Basal slip latent hardening by prism plane slip dislocations in sapphire (α-Al2O3)

The properties of sapphire (α-Al2O3) predeformed at T = 1450 °C by 〈101¯0〉{12¯10} prism plane slip and subsequently deformed in 〈12¯10〉(0 0 0 1) basal slip between 1050 and 1250 °C were investigated. The critical resolved shear stress in basal slip was increased with respect to the non-predeformed samples by a temperature-independent increment of Δτ = 43.6 MPa consistent with a forest dislocation mechanism. The deformation microstructures were investigated by transmission electron microscopy. Prism plane slip involves 〈101¯0〉 dislocations that are essentially decomposed into two 1/3〈12¯10〉 dislocations separated by a constant distance of 10 nm. No evidence of the dissociation into three partials could be found. Reactions between dislocations of the most favored P slip system (12¯10)[1¯010], and of the other two less preferred P systems were frequently observed forming dipoles and junctions at moderate and high temperatures, respectively. The samples subsequently deformed in basal slip exhibit junction reactions between dislocations of the two systems. The predeformation tests were designed to engender 1/3〈101¯0〉 partial dislocations presumed to be at the origin of basal twinning but no basal twins were observed.