Characterization of the α-Al(FeMn)Si nuclei on β-AlFeSi intermetallics by laser scanning confocal microscopy

Abstract

As-cast billets of aluminum extrusion alloys of the 6xxx Al alloys family require a homogenization treatment to make the material suitable for hot extrusion [1]. During this homogenization treatment several processes take place such as the transformation of interconnected plate-like monoclinic β-Al5FeSi intermetallics into more rounded discrete cubic α-Al12(FeMn)3Si particles [2–4], the dissolution of Mg2Si [5–8] and Si precipitates [8] and the precipitation of elements still in supersaturated state [9, 10]. Of these processes the phase transformation of the intermetallics and the accompanying shape change is deemed to be the most important for obtaining a good extrudability and in particular a good surface quality in the extruded product. However, little microstructural and topological information is available concerning the details of this transformation process and in the absence of such information realistic modelling of the kinetics of this transformation on a physical basis is not possible [11]. To enable the construction of such physical models quantitative information concerning the position of the α particles on the β plates, their 3D shape during early and later stages of growth, and their site density is required. In principle such information can be obtained by standard optical microscopy in combination with stepwise (step size <0.2 μm) sectioning, scanning electron microscopy (SEM) on deep etched samples, or transmission electron microscopy (TEM). While the SEM offers the advantage of an extremely high depth of field which enables deep etched samples to be readily imaged, this technique does not allow quantitative data regarding 3D topography to be obtained. This paper describes the very first results of a 3D characterization of such α-Al(FeMn)Si particles on βAlFeSi plates in a partially homogenized 6005A Al alloy using laser scanning confocal microscopy (LSCM) in combination with a deep etching technique to isolate the intermetallics from the matrix. The advantage of LSCM over the methods mentioned earlier is that a sufficient lateral and vertical resolution (typically 190 nm lateral and 300 nm in the z direction) is possible and this yields the required accuracy of topological detail. This topology can be described quantitatively in 3 dimensions using commercial software. The material used in this work is a commercial DC cast 6005A series Al alloy the composition of which is given in Table I. The as-cast material contained interconected plate like β-AlFeSi intermetallics, with a typical length of a few tens of microns and a mean thickness of a few tenths of a micron. Using conventional optical microscopy on as-cast polished samples, no signs of α-Al(FeMn)Si particles on the β-AlFeSi plates were observed. The as-cast material was homogenized at 540 ◦C for 8 h followed by rapid quenching. From an extensive analysis of the kinetics of the β-AlFeSi to α-Al(FeMn)Si transformation [12] it is known that this annealing treatment transforms approximately half the volume of monoclinic β-Al5FeSi into cubic α-Al12(FeMn)3Si. The advantage of using a sample with such a degree of transformation is that the initial network of β-AlFeSi plates is more or less intact and the original position of the α particles with respect to the parent phase can be determined. The annealed samples were subsequently etched in an electrolytic solution of 78 ml percloric acid, 300 ml H2O, 730 ml ethanol, and 100 ml butylglycol at 10 volts for one minute. This etching procedure resulted in the removal of a layer of matrix material to a depth of approximately 10 μm below the original surface. No signs of attack of the surface of the β-AlFeSi intermetallics were observed. The deep etched samples