Changes in Internal Structure of Bronze and Iron over Time at Ambient Temperature

Abstract

This study analyzes changes in the inner structure of metals (bronze and iron) resulting from aging and creeping processes. A remarkable disproportion was noticed during hardness analyses of original metal objects from archaeological finds and replicas likely made in the same way as the originals. The mechanism of such long-term changes at ambient temperature remain unknown. We examined the azimuthal (lateral) diffraction line profile, which includes the size, number and shape of individual diffraction spots comprising a discontinuous diffraction line. By x-ray analysis, all of the original bronze and iron artifacts were proven coarse–grained; coherent areas (mosaic blocks) were larger than 10 μm. The original structure of objects produced by the forging of sponge iron must have been much more fine–grained. The measurements revealed that the mechanism responsible for the change in the structure of forged artifacts over time is spontaneous recrystallization. We found that spontaneous recrystallization occurs much more quickly than usually expected at ambient temperature. We derived formulas explaining how the mechanism of relaxation changes on cooling from high to ambient temperature. Some low-energy mechanisms are blocked at elevated temperatures but released at ambient temperature. The activation energy of such mechanisms is much lower than the activation energy of relaxation processes at elevated temperatures. Our findings can be usefully exploited when monitoring processes that degrade the structure of a material in the course of its service.