Enhancing fatigue life of low-carbon ultra-high strength steel by inducing multi-component precipitates

Abstract

In this study, the low-carbon ultra-high-strength steels with precipitation-free were prepared using quenching processes, and the co-precipitation strengthening of multi-scale Cu-rich and NiAl were designed to enhance fatigue performance through quenching-tempering (QT) and quenching-partitioning-tempering (QPT) processes respectively. The microstructure of quenched steel shows a typical mixed microstructure of lath martensite (LM) and granular bainite (GB). After aging at 550 °C for 1 h, the high density (1.945 × 10²³ m^-3) of B2-NiAl and B2 core-9R shell nanoparticles were uniformly co-precipitated and greatly increased the yield strength and high-cycle fatigue strength from 965 MPa and 384.6 MPa to 1548 MPa and 510.7 MPa, respectively. The substantial improvement in fatigue performance is attributed to the large number of small-sized nanoparticles that hinder the movement of dislocations to form high-density dislocation tangles (HDDTs) and cell structures, reducing the stress concentration at grain boundaries. Furthermore, geometric phase analysis (GPA) revealed the existence of micro-strain around small-sized multi-component precipitates, which is less likely to cause micro-crack initiation, thereby enhancing the fatigue performance. After QPT treatment, the co-precipitated nanoparticles exhibited multi-scale distribution with a significantly reduced number density of 1.005 × 10²³ m^-3, and the typical large-sized FCC-Cu particles are identified, which weakens the precipitation strengthening and leads to the yield strength and fatigue strength reached 1396 MPa and 424.5 MPa respectively. Furthermore, the GNDs obviously accumulate at the interface between reversed austenite (RA) and matrix by the movement of dislocations and bypassed nanoparticles, which increases the tendency of microcrack initiation at the interface. In addition, the high strain accumulated at the interface of FCC-Cu particles increases the risk of fatigue damage and limits the improvement of fatigue performance.