Local measurement of geometrically necessary dislocation densities and their strengthening effect in ultra-high deformed pearlite

Abstract

The strength of pearlitic wires can be increased by cold-drawing to a world record level for bulk ductile materials of 7 GPa. The underlying strengthening mechanisms are not fully understood, as the application of usual characterization is challenging because of the small grain sizes and the high degree of deformation. Here we demonstrate that the microstructure of the wires can be directly probed by nano-beam diffraction (NBD) orientation mapping in a transmission electron microscope even after a drawing strain of 6.52. We observe a highly fragmented microstructure with a high density of low-angle grain boundaries (LAGBs) within the ferrite lamella. That makes it difficult to define grain sizes in the ordinary way. We thus calculate an equivalent grain size based on the density of high-angle grain boundaries (HAGBs) per measurement area and an average density of geometrically necessary dislocations (GNDs) calculated from all local misorientation gradients below 15° misorientation. Total strengths calculated from a summed Hall-Petch and Taylor effect of the latter values as well as carbon solid solution hardening are in good agreement with the strengths as measured by tensile tests. Our results show that the GNDs are similarly important as HAGBs in terms of their contributions to the total strength. On this basis, the experimental evidence of the strengthening mechanism with emphasis on GNDs, particularly in ultra-high deformed materials is highlighted. The present results also validate the application of NBD to assessing mechanical properties of other ultra-high deformed materials where mechanical tests often are not feasible.