Relatively few metallographers work with precious metals, other than those used in electronic devices.  Precious metals are very soft and ductile, deform and smear easily, and are quite challenging to prepare. Pure gold is very soft and the most malleable metal known. Alloys, which are more commonly encountered, are harder and somewhat easier to prepare.

Gold is difficult to etch. Silver is very soft and ductile and prone to surface damage from deformation. Embedding of abrasives is a common problem with both gold and silver and their alloys. Iridium is much harder and more easily prepared. Osmium is rarely encountered in its pure form; even its alloys are infrequent subjects for metallographers. Damaged surface layers are easily produced and grinding and polishing rates are low. It is quite difficult to prepare. Palladium is malleable and not as difficult to prepare as most of the precious metals. Platinum is soft and malleable. Its alloys are more commonly encountered. Abrasive embedment is a problem with Pt and its alloys. Rhodium is a hard metal and is relatively easy to prepare. Rh is sensitive to surface damage in sectioning and grinding. Ruthenium is a hard, brittle metal that is not too difficult to prepare.

Welding is a very important joining process and has been used extensively for at least the past 75 years. There is a need to control processes, such as welding, to insure a high quality end result. Over the years there have been many spectacular failures of welded structures, starting with Liberty ship and T-2 tanker failures during WWII, that emphasize this need. Many procedures involving non-destructive and destructive tests are used to study weldments.

Metallographic examination can be performed in-situ by grinding an area on the surface of the weld, its heat affected zones and adjacent base metal (the metal being joined that was unaffected by the temperature of the welding process). This is a reasonably non-destructive evaluation. Destructive examination, where a specimen is removed from either the welded assembly or test coupons, is quite commonly performed. Test coupons are often used to qualify the welder and ensure that the techniques and materials chosen will produce a weld with acceptable soundness and mechanical properties. Post mortems of failed weldments are also examined metallographically using sections removed from the welded assembly, generally after non-destructive examination is completed.

When I worked for Carpenter Technology Corporation in their research center, we encountered several cases where chart ratings of specimens by their production lab yielded grain size ratings between 4 and 5 for a number of specimens on an order (these orders required tests on 20 specimens from different bars). When we re-tested them in the R&D center, we got similar chart ratings. But, when we actually measured the grain size, all ratings were between 5 and 6 on the ASTM E 112 scale. As the criterion for pass/fail was a grain size of 5 or finer (higher), this bias was important. Consequently, at a subsequent ASTM E-4 committee meeting, I conducted a “round robin” test. 

Compared to many other metals and alloys and many other materials, such as carbides, ceramics and sintered carbides, aluminum and its alloys are low in strength and hardness. Aluminum is a soft, silvery metal with a face-centered cubic crystal structure, a hallmark of ductile metals. Its softness makes it somewhat difficult to prepare but the alloy is not sensitive to problems that plague preparation of magnesium and titanium, that is, a sensitivity to mechanical deformation that generates mechanical twins or Neumann bands. Aluminum, like chromium, niobium and titanium, is very corrosion resistant and a thin, transparent oxide film will form on a freshly polished surface. This film is responsible for its good corrosion resistance, but also makes etching difficult. Aluminum alloys contain a rather high content of intermetallic precipitates that contribute little to improving the alloys and may be detrimental. Contemporary four or five step preparation procedures are given for preparing aluminum and its alloys. Results are also shown for revealing grain size with Weck’s reagent, a useful alternative to anodizing. 

After we have properly prepared our tool steel specimens, it is generally best to examine them as-polished before etching – unless we are simply doing a routine test, for depth of decarburization, degree of spheroidization, and so forth. Examination of selectively etched tool steel microstructures by light microscopy provides more information than standard etchants, such as nital, picral or Vilella’s reagent. Further, the images are more suitable for quantitative measurements, especially by image analysis. Specimens must be properly prepared, damage free, if selective etchants, or color etchants, are to be applied successfully.

A number of etchants have been claimed to selectively etch certain carbides in tool steels. The response of these etchants has been evaluated using a variety of well-characterized tool steel compositions. While many are selective, they are often selective to more than one type of carbide. Furthermore, their use in image analysis must be evaluated carefully as measurements showed that the amount and size of the carbides are often greater after selective etching as many of these reagents outline and color or attack the carbides.