is less sensitive to the cooling rate changes due to the complexities in multi-components interaction and different diffusion efficiency of different elements. The addition of Mg to Al-1.2Fe-0.7Mn and Al-2Si-1.2Fe-0.7Mn alloys was found to lead to a morphology change of Fe-IMCs. Al6(Fe,Mn), the predominant Fe-IMC in the Al-1.2Fe-0.7Mn-xMg alloy, changed from needle morphology to interconnected lamellar morphology when Mg composition increased from 0.004wt.% to 6.04wt%. A Mg-rich layer at about 5-20nm in thickness was commonly observed on the Fe-IMC/α-Al interface in the alloys with Mg content. The eutectic lamellar spacing for Al6(Fe,Mn) increases from 1.8±0.3μm to 4.5±0.8μm when Mg content increased from 0.004wt.% to 6.04wt.%. In the case of α-Al12(Fe,Mn)3Si, the predominant Fe-IMC in Al-2Si-1.2Fe-0.7Mn-yMg alloys, its lamellar spacing of the eutectic increased from 1.4±0.3μm to 3.25±0.8μm when Mg increased from 0.04wt.% to 5.41wt.%. Owing to the strong anisotropy of the Fe-IMC crystals, the segregation of solute Mg on preferred growth orientation is higher, causing greater growth restriction on this orientation. Consequently, the growth velocity on other orientations becomes relatively more significant. To optimise the morphology of Fe-IMCs in Al alloys, a novel Αl-Ti-B(Fe) grain refiner for Fe-IMCs has been developed to enhance the heterogeneous nucleation of Fe-IMCs. The addition of the novel grain refiner to an Al-5Mg-2Si-1.2Fe-0.7Mn alloy under controlled solidification condition results in a considerable refinement of the primary Fe-IMCs from 251.3±75.3μm to 110.9±45.5μm and from 127.3±36.2μm to 76.5±18.2μm at cooling rates of 0.02K/s and 0.15K/s, respectively. TEM investigations on the refiner reveal a Fe-rich adsorption monolayer in a zigzag fashion on the prismatic planes on the boride particles. This surface modification is beneficial for the heterogeneous nucleation of the Fe-IMCs. Further investigation of the Al alloy with this grain refiner addition revealed that there existed specific orientation relationships (ORs) between TiB2 and Fe-IMCs: (001)[020]Al13Fe4 // (11-20)[10-10]TiB2, and (001)[120]Al13Fe4 ∠6.05˚ (11-20)[10-11]TiB2; (0-11)[100]α-AlFeMnSi // (0001)[-2110]TiB2, and (0-11)[111]α-AlFeMnSi ∠4.5˚ (0001)[10-10]TiB2. The Fe adsorption on substrate particle, the observed ORs between TiB2 and Fe-IMCs, and the refinement of primary α-AlFeMnSi with the addition of modified TiB2 provide evidence of structure templating and composition templating required by heterogeneous nucleation of Fe-IMCs. This research has delivered contribution to the understanding and new approach for optimizing the morphology of Fe-IMCs in the Fe-containing Al alloys. Using the slow cooling rates (≤0.15K/s), the formation compact primary α-AlFeMnSi can be considerably encouraged. With a lower casting temperature, the size and volume fraction of large Chinese-script α-AlFeMnSi can be significantly reduced. With addition of reasonable Mg content the morphology of Fe-IMC can be modified. Particularly, with the addition of the Al-Ti-B(Fe) grain refiner in well-controlled condition, the primary α-AlFeMnSi can be significantly refined. Thus, by implementing these approaches, the optimized Fe-IMC morphology in the microstructure of Fe-containing Al alloy is able to offer promising mechanical performance.
Supervisor: Fan, Z. ; Stone, I. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Grain refinement ; Heterogeneous nucleation ; Casting ; Aluminium recycling ; Electron microscopes