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316L and 304L stainless steels and a compositional gradient of both are fabricated using the same processing parameters via laser directed energy deposition additive manufacturing. In those alloys, the increase in chromium-to-nickel ratio is accompanied with grain refinement and formation of a high density of twin boundaries, i.e. sigma3 boundaries. By means of electron microscopy, crystallographic and thermodynamic calculations, we demonstrate that two mechanisms arising from the ferrite-to-austenite solidification mode are at the origin of twin boundary formation and grain refinement: 1) inter-variant boundaries emerging from the encounter of pairs of austenite grains formed from a common ferrite orientation with Kurdjumov-Sachs orientation relationship; 2) icosahedral short-range-ordering-induced (ISRO) nucleation of twin-related grains directly from the solidifying liquid. These findings define new routes to achieve grain boundary engineering in a single step in FeCrNi alloys, by tailoring the solidification pathway during the AM process, enabling the design of functionally graded materials with site-specific properties. 

We demonstrate the possibility of spatially controlling the degree of grain boundary serration in functionally graded stainless steels, by alloying powder mixtures on-the-fly during directed energy deposition additive manufacturing. Grain boundary serration is an attractive feature in polycrystalline microstructures, as it confers superior resistance to crack propagation and hot corrosion. Quantitative measurements at the microstructure scale coupled with thermodynamic calculations allow us to propose a mechanism to explain the origin of grain boundary serration. The formation of transient δ ferrite during solidification and its subsequent dissolution during cooling, governed by the Cr/Ni ratio, leads to the formation of remnant ferrite particles that hinder the growth of austenite grains in the solid state via a Smith-Zener pinning phenomenon. This finding opens new perspectives for grain boundary engineering, in-situ during additive manufacturing.