A systematics of grain boundary (GB) segregation transitions and critical phenomena has been derived to expand the classical GB segregation theory. This study uncovers when GB layering vs. prewetting transitions should occur and how they are related to one another. Moreover, a novel descriptor, normalized segregation strength, is introduced. It can represent several factors that control GB segregation, including strain and bond energies for both general and small-angle GBs, as well as misorientation for small-angle GBs, which had to be treated separately in prior models. In a strong segregation system with a large normalized segregation strength, first-order layering transitions occur at low temperatures and become continuous above GB roughing temperatures. With reducing normalized segregation strength, the layering transitions gradually merge and finally lump into prewetting transitions without quantized layer numbers, akin to Cahns critical-point wetting model. Furthermore, GB complexion diagrams with universal characters are constructed as the GB counterpart to the classical exemplar of Pelton-Thompson regular-solution binary bulk phase diagrams.
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