Unexpected precipitation hardening from interdendritic chemical-structural heterogeneity in laser powder bed fusion austenitic stainless steels

Abstract

Interdendritic chemical-structural heterogeneity (elemental segregation and dislocation aggregation) is a universal feature of additively manufactured metals and alloys arising from the thermal cycling inherent to layer-by-layer printing. Here we show that this mesoscale heterogeneity, rather than being merely a printing artefact, can unexpectedly trigger a precipitation phenomenon during conventional annealing of austenitic stainless steels, converting them from solid-solution-strengthened to precipitation-hardening materials. Mo/Si-rich precipitates nucleate preferentially at periodically arranged dislocation cell boundaries, especially at triple junctions where Mo and Cr segregate most strongly. In heat-treated samples, these submicron precipitates hinder dislocation motion during plastic deformation, while concurrently promoting stacking faults and, at larger strains, high-density nano-twins. As a result, the additively manufactured steels exhibit simultaneous gains in strength, ductility and strain-hardening capability. Our findings challenge the conventional view that printing-induced heterogeneities are detrimental, suggesting instead that they can be harnessed to design superior heat treatments for additively manufactured alloys.