November_December_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 1 2 5 surface and the core. The surface layer is about 0.6 mm thick. The core shows a fair amount of porosities and cracks corresponding to the veins revealed by x-ray radiography in Fig. 4. They seem to be internal cracks surrounded by Al-Ca-Mg-Si rich oxides (Fig. 7a). SEM observations backed with EDS analysis (Figs. 6b, 7a, and 7b) re- vealed silicon and oxygen rich aggre- gates (100-300 µm), thus more likely SiO 2 , embedded in an iron-base heavily oxidized matrix. CORE OF THE ARTIFACT Basedon these results, it canbe as- sumed that the material is a low-carbon wrought iron charged with inclusions elongated in the forming directions. Although the study did not go as far as needed to have a thorough identifi- cation of the inclusions, it is fair to say that they contain the main elements usually found in slags. These type of in- clusions—rich in elements such as Al, Ca, Mg, and Si (though finer and more controlled)—are still found in modern steels [2] and are due to compounds add- ed during the steelmaking process. Archaeometallurgy studies of iron artifacts [3-5] have shownmicrostructures that resemble those observed in this in- vestigation with vein-like features (as shown in Fig. 6) that seem to be char- acteristic of slag inclusions elongated in the direction of forming. The authors were able to demonstrate that the form- ingmethodwould involve bothwrought iron (sometimes carburized) and slag which are assembled by a forging op- eration, known as stratification, which includes heating-hammering-bending. Whatever the addition made by black- smiths, it is the stratification process that seems to have given the observed metal/slag composite structure to the nail. It provided the needed properties for a construction nail that wrought iron alone would not have. However, given the very different physical properties of metal and slag, mainly their melting temperature and coefficient of thermal expansion, cracking can occur, which explains the cracks observed in the core of the part. Fig. 7 — Corresponding EDS analysis of the microstructure in Fig. 6. (a) General view; (b) aggregates A; and (c) adjacent matrix B. (c) (b) (a)