Segregation of solute atoms (Y, Nb, Ta, Mo and W) in ZrB2 grain boundaries and their effects on grain boundary strengths: A first-principles investigation

ZrB2 based ultra-high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties that makes them promising candidates for applications in extreme environments. Evaluating the correlation between microscopic defects and macroscopic performance of these materials is crucial for the design of UHTCs. The present work deals with a first-principles investigation on segregations of solute atoms (Y, Nb, Ta, Mo and W) in ZrB2 grain boundaries and their influences on grain boundary strengths. Opposite segregation tendency between Y and Nb, Ta, Mo or W is obtained, where Y prefers sites with long M-B bonds, while Nb, Ta, Mo or W prefers sites with short M-B bonds. The short equilibrium M-B (M = Nb, Ta, Mo or W) bonds induce local contractions around grain boundaries, which in turn strengthens grain boundaries remarkably, thereby enhances the mechanical properties of ZrB2 at evaluated temperatures. In contrast, segregation of Y poisons grain boundaries due to local expansions induced by long Y-B bonds, which will deteriorate the performance of ZrB2 based UHTCs. The results provide useful guidelines for the design of ZrB2 based UHTCs, since grain boundaries play a key role in determining high temperature mechanical properties. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.