  Understanding the competition between brittleness and plasticity in
refractory ceramics is of importance for aiding design of hard materials with
enhanced fracture resistance. Inspired by experimental observations of crack
shielding due to dislocation activity in TiN ceramics [Int J Plast 27 (2011)
739], we carry out comprehensive atomistic investigations to identify
mechanisms responsible for brittleness and slip-induced plasticity in Ti-N
systems. First, we validate a semi-empirical interatomic potential against
density-functional theory results of Griffith and Rice stress intensities for
cleavage (KIc) and dislocation emission (KIe) as well as ab initio molecular
dynamics mechanical-testing simulations of pristine and defective TiN lattices
at temperatures between 300 and 1200 K. The calculated KIc and KIe values
indicate intrinsic brittleness, as KIc<<KIe. However, KI-controlled molecular
statics simulations - which reliably forecast macroscale mechanical properties
through nanoscale modelling - reveal that slip-plasticity can be promoted by a
reduced sharpness of the crack and/or the presence of anion vacancies.
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. 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. 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 (Iplast) of pristine
Ti-N crystals. We propose that the Iplast descriptor should be considered for
ranking the ability of ceramics to blunt cracks via dislocation-mediated
plasticity at finite temperatures.

Materials Science