Segregation Phenomena in Alloys Systems

Dear Colleagues,

The most important characteristic of an alloy is the presence of one or more elements in its composition. This presence is at the origin of the significant changes in the properties of the alloy, in particular, its mechanical properties. High strength steels, nickel-based superalloys, and titanium or aluminum alloys could contain up to 20 different alloying elements, which are critical engineering materials that owe their extraordinary properties to the interactions between the elements and their impact on phase formations, phase transformation, and microstructure evolution. However, the differences associated with the inherent nature of each element result in the formation of localized regions with different chemical compositions than the nominal one of the alloys during the subsequent processing operations, such as solidification, solid state forming, and heat treatment. Segregation refers to the formation of such local chemical heterogeneities, and is one of the biggest challenges faced when processing multicomponent systems. Segregation could be responsible for the formation of cracks; undesirable second phases; changes in the transition temperature; the modification of physical, microstructural, and mechanical properties of the final product; and result in quality reduction or even the rejection of the part.

Segregation is classified according to its scale of occurrence on microsegregation, which corresponds to the chemical inhomogeneities at the scale of the interfaces (e.g., dendrites and grain boundaries), while macrosegregation is usually observed in large-size cast products such as ingots and slabs, and can extend in the range of centimeters or meters. In recent years, with the advent of new manufacturing technologies, such as additive manufacturing, as well as the request for ever larger size castings, the impact of segregation on product quality has become highly important.

In this context, it is of great importance to better understand the mechanisms influencing the occurrence of segregation at different scales, especially knowing that there are very little data on the effect of the different process parameters on alloying element segregation behaviors. Segregation studies require both experimental and simulation investigations. The role of experimental methods is to provide information about segregation behaviors. However, this task is more complicated in the case of large-size ingots, where the experimental studies are very costly and time consuming, or at a very small scale, such as additive manufacturing, where several cycles of solidification and heat treatment operations are applied during the fabrication of one single part. The use of simulation tools would allow for reducing of cost and the rapid identification of the influence of each parameter. Nevertheless, simulation studies require the use of reliable and representative thermophysical and thermomechanical input data. However, the determination of these parameters is not easy, because of their dependence on the chemical composition, temperature, and cooling rate.

The main objective of this Metals Special Issue is to cover the significant scientific and technical challenges of experimental or simulation studies related to segregation phenomena in alloys.

Prof. Mohammad Jahazi
Guest Editor

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