Investigation On The Mechanism Of Al/MgAl₂O₄ Nucleation

Al Alloys are one of the most widely used materials in the national economy.Grain refining being as an important method to enhance the strength and ductility of materials also is a major means to improve the quality of aluminum.MgAl₂O₄ is a common oxide inclusions in Al-Mg alloy smelting process.It is considered as an effective nucleating agent for Al-based alloys and can play the role of grain refinement due to the same crystal structure as Al,face-centered cubic structure?FCC?,and a small lattice mismatch with Al?f Al/MgAl₂O₄ = 1.11%?.However,this kind of refiner has not been used in practical industrial applications.Several researches showed that chemical reaction occurs at the interface of Al/MgAl₂O₄ at high temperature to form Al₂O₃.Since Al₂O₃ has a hexagonal close-packed structure which is different from that of Al crystals and a large lattice mismatch with Al,it is considered that Al₂O₃ can decreases the nucleation efficiency of MgAl₂O₄ to Al therefore is maleficent for the grain refinement of Al.This paper attempts to investigate the chemical reaction conditions of Al/MgAl₂O₄ by adjusting the kinetic factors such as temperature and time,and to investigate the nucleation behavior of Al/MgAl₂O₄ without the chemical reaction reactant Al₂O₃.The observation of the undercooling and the preferred orientation of the nucleated grains of Al on MgAl₂O₄ substrate and the microscopic study of nucleation interface was conducted,the formation mechanism of the nucleation interface were discussed.The main results of this study are as follows:1.At high holding temperature long holding time?such as 1200?,160s?condition,the Al melt chemically reacts with MgAl₂O₄ to form Al₂O₃ at theinterface and cause a large undercooling phenomenon.However,under the condition of low holding temperature and short holding time?such as 690?,60s?,the undercooling is small and there is no reactant layer of Al₂O₃ at the interface.Al nucleates directly on MgAl₂O₄.2.The degree of undercooling is positively correlated with the holding time and the holding temperature.At the short holding time?1 min?condition,effect of holding temperature on the undercooling is divided into two intervals,when the holding temperature is below 750?,the increase of the undercooling is very significant,When the temperature is above 750?,the increase of undercooling tends to be gentle,and the curve of undercooling vs holding temperature forms the shape of a waterfall.3.The holding temperature has an influence on the Al crystal orientation.The preferred crystal planes are?111?and?200?when the holding temperature is 700?,while the preferred crystal faces are?311?,?111?and?200?when the holding temperature is 800?.4.The difference of undercooling and the different preferred orientation of crystal at different holding temperatures is resulted from the fact that the thermal stress of the droplet on the substrate leads to different degrees of micro-strain on the substrate surface,which leads to the undercooling of the nucleation and the preferential orientation of the crystal change at different temperatures.