Study On The Formation And Stability Of Quasicrystal Phase In Mg-Zn-Nd Alloys

Magnesium alloys is widely used with its excellent performance,however,low intensity and easy oxidation shortcomings limit its development.Rare earth elements have high solid solubility in magnesium alloy,and its solid solubility increases significantly with increasing of temperature,which will increase the hardness by the solid solution strengthening and aging strengthening.With appropriatethe alloy composition,rare earth elements Nd and Y can form quasicrystals in the alloy,which greatly improves the comprehensive mechanical properties of the alloys.There are a lot of studys about quasicrystal phase in Mg-Zn-Nd and Mg-Zn-Y alloys,but there is few study with both Nd and Y.Therefore,in order to explore the formation mechanism of multiple quasicrystal in this paper,Mg-Zn-Nd-Y quasicrystal phase will be producted,which plays an important role in exploring the diversity of Mg quasicrystal and riching the quasicrystal phase formation mechanism.In this article,Mg-Zn-Nd system alloys are producted by as-cast,sub-rapidly solidified and rapidly solidified methods.The differences of microstructure and the main phases under different components were analyzed,especially the formation and growth mechanism of quasicrystal.The effects of microstructure,composition and evolution of Mg-Zn-Nd system quasicrystal phase with different solidification conditions and solidification speed were explored.The effects on the micromorphology and the stability of quasicrystal with heat treatment were discussed.X-ray diffraction?XRD?,differential thermal analyzer?DSC?and scanning electron microscopy?SEM?were used in this process.The results show that:Mg-Zn-Nd-?Y?icosahedral quasicrystals have been prepared successfully from the Mg-Zn-Nd-?Y?by three different methods,i.e.,as-cast,sub-rapidly solidified and rapidly solidified methods.Different alloy compositions and cooling rates can form different microstructures.Mg-Zn-Nd quasicrystals are smooth spheroids with high roundness,and its composition is Mg₄₀Zn₅₅Nd₅.When the content of Y element is 1.25at%and Nd/Y is between1/2 and 1/3,there are the most petal quasicrystal phases in the alloy.The final morphology of Mg-Zn-Nd-?Y?quasicrystal is the consequence of two rare earth elements.The solidification path of the normal solidified as-cast Mg-Zn-Nd-?Y?alloy is complex.The primary quasicrystal is formed in the place with high content of rare earth elements and Zn elements,and grows along the main growth direction of the quasicrystal phase.The eutectic phase of the quasicrystal and?-Mg were formed around the primary quasicrystal phase and radiated in the direction of the five petals.The solidification path of subrapid and rapid solidification is relatively simple,and I-phase can precipitate directly from the undercooling alloy melt.In the process of rapidly solidification,Mg₇Zn₃ phase disappeared,because it can be existed in low temperature.Icosahedral short-range order?ISRO?exists in the alloy melt,for the nucleation of the crystal particles in the process of rapid solidification.The quasicrystal is grown in small planar growth in as-cast,while the quasicrystalline phase is mainly grown in a non-small plane with rapidly solidification.With 300?-24h heat treatment,quasicrystal still existed and did not change obviously.Quasicrystal phase were damaged the stability with 350?-24h heat treatment.Mg-Zn-Nd quasicrystal phases were melted,while Mg-Zn-Nd-Yquasicrystal phases all changed into D-phase or Z-phase.The matrix Mg₇Zn₃ phase changed into MgZn and?-Mg phase.By analyzing the XRD and EDS energy spectrum,the rod-shaped phase is approximate quasicrystal phase Z-phase,whose component is ideal Mg₂₈Zn₆₅RE₇.