Heterogeneous nucleation of Ni3Ti by Mo-enriched particles enhances strength and fracture toughness of maraging steel

Developing ultra-high strength and high toughness maraging steel requires an effective route of a well-designed heat treatment regimen to control the precipitates. In this study, we regulated the heterogeneous nucleation sites through Mo-enriched particles to increase the nucleation rate of Ni3Ti. The Mo-enriched particles as Ni3Ti precursors decrease the lattice mismatch between alpha-Fe and Ni3Ti, reducing crack formation at the interface. We tailored the maraging steel utilizing duplex aging treatment (DAT) to break the conventional precipitation pathway. The DAT comprises a solid solution treatment followed by a two-step aging treatment at 370 degrees C and 480 degrees C. The Mo-enriched particles precipitated after solid solution treatment due to the low solubility and diffusion rate at low temperatures. The aging treatment conducted at 370 degrees C enhances the density of the Moenriched particles and inhibits the formation of Ni3Ti. The pre-formed precipitates act as heterogeneous nucleation sites for Ni3Ti during aging at 480 degrees C, increasing the nucleation rate and reducing the size. As aging continues, Mo segregates to the interface of Ni3Ti and the matrix, inhibiting the growth of Ni3Ti and forming Fe7Mo2 precipitates. The DAT steel exhibits an excellent yield strength of 2093 MPa and fracture toughness of 77 MPa center dot m1/2 at room temperature. The well-regulated precipitates increase crack nucleation energy, forming a curved primary crack and multiple secondary cracks, which reduce stress concentrations and increase fracture toughness. The DAT strategy provides a new avenue for improving fracture toughness while maintaining high strength in maraging steel.