In-situ Whisker Growth In The Mg Alloy And Its Properties

As the most promising and emerging materials in 21st century,Magnesium alloys have a number of advantages including light weight,high specific strength,high specific stiffness,good electromagnetic shielding performance and good machining properties,however,it also has a series of disadvantages that restrict the application of magnesium alloys in industry,such as low Elastic Modulus,low strength,poor wear resistance and heat resistance etc.With the vigorous research in recent years,people find that alloying and addition of trace elements make contribution to improving the mechanical properties of Magnesium alloys.Meanwhile,the rising of composite materials offered another method to improve the properties of Magnesium alloys.As a kind of single crystal materials,whisker has a series of advantages including high strength,high Elastic Modulus and high melting point.It can be used as a reinforcement in composites to improving the mechanical properties of Magnesium alloys.In traditional composites materials,the reinforcement has be completed when it was embedded into matrix,this method also results the inhomogeneous distribution of reinforcements and bad bonding interface between reinforcement and matrix.If the whiskers can be in-situ synthesized in the matrix,the mechanical properties of Magnesium alloys will be benefited from both alloying and composite materials.In this thesis,La,Ce,Pr and Nd were respectively added into magnesium alloys and controlled melting process,finally,we got four kinds of Mg-6Al-8RE and finished the following work:Scanning electron microscope?SEM?,optical microscope?OM?,X-ray diffraction?XRD?and transmission electron microscopy?TEM?were used to research the chemical composition and crystal structure of the rod-like compound in Mg-6Al-8RE.The rod-like compound in Mg-6Al-8La could be considered to be Al4La and it has a structure of body-centered tetragonal,the research also made a certain result that Al4La rod-like compound was a phase of whisker.The distribution of Al4La had a characteristic that arranged in parallel in the micro-region and random on the whole.The rod-like compound in Mg-6Al-8Ce,Mg-6Al-8Pr and Mg-6Al-8Nd are Al2Ce,Al2Pr and Al2Nd,all of them had a structure of face-centered cubic and arranged random on the whole.Al4La,Al2Ce,Al2Pr and Al2Nd were thermostable.Growth mechanism of four kinds of rod-like compound were discussed by using metal solidification theory.The chemical composition of the rod-like compound resulted from the electronegativity difference between Al and RE.The solid-liquid interfaces of four kinds of rod-like phase were smooth interface and grew up depending on the aggregation of atomic on the side of step.The growth direction of smooth interface were also selective,which stemed from the difference of growth rate between high-index surfaces and low-index surfaces.Al₄La phase was rod-like which result from the the asymmetry of crystal structure,and the others resulted from the unstable factors in front of melting micro regions.The interaction between Al4La phase and matrix had two kinds of solidification mechanisms,while Al₂Ce,Al₂Pr and Al₂Nd only had one.Microstructure morphologies of four kinds of Mg-6Al-8RE were observed,each group of Mg-6Al-8RE was solidified under two different cooling rates.It indicated that the cooling rate make a big difference on the microstructure morphologies.We got Al4La,Al2Ce,Al2Pr and Al2Nd phase with a great aspect ratio under slow cooling rate.With the rising of cooling rate,the aspect ratio of Al4La decrease,and Al2Ce,Al2Pr and Al2Nd phase turned into massive phase.Meanwhile,there were some other phase appearing in Mg-6Al-8RE.Tensile strength and Elastic Modulus of four kinds of Mg-6Al-8RE were tested which are solidified under 5?/min.The results indicated the tensile strength and Elastic Modulus of four kinds of Mg-6Al-8RE all had improved,and the plasticity of alloy also had improved except Mg-6Al-8La,but it had the most highest Elastic Modulus.