Anodic etching technique for observation of microstructural change during the solidification and precipitation process in Al-Cu alloy

Abstract

In order to study the microstructure of metals and alloys in as-cast, work-hardened and recrystallized states with all of its manifestations, conventional polishing techniques were found to be inadequate. These techniques revealed the grains formed by primary dendrites and often failed to reveal the inside of a grain and the distribution of intermetallic phases in the complete section of a casting. The technique of anodic oxidation etching has been developed by the principal author for A1 and some binary A1 alloys [i, 2]. It has been used as a tool to study the process of solidification. The techique developed earlier by Lecombe and Mouflard [3] did not reveal complete details in the cast morphology, but was successful in revealing the distribution of secondary phases with polarized light and colour microscopy. The technique deals with the formation of an anhydrous film of alumina on A1 alloys b'y :electrolytic oxidation, which, when viewed under ordinary light, reveals the underlying microstructure of the sample. (For a single-phase homogeneous alloy the thickness of the film is a function of only the orientation of the underlying grains, other experimental factors remaining constant. If the alloy is not perfectly homogenized, the differences in composition are mapped out in the form of characteristic undulations.) The technique gives qualitative results in regions containing minute amounts of segregation and serves as an excellent guide for microprobe work. For a multiple-phase alloy the intermetallic compounds are generally oxidized at a different rate from the solid-solution matrix, thereby disturbing the continuity of the oxide film. A12Cu reportedly [4, 5] oxidizes faster than the matrix. It was therefore attempted to explore the possibility of studying the process of diffusion of Cu and A1 quantitatively by optical microscopy using this technique. A1 of 99.7 wt % purity was melted in a resistance furnace and electrolytic Cu was added to the required level under a Foseco flux (coverall-11). The melt was poured at a temperature of 800 °C into a preheated Cu mould 3 cm in diameter held together by a C-clamp. In order to have uniform cell size across the entire section of the casting and to observe the effect of supercooling, isothermal solidification was attempted by remelting a part of the chill-cast alloy, pouring it directly into molten-salt baths maintained at different temperatures of 500, 420 and 340 °C, respectively. The composition of the alloy as determined by chemical analysis was 6.6 wt % Cu. Six samples 1 cm thick were cut from chill-cast alloy and five such samples were homogenized at 500 °C for 10 h. Four samples were subsequently aged at 250°C for 2, 6, 12 and 24h, respectively. All six samples were ground on an L-belt, polished on SiC papers using kerosene oil, ending with 600-paper. Polishing on a wheel could be dispensed with, in the case of the anodic etching technique. The top oxidized layers were ground off for all samples. The buttons cast isothermally were ground to convenient sizes, polished on papers and anodically etched. The anodic etching conditions for A1 and AI-Cu alloys were standardized (Table I [2]). Using these electrolysis conditions, the polished samples were held by a Pt wire and a Pt plate was used as a cathode. The voltage drop during electrolysis was compensated by an increase in the applied current. The samples after anodizing were washed under running water to remove oxidation products (if any), and were then washed with alcohol and dried with hot air. The microstructures of all specimens were viewed in a Leitz microscope under monochromatic light. Point counting was performed to measure the dendritic cell size in isothermally cast alloys and the dark regions in homogenized and heat-treated samples by projecting the image on the ground glass screen of the microscope. The cell sizes and the intermetallic network sizes were measured as functions of the degree of supercooling by the line-intercept or Heyn procedure [7]. The intermetallic phases were extracted in a 10% HC1 acid plus methanol solution by electrolysis for 2 h or more using the sample as anode. The solution was found to remove A1 alloy matrix selectively by anodic dissolution, leaving intermetallic phases as