Manufacturing Multi-scale Architectured Materials with Composition and Grain Size Gradients by High-pressure Torsion

Abstract

There exist growing interests toward architecturing of materials among materials science community these days. Architectured materials, defined as combinations of two or more materials or of materials and space, open up a new possibility to fill out holes of material property space or Ashby chart, substantial parts of which have still remained empty. Especially, using severe plastic deformation (SPD) in an effort to manufacture architectured materials has gained much attention as a promising way to achieve architecturing as well as nanostructuring at the same time. Combining severe plastic deformation processing and diffusion-based architecturing, the present study aimed to produce multi-scale architectured materials (MS-ArchiMat) and explored a correlation between their microstructures and mechanical properties. The starting material was interstitial free steel and a carbon gradient was introduced by solid carburizing. The carburized samples were processed by high-pressure torsion (HPT), one of SPD processing techniques, and became fully nanostructured. The strength of the composition-gradient materials after HPT was much higher, compared to that of severely deformed homogeneous low carbon steels whose average carbon contents were in a similar range. This outstanding improvement of strength was attributable to the higher dislocation density in the high carbon region near the surfaces than that of the low carbon region near the center, which was attested by X-ray diffraction analysis. After annealing, the difference in thermal stability due to the carbon gradient led to the evident multi-scale microstructure, ultrafine grains near the surface and coarse grains close to the center.