Why Doping Strengthens Grain Boundaries

Ceramic engines have been a topic of research for decades because they are more lightweight and fuel-efficient than metal engines. So why aren’t we seeing cars and trucks with ceramic engines on the streets yet? The biggest problem with ceramics is durability. Their microscopic structure is granular, and under heat and pressure, the boundaries between the grains can act like tiny seismic faults, allowing small-scale slippage that may grow into cracks and fractures—a dangerous possibility in an engine.

Aluminum oxide or alumina (Al2O3) is one of the most promising ceramics for engines because of its hardness—it is widely used for abrasives, like sandpaper, and cutting tools. One drawback is that at high temperatures, alumina is prone to microscopic creep at grain boundaries. However, researchers have found that doping alumina with rare earth elements, such as yttrium (Y), improves its resistance to creep. The dopant has been shown to settle in the grain boundaries, but how it prevents creep at the atomic scale has been controversial.

Now a collaboration of researchers from the universities of Tokyo and Missouri–Kansas City may have settled the issue. They examined both undoped and doped grain boundaries with scanning transmission electron microscopy (STEM), then analyzed the grain boundary structure and bonding using a combination of static lattice and first principles calculations.[1]

Figure 1 shows STEM images of undoped (A, B) and yttrium-doped (C, D) alumina grain boundaries. The orange spots correspond to atomic columns of aluminum (the oxygen is not visible), and the yellow spots in C and D are yttrium columns. The schematic overlay in B and D highlights the periodic structural units along the boundary plane, with seven-member rings of Al ions forming a large open structure. These images reveal that Y doping does not alter the basic grain boundary structure; instead, Y simply replaces Al at the center of some seven-member rings in the grain boundary.

The theoretical part of the study was conducted by Wai-Yim Ching, Curators’ Professor of Physics at the University of Missouri–Kansas City, along with Japanese researchers and post-doctoral fellow Jun Chen. They first used static lattice calculations to determine the lowest energy structure of the undoped grain boundary. The calculated structure reproduced the experimentally observed seven-member ring structure at the grain boundary (Figure 2).

Figure 1. STEM images of undoped and yttrium-doped alumina grain boundaries. (A) Undoped alumina; (B) same image with overlay to illustrate the aluminum atomic column arrangement; (C) yttrium-doped alumina; (C) same image with structural overlay. (Click image for larger view)		Figure 2. Theoretical grain boundary structure obtained by static lattice calculations. Aluminum atoms are white, oxygen blue. Bold lines mark the grain boundary structure as observed in the STEM images. Yttrium segregation energies were investigated for columns a through p, and column m showed the lowest segregation energy. (Click image for larger view)

To locate the most energetically stable site for Y segregation, the theoretical researchers substituted a single Y ion in various columns at or near the grain boundary (Figure 2, a through p). Site m, in the middle of a seven-member ring, had the lowest segregation energy, just as the experiment showed. The main difference between Y and Al ions is their ionic radius: 67.5 picometers for Al, and 104 picometers for Y. The researchers believe the larger area within the seven-member ring can accommodate the larger ion better than a six-member ring.

To investigate local atomic bonding and charge distributions, the researchers then used ab initio calculations to construct a large periodic supercell with 700 atoms. Figure 3 shows charge density maps for the undoped (A) and Y-doped (B) grain boundaries. White circles show the location of Al ions; graduated blue spots show the charge density from O ions; and the yellow circle in B is a Y ion. Figure 3A shows sharp nodes between the O charge densities and the charge density from the Al ion in the center of the seven-member ring. In contrast, Figure 3B shows that the O electron densities are elongated toward the Y ion, indicating a stronger covalency (sharing of electrons) between the Y–O bonds. Further calculations showed that more bonds formed between Y and O ions than between Al and O ions; the larger number of bonds in the Y-doped case contributed to lowering the grain boundary energy.

Figure 3. Charge density map for undoped (A) and Y-doped (B) grain boundaries. (Click image for larger view)

Although the actual mechanism for grain boundary creep is still not well understood, this study advances our understanding of creep resistance. Creep requires the continuous breaking and reforming of atomic bonds as two grains move in opposite directions. Grain boundaries with more bonds and higher bond strength, will, therefore, be more resistant to creep. The undoped seven-member rings in this study have fewer bonds than the interior of the grain, which is why the grain boundaries are mechanical weak points. But the Y-doped rings have more and stronger bonds between Y and O ions, which explains why Y doping increases creep resistance in alumina.

Ching sees the significance of these results in a larger context. “This work demonstrates the importance of combining theoretical computation and experimental observation, the effectiveness of international collaboration, and the need of top-line supercomputers for modern materials research,” he said.

[1] J. P. Buban, K. Matsunaga, J. Chen, N. Shibata, W. Y. Ching, T. Yamamoto, and Y. Ikuhara, “Grain boundary strengthening in alumina by rare earth impurities,” Science 311, 212 (2006).