Nanoscale stacking fault-assisted room temperature plasticity in flash-sintered TiO2

Abstract

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO₂ prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO₂ can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO₂ is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO₂ deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed.

Fig. 1. Microstructures of TiO₂ prepared by flash sintering and comparison of temperature-dependent deformability of flash-sintered TiO₂ with other typical ceramic materials.

(A) Schematics of flash sintering of TiO₂. (B) SEM image of the unpolished flash-sintered TiO₂. (C) Bright-field TEM micrograph showing an area of the flash-sintered TiO2 containing stacking faults. (D and E) TEM micrographs showing high-density dislocations within TiO₂ grains. (F) Ultimate/fracture strain (%) as a function of testing temperatures for flash-sintered TiO₂, conventional TiO₂ (28, 30), and other conventional ceramic systems, including SiCN (31), YSZ (32), and TiC (33). Fracture strain is very low at low temperatures and increases with temperatures for most of the conventional ceramics. However, the flash-sintered TiO₂ exhibits significantly enhanced deformability even at RT (solid magenta circle data points).

Fig. 2. Uniaxial in situ microcompression tests on the conventional and flash-sintered TiO₂ at different temperatures, RT versus 400°C, at a constant strain rate of 5 × 10⁻³ s⁻¹.

(A1 to A6) Representative stress-strain curves of conventional sintered TiO₂ tested at RT and 400°C; all pillars experienced brittle catastrophic fractures at an average true strain of 2% for RT tests and 3% for 400°C tests. (B1 to B6) Stress-strain curves of flash-sintered TiO₂ tested at RT showing work hardening to a maximum flow stress of 2 to 2.5 GPa and relatively continuous flow stress-strain curves with small serrations. Stress-strain curves of different colors were obtained from different individual pillars. The in situ movie SEM snapshots of a pillar (red data in Fig. 2B1) compressed to different strain levels show the formation of successive high-density slip bands. No fracture was observed up to a strain of 8%. (C1 to C6) At 400°C, large isolated serrations manifested by sharp load-drops were observed. Each load-drop usually corresponds to the formation of a major shear band as indicated by arrows. The inset of (C6) shows a magnified view of multiple shear bands generated during deformation (see movies S1 and S2 for more details on conventional sintered pillars and movies S5 and S6 for more details on flash-sintered pillars).

Fig. 3. TEM micrographs of several flash-sintered TiO₂ pillars after compression tests at different temperatures to a similar strain (8 to 10%).

Loading direction is indicated by yellow arrows in each figure. All images were taken from the [010] zone axis. (A1) Low-magnification TEM image of a pillar compressed at RT. Most of the wide straight shear bands are stacking faults (SFs). (A2 and A3) Examples of the high-density stacking faults formed on two sets of {101} planes with an intersection angle of 66˚. (A4) HRTEM micrograph showing a twin boundary decorated with stacking faults. (B1) Low-magnification TEM image of a pillar compressed at 400°C. Most of the wide straight shear bands are twin boundaries. (B2) Examples of deformation twins as confirmed by the insert SAD pattern. (B3) HRTEM images of typical twin boundaries that are either sharp or decorated with stacking faults. (B4) Dark-field TEM image of stacking fault segments formed after compression. (C1) Low-magnification TEM image of the top part of a pillar (~2 μm from the surface) compressed at 600°C. No shear bands were observed. (C2) High-density dislocations and (C3) stacking fault segments formed near the pillar top surface. (C4) Example of a stacking fault segment. Partials cross-slipped on two different {101} planes with an intersection angle of 66°.

Fig. 4. High-resolution STEM micrographs of twin boundaries in the flash-sintered TiO₂ after compression at 400°C and DFT calculations of generalized stacking fault energy in TiO₂.

(A) iDPC image of a twin boundary showing the atomic arrangement of both Ti and O columns. For clarity, Ti columns above and below the twin boundary are shown in yellow and green, respectively. Further analyses of a magnified area as shown by red dashed lines are shown in (B) to (D). (B) Estimated atomic structure. (C) Simulated DPC image without aberrations. (D) Experimental observation. Selected Ti and O columns close to the twin boundary are shown by open circles. O columns surrounding Ti on the plane TB1 (next to the real twin interface TB0) have relatively dimmer contrast as shown by small open red circles, indicating the O deficiency close to the twin boundary. (E) Variation of generalized stacking fault (GSF) energies with displacement. The unstable stacking fault energy is high, ~1.9 J/m², while the stable stacking fault energy is much lower, 30 to 40 mJ/m². (F) Stacking fault and twin fault formation energy versus O vacancy position. The embedded figures show (left) a basic stacking fault (unit cell vector shift) and (right) a basic twin fault (supercell). The solid line indicates the fault energy without a vacancy.