Microstructure Evolution and Strengthening Mechanism of V-N Microalloyed Invar Alloy Processed by Mechanical Heat Treatment

Invar alloy is widely used for manufacturing precision instruments owing to its exceptionally low thermal expansion property. Nevertheless, conventional invar alloys usually lack the sufficient strength required for engineering applications, so there is an urgent need for innovative approaches to enhance the strength. In this study, the possibility of elevating the strength and preserving the low coefficient of thermal expansion (CTE) of invar alloy is investigated by combining deformation strengthening with the V (C, N) precipitation strengthening mechanism. The increase in dislocation density resulting from deformation treatment promotes the V (C, N) precipitation. This leads to a significant enhancement in the strength of alloys after cold-rolling aging compared to direct aged counterparts, while preserving the low CTE. After a cold-rolling deformation with a 40% reduction and subsequent aging at 650 °C for 3 h, the V-N invar alloy exhibits a tensile strength of 907 MPa and an elongation of 6.9%, demonstrating the optimal mechanical properties. In addition, the CTE value maintains a low value of 1.31 × 10^-6/°C within the temperature range of 20 to 100 °C. These findings are vital for developing high-strength, low-CTE invar alloys.