Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.