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.

The paper describes the approach taken in the author’s first metallographical study of plasma sprayed coating. After consideration of the preparation problems known to be associated with porous materials, the coating materials and the substrate material, and coatings in general, literature on metallographic studies of plasma sprayed coatings was identified, obtained and reviewed.

A coating section of each type was carefully removed from the substrate, refrigerated in liquid nitrogen, broken and examined with the SEM to reveal the nature and degree of porosity and bonding defects. Specimens were prepared using two different approaches and two variations of one of the approaches. Polishing was conducted using DP-Plan cloths and two different diamond sizes followed by two stages of diamond polishing with conventional cloths and finally using colloidal silica on a vibratory polisher. Vibratory polishing time must be critically controlled for two of the three coatings. Conventional etchants were tried to enhance microstructural detail but the Pepperhoff interference layer method was found to be more useful.

The microstructure of iron-base alloys is very complicated and diverse, being influenced by chemical composition, material homogeneity, processing and section size. This article offers a brief explanation of the terminology describing the constituents in ferrous alloys, and offers a basic review of steel microstructures.

Microstructures of castings look different from those of wrought products, even if they have the same chemical composition and are given the same heat treatment. In general, it is easiest to identify heat-treated structures after transformation and before tempering. For example, if a mixed microstructure of bainite and martensite is formed during quenching, these constituents will become more difficult to identify reliably as the tempering temperature used for the product increases toward the lower critical temperature. Further, ferrous metallographers tend to use nital almost exclusively for etching, but nital is not always the best reagent to use to properly reveal all microstructures.

While specimen preparation of superalloys for metallographic examination is relatively straightforward, the metallographer must take into consideration some of the inherent characteristics of these complex alloys, such as high toughness, presence of large amounts of strengthening phase and high corrosion resistance, to ensure getting samples that clearly reveal their complex microstructures.

Superalloys are complex alloys of Fe-Ni, Ni-, and Co-base compositions. Their microstructure can be quite complex due to the potential for a variety of phases that can form in heat treatment or service exposure conditions. This article discusses the use of new metallographic materials to prepare these alloys and the different etchants required to reveal the structure of these alloys properly as a function of alloy composition, heat treatment and microstructural phases. This discussion is limited to iron-nickel and nickel-base alloys, but most of the comments are also applicable to Co-base alloys.