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rials are generally brittle and would suffer from catastrophic fracture upon mechanical loading beyond their elastic limit.[1,2] Various studies have shown that a brittle-to-ductile transition (BDT)[1,3–8] may take place when the ceramic is deformed at elevated temperatures[9] or when a high hydrostatic pressure is superimposed on the sample during room temperature deformation.[10] The underlying mechanisms for BDT have been proposed to be structural transformation (amorphization[11,12] or and/or dislocation phase transformation[8,13,14]) movement.[1,14] Recent developments on small-scale mechanics revealed that, in addition to temperature and pressure confinement, reducing the external dimension of the

XIAOLEI GUO, QIANG GUO, ZHIQIANG LI, GENLIAN FAN, DING-BANG XIONG, YISHI SU, JIE ZHANG, ZHANQIU TAN, CUIPING GUO, and DI ZHANG are with the State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China. Contact e-mails: [email protected]; [email protected] Manuscript submitted May 8, 2017

METALLURGICAL AND MATERIALS TRANSACTIONS A

specimen down to the micro-/nano-scale is another way to achieve moderate ductility in ceramics,[3,4,7] which has been demonstrated in materials such as silicon,[3] MgO,[5] MgAl2O4[6] and SiC.[1,10,14] For example, in the case of SiC, Shin et al.[10] reported that 3C-SiC pillars would show a transition from elastic response to discrete plastic flow under uniaxial compression when its diameter decreases below 650 nm, and traces characteristic of particular slip systems were observed on the surface of the deformed pillars. To further probe the plastic deformation mechanism of SiC at small scales, both Chen et al.[14] and Kiani et al.[1] carried out in situ compression tests of a-SiC (4H and 6H type, respectively) single crystalline nano-pillars in transmission electron microscopes (TEM). For the 180-nm-diameter 4H-SiC pillar compressed along 45 deg off the basal plane (so that the Schmid Factor, SF, is non-zero and has a large value),[14] a non-linear stress-strain response occurred once the stress reached 10.2 GPa. Electron diffractometry analysis showed that a 4H-3C phase transformation took place in some of the regions in the deformed pillars, which was supposed to be triggered by a stick–slip process on the basal plane, as revealed by molecular dynamics (MD) simulations. In comparison, for 6H-SiC nano-pillars with diameters ranging from 170 to 280 nm, basal slip on the shuffle set along h1 100i were argued to be responsible for their plastic deformation[1] when the pillar had a non-zero SF, while for pillars having SF = 0, they all failed catastrophically via brittle fracture. The above-mentioned studies indicate that the plastic deformation of small-volume ceramics depends on both the size of the specimen and its crystallographic orientation, i.e., the resolved shear stress on the slip system. However, whether the sample size and its crystallographic orientation have a combined effect on the mechanical behavior is still unknown. In this