Achieving room temperature plasticity in brittle ceramics through elevated temperature preloading

Abstract

Ceramic materials with high strength and chemical inertness are widely used as engineering materials. However, the brittle nature limits their applications as fracture occurs before the onset of plastic yielding. There has been limited success despite extensive efforts to enhance the deformability of ceramics. Here we report a method for enhancing the room temperature plastic deformability of ceramics by artificially introducing abundant defects into the materials via preloading at elevated temperatures. After the preloading treatment, single crystal (SC) TiO₂ exhibited a substantial increase in deformability, achieving 10% strain at room temperature. SC α-Al₂O₃ also showed plastic deformability, 6 to 7.5% strain, by using the preloading strategy. These preinjected defects enabled the plastic deformation process of the ceramics at room temperature. These findings suggest a great potential for defect engineering in achieving plasticity in ceramics at room temperature.

Fig. 1.. Schematics of the comparison of micropillars compressed at RT with and without the treatment of preloading at elevated temperatures (the pillar top is upright).

HT, high temperature.

Fig. 2.. Uniaxial in situ microcompression tests on the SC TiO₂ at RT, 600°C, and 600°C preloading/RT compression at a constant strain rate of 5 × 10⁻³ s⁻¹.

(A to D) A representative stress-strain curve of SC TiO₂ tested at RT. The pillars experienced brittle failure at the strain of ~3% accompanied by the propagation of cracks. (E to H) For micropillars tested at 600°C, the shear band emerged at the strain of 6%. Evident shear bands were generated with successive compression without brittle failure. The pillar has a flow stress of ~1.0 GPa. (I to L) Micropillars were first compressed at 600°C to the yield point and cooled to the RT. During the RT compression test, flow stress increased continuously to 6.5 GPa, accompanied by serrations and load drops. Shear bands were generated, and the compressive strain reached 10% without brittle failure.

Fig. 3.. TEM micrograph showing defects formation in SC TiO₂ micropillar after elevated temperature preloading and elevated temperature preloading/RT compression.

(All the pillar top is upright). (A) Low-magnification TEM micrograph of a pillar preloaded to 2.5% strain at 600°C. A high density of dislocations nucleated on the pillar top. (B and C) Magnified TEM images from the [100] zone axis shows high-density dislocations. (D) Low-magnification TEM micrograph of a pillar compressed to ~5.5% strain at RT after elevated temperature preloading. Shear bands formed throughout the entire pillar. A higher density of defects was generated near the pillar top compared to the pillar bottom. (E and F) TEM micrograph of a pillar compressed to ~11% strain at RT after elevated temperature preloading. A portion of the SC TiO₂ on the bottom half of the pillar underwent grain fragmentation.

Fig. 4.. Uniaxial in situ microcompression tests on the SC Al₂O₃ at RT, 740°C, and RT compression after elevated temperature preloading at a constant strain rate of 5 × 10⁻³ s⁻¹.

(A to D) A representative stress-strain curve of SC Al₂O₃ tested at RT. SEM snapshots show that the pillars experienced a catastrophic brittle failure at the strain of 4% throughout the entire pillar. (E to K) For micropillars tested at 740°C, cracks initiated at a strain of ~7%. The pillar has a flow stress of 1 GPa at a strain of 12%. (L to P) Micropillars were first preloaded at 740°C to the yield point and cooled to the RT to continue the RT compression test. Many microcracks were generated under further RT loading. The pillar experienced failure at a strain of 6%.

Fig. 5.. TEM and SEM micrograph showing defects formation in SC Al₂O₃ micropillar after elevated temperature preloading and elevated temperature preloading/RT compression (the pillar top is upright in all images, and the compression direction is from pillar top to the bottom).

(A) A BF TEM micrograph of the micropillar shows the formation of twin boundaries from the top left corner to the middle. (B) A magnified TEM image of the parallel twin boundaries. (C) A high-resolution TEM micrograph shows the twin boundary decorated with SFs. (D) The corresponding IPF exhibits the defective twin boundaries with serrated steps. (E) Low-magnification TEM micrograph of a pillar compressed to ~7% strain at RT after elevated temperature preloading showing the intersection of two slip bands. (F) A magnified TEM image of the high-density dislocations formed near the pillar top and the middle of the pillar. (G) A DF TEM micrograph showing the dissociation of full dislocations into two partials. (H) An IPF exhibiting some subgrain rotation near the pillar top. (I) SEM micrograph shows a pillar compressed to 5% with a dilated top, and the intersected twin boundaries appeared on the peripheral surface. (J and K) The BF TEM micrograph shows more dislocations concentrated near the twin boundary located at the pillar bottom. (L) The BF TEM micrograph shows that a deflected microcrack was decorated with a high density of dislocations.