Citation: Straumal, B.; Lepkova, T.; Korneva, A.; Gerstein, G.; Kogtenkova, O.; Gornakova, A. Grain Boundary Wetting by the Second Solid Phase: 20 Years of History. Metals 2023, 13, 929. https:// doi.org/10.3390/met13050929 Academic Editor: Xiao-Wu Li Received: 17 March 2023 Revised: 30 April 2023 Accepted: 8 May 2023 Published: 10 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). metals Review Grain Boundary Wetting by the Second Solid Phase: 20 Years of History Boris Straumal 1,2, *, Tatiana Lepkova 2 , Anna Korneva 3 , Gregory Gerstein 4 , Olga Kogtenkova 1 and Alena Gornakova 1 1 Osipyan Institute of Solid State Physics, Russian Academy of Sciences, Ac. Osipyan Str. 2, 142432 Chernogolovka, Russia; kogtenkova@issp.ac.ru (O.K.); alenahas@issp.ac.ru (A.G.) 2 Department of Physical Chemistry, National University of Science and Technology “MISIS”, 119049 Moscow, Russia; lepkovatl@mail.ru 3 Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Reymonta St. 25, 30-059 Cracow, Poland; a.korniewa@imim.pl 4 Institut für Werkstoffkunde (Materials Science), Leibniz Universität Hannover, An der Universität 2, 30823 Garbsen, Germany; gerstein@iw.uni-hannover.de * Correspondence: straumal@issp.ac.ru; Tel.: +7-916-6768-673 Abstract: Grain boundaries (GBs) can be wetted by a second phase. This phase can be not only liquid (or melted), but it can also be solid. GB wetting can be incomplete (partial) or complete. In the case of incomplete (partial) wetting, the liquid forms in the GB droplets, and the second solid phase forms a chain of (usually lenticular) precipitates. Droplets or precipitates have a non-zero contact angle with the GB. In the case of complete GB wetting, the second phase (liquid or solid) forms in the GB continuous layers between matrix grains. These GB layers completely separate the matrix crystallites from each other. GB wetting by a second solid phase has some important differences from GB wetting by the melt phase. In the latter case, the contact angle always decreases with increasing temperature. If the wetting phase is solid, the contact angle can also increase with increasing temperature. Moreover, the transition from partial to complete wetting can be followed by the opposite transition from complete to partial GB wetting. The GB triple junctions are completely wetted in the broader temperature interval than GBs. Since Phase 2 is also solid, it contains GBs as well. This means that not only can Phase 2 wet the GBs in Phase 1, but the opposite can also occur when Phase 1 can wet the GBs in Phase 2. GB wetting by the second solid phase was observed in the Al-, Mg-, Co-, Ni-, Fe-, Cu-, Zr-, and Ti-based alloys as well as in multicomponent alloys, including high-entropy ones. It can seriously influence various properties of materials. Keywords: grain boundaries; wetting; melt; solid phase; phase transitions; phase diagrams 1. Introduction The phenomenon of wetting of solid surfaces with liquids has been known for many centuries. The study of wetting metal or ceramic surfaces with melts has a shorter history. In reality, this issue began to be investigated shortly after the Second World War, when the technology of liquid-phase sintering of metal or ceramic powders was developed [1]. With incomplete (or partial) wetting of the free surface, a droplet of the liquid phase lies on a solid surface and forms a non-zero contact angle θ with it. Incomplete wetting occurs if σ SG < σ SL + σ LG where σ SG is the energy of the solid-gas interface, σ sl is the energy of the boundary between solid and liquid phases, and σ lg is the energy of the boundary between the liquid phase and gas. It follows from the conditions of mechanical equilibrium that σ SG = σ SL + σ LG cos θ. If σ SG > σ SL + σ LG , complete wetting occurs, the liquid spreads over the surface, and the contact angle between the liquid droplet and the solid substrate is formally equal to zero. Metals 2023, 13, 929. https://doi.org/10.3390/met13050929 https://www.mdpi.com/journal/metals