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Ab initio-calculated ideal strength and toughness describe the upper limitsfor mechanical properties attainable in real systems and can, therefore, beused in selection criteria for materials design. We employ density-functionalab initio molecular dynamics (AIMD) to investigate the mechanical properties ofdefect-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 roomtemperature. We determine the alloys' ideal strength and toughness, elasticresponses, and ability to plastically deform up to fracture as a function ofthe Al content. Overall, TiN exhibits greater ideal moduli of resilience andtensile strengths than TiAlN solid solutions. Nevertheless, AIMD modelingshowsthat, irrespective of the strain direction, the binary compound systematicallyfractures by brittle cleavage at its yield point. The simulations also indicatethat Ti0.5Al0.5N and Ti0.25Al0.75N solid solutions are inherently moreresistant to fracture and possess much greater toughness than TiN, due to theactivation of local structural transformations (primarily of B1 -> wurtzitetype) beyond the elastic-response regime. In sharp contrast, TiAlN alloys with25% Al exhibit similar brittleness as TiN. The results of this work areexamples of the limitations of elasticity-based criteria for prediction ofstrength, brittleness, ductility, and toughness in materials able to undergophase transitions with loading. Furthermore, comparing present and previousfindings, we suggest a general principle for design of hard ceramic solidsolutions that are thermodynamically inclined to dissipate extreme mechanicalstresses via transformation toughening mechanisms.

Strength, transformation toughening and fracture dynamics of rocksalt-structure Ti1-xAlxN (0 <= x <= 0.75) alloys