Key Factors Influencing Eutectic Si Modification in Al-Si Hypoeutectic Alloy by Trace La
ZHANG Lili1, JI Zongwei2, ZHAO Jiuzhou1(), HE Jie1, JIANG Hongxiang1
1.Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China
Cite this article:
ZHANG Lili, JI Zongwei, ZHAO Jiuzhou, HE Jie, JIANG Hongxiang. Key Factors Influencing Eutectic Si Modification in Al-Si Hypoeutectic Alloy by Trace La. Acta Metall Sin, 2023, 59(11): 1541-1546.
The modification of eutectic Si to fiber morphology from coarse plate-like morphology is essential for producing an Al-Si hypoeutectic alloy. Furthermore, chemical modification through the addition of modifying elements, such as Na and Sr, to melt is the most widely used method in industrial production to improve microstructures. Recently, the effect of rare earth metals on the eutectic Si modification has also attracted considerable attention, especially for the economical element La. Key factors influencing eutectic Si modification in Al-Si hypoeutectic alloy by trace La are theoretically explored. The results demonstrate that the solubility of La in the primary α-Al phase and interaction parameter between La and Al (or Si) primarily contribute to the eutectic Si modification. When the addition level of trace La is within its solubility in the primary α-Al phase, La distributes in α-Al and eutectic Si, and the modification effect increases with the La addition level. When the addition level of trace La is greater than its solubility in α-Al, a ternary compound containing Al, Si, and La exists before the eutectic reaction due to the significant value of the interaction parameter between La and Al (or Si). Calculated results further prove that the composition of the ternary compound is AlSiLa due to the substantial value of heat for the formation of AlSiLa and the small value of interfacial energy between Al melt and AlSiLa. Under this condition, La distributes in α-Al, AlSiLa, and eutectic Si, and the La content in α-Al and eutectic Si almost remain constant. Thus, the modification effect almost stays unchanged with La addition. A suitable modification effect is achieved when the La addition level is around its solubility in the primary α-Al phase.
Fund: National Key Research and Development Program of China(2021YFA0716303);China's Manned Space Station Project;National Natural Science Foundation of China(51901231);National Natural Science Foundation of China(51971227)
Corresponding Authors:
ZHAO Jiuzhou, professor, Tel: (024)23971918, E-mail: jzzhao@imr.ac.cn
Fig.1 Solute M (La or Si) concentration profile around an α-Al grain ( is the initial concentration of solute M; kM and are the equilibrium partition coefficient and eutectic composition in Al-M binary system, respectively; is the maximum solubility of M in the α-Al phase; and are the concentration of solute M in the melt and in the α-Al at α-Al/melt interface, respectively; (T1) and (T2) are the solute M concentration in the melt ahead of α-Al/melt interface at temperatures T1 and T2, respectively; m is stoichiometric ratio)
Element
Tm / K
V 2/3 / (cm2·mol-2/3)
ϕ / V
(r / p) / V
u
Al
933
4.60
1.39
4.20
1.9
0.07
Si
1685
4.20
1.50
4.70
2.1
0.04
La
1193
7.98
1.18
3.17
0.7
0.07
Table 1 Parameters used in the calculations[20,21]
Fig.2 Calculated results for the interaction parameters of La-i melt (ΩLa-i ) as a function of La concentration in the i-M system at α-Al/melt interface() (ΩLa-Al, ΩLa-Si, and ΩAl-Si are the interaction parameters of La-Al melt, La-Si melt, and Al-Si melt at 850 K, respectively. Inset shows the enlarged view of ΩLa-i in the i rich corner)
m
/ (kJ·mol-1)
/ (J·m-2)
1
-66.1
0
2
-40.3
0.021
Table 2 Heat for the formation of Al m Si m La () at 0 K and interfacial energy between the melt and Al m Si m La compound () at 850 K
Fig.3 Dependences of the La concentration in eutectic Si () on the La initial concentration () in Al-6Si alloy melt
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