It is well established that the rapid cooling rates during Laser Powder Bed Fusion (LPBF) processing of metals and alloys, leads to the formation of unique structures in the micro and nanoscale, which are often referred to as dislocation cells. As such, the microstructure consists of a complex substructure of high dislocation density walls with low dislocation density cell interiors. Often secondary alloying elements are also observed to enrich the dislocation wall areas, as a result of chemical microsegregation during the solidification process and intrinsic cyclic heating of the LPBF process. Consequently, the size of the cells can be manipulated by altering the printing parameters, thus the cooling rates, or completely annihilated by post built heat treatments. Therefore, LPBF processing bares the promise of achieving an optimum combination of strength and ductility by controlling the dislocation cell characteristics.

When investigating wrought materials, their strength depends on the grain size which is expressed by the well-known Hall-Petch relationship. However, this relationship does not work well in the case of LPBF materials, when one considers that the macroscopic grains consist of fine dislocation wall structures. Therefore, the mechanical properties of LPBF materials consisting of a complex hierarchical microstructure are predominantly determined by the dislocation cell structure. However despite the fact that the TRIP effect is known to depends on the grain size in wrought materials, the role of the dislocation cell size has not been unveiled yet in LPBF-processed materials.

The present contribution aims at shedding light on the effect that the dislocation cell structures have on the TRIP effect, on a highly metastable 304L stainless steel processed by LPBF through a synergy of neutron diffraction, TEM and EDX characterization methods.