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T. Wojcik, E. Kozeschnik:
"Shell-wise growth of (Nb,Ti)(C,N) precipitates during cyclic cooling of microalloyed steel";
Poster: 18th International Microscopy Congress, Prag; 09-07-2014 - 09-12-2014; in: "18th International Microscopy Congress", (2014), ISBN: 978-80-260-6720-7; 2877.

The nucleation and growth characteristics of NbC- and TiN-precipitates, as well as complex
Ti-Nb-carbonitrides in microalloyed steel, have been investigated numerous times. Due to
different solubility products, TiN nucleates at higher temperatures than NbC in austenite. Both
phases have the same fcc crystal structure with similar lattice constants and are able to form
complex Nb-Ti-carbonitrides. In the present work, Nb-Ti-microalloyed steel samples are
exposed to a cyclic cooling profile with nine quenching and reheating cycles between 1200°C
and 800°C. The temperature oscillations have amplitudes of over 100K. The actual
composition of the precipitates depends on the temperature and chemistry of the matrix. A
TEM-investigation of samples treated in this way shows various particle populations. The
largest ones, which nucleated at the first stages of the heat treatment, are platelet shaped
with diameters of over 200 nm. The EDX and EFTEM characterisation shows an alternating
structure of shells with local Nb- and Ti-enrichments. In HRTEM micrographs of these particles,
local variations in the lattice parameter are measured. The number of shells corresponds to
the number of applied thermal cycles. The smallest precipitates are spheroidal NbC
nanoparticles with diameters up to 10 nm. The experimental setup is modelled with the
thermo-kinetic software MatCalc, which allows for the simulation of the precipitate evolution in
the course of this cyclic treatment. The calculated results are in good agreement with the
experimental data.