The Aluminum-Silicon Phase Diagram and Eutectic Modifications
Preparation of Composites with Al Matrix
- It is probably best to cut the specimens with a precision saw, or lowspeed diamond saw. I would try a 15 LC diamond blade, but you will need to dress it occasionally during cutting as the Al matrix is “gummy” and will load up on the blade.
- Use plenty of water coolant when grinding with the resin-bonded DGD Color disks
- Charge the cloths with diamond in paste form; press the diamond into the cloth surface with your finger tips and spread it around the surface; then add MetaDi Fluid lubricant. During the polishing cycle, you can squirt on diamond of the same size in suspension form to keep the surface covered. Do not let the cloth surface get dry.
- With 1-um diamond, use it only in paste form to avoid embedding. One of my colleagues is polishing 1100 Al containing B4C and diamond embedding is a serious problem, even with sizes >1 m, which is not a common problem.
Picric acid (2,4,6-trinitrophenol, [(NO2)3C6H2OH]) is widely used in metallography labs for the common steel etchants known as picral, a 4% solution in ethanol, Vilella’s reagent, 1 g picric acid and 5 mL HCl and 100 mL ethanol, and alkaline sodium picrate (2 g picric acid, 20 g NaOH, 100 mL water) for coloring M3C and M6C carbides, as well as several other formulations. Picric acid was formulated by Peter Woulfe, a British chemist, in 1771, although Glauber is claimed to have written about it in 1742. The name comes from the Greek word pikros which means bitter, as picric acid has a bitter taste (it is toxic). Initially it was used to dye fabrics yellow. In the early 20th century, workers producing picric acid were sometimes called canaries, because their skin also became stained yellow
High-purity Pb and Sn are very difficult metallographic subjects and the alloys of Pb and Sn are somewhat easier to prepare, but still rather difficult. This Poster lists our preferred preparation procedure. Vibratory polishing is essential for best results. Pollack’s reagent (100 mLwater, 10 g citric acid, 10 g ammonium molybdate) is one of the best etchants for Pband Pb-Snalloys; 2% nitalis a good etch for pure tin..
A three-step preparation procedure was developed for titanium and its alloys. Attack polishing is utilized in the third step for optimal results, particularly for imaging alpha-Ti with polarized light. Two-phase α-β alloy specimens and all β alloys are easier to prepare than single-phase α specimens. Kroll’s reagent appears to be adequate for most alloys. A modification of Weck’s reagent was used for color metallography.
Sand Cast Alloys
Microstructure of sand-cast AZ91D (Mg – 9% Al – 0.25% Mn – 0.7% Zn – 0.0008% Be) revealed (left) using the glycol etch and viewing with polarized light plus a sensitive tint filter; and (right) after etching with the acetic-glycol reagent and viewing in bright field illumination. The magnification bars are 100 and 50 μm, left and right, respectively. Note the mechanical twins at the surface.
Native Copper – Microstructure of native copper (112 HV). Note the extensive slip lines. Color micrograph was taken with crossed polarized light plus sensitive tint.
High-Purity Copper (99.89%) – Microstructure of hot extruded, cold worked and annealed (500 °C) high-purity copper etched with equal parts ammonium hydroxide and hydrogen peroxide (3% conc), 46 HV.
High-Purity Copper (99.89%) – Microstructure of hot extruded, cold worked and annealed (500 °C) high-purity copper etched with klemms III and Beraha’s PbS tint etches and viewed with polarized light plus sensitive tint, 46 HV.
GENERAL SUGGESTIONS ON PREPARATION METHODS
- Use the gentlest possible cutting method to minimize cutting damage and yield a smooth surface
- SiC paper may be used for first grinding step
- Start with the finest possible SiC grit size, P280 or P400, to minimize grinding damage
- Diamond in slurry or suspension form in small sizes (< 6 μm) may become embedded in the surface of soft metals
- Use diamond in paste form for sizes < 6 μm for soft metals To reduce relief, use napless cloths