It is well known that surface energy differences thermodynamically stabilize nanocrystalline γ-Al2O3 over α-Al2O3. Here, through correlative ab initio calculations and advanced material characterization at the nanometer scale, we demonstrate that the metastable phase formation of nanocrystalline TiAlN, an industrial benchmark coating material, is crystallite size-dependent. By relating calculated surface and volume energy contributions to the total energy, we predict the chemical composition-dependent phase boundary between the two metastable solid solution phases of cubic and wurzite Ti1−xAlxN. This phase boundary is characterized by the critical crystallite size d critical. Crystallite size-dependent phase stability predictions are in very good agreement with experimental phase formation data where x was varied by utilizing combinatorial vapor phase condensation. The wide range of critical Al solubilities for metastable cubic Ti1−xAlxN from x max = 0.4 to 0.9 reported in literature and the sobering disagreement thereof with DFT predictions can at least in part be rationalized based on the here identified crystallite size-dependent metastable phase formation. Furthermore, it is evident that predictions of critical Al solubilities in metastable cubic TiAlN are flawed, if the previously overlooked surface energy contribution to the total energy is not considered.
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机译:众所周知,表面能差异在热力学上使纳米晶γ-Al2O3稳定在α-Al2O3之上。在这里,通过相关的从头算和纳米级的先进材料表征,我们证明了纳米晶TiAlN(一种工业基准涂料)的亚稳态相形成与晶粒尺寸有关。通过将计算出的表面和体积能量贡献与总能量相关联,我们预测了立方和纤锌矿Ti1-xAlxN的两个亚稳态固溶体相之间的化学成分相关的相界。该相界的特征在于临界微晶尺寸dcritical。微晶尺寸相关的相稳定性预测与实验相形成数据非常吻合,其中通过利用组合气相凝结来改变x。文献中报道,亚稳态立方Ti1-xAlxN的临界Al溶解度范围从x max = 0.4到0.9很大,并且其与DFT预测的清醒分歧可以至少部分地基于此处确定的与晶粒尺寸有关的亚稳态相形成来合理化。此外,很明显,如果不考虑先前忽略的表面能对总能的贡献,则对亚稳立方TiAlN中临界Al溶解度的预测是有缺陷的。
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