Effect Of Cooling Rate On Microstructure Evolution Of Mg-zn-y/iqc In Situ Composites

Magnesium alloys have widely applied to automobiles, airplane, electrical products,communication equipment, defense and so on, due to their excellent properties suchas low density, high specific strength, superior damping characteristics, excellentelectromagnetic shielding performance, etc. Icosahedral quasicrystalline (I-phase) hasmany excellent properties, such as high strength and hardness, good corrosionresistance, low friction coefficient and low interfacial energy, etc, which can increasemechanical properties of magnesium at room temperature and elevated temperature,respectively, and improve corrosion resistance of magnesium. Research shows thatwhen the Zn/Y atom ratio is 6:1 in Mg-Zn-Y alloys, the microstructure is justα-Mgand I-phase. In this study, Mg/I composites are fabricated by in situ I-phase inMg-Zn-Y alloys. Microstructure, phase composition and I-phase about its pattern, sizeand distribution in Mg-Zn-Y alloys are analyzed via optical mcroscape (OM),scanning electron microscopy (SEM), energy dispersive spectrum(EDS), X-raydiffraction (XRD), differential scanning calorimetry(DSC) and transmission electronmicroscopy (TEM), etc. Further, it's investigated that microstructure evolution,solidified path and solidified kinetics of Mg-Zn-Y alloys, in which Zn/Y atom ratio is6:1, to develop new Mg alloys reinforced by I-phase providing mechanism basis.The results show thatα-Mg, fish bone eutectic and lamellar eutectic structure(α-Mg+I-phase) and MgZn phase bars are identified both in as-cast Mg12Zn2Y alloysand as-cast Mg18Zn3Y alloys, while there areα-Mg dendritic, bulk I-phase, fish boneeutectic and lamellar eutectic structure (α-Mg+I-phase) and MgZn phase bars inas-cast Mg24Zn4Y alloys.The phase compositions both of sub-rapidly solidified Mg12Zn2Y alloy andMg18Zn3Y alloy areα-Mg and lamellar eutectic structure (I-phase+α-Mg), in thesame time, MgZn phase is inhibited and fish bone eutectic disappear.α-Mg, lamellareutectic structure (I-phase+α-Mg) and petal I-phase are identified in sub-rapidlysolidified Mg24Zn4Y alloys. In addition, bulk I-phase disappears and 5 petals I-phase is formatted in the sub-rapidly solidified Mg24Zn4Y alloys. With the cooling rate increasing, petal I-phase is more and fine. Globosity I-phase is formatted under higher cooling rate conditions.The microstructure is mainly cell crystals in rapidly solidified Mg12Zn2Y ribbons and Mg18Zn3Y ribbons. XRD results show that the number of I-phase decreases with the cooling rate multiplication. The patterns of I-phase are bulk, lamellar eutectic structure and petals in lower rotate rapidly solidified Mg24Zn4Y ribbons. With the cooling rate increasing, the number of globosity I-phase is raising. Once counts of I-phase is coming to a certain value, the number of I-phase is reduced when the cooling rate arguments.When content of Zn and Y elements and cooling rate change, pattern of I-phase is also changed by analysis the microstructure and phase composition of different composite and different cooling rate Mg-Zn-Y alloys, which can be described as: lamellerâ†'bulkâ†'petalâ†'globosity. Also, as the cooling rate raises, I-phase is further fine.Solidification path of as-cast Mg12Zn2Y alloy can be descried as follow:Solidification paths of sub-rapidly solidified Mg12Zn2Y alloy withΦ8mm,Φ4mm andΦ2mm can be described as follow, respectively:Solidification path ofΦ6mm sample is similar to that ofΦ8mm sample, and the slightly difference of 8mm sample and 6mm sample is just transformation reaction temperature.In addition, solidification path of rapidly solidified Mg12Zn2Y alloy is similar to that ofΦ2mm sample. Solidification paths of as-cast and sub-rapidly solidified Mg18Zn3Y alloy issimilar to that of as-cast and sub-rapidly solidified Mg12Zn2Y alloy, respectively, andthe difference is that eutectic structure (I-phase+α-Mg) directly forms in theΦ4mmsub-rapidly solidified Mg18Zn3Y melt. When the rotate of rapidly solidifiedMg18Zn3Y ribbons is less than 1200r/min, its solidified path is as follow:And when the rotate of rapidly solidified Mg18Zn3Y ribbons is 1600r/min and2000r/min, respectively, the solidified path of them is as follow:In the solidification path, T is the corresponding temperature of different rotateMg18Zn3Y ribbons.Compare to the solidification path of as-cast Mg12Zn2Y alloy and sub-rapidlysolidified Mg12Zn2Y alloy, the difference of solidified path of as-cast Mg24Zn4Yalloy and sub-rapidly solidified Mg24Zn4Y alloy is that W phase directly forms in themelt, not YZn₂ andα-Mg, respectively. When the rotate of rapidly solidifiedMg24Zn4Y ribbons is 400r/min, 800r/min and 1600r/min,respectively, thesolidification path of them is as follow:The relationship between incubation time and temperature of rapidly solidifiedMg-Zn-Y alloys is calculated via transient nucleation theory related to time and theÏ„-T curves of different composite rapidly solidified Mg-Zn-Y alloys are got. Theresults show that the calculated result is accord with the DSC curves.