Abstract
In this review, the phenomenon of grain boundary (GB) wetting by the second solid phase is analyzed for the high entropy alloys (HEAs). Similar to the GB wetting by the liquid phase, the GB wetting by the second solid phase can be incomplete (partial) or complete. In the former case, the second solid phase forms in the GB of a matrix, the chain of (usually lenticular) precipitates with a certain non-zero contact angle. In the latter case, it forms in the GB continuous layers between matrix grains which completely separate the matrix crystallites. The GB wetting by the second solid phase can be observed in HEAs produced by all solidification-based technologies. The particle chains or continuous layers of a second solid phase form in GBs also without the mediation of a liquid phase, for example by solid-phase sintering or coatings deposition. To describe the GB wetting by the second solid phase, the new GB tie-lines should be considered in the two- or multiphase areas in the multicomponent phase diagrams for HEAs. The GB wetting by the second solid phase can be used to improve the properties of HEAs by applying the so-called grain boundary engineering methods.
Highlights
The idea of high-entropy alloys (HEAs) was first proposed in 2004 in seminal works of Cantor et al and Yeh et al [1,2]
In the majority of cases, HEAs are manufactured by crystallization from the melt after arc or induction melting [5,6,7,8,9,10,11,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34], electric current assisted sintering [35,36], plasma spark sintering [37], additive manufacturing by the laser powder bed fusion [38,39] or laser metal deposition [40], laser or plasma cladding deposition of coatings [41,42,43,44,45,46,47,48,49,50,51,52], selfpropagating high-temperature synthesis (SHS) [53], or even by brazing of dissimilar materials within the brazing joints [54,55])
The formation of such thick grain boundary (GB) layers is due to the phenomenon of complete or incomplete GB wetting by a second solid phase
Summary
The idea of high-entropy alloys (HEAs) was first proposed in 2004 in seminal works of Cantor et al and Yeh et al [1,2]. Researchers’ interest shifted from single-phase HEAs to alloys with numerous elements of heterogeneity They may include the inhomogeneous distribution of elements in the bulk, the presence of other phases, various grain-boundary layers, etc. The appearance of these heterogeneous structures in HEAs instead of a uniform solid solution can be treated using the ideas of grain boundary (GB) phase transformations. These phase transitions may include the GB wetting with a liquid phase or a second solid phase, as well as the formation of various thin GB layers of the second phase at the boundaries [14,15,16]. This review paper is devoted to the GB wetting in HEAs with a second solid phase
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