Co-Precipitation Kinetic Pathways in a Blast Resistant Steel for Naval Applications

Nanoscale co-precipitation is studied in detail after isothermal and isochronal aging. Atom-probe tomography is utilized to quantify the co-precipitation of co-located Cu precipitates and M2C carbide strengthening precipitates. Coarsening of Cu precipitates is offset by the nucleation and growth of M 2C carbide precipitate, resulting in the maintenance of a yield strength of 1047 +/- 7 MPa (152 +/-1 ksi) for as long as 320 h of aging time at 450 °C. The co-location of Cu and M2C precipitates results in non-stationary state coarsening of the Cu precipitates. Synchrotron-source x-ray diffraction studies reveal that the measured 33% increase in impact toughness after aging for 80 h at 450 °C is due to dissolution of cementite, which is the source of carbon for the nucleation and growth of M2C carbide precipitates. Only small austenite volume percentages (<1.5%) were measured after aging at temperatures up to 625 °C for 5 h.;The differences in artifacts associated with voltage-pulsed and laser-pulsed (atom-probe tomographic (APT) analyses of nanoscale precipitation are assessed using a local-electrode atom-probe (LEAP) tomograph. It is found that the interfacial width of Cu precipitates increases with increasing specimen apex temperatures induced by laser pulsing. This effect is probably due to surface diffusion of Cu atoms. Laser pulsing is also found to increase the severity of the local magnification effect for nanoscale M2C metal carbide precipitates, which is indicated by a decrease of the local atomic density inside the carbides. Methods are proposed to solve these problems based on comparisons with the results obtained from voltage-pulsed APT experiments.;Based on detailed three-dimensional (3-D) local-electrode atom-probe (LEAP) tomographic measurements of the distributions of Cu and M2C precipitates, the yield strength as a function of aging time is predicted using a newly developed 3-D yield strength model. Contributions from each strengthening constituent are evaluated with the model and superposition laws are applied to add each contribution. Prediction of the yield strength entirely based on 3-D microstructural information is thus achieved. The accuracy of the prediction depends on the superposition laws and the LEAP tomographic measurements, especially the mean radius and volume fraction of M2C precipitates.