Grain Boundary Strengthening in Alumina by Rare Earth Impurities

Abstract

Impurity doping often alters or improves the properties of materials. In alumina, grain boundaries play a key role in deformation mechanisms, particularly in the phenomenon of grain boundary sliding during creep at high temperatures. We elucidated the atomic-scale structure in alumina grain boundaries and its relationship to the suppression of creep upon doping with yttrium by using atomic resolution microscopy and high-precision calculations. We find that the yttrium segregates to very localized regions along the grain boundary and alters the local bonding environment, thereby strengthening the boundary against mechanical creep.

Atomistic modeling of dopant segregation in α-alumina ceramics: Coverage dependent energy of segregation and nominal dopant solubility

“…La, Gd and Yb show a tendency for formation of complete layers at low-energy, low Σ grain boundaries, as opposed to previously reported patterns for the slightly smaller Y. 6,21 For more general grain boundaries these high concentrations are not observed although local minima may exist.…”

“…Grain boundary segregation rather than second phase precipitation is the cause of these effects, as they occur below the solubility limit. 43,46,47,50 Initial suggestions that La dopants promote the formation of low-energy, low-Σ boundaries could not be substantiated 38 and the prevalent opinion in recent literature is that both dopants reduce grain boundary diffusion 21,38,47,48 and grain boundary dislocation climb/slide 33,49 due to site blocking. Due to the higher segregation energy calculated here and elsewhere 35,36 the interface concentration of La at the same overall dopant concentration and grain size is expected to be higher than for Y.…”

“…As high resolution TEM images of doped ␣-alumina grain boundaries [19][20][21] are rare, one challenge in the present study was to link the obtained simulation results to experimental observations on ceramics. Moreover as TEM experiments are usually carried out on well-defined bicrystals, they can be used to validate the atomistic simulation approach but do not allow a link with sintered ceramic microstructures containing a series of different grain boundaries.…”

“…It is however interesting to note that the calculated Mg-O coordination number is lower than the one for La-O, which will further contribute to a higher mobility compared to La dopants and thus to higher diffusion rates. 21 The average predicted equilibrium solubility given in Table 4 is 4.32 Mg/nm 2 for surfaces and 2.18 Mg/nm 2 for grain boundaries (2.61 Mg/nm 2 for high-energy, high-Σ grain boundaries -see Supporting Information Table S4 for coordinative environment). This is considerably lower than the values calculated for La but is close to the predicted solubility for Y.…”

Prospects for 3D Imaging of Dopant Atoms in Ceramic Materials

“…This has been used recently to explore the detailed configuration of impurity atoms segregated at grain boundaries in ceramic materials [3], where such dopant addition can significantly change the physical properties of the material, particularly its resistance to creep deformation [4]. At best, a single image only gives a clear indication of the projected structure along one direction.…”