Mechanism Of La Effect On Morphology Of Al-Re In AZ80-Ce Alloy

Adding rare earth elements is an important way to improve the mechanical properties of AZ magnesium alloys.However,it is easy to form coarse Al-RE intermetallic phase with high melting point during the solidification.How to improve this kind of Al-RE compound phase is one of the research directions in magnesium alloy field.In this paper,AZ80-Ce-x La(x=0,0.5,1)alloys with different La contents were prepared by the alloying method based on AZ80-Ce alloy.XRD,SEM,EDS,EPMA,and TEM were used to analysis the effect of La on the second phase in AZ80-Ce alloy.Furthermore,the extraction method was used to extract the tubular-like Al-RE phase in the AZ80-Ce-La alloy,and the morphology evolution mechanism of the tubular-like Al-RE phase was discussed.The results showed that the microstructure of as-cast AZ80-Ce-x La alloys were mainly composed of?-Mg,?-Mg₁₇Al₁₂ and Al-RE(Al₁₁Ce₃ or Al₁₁(Ce,La)₃)phases.In the AZ80-Ce alloy,the?-Mg₁₇Al₁₂ phases were mainly in the shape of a broken network,and the Al₁₁Ce₃ phase was mainly needle-like.When a trace amount of La was added to the alloy,the tubular-like Al-RE phase appeared in the alloys.With the increase of La content,the needle-like Al-RE phase decreased,but the tubular-like Al-RE phase increased.When the amount of La was added at 1.0wt.%,most of the Al-RE phases in the alloy were tubular-like,and the?-Mg₁₇Al₁₂ phases were granular.In the AZ80-Ce-x La alloys,the Al element was mainly distributed in?-Mg₁₇Al₁₂,Al-RE and Al-Ce-Mn phases.The Ce and La elements were mainly distributed in the needle-like or tubular-like Al-RE phases.The Zn element was mainly dissolved in?-Mg and?-Mg₁₇Al₁₂ phases.The Mn element was mainly in Mn simple,solid dissolved in the Al-RE phases or existed in the Al-Ce-Mn phases.The tubular-like Al-RE phases were mainly composed of Al,Ce and La elements.The ratio of the aluminum element and the total rare earth element was about 3.6,closed to the stoichiometric ratio of Al₁₁RE₃(Al₁₁(Ce,La)₃).TEM analysis results showed that:compared with the Al₁₁La₃ phase,the crystal structure and unit cell parameters of the tubular-like Al-RE phase were closer to the Al₁₁Ce₃ phase.The tubular-like Al-RE phase were based on the Al₁₁Ce₃ unit cell,and La atoms occupied some positions of the lattice sites originally belonging to the Ce atoms.The Materials Studio software was used to predict the growth morphology of the crystal.The calculation results showed that the Al₁₁Ce₃ phase were likely grow into a hexagonal prism shape along the[001]direction.The sides of the hexagonal prism were{011}and{002}family planes.The{011}family planes were the main exposed face of the crystal,and their growth rate were the slowest.The extraction method was further used to extract the tubular-like Al-RE phases in the AZ80-Ce-La alloy to analysis their morphology evolution mechanism.The research results showed that the doping of La would cause the lattice distortion in crystal and increased the energy of the growth surface,and then the growth rate of Al₁₁RE₃ phase increased.With the condition of rapid growth,the growth rate difference between the central region and the edge region of(001)plane is aggravated,resulting in the formation of tubular structure of Al₁₁RE₃phase.After the tubular structure was formed,the growth of Al₁₁RE₃ phase were divided into the two stages in the following growth process.During the first stage of crystal growth,the tubular-like Al₁₁RE₃ phase grew rapidly toward both ends.Since the growth rate of the{011}family planes were lower than the{002}family planes.At the first stage of the crystal growth,some of the{001}family planes in the Al₁₁RE₃ phase would form the incomplete regions with unclosed sides.When the Al₁₁RE₃ phase grew larger,the effect of surface energy on the growth of planes decreases rapidly.The growth of the Al₁₁RE₃ phase entered the second stage.According to the PBC theory,the solute atoms were more likely to adsorbed in the incomplete area of tubular-like Al₁₁RE₃ phase,and gradually reduced the incomplete area.When this incomplete area nearly disappeared,the growth rate of the Al₁₁RE₃ phase on both ends became very slow.The solute atoms would slowly deposit in the tube of the Al₁₁RE₃phase and closed the tube structure,and a small amount of tubular Al₁₁RE₃ phase grew into the solid-like.