In-situ nitriding of Fe2VAl during laser surface remelting to manipulate microstructure and crystalline defects

TL;DR

This paper presents a novel laser surface remelting technique in a reactive nitrogen atmosphere to modify the microstructure and defect chemistry of Fe2VAl, a thermoelectric material, enhancing its properties through nitrogen segregation and defect manipulation.

Contribution

It introduces a new in-situ nitriding method during laser remelting to control microstructure and defect chemistry in Fe2VAl, with potential applications in additive manufacturing.

Findings

High density of stable dislocations observed

Nitrogen and vanadium segregate at dislocations and grain boundaries

Repeated remelting enhances segregation effects

Abstract

Tailoring the physical properties of complex materials for targeted applications requires optimizing the microstructure and crystalline defects that influence electrical and thermal transport, and mechanical properties. Laser surface remelting can be used to modify the sub-surface microstructure of bulk materials and hence manipulate their properties locally. Here, we introduce an approach to perform remelting in a reactive nitrogen atmosphere, in order to form nitrides and induce segregation of nitrogen to structural defects. These defects arise from the fast solidification of the full-Heusler Fe2VAl compound that is a promising thermoelectric material. Advanced scanning electron microscopy, including electron channelling contrast imaging and three-dimensional electron backscatter diffraction, is complemented by atom probe tomography to study the distribution of crystalline defects and their local chemical composition. We reveal a high density of dislocations, which are stable due to their character as geometrically necessary dislocations. At these dislocations and low-angle grain boundaries, we observe segregation of nitrogen and vanadium, which can be enhanced by repeated remelting in nitrogen atmosphere. We propose that this approach can be generalized to other additive manufacturing processes to promote local segregation and precipitation states, thereby manipulating physical properties.