Study On The Structure And Liquid-solid Correlation Of Al-Mg Alloy Melts

Structures of metal/alloy melts are heritable and have an effective effect on the solidification structure and properties. So it makes the most sense to study their microstructure and the liquid-solid correlation which have not been clear so far due to the high temperature and the uncertainty of the structure of alloy melts. In this paper, Al-Mg alloys which have excellent properties and are wildly used but rarely researched on the melt were chosen for this study. Firstly, the melt-related theory and the research status of Al-Mg alloy melts were systematically studied and briefly reviewed. Then, Al(100-x)Mgx(x=0, 10, 20, 30, 40, 50) alloys were chosen after analyzing the phase diagram of Al-Mg alloy and then a series of experiments and simulations were carried out to study the structure of melts and the liquid-solid correlation aiming at further awareness of the heredity of metal/alloy melts, development of the relative theories and new craft as well as improvement of the quality of castings and production efficiency.Based on the available structural models and theories on the electrical resistivity of liquid alloys, the structure and the liquid–solid correlation of Al(100-x)Mgx(x = 0, 10, 20, 30, 40, 50) alloy melts have been qualitatively studied by measuring the electrical resistivity during the heating/cooling process using the direct-current four-probe method, as well as by characterizing the solidification morphology and testing the hardness. The result shows that the electrical resistivity of Al–Mg alloys increases with the increasing temperature as well as the increasing content of Mg but exhibits negative temperature coefficient in the solid-liquid zone with the presence of ? phase; with the increasing content of Mg, the number of relative Al-Mg clusters in melts increases and then makes the melts and solidification structure more uniform; thermal state and history have an effect on the solidification structure and properties: the electrical resistivity of Al–Mg alloys exhibits a lag phenomenon of structure change during the heating/cooling process; a higher heating/cooling rate contributes to the more obvious relaxation effect of electrical resistivity and the more uniform structure. Furthermore, higher pouring temperature leads the melts and solidification structure to be more homogeneous, which increases the hardness. There is a good agreement among the resistivity changes, the solidification structure, and hardness differences.To directly gain the information on the microstructure during the solidification process, molecular dynamics simulation were carried out on the rapid solidification processes of Al(100-x)Mgx(x = 0, 10, 30) alloys. The pair diffraction function, kinetics and thermodynamics as well as clusters were compared and analyzed to study the effect of cooling rate and composition on the rapid solidification process. Meanwhile the quasi-transition state model of clusters was presented. It shows that cluster transition and selection take place in thermodynamics and kinetic during the evolution of structure during the solidification process. As a result, those clusters in quasi-transition state exist. The content of icosahedron-related clusters which are viewed as metastable clusters in supercooling liquid increases firstly and then decreases with the increasing cooling rate while that of face-centered cubic-related clusters decreases all the time, which proves the existence cluster transition. Thermal history has little effect on the thermodynamics of melts above liquidus during rapid solidification process and obviously works in undercooled liquid. With the increasing degree of the relaxation, the number of the retained clusters in quasi-transition state increases and degree of supercooling of the homogeneous melt enhances during the solidification process, which results in more nucleus at a certain moment in thermodynamic and suppression of the growth; if the cooling rate is high enough, supercooling reaches the glass transition zone resulting in lower nucleus and higher content of glasses as well as higher energy retained in the system. Holding temperature can reduce the relaxation of the transition in microstructure. The content of Mg has an effective on the structure of melts and the rapid solidification process of Al-Mg alloys. With the increasing content of Mg, average atomic free volume of atoms in system decreases as a result of the bigger volume of Mg than Al and stronger interaction of Al-Mg than that of Al-Al and Mg-Mg, which promotes the relaxation and leads to the stronger ability of glass-transition during rapid solidification. There are mainly Al-centered clusters in Al-Mg alloy melts and Al-centered icosahedral clusters in the glasses. The origin of the splitting in the first peak of g(r) of amorphous Al70Mg30 alloy is from the Al-centered icosahedron-related clusters consisting of Al and Mg, while splitting of the other peaks is mainly caused by the increasing content and various connection styles of the short-range orders and medium-range orders.Lastly, according to the results above, experiments on studying the mechanism of the thermal-rate treatment were carried out for further study of the effect of the thermal history on the solidification structure by comparing and analyzing the cooling rate, solidification structure and mechanical properties of pure Al and Al90Mg10 alloy gained under different thermal histories. Meanwhile the mechanism of the heredity of alloy melts was systematically analyzed after considering relative results of studies and theories as well as comparing the growth process of creatures. It shows microheterogeneity exists in melts as a result of thermal fluctuation and electrical structure of clusters, acts as a part of the genes of the solidification structure by taking part in the process of nucleation and then affects the properties of castings. Higher melt temperature decreases the content of microheterogeneity by enhancing the migration of atoms and the transition of clusters, which leads to more homogeneous melts and solidification structure. With the increase of the temperature of melts, microheterogeneity is reduced due to the enhancement of the atomic migration and the cluster transition, which in turn leading to the relative microhomogeneous melts and the uniform solidification structure. The strength of castings increases firstly and then decreases with the increasing pouring temperature, which is on due to the enlargement of the supercooling caused by the decrease of the microheterogeneity and enhancement of the relaxation as well as due to the decrease of heteronuleus and the increase of the crystallization time. Higher cooling rate leads to less size difference of nucleuses and more nucleuses at a certain time and then the suppression of the process growth. Thermal-rate treatment is effective technique to improve the properties of castings mainly in the followed three aspects: decreasing the degree of microheterogeneity in melts by increasing the maximum temperature of melts, decreasing the maximum temperature to pouring temperature in a short time to enhance the relaxation and avoid the decrease of crystallization time by adding cold feeding and the appropriate choices of the combination of maximum temperature, pouring temperature, holding time as well as the form and content of cooling feeding.