Metallographic approach to the investigation of metallic archaeological objects.

Metallic objects are considered among the most significant findings in Cultural Heritage and represent the 'culture of Materials' and the habits of an historical period and of a population. They also preserve traces of time: from the transformation of the ores in metal (by smelting) to the degradation from metal to oxidised compounds (by corrosion processes). Metallography, historically devoted to connect the microstructural features to production processes and to chemical-physical-mechanical properties is a powerful and relatively easy approach to characterise metallic findings.

Metal objects mapping after small charge explosions. A study on AISI 304Cu steel with two different grain sizes.

Evidence of exposure of a metal component to a small charge explosion can be detected by observing microstructural modifications; they may be present even if the piece does not show noticeable overall plastic deformations. Particularly, if an austenitic stainless steel (or another metal having a face-centered cubic structure and a low stacking fault energy) is exposed to an explosive shock wave, high-speed deformation induces primarily mechanical twinning, whereas, in nonexplosive events, a lower velocity plastic deformation first induces slip. The occurrence of mechanical twins can be detected even if the surface is damaged or oxidized in successive events.