Interconnection between microstructure and microhardness of directionally solidified binary Al-6wt.%Cu and multicomponent Al-6wt.%Cu-8wt.%Si alloys

An experimental study has been carried out to evaluate the microstructural and microhardness evolution on the directionally solidified binary Al-Cu and multicomponent Al-Cu-Si alloys and the influence of Si alloying. For this purpose specimens of Al-6wt.%Cu and Al-6wt.%Cu-8wt.%Si alloys were prepared and directionally solidified under transient conditions of heat extraction. A water-cooled horizontal directional solidification device was applied. A comprehensive characterization is performed including experimental dendrite tip growth rates (VL) and cooling rates (TR) by measuring Vickers microhardness (HV), optical microscopy and scanning electron microscopy with microanalysis performed by energy dispersive spectrometry (SEM-EDS). The results show, for both studied alloys, the increasing of TR and VL reduced the primary dendrite arm spacing (l1) increasing the microhardness. Furthermore, the incorporation of Si in Al-6wt.%Cu alloy to form the Al-6wt.%Cu-8wt.%Si alloy influenced significantly the microstructure and consequently the microhardness but did not affect the primary dendritic growth law. An analysis on the formation of the columnar to equiaxed transition (CET) is also performed and the results show that the occurrence of CET is not sharp, i.e., the CET in both cases occurs in a zone rather than in a parallel plane to the chill wall, where both columnar and equiaxed grains are be able to exist.

本研究开展了一项实验，以评估定向凝固二元铝铜（Al-Cu）及多组分铝铜硅（Al-Cu-Si）合金的微观组织与显微硬度演变规律，以及硅合金化的影响。为此，本研究制备了Al-6wt.%铜（Al-6wt.%Cu）与Al-6wt.%Cu-8wt.%硅（Al-6wt.%Cu-8wt.%Si）合金试样，并在瞬态热提取条件下进行定向凝固。实验采用水冷式水平定向凝固装置。本研究开展了全面的表征工作：通过测试维氏显微硬度（HV）、光学显微分析及配备能量色散光谱（EDS）的扫描电子显微镜（SEM-EDS）微观组分分析，获取实验测得的枝晶尖端生长速率（VL）与冷却速率（TR）。结果表明，对于两种研究合金而言，冷却速率（TR）与枝晶尖端生长速率（VL）的提升均会减小一次枝晶臂间距（l1），并提升合金的显微硬度。此外，在Al-6wt.%Cu合金中引入硅以制备Al-6wt.%Cu-8wt.%Si合金，可显著改变合金的微观组织与显微硬度，但不会影响一次枝晶生长规律。本研究同时开展了柱状晶-等轴晶转变（CET）形成机制的分析，结果显示CET的发生并非突变式：两种合金的CET均发生于一个区域内，而非平行于激冷壁的单一平面，该区域内可同时存在柱状晶与等轴晶。