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G. Stechauner, E. Kozeschnik:
"Tailoring copper precipitates in a 17-4 PH steel by using thermo-kinetic simulation software";
Talk: 61. Metallkunde-Kolloquium, Lech am Arlberg; 2015-04-15 - 2015-04-17.

High alloyed, precipitation hardening steel of the 17-4 grade is of great technical interest due to its high strength and ductility. Strength however depends on an appropriate heat treatment: the as-cast condition can be improved by 20 - 60% in strength depending on annealing temperatures. The greatest increase in strength comes from coherent, nanometer sized Cu precipitates of bcc crystal structure. Prolonged annealing leads to growth and, ultimately, coarsening, which involves a structural transformation of the copper precipitates to 9R and fcc crystal structure, respectively. The loss of coherency is accompanied by a decrease in yield strength. With the aid of thermo-kinetic simulation software MatCalc, we simulate the radius and particle density evolution as a function of annealing time and temperature and predict a multi-modal size distribution for multi-step heat treatments. The simulation results are backed up by transmission electron microscopy (TEM) and atom probe tomography (APT) experiments. This approach is based on CALPHAD-type Gibbs energies, a mean-field model for precipitate growth, parameter-free interfacial energy calculation and advanced classical nucleation theory.