Study On The Evolution,Precipitation And Growth Mechanism Of Mg₂Si In Al Alloys

The evolution, precipitation and growth mechanism of Mg2Si in Al alloys were studied by high scope video microscope (HSVM), electron probe micro-analyzer (EPMA), field emission scanning electron microscopy (FSEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC), conventional transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM), atom probe tomography (APT), etc. The solidification process and microstructure evolution of Al-Mg2Si-(Si) were researched. The nano-precipitates sequence in Al-4%Mg-0.5%Si-1%Cu and the effect of Ag on precipitation phenomena were analyzed. Meanwhile, the growth pattern of primary Mg2Si was established, and the evolution mechanism of different morphologies was revealed which leads to realizeing the control of growth of Mg2Si. Furthermore, the growth of eutectic Mg2Si was transformed by the addition of Ni.The main results can be described as follows:(1) Solidification process and microstructure evolution of Al-Mg2Si-(Si)The solidification process of hypoeutectic Al-Mg2Si alloys can be expressed as Lâ†'(α-Al)P+(Al+Mg2Si)E, and eutectic Mg2Si is plate-like or rod-like. In the hypereutectic Al-Mg2Si alloys, primary Mg2Si first precipates from the melt and exhibits various typical morphologies:perfect octahedron, truncated octahedron, hopper and dendrite. Its solidification path is indicated as Lâ†'Mg2Sip+(Al+Mg2Si)E. When Si is excess in the alloy, the ternary eutectic reaction occurs and the solidification sequence is considered to be:Lâ†'Mg2SiP+(Al+Mg2Si)E+(Al+Mg2Si+Si)E.(2) Nano-precipitate formation in Al-4%Mg-0.5%Si-1%Cu and effect of Ag on precipitation phenomenaAfter solution treated at520℃and cold water quenched, part of eutectic Mg2Si is dissolved in the matrix. Aged at200℃, the dominated phases are rod-like GPB (Guinier-Preston-Bagaryatsky) zones and lath-like S-phase particles (Al2CuMg). The dissolved Si stabilises GPB zones. Meanwhile, Ag addition obviously changes precipitation sequence and kinetics by promoting the formation of plate-like Z-phase (on {111} Al habit planes) and suppressing the transformation of GPB zone into S-phase. The composition of Z phase is determined to be～38at.%Al,～32at.%Mg,～20at.%Cu and～10at.%Ag. The increase of solution temperature from520℃to560℃leads to more Si dissolved into the Al matrix (from0.06at.%to0.13at.%). In the precipitation process, high Si content dissolved in the matrix promotes precipitation of more GPB zones and the existence of Ag prohibits the continuous transformation of GPB zones into S-phase. The Z-phase and increase of number density of GPB zones result in the enhanced age-hardening response.(3) Growth pattern of primary Mg2Si and transformation mechanism of morphologyThe intermetallic compound Mg2Si is a face-centered cubic (anti-fluorite type) structure and tends to form their equilibrium shape (faceted octahedron) with minimized total surface free energy. As {100} faces have the highest growth rate and the preferential growth directions are <100>, perfect octahedron Mg2Si is bounded by {111} surfaces, and {100} and {110} planes degrade to corners and edges, respectively.The variations of growth conditions, such as mass transfer and adsorption of impurities, usually change the growth rates along the <100> and<111> directions during solidification, which leads Mg2Si to develop into other morphologies, such as hopper, truncated octahedron and cube. In addition, interesting enormous Mg2Si dendrites tend to be formed in alloys with high content of Mg2Si. The primary dendrite trunk is formed along <100> direction of which the growth rate is accelerated, and develops the secondary branches in the <100> directions perpendicular to the main stem. Connection and overlapping of growth units lead to forming various interesting dendrites with inconspicuous crystallographic features of surfaces.The coarse Mg2Si dendrites can be turned to numerous fine octahedron or truncated octahedron particles with the addition of Al-P or Al-Ti-B master alloys increasing the Mg2Si crystal nuclei. According to Turnbull-Vonnegut equation, it is calculated that the disregistry between (220) crystal face of A1P and the (311) crystal face of Mg2Si is only6.58%, and the disregistry between (001) crystal face of TiB2and the (200) crystal face of Mg2Si is4.64%. Crystal lattice correspondence indicates A1P and TiB2can act as the nuclei of the primary Mg2Si. Furthermore, it is also found that coupling particles of A1P and TiB2exist in the centre of primary Mg2Si and promote the precipitation of Mg2Si. (4) Microstructure evolution of Al-Mg2Si-NiAl3alloys and effect of NiAl3on the growth of eutectic NiAl3The Al-Mg2Si-NiAl3phase diagram is calculated using Thermo-calc software, and its solidification process and microstructure are analyzed. In Al-Mg2Si-NiAl3pseudo-ternary phase diagram, the composition of ternary eutectic is Al-12.1%Mg2Si-8.4%NiAl3and the calculated temperature in equilibrium state is587.05℃. For Al-15%Mg2Si-NiAl3system, two critical compositions were detected at7.9%and13.1%N1Al3. The NiAl3phase first appears only in the ternary eutectic zone for the composition of NiAl3up to7.9%. With NiAl3contents between7.9%and13.1%, NiAl3appears in both the binary and ternary reactions. Above13.1%NiAl3, it solidifies as a primary phase as well as during the binary and ternary reactions. When (Al+Mg2Si)E binary eutectic completely evolves into (Al+Mg2Si+NiAl3)E ternary eutectic, eutectic NiAl3phase tends to form rods that have lower interfacial energy, compared with lamellar structure. The growth of NiAl3in rod-like manner strongly induces the formation of rod-like Mg2Si in order to decrease the total interfacial energy. Finally, it is formed the unique double rod ternary eutectic structure with rod-like Mg2Si and NiAl3uniformly distributing in Al matrix.