Preparation and Performance Research of Mg-Zn-Nd Quasicrystals/AZ91Composite
|Course||Materials Science and Engineering|
|Keywords||AZ91composite Mg-Zn-Nd spherical quasicrystals mechanical properties cooling rate corrosion resistance|
Magnesium alloys as a lightweight engineering material have been widely used inmilitary, aerospace industry,3C, transportation and other areas. However, as-cast magnesiumalloys have a series of shortcomings such as coarse grains, low the mechanical properties,relatively poor corrosion resistance and low high temperature strength, which severely limitsthe application of magnesium alloy. Magnesium matrix quasicrystals possess advantages ofhigh hardness, low surface energy, good corrosion resistance and good wettability withmagnesium matrix. So adding Mg-based quasicrystals into magnesium alloy can improve themechanical properties and corrosion resistance.by grain refinement and microstructureimprovement. Therefore, the development of new high-performance magnesium alloys hasbecome a hot research field of magnesium alloy.According to this subject, Mg-Zn-Nd quasicrystal master alloy was prepared by normalcasting. Choosing MZN quasicrystal master alloy containing a high volume fraction ofspherical quasicrystals as reinforcements add into AZ91alloys was successively performed.Moreover, the influences of cooling rate on the morphology, distribution, quantity,quasicrystalline phase composition of MZN quasicrystalline phase were studied respectively.To investigate the microstructures and compositions of the alloys, scanning electronmicroscopy （SEM）, transmission electron microscopy （TEM） and X-ray diffraction （XRD）methods were employed. Brinell hardness and tensile strength tests were carried out toinvestigate the influences of the quantity of MZN quasicrystal master alloy on mechanicalproperties of AZ91alloys. The corrosion behaviour of AZ91alloys was investigated byelectrochemical test.The study results show that: MZN quasicrystal master alloy with high volume fraction ofspherical quasicrystals were prepared by conventional casting methods. The composition ofMZN spherical quasicrystals is Mg40Zn55Nd5. The number of spherical quasicrystalline phasespresents the trend of the normal distribution with Nd content increasing. When the additionlevel of Nd reaches1.3at%, spherical quasicrystalline phases are homogeneously distributedin the matrix, and the volume fraction of quasicrystalline phases is the most. The cooling rate experiments show that there forms spherical quasicrystalline phases under the experimentalconditions solidification modulus of2.38. In the rapid cooling rate, nucleation rate increasesand the initial morphology of the quasicrystalline phase growth can be preserved. The largestnumber of quasicrystalline phases which are distributed uniformly in the matrix is obtained.However, the solidification time is extended in the slow cooling rate. Because of the largegrowth space and much time, the spherical quasicrystalline phases have sufficient conditionsto grow and get roughened and even mutated. The cooling rate also affect the composition ofthe quasicrystalline phases, under the experimental conditions solidification modulus of2.38,the spherical quasicrystalline phases (Mg40Zn55Nd5) transform into the petal-likequasicrystalline phases （Mg30Zn60Nd10） in the growth process. Thermal stability experimentsprove spherical quasicrystalline phases at300℃can stably be present.The formation of the spherical quasicrystalline phases follows the nucleation and growththeories. Because the nucleation of quasicrystalline phase needs to overcome the smallnucleation energy, quasicrystalline phases are more easily to nucleate than other crystalphases in the liquid. During the nucleation process of the spherical quasicrystal, it first formspentagonal dodecahedron morphology. The growth process and the final growth morphologyof spherical quasicrystalline phases are restricted by the Nd element. There is a differentdistribution of Nd concentration on each tiny quasicrystal grain plane. The faster of crystalplane grows, and the adsorption content of Nd is bigger. As the growth rate is slow, the crystalplane becomes smaller under the interfacial tension and the adsorption content of Nd issmaller. Thus the growth rate of the crystal surface becomes relatively fast. Finally, thegrowth rates of all quasicrystal growth surfaces tend to be consistent, and in the cooling rateof solidification modulus of2.38, which form the spherical morphology of thequasicrystalline phases.According to the structure of the liquid metal hereditary and the XRD analysis results ofadding MZN quasicrystal master alloy into AZ91alloys, it can prove quasicrystalline phasesare retained well in AZ91alloys. The microstructure of AZ91alloys are significantlyimproved by adding MZN quasicrystal master alloy, i.e., the morphology of β-Mg17Al12phasechanges from continuous nets to discrete nets or even particles. Moreover, the quantity ofβ-Mg17Al12phase is reduced. When the addition level reaches6wt%, the β-Mg17Al12phase is thoroughly disconnected into minimum grain size. The microstructure of the alloy isobviously refined. When the addition level of MZN quasicrystal master alloy surpasses6wt%,the β-Mg17Al12phase begins to coarsen and reconnect into nets.After adding MZN quasicrystal master alloy into AZ91alloys, the tensile strength ofAZ91composite is significantly increased, and the tensile strength reaches the maximumvalue of210.68MPa, which is about30%higher than that of AZ91magnesium alloy. Whenthe addition of MZN quasicrystal master alloy is up to4wt%, the elongation of AZ91composite reaches the maximum value of3.3%, which is about1.5times that of AZ91alloy.This is because the dispersed quasicrystalline phases can prompt the refinement of themicrostructure of alloy. At the same time, the dispersed quasicrystalline phases can restrict thedeformation of alloy matrix and pin the grain boundaries to prevent grain boundary fromslipping, which improves the strength and ductility of the alloy. The micro-hardness value ofquasicrystalline phase （about557HV） is much higher than that of β-Mg17Al12phase （153HV）.The hardness of AZ91composite can reach the peak value of71.3HB, which is nearly26%higher than the hardness value of AZ91magnesium alloy.MZN quasicrystal master alloy can significantly improve the corrosion resistance ofAZ91alloys. When the addition of MZN quasicrystal master alloy is up to6wt%, the corrosionrate reaches the minimum value of0.8mg/（cm2·d）, which is about1/15of the value of11.9mg/（cm2·d） of AZ91magnesium alloy. Due to the high corrosion resistance of quasicrystallinephases, dispersed quasicrystalline phases can improve the stability and corrosion resistance ofthe matrix. The grain size of AZ91alloys was significantly refined after adding MZNquasicrystal master alloy. The grain boundaries act as a physical corrosion barrier formagnesium alloys. The increasing quantity of grain boundaries cause the corrosion resistance ofalloys is enhanced. In addition, adding MZN quasicrystal master alloy into AZ91alloy alteredthe amount and distribution of β-Mg17Al12phase. The amount of β-Mg17Al12phase reduce,which effectively reduce the number of micro-galvanic. So the reduction of β-Mg17Al12phasefurther weakens the degree of corrosion of the alloy.