Region of crystal under slip plane contains two extra ( 10 1 ¯) atomic layers. Process generates 〈 1 1 ¯ 0 〉 edge dislocation. Increase in strain to 9% (d), (d′) assists penetration of stacking fault into lattice via (101) slip. In (c), (c′), portion of crystal under slip plane enters lattice and creates stacking-fault defect: note that and are no longer aligned. Plots in (b), (b′) evidence concerted breakage of Ti–N bonds linking A-B lattice rows at notch front. Snapshots (a), (a′) show two-atom-height notch tip, that is, one dislocation was previously emitted from one-atom-sharp tip. In (a′), dotted green lines mirror solid blue lines across slip plane, indicating aligned Ti and N lattice rows prior to slip. Typical atomistic pathway for nucleation and emission of edge dislocation at notch tip of TiN subject to tensile strain (a)–(f), with corresponding magnifications in panels (a′)–(e′).
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We propose that the I plasticity slip descriptor should be considered for ranking the ability of ceramics to blunt cracks via dislocation-mediated plasticity at finite temperatures. Furthermore, we show that the probability to observe slip-induced plasticity leading to crack blunting in flawed Ti-N lattices correlates with the ideal tensile/shear strength ratio ( I plasticity slip ) of pristine Ti-N crystals. Although crack growth occurs in most cases, a sufficiently rapid accumulation of shear stress at the notch tip may postpone or prevent fracture via nucleation and emission of dislocations. Classical molecular dynamics simulations of notched Ti-N supercell models subject to tension provide a qualitative understanding of the competition between brittleness and plasticity at finite temperatures. However, K I-controlled molecular statics simulations-which reliably forecast macroscale mechanical properties through nanoscale modeling-reveal that slip plasticity can be promoted by a reduced sharpness of the crack and/or the presence of anion vacancies. The calculated K Ic and K Ie values indicate intrinsic brittleness, as K Ic ≪ K Ie. First, we validate a semiempirical interatomic potential against density-functional theory results of Griffith and Rice stress intensities for cleavage ( K Ic) and dislocation emission ( K Ie) as well as ab initio molecular dynamics mechanical-testing simulations of pristine and defective TiN lattices at temperatures between 3 K. Inspired by experimental observations of crack shielding due to dislocation activity in TiN ceramics, we carry out comprehensive atomistic investigations to identify mechanisms responsible for brittleness and slip-induced plasticity in Ti-N systems.
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Understanding the competition between brittleness and plasticity in refractory ceramics is of importance for aiding design of hard materials with enhanced fracture resistance.