Characterization of nano-sized precipitation and dislocations and the correlation with mechanical properties of a low alloy TRIP-aided steel

The present work focuses on structure-property correlation in a new low alloy multi-phase microstructure steel engineered with retained austenite via Cu alloying plus a three-step heat treatment, namely, intercritical annealing, intercritical tempering and sub-critical tempering of an as hot rolled structure. Particular attention was paid to characterizing nano-sized precipitates and dislocations after the three-step heat treatment by atom probe tomography (APT) and transmission electron microscopy (TEM), in order to establish their respective contribution to the yield strength. Cu-rich atomic clusters and ultra-fine precipitates were obtained in intercritical ferrite after the three-step heat treatment. The average radius of clusters and precipitates was 1.3±0.4 nm and 5.6±0.7 nm, respectively. The number density of clusters and precipitates was estimated as 2.55 × 1023 m-3 and 1.68 × 1021 m-3, respectively. The strength contribution from clusters and precipitates was ~72 MPa and ~101 MPa, respectively, assuming the number density of the APT dataset to represent the whole material. In addition, a large number of dislocations were observed in the intercritical ferrite by TEM study. The average density of dislocations was measured to be ~1.66 × 1014 ± 7.12 × 1013 m-2, which contributed ~250 MPa to yield strength according to the Bailey–Hirsch relationship.