Effect of heat treatment on the microstructure and properties of FeSi-NiAl composite prepared by induction melting

Kaloč Václav - VSB - Technical University of Ostrava (Czech Republic)

This study explores the development of FeSi–NiAl-based intermetallic compounds, which hold promise as advanced tool materials for future applications. These alloys, characterized as in-situ composites, have the potential to replace conventionally used materials such as cemented carbides and high-speed tool steels. Current tool materials often rely on tungsten and cobalt, both classified as critical raw materials by the European Union. Reducing dependency on these scarce and strategically important resources is crucial from the perspective of sustainable development, as well as for fostering economically, environmentally, and ecologically responsible metallurgical practices.

The primary objective of this research was to design and prepare FeSi–NiAl alloys with composite properties in weight ratios of 1:1 and 1:2. The microstructure, chemical and phase composition, and microhardness of the alloys were investigated. To improve hardness and homogenize the chemical composition, the composite samples were subjected to heat treatment. The first heat treatment was carried out in air at 900 °C for 3 hours. The second heat treatment was performed in a vacuum, where the samples were sealed in quartz ampoules and annealed at 900 °C for 8 hours.

The microstructure of the FeSi–NiAl 1:1 alloy in its as-cast state consisted of dendrites and a eutectic structure, with Ni and Al as the predominant components. The interdendritic regions were primarily composed of Fe and Si. Heat treatment refined the dendrites, leading to a slight reduction in their volume fraction. In contrast, the microstructure of the FeSi–NiAl 1:2 alloy in its as-cast state was entirely dendritic, with no eutectic phase observed. The dendrites predominantly contained Ni and Al, while the interdendritic regions were rich in Fe and Si.

Following heat treatment, the dendritic structure was further refined, resulting in a noticeable decrease in the dendrite volume fraction.

In terms of phase composition, the samples predominantly consisted of the intermetallic phases FeSi and NiAl, along with minor phases of AlNi2Si and Al2FeSi. Heat treatment led to the elimination of these secondary phases, resulting in a structure composed exclusively of FeSi and NiAl phases. The presence of the AlNi2Si phase in the as-cast samples can be attributed to non-equilibrium solidification, as indicated by modeling results. During solidification, iron solidifies first, enriching the remaining melt with other elements. This localized enrichment subsequently facilitates the formation of the AlNi2Si phase.

Heat treatment negatively impacted the microhardness of the material, causing reductions ranging from slight to significant, attributable to minor alterations in the microstructure. Nevertheless, the newly developed alloys demonstrate notable advantages, including exceptional phase stability and high hardness. These properties, however, are intrinsically linked to the material’s brittleness, which limits its practical applicability. Future investigations should prioritize strategies to enhance the toughness and machinability of these alloys while maintaining their requisite hardness and phase stability. This could involve systematic modifications to the FeSi:NiAl compositional ratios and the refinement of heat treatment protocols to achieve an optimal balance between mechanical performance and material stability.

Key words: intermetallic compound, iron silicide, nickel aluminide, tool material, vacuum induction melting, composite material.