Multicomponent alloying can be utilized to enhance the thermal stability of nanocrystalline alloys. The grain boundary energy can be reduced significantly via both bulk and grain-boundary high-entropy effects with increasing temperature at/within the solid solubility limit, thereby reducing the thermodynamic driving force for grain growth. Moreover, grain boundary migration can be hindered by sluggish kinetics. Although nanocrystalline metals can exhibit superior properties such as high strength and hardness [1][2][3][4], their applications are often hindered by the extreme susceptibility to grain growth. For example, nanocrystalline Al, Sn, Pb, and Mg are subjected to grain growth even at room temperature [5][6][7]. There are two general approaches to stabilize nanocrystalline materials against grain growth [8][9][10][11]. First, kinetic stabilization by the solute-drag effects and/or Zener (particle) pinning can slow down grain boundary (GB) migration [12][13][14], which become less effective at high temperatures. Second, thermodynamic stabilization can be achieved by reducing the GB energy (γ GB ) via solute segregation (a.k.a. GB adsorption) to reduce the thermodynamic driving force for grain growth. Specifically, Weismuller originally proposed that an "equilibrium" grain size can be reached when the effective γ GB vanishes [15,16]. Yet, Kirchheim [17] showed that the equilibriumgrain-size binary nanocrystalline alloys generally represent metastable states in supersaturated regions if (and only if) the precipitation is hindered kinetically. In such cases, the kinetic inhibition of the precipitation also becomes more difficult with increasing temperature, triggering abrupt grain growth. These challenges motivate the present study to develop and test new theories and strategies to enhance the thermal stability of nanocrystalline alloys at high temperatures via utilizing highentropy GB complexions (where the term "complexion" refers to an interfacial "phase" that is thermodynamically two-dimensional [18] The Gibbs adsorption theory states:where S XS (entropy) and Γ i (adsorption) are the GB excess quantities, T is temperature, and μ i is the chemical potential of the i-th element. Eq.(1) implies that the GB energy (γ GB ) can be reduced by segregation at a constant T and this effect can be enhanced for multicomponent (high-entropy) GBs under certain conditions. We should note that generally (∂γ GB /∂ T) P , X i N 0 for a specimen of a constant bulk composition (X Bulk i ) due to thermally-induced desorption. In a recent article [26], we proposed that a high-entropy GB effect can be achieved for a saturated specimen (in equilibrium with precipitates), when the bulk composition moves long the solvus line, where the solutes' bulk chemical potentials are pinned by the precipitates so that (∂ γ GB / ∂ T) P , X Bulk i on solvus ≈ (∂ γ GB /∂ T) P , μ i = − S XS ; thus, γ GB can be reduced with increasing temperature for a high-entropy GB with positive and