Abstract
In this review, the phenomenon of grain boundary (GB) wetting by melt is analyzed for multicomponent alloys without principal components (also called high-entropy alloys or HEAs) containing titanium. GB wetting can be complete or partial. In the former case, the liquid phase forms the continuous layers between solid grains and completely separates them. In the latter case of partial GB wetting, the melt forms the chain of droplets in GBs, with certain non-zero contact angles. The GB wetting phenomenon can be observed in HEAs produced by all solidification-based technologies. GB leads to the appearance of novel GB tie lines Twmin and Twmax in the multicomponent HEA phase diagrams. The so-called grain-boundary engineering of HEAs permits the use of GB wetting to improve the HEAs’ properties or, alternatively, its exclusion if the GB layers of a second phase are detrimental.
Highlights
We describe the grain boundary (GB) in a micrograph as completely wetted in the case when the melted layer is continuous from one GB triple junction (TJ) to another
If the melt layer between two TJs is broken and the portions of dry GB remain in between, we describe the GB as partially wetted
high-entropy alloys (HEAs) during crystallization of the melt in all synthesis technologies. These thick GB layers are liquid during cooling in the S+L area of the HEA
Summary
In the majority of cases, multicomponent alloys are synthesized by crystallization from the melt (arc or induction melting in vacuum or argon [5,6,7,13,14,15,16,17,18,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43], plasma spark sintering [19], electric current assisted sintering [20,21], laser or plasma cladding deposition of coatings [44,45,46,47,48,49,50,51,52,53,54,55], additive manufacturing by the laser-powder bed fusion [56,57] or laser-metal deposition [58], self-propagating high-temperature synthesis (SHS) [59], and by brazing within the brazing joints [60,61]). Higher than the energy of the two solid/liquid interphase boundaries, σGB > 2σSL, the the minimum wetting phase-transition temperature, Twmin , the completely wetted GBs contact angle will be equal to zero In this case, the GB must be replaced by a sufficiently thick layer of the liquid phase. If the two-phase S + L area of the phase diagram does not contain any tie lines, Twmax or Twmin, of the GB wetting transition, the one-phase solid solution polycrystal with typical dendritic structure forms after solidification of the melt This is the case of the equiatomic HfNbTaTiZr high-entropy alloy with bcc structure prepared by plasma arc melting (Figure 2a) [5].
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