Achieving high strength and low thermal expansion coefficient in additively manufactured Invar alloys by leveraging the effect of alloying elements

Strengthening of Invar alloys is often accompanied by compromising their low coefficient of thermal expansion (CTE), which has been restricting the development of high-strength yet low CTE Invar alloys. Herein, by leveraging the opposite effect of Co and V on CTE, we designed a novel high-strength and low CTE Invar alloy. Laser powder bed fusion (LPBF) technique was used to fabricate the material. Results showed that this novel Invar alloy demonstrates excellent printability, which can achieve near-full density (>99.5 %) in a wide processing window. An optimal direct aging treatment process of 3 h at 650 °C was determined to maximize its strength. TEM and in situ small-angle neutron scattering (SANS) analysis showed that VC nanoparticles as fine as 10 nm precipitate upon heat treatment. Tensile tests revealed that the peak-aged specimen possesses a high yield strength of 600 MPa, reflecting about 50 % enhancement compared with LPBF-processed conventional Invar alloys. Such a strength increase is primarily contributed by Orowan strengthening of those ultra-fine VC nanoparticles. Meanwhile, a relatively low CTE (1.41 × 10⁻⁶ °C⁻¹, 30–150 °C) has been maintained. The combination of high strength and low CTE of this novel Invar alloy is superior to that of additively manufactured Invar in the literature. This work offers a pathway to increase the strength of Invar alloys without sacrificing their low CTE.