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Study the metastable phases of the Al-Cr system

ФИО: Antoine Rousseau

Направление: Материаловедение

Научный руководитель: Dr. Alexandra KHVAN

Институт: Институт экотехнологий и инжиниринга

Кафедра: Кафедра Металловедения цветных металлов

Академическая группа: MTM-14-1

It is well known fact that the mechanical properties of aluminium alloys can be enhanced by up to 20% by better control of the microstructure formed during the casting process. This is particularly important for the automobile industry. For this reason big efforts are made in order to understand and model factors influencing microstructure of the alloys. In spite of the fact that transition metals have been used for many years as key alloying elements in commercial aluminium alloys, many questions remain unanswered about how the various phases containing these elements are formed. In contrast to «normal» non-equilibrium solidification, which may be described well by the Scheil model using stable phase diagrams, rapid solidification of such alloys leads to an increase in the concentration of these alloying elements in the terminal solid solutions and is accompanied by both the formation of metastable intermetallic phases and the movement of the eutectic point to higher concentrations of the alloying element and lower temperatures. In such cases solidification is controlled by «phase selection» which must be taken into account before any analysis using the Scheil model can be carried out.

In this scientific research work, our goal is to determine possibility of formation of metastable phases during none-equilibrium solidification of aluminium alloys containing Cr.

The solidification of the Al-rich Al-Cr alloys (0.5%, 1% and 1.5%wt) has been studied both experimentally (LM, SEM, EPMA analysis) and by calculation of the phase equilibria using Thermo-Calc software. The results of previous experimental studies, which indicated that one or more stable intermetallic phases are suppressed on solidification from the liquid, have been confirmed. The additional problem which was faced during the present investigation is that despite the Al-Cr system was assessed many times, since the first investigation of Fink and Reche in 1933, there are still many unsolved questions about constitution of the phase diagram. The one of the important for this work questions is the type of the reaction between Liquid phase with (Al) and Al7Cr.

The early works describes the reaction as peritectic in nature. However, more recent investigations describe the reaction as eutectic. The question of the type of the reaction has been under discussion for several reasons: first of all there is a very narrow solidification range, especially for hypo-eutectic alloys. So the precise determination of temperature depends on the precision of the equipment used. Secondly, the (Al) + Al7Cr eutectic has an unusual morphology owing to the extremely small volume fraction of the intermetallic phase present. The temperature of the eutectic reaction reported varies from 656°C to 659.7°C. The difference in the data obtained could be the result of an inclination for the alloys to undercool as well as the measurements being made with insufficient precision. Presented a detailed analysis of the eutectic microstructures.

For this reason additional investigations in order to determine type of the reaction and it’s temperature and position of solvus line of the aluminum solid solution in this work were needed.