You can try for modified Poulton’s reagent to etch this alloy. Use freshly prepared reagent for better results. The composition of modified Poulton’s reagent is as follows:-

50ml Poulton reagent + 25ml HNO3 +1ml HF+1ml H2O.

(Poulton reagent : 12mlHCl concentrated+6mlHNO3+1ml HF(48%)+1mlH2O).

time 5 to 10 s

I think you should try another chemical solution instead of Kellar's reagent. However, it will be more or less similar to the chemical constituents of Kellar's.

The following chemicals can be used to make the solution;

HNO3 - 25 ml

Methanol - 25 ml

HCl - 25 ml

HF - 1-2 ml

Hope it will work for you.

And, there is no need to use separate etchants for SEM analysis. The same etchant will work satisfactorily.

Try Keller's reagent (82 ml water, 4 ml HF, 6ml HCL, 8 ml HNO3).

Hi

The best way is electro-etching with Barker reagent

Good luck

MP

Dear doctor

"THE MAIN CHARACTERISTICS OF ALUMINUM AND ITS ALLOYS

Aluminum is a multifaceted material with multiple uses, including as a matrix metal for composites. It has a silvery white appearance and is used as either a pure metal or as an alloy. It is extremely light and just small amounts of alloying elements can increase its strength. It is also highly resistant to corrosion. This is due to a passive film of aluminum oxide that is intimately connected to the surface and capable of renewing itself spontaneously when the surface is damaged. Aluminum’s other significant properties include its high heat conductivity and easy formability – either by casting, hot or cold working or machining – as well as its neutral taste and non-toxicity. Common uses of aluminum or its alloys:

• High-strength/low-weight applications in the aircraft, aerospace and automobile industries
• Polished and brushed surfaces, as well as anodized colors, in the building industry
• Non-toxic/taste-free packaging and machinery in the food industry.

THE PRODUCTION OF ALUMINUM

Economical extraction of aluminum is only possible from bauxite. The production process involves two basic steps. Extraction of pure alumina Alumina recovery begins by crushing and finely grinding the bauxite and heating it with sodium hydroxide under pressure. In this process, a water-soluble sodium aluminate is formed together with undissolved residues of iron, titanium and silicon. ‘Seed crystals’ of fresh aluminum hydroxide are added to initiate the precipitation of pure aluminum hydroxide (Al(OH)3). Through calcination at 1200 °C, the water is then removed and pure anhydrous alumina (aluminum oxide) remains. Converting alumina to aluminum (the Hall-Heroult process) The reaction chemistry of pure alumina requires an electro-chemical process to extract aluminum from its oxide. As the melting point of aluminum oxide is very high (2050 °C), it is mixed with cryolite to reduce the melting point. Electrolysis takes place in a large carbon or graphite lined steel container that contains steel rods for conducting electricity and carbon blocks as anodes. During electrolysis, the carbon of the anode reacts with the oxygen of the alumina and, in a secondary reaction, metallic aluminum is produced with the formation of carbon dioxide: 2Al2O3 + 3C → 4Al + 3CO2. This process produces aluminum of 99-99.9 % purity. Much of this is used for aluminum alloys.

ALUMINUM ALLOYS

Adding very small amounts of alloying elements to aluminum can increase tensile strength, yield strength and hardness compared to pure aluminum. The most important alloying elements are Si, Mg, Cu, Zn and Mn. These mostly eutectic compounds must be finely dispersed through a hot working process before the alloy can be cold worked. Ageing of aluminum alloys Many aluminum alloys are age hardened to improve the mechanical properties. This can be done either naturally or artificially.

• Natural age hardening (example AlCuMg). After solution annealing, the workpiece is quenched and consequently the precipitation of the Al2Cu in the solid solution is sup- pressed. The workpiece is then left to age in ambient temperature. During this process the aluminum lattice precipitates the copper from the supersaturated solution. The resultant strain produced in the aluminum lattice leads to an increase in strength and hardness. The process takes 5-8 days.
• In artificial age hardening, ageing takes place at an elevated temperature, which reduces process time. With an AlMgSi alloy, for example, ageing occurs in 4-48 hours at 120-175 °C after solution annealing and quenching. The precipitation of the Mg2Si phase produces internal strain in the aluminum lattice, which results in an increase in strength and hardness.

PREPARATION OF ALUMINUM AND ITS ALLOYS: MECHANICAL GRINDING & DIAMOND POLISHING

When working with aluminum and its alloys, we recommend mechanical grinding, followed by diamond polishing. For many pure aluminum and wrought alloy specimens, electrolytical polishing is also recommended.

Mechanical grinding

Plane grinding should be carried out with the finest possible grit to avoid excessive mechanical deformation.

• The hardness, size and number of specimens should be considered. However, even with large specimens of pure aluminum, plane grinding with 500# SiC Foil or Paper is usually sufficient.
• Large cast parts of aluminum alloys can be ground with 220# SiC or 320# SiC Foil. It is important that the grinding force is low to avoid deep deformation and to reduce friction between the grinding SiC Foil or Paper and specimen’s surface.

Diamond polishing

Diamond polishing should be carried out until all deep scratches from grinding have been removed. If water soluble constituents must be identified, we recommend polishing with water-free diamond suspension and lubricant.

Final polish for pure aluminum and aluminum alloys: The polish/check sequence

• Begin polishing. After 1 minute of polishing with OP-U suspension, check the specimen under the microscope.
• If necessary, continue polishing for another minute and check the specimen again.
• Continue this polish/check sequence until the required quality has been achieved.
• If diamond particles have been pressed into the surface during polishing, they can lead to erroneous interpretations of the structure. Therefore, the polish/check sequence may need to be relatively long. Continue the sequence until you can no longer see bright and dull areas on the surface of the specimen with the naked eye.
• Approximately 30 seconds before the end of polishing, pour water onto the polishing cloth to rinse the specimen and cloth.
• Finally, wash the specimen again with clean water and then dry it.

ETCHING OF ALUMINUM AND ITS ALLOYS

When working with aluminum and its alloys, macro etchants are used for grain size evaluation, also to show flow lines from extrusion and to reveal weld seams. Before etching, the specimen has to be ground with 1200# SiC Foil or 2400# SiC Foil. Due to the many alloying possibilities of aluminum, the different phases cannot always be clearly identified in some of the multi-component alloys. However, the eutectic phases can sometimes be recognized by the typical shape of their eutectic. Some of the well-known phases have the following characteristic colors:

• Si: Grey
• Mg2Si: Tarnished dark blue during polishing (in cast: Chinese script)
• Al2Cu: Pinkish-brown, copper colored
• Al6Mn: Light grey

Etching solutions

When working with chemicals the standard safety precautions must be observed.

Aluminum cast alloys are polished relatively easily. For grain size evaluation, anodizing with Barker’s reagent will result in a better contrast than chemical etching. Different phases in cast alloys can either be identified by their characteristic color or by etching with specific solutions that attack certain phases preferentially."