Precipitation and TRIP enhanced spallation resistance of additive manufactured M350 steel

This work investigates the spall damage and microstructural deformation behaviors of a heat-treated, hierarchical structured 18 wt% Ni-350 maraging steel (M350) produced by laser powder bed fusion (LPBF) under shock loading. The samples were shock-loaded along different orientations with peak shock stresses ranging from 7.0 GPa to 10.5 GPa. Experimental results demonstrate that the M350 exhibits ultra-high spall strength of 5.01–5.89 GPa and 4.53–4.99 GPa when loading perpendicularly and parallel to the building direction, respectively. Spall damage is characterized as a typical transgranular brittle fracture with {100} cleavage planes within the block. The observed superior mechanical performance is attributed to the precipitation strengthening and the transformation-induced plasticity (TRIP) effect. Dislocation slip cuts through the Ni₃Ti precipitates, causing them to fracture, simultaneously, high density precipitates impede dislocation movement according to the Orowan mechanism, preventing the formation of microcracks. The residual austenite undergoes martensitic transformation with the formation of new secondary laths with widths of 20–60 nm to accommodate localized plastic deformations, which creates a large number of grain boundaries and leads to grain refinement.