Size effects with regard to strength in cubic metallic systems have been widely studied using micromechanics [1,2]. In more complex oxide systems, such as trigonal Hematite (Fe2O3), plasticity is limited at large length-scales by the inherent brittleness owing to the covalent bonding and insufficient deformation modes due to low crystal symmetry. The present study aims to identify the active deformation modes of Hematite via testing at small length-scales and reporting the size effects associated with each deformation mode under ambient conditions. Hematite single crystals with 3 different surface orientations (0001), (0-110) and (1-102) were used to activate various deformation systems using a spherical nanoindentation counterbody. Orientation co-related electron microscopy imaging was done to characterise the linear surface features around the indents. Basal {c}<a>, prismatic {a}<m> and Pyramidal <c+a> slip along with rhombohedral {R}-twinning were identified as active deformation modes. The critical resolved shear stress (CRSS) values in conjunction with surface trace analysis and transmission electron microscopy showed both slip and twinning can be activated individually in micropillar compression. The CRSS values for R twin and {a}<m> slip decreased with increasing pillar diameter from 0.85 to 4.5 μm, following an inverse power law relationship with power law exponents of 0.30 and 0.51, respectively. The difference in power law exponent between slip and twinning is understood with respect to initial dislocation density and a ‘stimulated slip’ model [3].

References:

1. Lee, Seok-Woo, and William D. Nix. "Size dependence of the yield strength of fcc and bcc metallic micropillars with diameters of a few micrometers." Philosophical Magazine 92 (2012): 1238-1260.

2. Parthasarathy, Triplicane A., et al. "Contribution to size effect of yield strength from the stochastics of dislocation source lengths in finite samples." Scripta Materialia 56 (2007): 313-316.

3. Yu, Qian, et al. "Strong crystal size effect on deformation twinning." Nature 463 (2010): 335-338.