Twin structure in laser-melted Al-Si alloy containing rare earths

Abstract

Aluminium and its alloys have been known as kinds of materials with high stacking-fault energy. It is therefore difficult to find stacking faults or twins in them. It was shown by our early studies that apart from a pearlite-like A1-Si eutectoid structure [1], a large amount of stacking faults [2] was found in laser-melted A1-Si alloy containing rare earths (RE). Following these results, we performed experiments with both the same material as in [1, 2] and laser-melted material coated with alloy powder containing RE. A twin structure was observed by transmission electron microscopy (TEM). The purpose of this letter is to display the twin morphology and to discuss possible mechanisms for twin formation. The materials used for laser melting were divided into two groups. The composition of the first group (FG) was (wt %): 12 Si, 1.23 Cu, 0.5 Mn, 0.93 Mg, 1.0 Fe and 0.1 RE. The RE contained 10 wt% Ce, the remainder being A1. The second group (SG) were prepared from laser alloying. Alloy powder was predeposited on the surface of the material, which was similar to the first material but without RE. The powder contained (wt %): 55 A1, 25 Si, 10 Cr and 10 RE (Ce-rich). Laser melting or alloying was carried out with an HG1-CO2 laser. The output power was 2.8 kW and the beam diameter 5 mm with a scanning speed of 54 mms -1 . Slices for TEM were cut from laser-melted alloyed zones with a thickness of 0.4 mm parallel to the melted surface. After thinning by a series of SiC papers, the thin foils were then thinned by ion-miiling or electrolyte (5% perchloric acid and 75% ethanol). A model H-800 TEM was employed to study microstructure, operated at 200 kV. The laser melted/alloyed region was 0.8 mm deep and 4.0ram wide, and the secondary dendritic distance of the melted region ranged from approximately 2 to 3.5 ram. Growth of the eutectic and 0~-A1 was based on the interface between liquid and solid (matrix). The growth direction of ol-A1 was in agreement with [0 0 1] orientation. Fig. 1 shows the o~-A1 and AI-Si eutectic for the laser-melted region (FG). The typical twin morphology in o~-A1 is shown in Fig. 2a and b, using brightand dark-field techniques. The electron diffraction pattern and its