| Mg2Si, an intermetallic compound, has high melting temperature, low density, highhardness, low thermal expansion coefficient and reasonably high elastic modulus. Owing tothe unique combination of physical and mechanical properties, Mg2Si can be used as areinforced phase to prepare high-temperature-resistant light alloys, which have potential asstructural materials for industrial application. However, primary Mg2Si is prone toundesirable coarse dendrite phase in as-cast Al–Mg–Si or Mg–Si alloys which willdeteriorate the mechanical properties of the alloys and thus severely limit their industrialapplications. Therefore, it is essential to modify the morphology and size of Mg2Si.Modification treatment is a common method to regulate the morphology and size of Mg2Si.Notably, the adsorption-poisoning mechanism has become a generally accepted modificationtheory. However, in the past, researchers mainly concentrated on single substance phase (e.g.Si), and only limited results have been reported about compounds such as Mg2Si. Moreover,the selection criterion of modification element is mainly focused on the radius ratio of themodifying element atom and simple substance phase (RE/RSi). Hence, it has great practicalsignificance to develop a reliable thermodynamic description of different modificationeffects of modifying elements and then predict the modification effect for new modifyingelements. There are apparent composition segregation and different undercooling degree inthe solidification process of Al–Mg–Si melt, and the factors influencing the growth behaviorof primary Mg2Si are very complicated. So far, some work on the growth process of primaryMg2Si in melt has been performed and has made some encouraging progress. However,subject to the choice of modifying element and effective method to observe the morphologyof primary Mg2Si in3-D space, these researches are mainly focused on the growth process ofoctahedral and dendritic primary Mg2Si. So far, limited results have been reported aboutcubic and truncated octahedral primary Mg2Si.Therefore, in this thesis, the growth evolution of the primary Mg2Si inAl–20wt.%Mg2Si alloys modified by modifying element were studied, and the growthprocess of primary Mg2Si was revealed. A regularity of morphology evolution of primaryMg2Si was revealed by comparing the weighted ratio of adsorption energy (δ) of {100} and{111}. The influence rule of the morphology of primary Mg2Si and the elastic modulus (E) and tensile properties of Al–Mg–Si alloys was presented.The main results are as follows:1) A series of modified primary Mg2Si with various morphologies, including dendritic,octahedral, nearly spherical truncated octahedral, truncated octahedral, truncated cubicand cubic shapes, were successfully fabricated in Al–Mg–Si melt via the additions of Li,Ca, P, Sb or Sr elements, and the morphology regulation of primary Mg2Si was achieved;the common frame-type growth processes of primary Mg2Si were found inthree–dimensional space; it was found for first time that growth process of cubicprimary Mg2Si will successively go through six growth steps, asymmetrical dendriteâ†'similar cubic profileâ†'cubic frameâ†'defective cubeâ†'incomplete cubeâ†'perfect cube,respectively; The dodecahedral primary Mg2Si were successfully synthesized, and thegrowth process of dodecahedron is as follow: asymmetrical dendriteâ†'dodecahedralframeâ†'defective dodecahedronâ†'perfect dodecahedron, respectively.2) The influence rule of heterogeneous nucleation and the growth process of truncatedoctahedron were revealed; the growth of truncated octahedral with heterogeneousnucleation (Mg3Sb2) will proceed successively through four grow steps: similartruncated octahedral profileâ†'truncated octahedral frameâ†'defective truncatedoctahedronâ†'perfect truncated octahedron, respectively, while the truncated octahedronwithout heterogeneous nucleation may proceed successively through six grow steps:asymmetrical dendriteâ†'similar truncated octahedral profileâ†'truncated octahedralframeâ†'defective truncated octahedronâ†'truncated octahedronâ†'perfect truncatedoctahedron, respectively; increasing with the contents of Ca element, the morphologyevolution of primary Mg2Si is as follow: dendrite (0)â†'dodecahedron(0.08wt.%)â†'truncated cube (0.1wt.%)â†'cube (0.2wt.%).3) The modifying elements are divided into two types: one is adsorption element, includingLi, Be, Ca, Sr and Pb; the other is displacement-adsorption element, incorporating Sn, P,Sb, Bi and Te elements; adsorption element and displacement-adsorption element wereboth preferentially absorbed on {100} of Mg2Si, which could inhibit the growth along<100> crystal orientations and change the ratio of the growth rate of <100> and <111>direction (R); The morphology evolution mechanism can be called adsorption andpoisoning; the over-modification of Sc was ascribed to the formation of Al3Sccompound when the content of Sc exceeded0.4wt.%.4) A regularity of morphology evolution of primary Mg2Si is revealed by comparing theweighted ratio of adsorption energy (δ) of {100} and {111} surfaces, which is presentedas the following aspects：the higher the δ, the more positive the modification effect, and the more areas of {100} surfaces will be reserved in the final morphology; foradsorption elements, if2.3≤δ≤3.9, the primary Mg2Si will growth as a perfectoctahedron, and if9.1≤δ≤16.6, it will appear as a cube; for displacement-adsorptionelements, if δ≤1.6, the primary Mg2Si appears as a perfect octahedron, while if1.8≤δ≤3.1, the primary Mg2Si exhibits a perfect truncated octahedral morphology.5) When the average size of primary Mg2Si was close, the relationship between themorphology of primary Mg2Si and the tensile strength of hot-extrudedAl–20wt.%Mg2Si–4wt.%Cu alloy is established, that is, truncatedoctahedron>cube>octahedron; the tensile strength (σ b) of alloys with truncatedoctahedral primary Mg2Si was increased from220MPa (unmodified alloys) to281MPa,and the elongation-to-failure (ε f) was raise from2.6%to4.1%at room temperature,while theσ bof alloys with truncated octahedral primary Mg2Si was increased from185MPa (unmodified alloys) to264MPa, andε fwas raise from2.6%to3.5%at150oC.In summary, in this thesis, the growth behaviors of primary Mg2Si in Al–20wt.%Mg2Sialloys modified by modifying element were studied; the morphology evolution of primaryMg2Si was revealed; a common regularity of morphology evolution of primary Mg2Si hasbeen proposed; The result achieved here will help to guide the development of newhigh-temperature-resistant light alloys. |
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