Phase Equilibria and Solidification of Al-Rich Ai-Li-Cu Alloys
- PDF / 428,643 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 33 Downloads / 183 Views
PHASE EQUILIBRIA AND SOLIDIFICATION OF Al-RICH AI-Ui-Cu ALLOYS SINN-WEN CHEN AND Y. AUSTIN CHANG Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706 ABSTRACT A liquidus projection of the Al-rich Al-Li-Cu system is proposed. The proposed liquidus projection was based on DTA, X-ray diffraction, metallography, EPMA, SEM, and chemical analysis of 50 ternary alloys. Using these data and those reported in the literature, and thermodynamic models of Al-Li, AI-Cu and Cu-Li, a thermodynamic description of the Al-rich AI-Li-Cu system was developed. The calculated isothermal sections at several temperatures and the liquidus projection are in agreement with the experimental determinations. Combining the thermodynamic models and a Scheil-type equation, quantitative solidification paths were described. The calculated amount of primary solidification phase was compared to the experimental determination. 1.0 INTRODUCTION Al-rich AI-Li-Cu ternary alloys are the basis for many new light alloys used in aerospace applications. An understanding of the Al-Li-Cu phase diagram in the Al-rich corner is essential for alloy and process development, such as the optimum conditions for casting processes. Hardy and Silcock [1] pioneered a study of this system and established two isothermal sections at 5000 C and 350 0 C. Mikheeva et al. 121, Shamral et al. 13] and Bochvar et al. [41 also worked on this system in the early 1960's. Their studies extended to the liquid phase region. Recently, Dubost et al. [5] studied this system and extensively examined the area where the primary solidification phase is either T 2 or R. However, the data reported are still very limited. Furthermore, they are not in agreement; e.g., the temperature and composition of the ternary eutectic determined by these investigators as given below varied by 10 0 C and 5 atomic percent. There is even disagreement about the type of the ternary invariant reaction reported. Investigators Ternary reaction Mikheeva et al. [2] L=a+T 1 +O Shamrai et al. [31 L=a+TI+TB L=a+Tl+TB Bochvar et al [41 Dubost et al. [51 L=a+TI+TB
Temp 526 0 C 528 00 C 518 0 C 521 C
Liquid Composition Al-9.atLi- 19.atCu Al-10.at%LI-14.4atCu
From our previous studies [6. 7], it was found that alumina and graphite are not inert to Al-Li-Cu alloys. The lithium loss became appreciable when molten Al-Li-Cu alloys were contained in these crucibles. The three groups of Russian investigators [2,3,4] used Al2 0 3 and Dubost et al. 15] used steel coated with graphite as their container materials. The lithium losses encountered by these investigators might be the cause for the discrepancies of the reported data. The objective of this study is to re-determine the solid-liquid phase equilibria for the Al-rich comer using techniques which were employed successfully in determing the phase equilibria of Al-P3(AlLi) 161. On the basis of these data, and the data for the three binary systems, Al-Li, Al-Cu, and Cu-Li, the Al-rich Al-Li-Cu system was modelled thermodynamically. By
Data Loading...
