铝钇共晶反应及其产物对亚共晶铝硅合金初生相的细化研究

Al-Y Eutectic Reaction and Its Products on Refining Primary α Phase in Hypoeutectic Al-Si Alloy

研究了稀土Y在亚共晶铝硅合金中诱发稀土-铝共晶反应对铝合金初生α相的细化效应;应用Bramfitt提出的方法,计算了Al-Y共晶反应产物Al3Y与α-Al界面的二维点阵错配度,结果显示两者的二维错配度<6%,即Al3Y可作为α-Al的异质形核质点,且能达到中等有效形核而起到细化晶粒的作用,并通过实验验证计算结果如下:选用稀土元素Y作为A356合金细化剂的同时,将A356-Y熔体分别在稀土钇-铝共晶温度上、下10℃左右保温2 min后快速冷却,获得试样的金相组织照片后利用图像分析工具得到A356-Y合金初生α相的平均晶粒尺寸和平均形状因子。实验结果表明:A356-Y合金在Al-Y共晶温度之下保温,可获得较理想的初生α相形貌和较小的晶粒尺寸;结合二维错配的计算结果,可推断初生α相细化的主要原因为异质形核质点的增加:Al-Y共晶反应产物Al3Y与α-Al的二维点阵错配度在中等有效形核范围内,具有细化合金初生α相的作用;另一共晶产物α-Al与初生α相具有相同的晶体结构和点阵常数,则其也可作为异质形核质点而起到细化合金的作用。稀土Y可作为半固态A356合金中初生α相的优质细化剂。

The refining effect of RE-A1 eutectic reaction on primary α phase induced by yttrium in hypoeutectic aluminum-silicon al- loy was studied. The planar disregistry between A1-Y eutectic reaction product A13Y and nucleated phasesα-Ai during liquid A356 al- loy solidification was calculated according to the method put by Bramfitt. The results showed that the planar disregistry between the two phases was less than 6%, which implied that AI3Y could be used as the site of the heterogeneous nucleation for α-A1 to play the role of grain refinement owing to its moderate effectiveness in nucleation. Some experiments were designed to check the calculation result. Yt- trium was selected as the refiner for A356 alloy, meanwhile, the A356-Y melt was isothermally held above the eutectic temperature 10 ℃ and below the eutectic temperature 10 ℃ for 2 min, respectively. The average grain size and the shape factor of the primary α phase in A356-Y alloy were calculated by the image analysis tools after ihe microstructures of specimens were obtained. The results indicated that the desirable morphology and smaller grain size of primary α phase could be obtained when the A356-Y alloy was isothermally held below the A1-Y eutectic temperature. The reason that the primary α phase refined was resulted from the increase of the site of heteroge- neous nucleation in A356-Y alloy combined with calculation on the planar disregistry between A13Y and α-A1. A13Y, which was a kind of the product in A1-Y eutectic reaction, could be used to refine the primary α phase in A356-Y alloy because their planar disregistry was within the range of effective nucleation. And or-A1, another product of A1-Y eutectic reaction, also could be the site of heterogene- ous nucleation to refine the alloy due to its crystal structure and lattice constants were the same as the primaryα phase in A356 alloy. Further, this result could be used to refine the primary α phase of semisolid A356 alloy.