Origin of additive manufactured ultrafine stainless steel composites

Abstract

Cracking, anisotropy, texture, brittleness, coarse-reinforcing particles, and columnar grains often characterize ultrahigh small-radii-containing alloys processed via additive manufacturing. In this study, we demonstrate how Laser Powder Bed Fusion (L-PBF) processing, combined with massive boron addition (>1000 ppm), enables the formation of a refined and complex microstructure composed of an ultrafine stainless steel matrix and nanoborides decorating grain boundaries (GBs). Boron in L-PBF-produced stainless steel effectively transforms coarse columnar grains (∼200 μm) into equiaxed ultrafine grains (∼1 μm). Beyond exceptional grain refinement, colossal boron content promotes the formation of nanometric Cr₂B particles along GBs. Such surprising grain refinement arises from the extended thermal undercooling caused by a lag between the real and theoretical grain growth rate caused by the segregation of boron, forming a boundary layer ahead of the solid-liquid interface. This extended undercooling increases the nucleation rate on the underlying layer, suppressing the columnar-like grain growth of the primary phase. These findings provide a mechanistic understanding of grain refinement in boron-modified alloys, offering insights that are transferable to other materials and additive manufacturing conditions.