Precipitation behavior of Cu-NiAl nanoscale particles and their effect on mechanical properties in a high strength low alloy steel

Abstract

The precipitation behavior of Cu-NiAl nanoscale particles in a high strength low alloy steel and their effect on mechanical properties are studied systemically by atom probe tomography and tensile testing. The results display that the Cu-rich zones will nucleate firstly from the supersaturated solid solution for the experimental high strength low alloy steel tempered at 500 °C for 0.08 h, which will transform into Cu-rich particles and also promote the heterogeneous nucleation of isolated NiAl particles. The precipitation sequence in the studied high strength low alloy steel can be expressed as: supersaturated solid solution → Cu-rich zones → Cu-rich particles + isolated NiAl → Cu-NiAl + isolated NiAl. With increasing tempering time, the NiAl (Cu-NiAl + isolated NiAl) particles will coarsen continuously and the Cu-NiAl particles have larger radius and higher number density than the isolated NiAl particles. The tensile testing results reveal that NiAl particles exhibit strong precipitation strengthening and result in a strength plateau of about 1423–1486 MPa at 2–310 h. With tempering time increasing, the elongation decreases firstly and attains the minimum value of about 7.5% in the 5 h tempered condition due to the high number density (1.95 × 10 24 m −3 ) of NiAl particles, and then the elongation increases gradually to 13.0% at 310 h. Obviously, the sample tempered for 310 h displays a beneficial combination of yield strength (1423 MPa) and elongation (13.0%). Schematics of the precipitation behavior of Cu-NiAl co-precipitates for the experimental steel tempered at 500 °C. • The Cu-rich zones will nucleate firstly from the supersaturated solid solution. • The precipitation sequence can be shown as: Cu-rich zones → Cu-rich particles + isolated NiAl → Cu-NiAl + isolated NiAl. • The precipitation strengthening of NiAl particles is studied thoroughly. • The steel exhibits a beneficial combination of yield strength and elongation for the sample tempered at 500 °C for 310 h.