Effect of Micro-Alloying Element La on Solidification Microstructure and Mechanical Properties of Hypoeutectic Al-Si Alloys
ZHENG Qiuju1,2, YE Zhongfei3, JIANG Hongxiang1, LU Ming3, ZHANG Lili1, ZHAO Jiuzhou1,2()
1.Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 3.Electric Power Research Institute of State Grid Henan Electric Power Company, Zhengzhou 450052, China
Cite this article:
ZHENG Qiuju, YE Zhongfei, JIANG Hongxiang, LU Ming, ZHANG Lili, ZHAO Jiuzhou. Effect of Micro-Alloying Element La on Solidification Microstructure and Mechanical Properties of Hypoeutectic Al-Si Alloys. Acta Metall Sin, 2021, 57(1): 103-110.
Hypoeutectic Al-Si alloys are extensively used in the welding industry owing to their excellent cast-ability, low coefficient of thermal expansion, and good weldability. Unfortunately, Al-Si alloys solidify under conventional cooling conditions, forming coarse dendritic α-Al grains with an eutectic structure and a flake-like morphology that has poor mechanical properties. Chemical inoculations are often used to control the size of α-Al grains and the morphology of the eutectic Si particles. The grain refiner Al-Ti-B master alloy and eutectic Si modifier Sr are commonly used in industry. In recent years, great attention has been paid to controlling the microstructure and mechanical properties of hypoeutectic Al-Si alloys through the use of the cost-effective rare earth element La. Previous studies have mainly focused on the effects of La addition on the microstructural evolution and improvements of the mechanical properties. However, to date there have been no studies on the effects of combined addition of La, Al-Ti-B master alloy and Sr on the microstructure and mechanical properties of Al-Si alloys. In this work, solidification experiments were performed to investigate the effects of the micro-alloying element La, Al-Ti-B master alloy, and Sr on the solidification microstructure and mechanical properties of hypoeutectic Al-Si alloys. These results show that synergistic effects are achieved by combinations of La, Al-Ti-B master alloy, and Sr. An addition of 0.06%La was sufficient for effective α-Al grain refinement, eutectic Si particle modification, and improved the ductility of the alloys. Excess La addition formed a coarse LaAlSi intermetallic compound, which deteriorated the ductility of the alloy. The micro-alloying element La refined the α-Al grains by acting as a surfactant that decreased the wetting angle between the TiB2 nucleation substrate and the α-Al nucleus. It modified the eutectic Si particles by promoting the formation of the multiple Si twins and changing the growth behaviors of the Si particles.
Fund: National Natural Science Foundation of China(51771210);Science and Technology Project of the Headquarters of Sate Grid Company of China(5500-201924129A-0-0-00)
Fig.1 OM images of Al-6Si alloys with La additions of 0 (a), 0.02% (b), 0.06% (c), and 0.10% (d)
Fig.2 Average grain size (<Dα-Al>) of Al-6Si alloys with different additive amounts of La
Fig.3 Low (a, c) and high (b, d) magnified SEM images of Al-6Si alloys without La (a, b) and with addition of 0.06%La (c, d)
Fig.4 TEM bright-field images (a, b) of eutectic Si particle and their SAED patterns (c, d) in the zone axis of [011]Si in the Al-6Si alloys without La (a, c) and with addition of 0.06%La (b, d)
Fig.5 Backscattered electron image (BEI) and EPMA mappings of the eutectic Si and La-rich phase particles in the Al-6Si alloy with 0.10%La addition
Fig.6 TEM bright-field image and EDS result (a), and SAED pattern in the zone axis of [100] (b) for the LaAlSi phase in the Al-6Si alloy with 0.10%La addition
Fig.7 Differential thermal analysis (DTA) cooling curves for the Al-6Si alloys with different additive amounts of La (a), and the change of the nucleation undercooling for the α-Al () and the eutectic Si () with different additive amounts of La (b)
Fig.8 Tensile properties of Al-6Si alloys with different additive amounts of La (YS—yield strength, UTS—ultimate tensile strength, El—elongation)
Element
Al
Si
La
Sr
Ti
B
Al
-
-
-
-
-
-
Si
-19
-
-
-
-
-
La
-38
-73
-
-
-
-
Sr
-18
-49
14
-
-
-
Ti
-30
-66
20
53
-
-
B
0
-14
-47
-18
-58
-
Table 1 The enthalpy of mixing between various elements[23]
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