Bernd Kieback: Fundamentals of solid state sintering in multicomponent High Entropy Alloys
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Bernd Kieback, Nadine Eißmann*

Technische Universität Dresden, Institute for Materials Science, Dresden, Germany;
*Fraunhofer Institute for Manufacturing and Advanced Materials (IFAM), Dresden, Germany

 

Solid state sintering is driven by the vacancy gradients created by surface curvatures of the powder particles during sintering or of the pores at the final state of sintering. Contact formation and densification are theoretically calculated for model systems and give quite good description of the changes that can be observed in sintering experiments. Normally the model theories assume elemental systems with on sort of atoms to describe the diffusion processes that lead to contact growth and shrinkage of the system. Surprisingly there is no literature about multicomponent system, obviously because automatically these systems are assumed to behave completely similar to elemental systems with some effective diffusion coefficient. However, already in early publications about sintering fundamentals the possibility of segregation during sintering was discussed, if the atoms in the alloy would have large differences in their diffusion coefficients. Motivated by the activities in High Entropy Alloys that normally consist out of at least 5 elements in nearly equiatomic proportions the authors analyzed the problem of sintering in multicomponent systems. The theoretical analysis shows, that at standard conditions no segregation happens and very small gradients of the elemental concentrations are built up during diffusion. These gradients adjust the mobility of all elements to the same value ensuring that neck regions and pores are filled with a homogeneous solid solution of the initial composition. It will be shown, that the effective diffusion coefficient can be calculated by a combination of the partial diffusion coefficients of the elements. Since the concentration gradients are defined by the gradient of vacancy concentration in extreme conditions of nanometer particles more pronounced effects can be expected.