Effect of Rare Earth Elements on the Morphology of Eutectic Carbides in AISI D2 Tool Steels: Experimental and Modelling Approaches

Klemen Zelič,Paul John Mcguiness,Matjaž Godec,Jaka Burja

doi:10.1038/s41598-018-27658-w

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 rare-earth elements (REEs) 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. Here, 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 AISI D2 tool steel.

Highlights

The morphology of the eutectic chromium carbides in the microstructure of as-cast AISI D2 tool steel was modified by adding small amounts of rare-earth elements (REEs) to the melt
The most popular theoretical explanation for the modification is based on the effect of modified non-metallic inclusions in the melt acting as heterogeneous nuclei for the nucleation of the eutectic carbide[14,15]
The differences in the morphologies of the eutectic carbides are clearly seen in this figure as the carbide in the REE-modified sample is much finer

Summary

Introduction

The morphology of the eutectic chromium carbides in the microstructure of as-cast AISI D2 tool steel was modified by adding small amounts of rare-earth elements (REEs) to the melt. 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 AISI D2 tool steel. Calculations of the misfit between the crystal lattices of the hexagonal Cr7C3 carbide and all the inclusions that were found inside the REE-modified samples show that heterogeneous nucleation is unlikely to happen on modified inclusions This is the reason why we decided to propose a new theoretical explanation for the modification of the eutectic carbide morphology that explains the phenomenon on the basis of first-principles thermodynamic calculations in terms of the phase-field modeling of the eutectic reaction[16,17]. This principle turns out to be essential for describing the transition from a lamellar to a globular eutectic carbide morphology as a result of REE modification

Methods

Results

Discussion

Conclusion