Doctoral dissertation
Effect of the rare-earth elements Ce, La, Nd and Pr on the morphology of eutectic carbides in tool steels: experimental and modelling approach
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Abstract
The morphology of the eutectic chromium carbides in the microstructure of as-cast AISI
D2 tool steel was modified by adding small amounts of the rare-earth elements (REEs) Ce,
La, Nd and Pr to the melt. As a result of these REE additions, the eutectic carbide morphological
type was changed from lamellar to globular. Similar phenomena have already
been reported for various tool steels, but no complete theoretical explanation has been
provided. In this thesis the successful experimental modification of the eutectic carbide
morphology in AISI D2 tool steel by the REEs Ce, La, Nd and Pr is described. Further,
we propose a new model that is derived from first-principles thermodynamic calculations
based on the phase-field modeling of the eutectic reaction. Using this new approach, where
the decomposition of the phase-boundary surface-energy term is divided into the isotropic
and anisotropic parts, we were able to account for the transition from a lamellar to a
globular eutectic morphology in REE-modified (Ce, La, Nd and Pr) AISI D2 tool steel.
From the comparison of the experimental and modeling results we were able to explain the
origins of the differences between the REE-modified and non-modified steel. We showed
that the modification of the non-metallic incisions in REE-modified steel does not influence
the heterogeneous nucleation of eutectic carbides. Furthermore, we theoretically explained
mechanism that governs the morphological evolution of the eutectic carbides and explains
the observed differences.