Corresponding authors:ZHAO Jiuzhou, professor, Tel:(024)23971918, E-mail:jzzhao@imr.ac.cn
Received:2021-11-12Revised:2022-01-14
Fund supported:
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)
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.
Keywords：Al-Si hypoeutectic alloy;
modification;
interaction parameter;
trace element La;
solubility
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)
对于α-Al的稳态生长阶段来说，当La的初始浓度小于某一临界值xLaC时，La在α-Al(S)/Al(L)界面处熔体中富集的浓度为，熔体中无Al m Si m La (m为化学计量比)化合物形成；当大于xLaC时，La在α-Al(S)/Al(L)界面处熔体中的浓度大于，导致在α-Al(S)/Al(L)界面处形成Al m Si m La化合物(如图1所示)，消耗La浓度。前期研究[12,14]表明，临界值xLaC约等于La在α-Al中的最大固溶度， (为共晶成分)。
热力学上，Al m Si m La化合物的沉淀析出行为取决于熔体中La和原子i (Al或Si)间的键能εLa-i，εLa-i 数值越大，Al m Si m La越容易析出。εLa-i 可由熔体中La和i原子间的相互作用参数ΩLa-i 确定[18,19]：
键能体现的另一种形式是元素间化合物的形成热。键能越高，元素间化合物形成热越负，越容易析出。本工作采用密度泛函理论对Al m Si m La形成热进行计算，具体参数如下：Al、Si和La原子的价电子构型分别设置为Al(3s23p1)、Si(3s23p2)和La(5s25p65d16s2)；平面截断能设定为294 eV；Brillouin区k点取样间距低于2π × 0.3 nm-1；每个原子上的作用力< 0.1 eV/nm作为收敛判据[22]。
Al m Si m La化合物析出与否还与熔体和Al m Si m La间的界面能密切相关。越小，Al m Si m La析出所需的过冷度越低。可用下式计算[23]：
式中，ΩAl-Si和ΩAl-La分别为Al-Si和Al-La熔体中Al和Si、Al和La原子间的相互作用参数；fb = 0.74是体堆垛系数；为熔体和Al m Si m La界面的摩尔面积，ωAl和ωSi分别为元素Al和Si的摩尔面积，VAl和VSi分别为元素Al和Si的摩尔体积；ΔmHAl = 10711 J/mol、ΔmHSi = 50208 J/mol和ΔmHLa = 6196.5 J/mol分别为Al、Si和La的熔化焓[24]。
1.2 共晶Si中La浓度
在亚共晶Al-Si合金的凝固组织中，La以3种形式存在：固溶在α-Al中、可能形成Al m Si m La三元化合物及在共晶Si中。其中只有后者对共晶Si的变质有作用。根据如下溶质守恒公式可求得共晶Si中La的浓度：
式中，Xα-Al和XTe分别为α-Al和Al m Si m La化合物的摩尔分数，为共晶Si浓度，和分别为La在Al m -Si m La化合物和共晶Si中的浓度，为Si在Al m Si m La化合物中的浓度，和分别为La和Si在α-Al中的平均浓度，对于α-Al的稳态生长阶段来说，。
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)
The rare earth influence on the as-cast microstructures and mechanical properties of aluminum alloys attracted great attentions in the last decades. But up to date no reports can be found on the effect of micro-alloying element La (La addition is below 0.1 wt.%) on the solidification of hypoeutectic Al-Si alloys. This study carried out solidification experiments with Al-6Si alloys micro-alloyed by element La. The α-Al grain refinement, the eutectic Si modification and the tensile properties improvement caused by micro-alloying element La were investigated. The effect mechanisms of La were discussed. It is demonstrated that the addition of La as low as 100 ppm can deprave the more effective heterogeneous nucleation conditions for the eutectic Si caused by the impurity P. The addition of 0.06 wt.% La is sufficient to achieve an ideal α-Al grain refinement, eutectic Si modification and ductility improvement of the alloys. LaAlSi phase forms in the Al-Si alloy with the additive amount of La higher than 0.06 wt.%. It has a tetragonal structure. Micro-alloying element La refines the α-Al grains by working as a surfactant and modifies the eutectic Si by promoting the formation of the significant multiple Si twins.
ZhengQ J, YeZ F, JiangH X, et al.
Effect of micro-alloying element La on solidification microstructure and mechanical properties of hypoeutectic Al-Si alloys
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.
Effect of La addition on microstructure evolution of hypoeutectic Al-6Si alloys
4
2020
... 对于α-Al的稳态生长阶段来说，当La的初始浓度小于某一临界值xLaC时，La在α-Al(S)/Al(L)界面处熔体中富集的浓度为，熔体中无Al m Si m La (m为化学计量比)化合物形成；当大于xLaC时，La在α-Al(S)/Al(L)界面处熔体中的浓度大于，导致在α-Al(S)/Al(L)界面处形成Al m Si m La化合物(如图1所示)，消耗La浓度.前期研究[12,14]表明，临界值xLaC约等于La在α-Al中的最大固溶度， (为共晶成分). ...
The influence of rare earth element lanthanum on the microstructures and properties of as-cast 8176 (Al-0.5Fe) aluminum alloy
0
2021
Effect mechanisms of micro-alloying element La on microstructure and mechanical properties of hypoeutectic Al-Si alloys
4
2020
... 对于α-Al的稳态生长阶段来说，当La的初始浓度小于某一临界值xLaC时，La在α-Al(S)/Al(L)界面处熔体中富集的浓度为，熔体中无Al m Si m La (m为化学计量比)化合物形成；当大于xLaC时，La在α-Al(S)/Al(L)界面处熔体中的浓度大于，导致在α-Al(S)/Al(L)界面处形成Al m Si m La化合物(如图1所示)，消耗La浓度.前期研究[12,14]表明，临界值xLaC约等于La在α-Al中的最大固溶度， (为共晶成分). ...
... Parameters used in the calculations[20,21] ...
Mechanical properties and chemical bonding of the Os-B system: A first-principles study
1
2012
... 键能体现的另一种形式是元素间化合物的形成热.键能越高，元素间化合物形成热越负，越容易析出.本工作采用密度泛函理论对Al m Si m La形成热进行计算，具体参数如下：Al、Si和La原子的价电子构型分别设置为Al(3s23p1)、Si(3s23p2)和La(5s25p65d16s2)；平面截断能设定为294 eV；Brillouin区k点取样间距低于2π × 0.3 nm-1；每个原子上的作用力< 0.1 eV/nm作为收敛判据[22]. ...
Modelling interfacial energies in metallic systems
1
2005
... Al m Si m La化合物析出与否还与熔体和Al m Si m La间的界面能密切相关.越小，Al m Si m La析出所需的过冷度越低.可用下式计算[23]： ...
SGTE data for pure elements
1
1991
... 式中，ΩAl-Si和ΩAl-La分别为Al-Si和Al-La熔体中Al和Si、Al和La原子间的相互作用参数；fb = 0.74是体堆垛系数；为熔体和Al m Si m La界面的摩尔面积，ωAl和ωSi分别为元素Al和Si的摩尔面积，VAl和VSi分别为元素Al和Si的摩尔体积；ΔmHAl = 10711 J/mol、ΔmHSi = 50208 J/mol和ΔmHLa = 6196.5 J/mol分别为Al、Si和La的熔化焓[24]. ...
The sequence of intermetallics formation during the solidification of an Al-Mg-Si alloy containing La
