Solidification Process And Phase Transformation In Mg-Zn-Y Quasicrystal Alloy

Mg-Zn-Y quasicrystal are thermodynamically stable and3D icosahedral quasicrystal,because of their excellent mechanical properties with a powerful combination of Mg and canbe widely used to reinforce Mg alloys. Therefore, Mg-Zn-Y icosahedral quasicrystal has goodprospects for developing advanced Mg alloys. However, the growth mechanism oficosahedral quasicrystal and equilibrium phase transformation during heat treatment inMg-rich Mg-Zn-Y alloy are rarely researched. Since the Mg-Zn-Y quasicrystal existpseudo-gap at the Fermi level, which has a large resistivity and negative temperaturecoefficient, electrical resistivity as one of the sensitive physical parameters can use to analyzethe melting process and nucleation of quasicrystal alloy.The as-cast, sub-rapidly solidified and rapidly solidified methods were used to prepareMg-Zn-Y quasicrystal alloy in this paper, and heat treatment was carried out simultaneously.The alloy microstructures and solidification process were analyzed to research the formationand growth mechanism of quasicrystal as well as the equilibrium phase transition during theheat treatment. X-ray diffraction (XRD), high-resolution transmission electron microscopy(HRTEM), differential thermal analyzer (DSC) and scanning electron microscopy (SEM)were used in this process. The relationship between the formation of icosahedralquasicrystalline phaseThe results show that Mg-Zn-Y icosahedral quasicrystals have been prepared successfullyfrom the Mg-Zn-Y by three different methods, i.e., as-cast, sub-rapidly solidified and rapidlysolidified methods. Because of the large temperature range of I-phase formation, underdifferent cooling rates, I-phase can precipitate directly from the undercooling alloy melt.Attributing to the smaller degree of undercooling, there are not only much more primaryI-phase particles, but also developed laminar (α-Mg+I-phase) eutectic structures form in theas-cast Mg-Zn-Y alloy. However, there are only the smaller sized primary I-phase particles inthe sub-rapidly solidified and rapidly solidified alloys. Due to the difference in diffusion ratesof atoms and enrichment of solute element Y, solute partitioning takes place at the solid/liquidinterface. Thus, the partition among the molten pools is Y concentration, while Mgconcentration increases in the pools and ahead of the solid/liquid interface. Resistivity of the quasicrystal alloy and normal alloy with the same composition wasmeasured by a direct current four-probe method. The temperature dependence of resistivityρ(T) for two samples is completely different. It shows that the quasicrystal alloy have anegative temperature coefficient of resistivity (TCR) before the icosahedral quasicrystaldecomposes. In the liquid quasicrystal alloy, the TCR shows anomalous changes at~1073Kduring heating and cooling processes, which indicates that there occurs structural transitions.It concludes that the icosahedral short-range order (ISRO) with five-fold symmetry shouldexist in melt, which inherits from the solid I-phase structure. These ISRO will decomposeafter1073K, and it is a reversible transition during heating and cooling processes. As for themicrostructures of two samples after resistivity measuring, I-phase present in quasicrystalalloy, while there are Z-phase and α-Mg in the normal sample. It is concluded that the liquidstructure inherits the ISRO from icosahedral quasicrystal. Moreover, icosahedral quasicrystalshows the heredity during the remelting process. The pre-existing ISRO structure acts as atemplate in the liquid and promotes the nucleation of icosahedral quasicrystal. Increase thecooling rate appropriately can make a greater number of ISRO preserved, forming a highvolume fraction of icosahedral quasicrystals. On the contrary, when the cooling rate is slowenough, these atomic clusters will evolve into crystal structure.It is found that the preferred growth directions of icosahedral quasicrystalline phase(I-phase) are along five-fold axes and the planes perpendicular to the five-fold axes grow in afacet manner. Due to the local compositional change at the solid/liquid interface, the planargrowth is gradually replaced by cellular growth. The I-phases in the as-cast alloy get coarsenand smooth undergoing rough edges corners.The I-phases in all samples can be stable and coarsened after the heat treatment at623Kfor50h, and in the cast alloy Mg7Zn3decompose into MgZn. Moreover, the volume fractionof I-phase is effectively increased. Solid equilibrium phase transformation occurs at693Kafter30h. The W-phase with average composition of Mg22.4Zn51.9Y25.7form in the inner ofprimary I-phase, while H-phase with the average composition of Mg25.5Zn60.3Y14.2form in theregion of (I-phase+α-Mg) eutectic structures. Moreover, small amount of D-phase is formedat the I-phase/Mg7Zn3interface five-fold along the direction with the average composition ofMg35Zn61Y4. Electron diffraction analysis show that the orientation relationship exists between two phases, i.e.,[111]W//2-f,[113]H//[001]Mgand [110]H//[121]Mg. The coarseningprocess of I-phase in rapidly solidified alloy is along five-fold direction at623K, evolvinginto five-petals like I-phase. But, no ternary equilibrium phase transformation in rapidlysolidified alloy at693K for30h. Owing to the rapid solidification, the solubility limit of Znincreases, which cause that Mg7Zn3form in the the matrix.Ternary solid equilibrium phase transformation is attributed to the atomic diffusion.Especially, the diffusion of Y atom is the key factor responsible for them. During this process,solute atoms redistribution occurs that Y atoms concentrate in the nucleation position or theinterface of (I-phase+α-Mg) eutectic structures. At the same time, the remaining Mg atomsdiffuse from I-phase to matrix. It causes that H-phase, W-phase and D-phase form.