Strength, transformation toughening, and fracture dynamics of rocksalt-structure Ti1−xAlxN

Davide Sangiovanni

¹

,

Ferenc Tasnádi

²

,

Lars Johnson

³

et al.

Abstract

3Sandvik Coromant, 126 80 Stockholm, Sweden Ab initio-calculated ideal strength and toughness describe the upper limits for mechanical properties attainable in real systems and can, therefore, be used in selection criteria for materials design. We employ density-functional ab initio molecular dynamics (AIMD) to investigate the mechanical properties of defect-free rocksalt-structure (B1) TiN and B1 Ti1-xAlxN (x = 0.25, 0.5, 0.75) solid solutions subject to [001], [110], and [111] tensile deformation at room temperature. We determine the alloys' ideal strength and toughness, elastic responses, and ability to plastically deform up to fracture as a function of the Al content. Overall, TiN exhibits greater ideal moduli of resilience and tensile strengths than (Ti,Al)N solid solutions. Nevertheless, AIMD modeling shows that, irrespective of the strain direction, the binary compound systematically fractures by brittle cleavage at its yield point. The simulations also indicate that Ti0.5Al0.5N and Ti0.25Al0.75N solid solutions are inherently more resistant to fracture and possess much greater toughness than TiN due to the activation of local structural transformations (primarily of B1 → wurtzite type) beyond the elasticresponse regime. In sharp contrast, (Ti,Al)N alloys with 25% Al exhibit similar brittleness as TiN. The results of this work are examples of the limitations of elasticity-based criteria for prediction of strength, brittleness, ductility, and toughness in materials able to undergo phase transitions with loading. Comparing present and previous findings, we suggest a general principle for design of hard ceramic solid solutions which are thermodynamically inclined to dissipate extreme mechanical stresses via transformation toughening mechanisms.

Bulk high-entropy nitrides and carbonitrides

“…Increasing metallic bond character, which is associated with increasing anion vacancy concentration and increasing VEC, is found to have a negative effect on hardness and elastic modulus. The finding of a negative correlation of both hardness and modulus with VEC in bulk HENs and HECNs fits well with both existing computational 40 , 41 , 51 , 65 , 66 and experimental 7 , 9 results, which demonstrate that bonding and electronic structure govern the inherent mechanical properties in bulk rock-salt structured high-entropy carbides and nitrides. Purposeful design of high-entropy ceramic compositions’ configurational entropy and VEC provides the ability to tune mechanical properties and high temperature phase stability in cubic high-entropy ceramics.…”

Temperature-dependent elastic properties of binary and multicomponent high-entropy refractory carbides

“…The procedure for AIMD stress/strain calculations is detailed in Refs. [32][33][34]. The B1 TMC supercells used in our simulations have three different crystallographic orientations (Fig.…”

“…Recently, we have reported on an ab initio molecular dynamics (AIMD) approach for computing stress/strain curves of single-crystal B1 ceramics subject to tensile and shear deformation at any temperature of interest [32][33][34]. The simulations employ deformations well beyond the elastic limit conventionally used to calculate the elastic constants [28][29][30][31], and allow evaluating mechanical properties including, e.g., ideal strengths, moduli of resilience, toughness, and elongation at fracture.…”

“…The method detailed in Refs. [32][33][34] has been used to produce the full stress/strain curves in rocksaltstructure (B1) transition-metal carbides (TMC) at temperatures 300 ≤ T ≤ 1200 K [35]. In order to determine the most essential parameters that characterize the mechanical response of the studied systems, we here exploit the stress/strain relationships evaluated in the elastic regime [35] to extract the second-order elastic constants (SOEC) of B1 TiC, ZrC, HfC, VC, and TaC, as well as highentropy (Ti,Zr,Hf,Ta,W)C and (V,Nb,Ta,Mo,W)C. The fact that our approach utilizes six different deformation pathways (see Section 2.1) provides SOEC with reduced numerical dependence on the specific choice of strain matrixes and strain intervals [4].…”

Adaptive hard and tough mechanical response in single-crystal B1 VNx ceramics via control of anion vacancies

“…Our model supercells are oriented in a convenient manner for investigating the response of Tensile testing is modeled for VNx(001) and VNx(110) supercells with 768 lattice positions (24 layers orthogonal to the strain direction; 32 sites per layer), following the procedure of Ref. [36,37]. Briefly, at each zz strain step (1% of the supercell vertical size), the systems are first coupled to an isokinetic, and then to a Nosé-Hoover, thermostat.…”