Brittle-to-ductile transition and slip band structure in ionic crystals

Abstract

Abstract The interrelation between the brittle-to-ductile transition (BDT) and slip structure has been examined using NaCl and KCl crystals. In pre-cracked crystals of NaCl and KCl having a ⟨100⟩ axis, the fracture toughness K*ic increases above 300 and 220 K, and the BDT occurs at 510 and 430 K respectively. Scale-like wavy lines are revealed ahead of a pre-crack on etched fracture faces of NaCl tested above 300 K. As the temperature increases, the step height and density of the lines are increased but the lines are more restricted near the pre-crack front. In NaCl and KCl crystals without pre-cracking, the critical resolved shear stress for {110} main slip is very low above 77 K, while that for {010} slip markedly increases as temperature decreases below 500 and 300 K respectively. In ⟨100⟩-uniaxial tests for non-pre-cracked NaCl crystals, the BDT takes place at around 380 and 480 K at strain rates of 5.5 × 10−6 and 5.5 × 10−5s−1 respectively. Above the BDT temperature, wavy slip lines are abundantly produced by the marked operation of cross-slip. A mechanism based on the crack tip shielding through the cross-slip of dislocations along a crack front is proposed to explain observed crack face morphology as well as the BDT in ionic crystals of NaCl structure.