Descriptor for slip-induced crack-blunting in refractory ceramics

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.