Semi-coherent and coherent nanoprecipitation induced superior strength-ductility combination in additively manufactured CoCrMoW alloy.

Abstract

The interface coherence between precipitates and the matrix—whether coherent, semi-coherent, or incoherent—significantly impacts both the strength and ductility of metallic materials. Achieving an optimal balance between these properties can be realized through manipulation of the precipitate/matrix interface. In this study, 1 wt.% and 2 wt.% TiN nanoparticles were introduced into a laser powder bed fusion (LPBF) fabricated CoCrMoW alloy as a reinforcing phase. Compared to the unreinforced alloy, the alloy with 1 wt.% TiN exhibited substantial improvements in yield strength, ultimate tensile strength, and fracture elongation, increasing from 876 MPa, 1190 MPa, and 15% to 1071 MPa, 1421 MPa, and 17%, respectively. Although the addition of TiN nanoparticles did not significantly affect the average grain size, it inhibited the formation of fine grains at the fusion line and influenced the overall grain size distribution. More importantly, the incorporation of TiN nanoparticles during the LPBF process led to the in-situ formation of semi-coherent precipitates rich in Ti and N, as well as coherent precipitates rich in Ti, N, and O. The semi-coherent precipitates, which exhibit well-defined orientation relationships and periodic misfit dislocations, enhanced strength by blocking stacking faults and ε laths. They also preserved plasticity by promoting self-relaxation through twinning at intersected areas, thus preventing crack nucleation at the interface. Furthermore, the chemical inhomogeneity between the coherent phase and the matrix increased resistance to dislocation cutting of the precipitates, thereby improving the material’s strength without compromising ductility.