Abstract

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 TiAlN solid solutions. Nevertheless, AIMD modelingshows 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 elastic-response regime. In sharp contrast, TiAlN 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. Furthermore, comparing present and previous findings, we suggest a general principle for design of hard ceramic solid solutions that are thermodynamically inclined to dissipate extreme mechanical stresses via transformation toughening mechanisms.

Highlights

  • Hard, refractory rocksalt-structure (B1) titanium aluminum nitride [(Ti,Al)N] ceramics are extensively applied as wear and oxidation resistant protective coatings on cutting tools and engine components [1,2]

  • Comparing present and previous findings, we suggest a general principle for design of hard ceramic solid solutions that are thermodynamically inclined to dissipate extreme mechanical stresses via transformation toughening mechanisms

  • ab initio molecular dynamics (AIMD) simulations at 300 K are used to determine the inherent tensile strength, toughness, and resistance to fracture of defect-free B1 Ti1−xAlxN solid solutions (0 x 0.75)

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Summary

Introduction

Refractory rocksalt-structure (B1) titanium aluminum nitride [(Ti,Al)N] ceramics are extensively applied as wear and oxidation resistant protective coatings on cutting tools and engine components [1,2]. Far-from-equilibrium synthesis methods as, e.g., vapor deposition techniques [5], allow the kinetic stabilization of single-phase B1 Ti1−xAlxN over wide metal compositional ranges (up to x ≈ 0.9) [6,7]. During high-temperature operation (≈1000−1200 K), B1 (Ti,Al)N alloys undergo spinodal decomposition into strained, coherent B1 AlN-rich / B1 TiNrich domains. This, in turn, greatly enhances the material’s hardness improving the performance of the coating [8]. Single-phase materials generally become softer with temperature [9,10], alloys such as (Ti,Al)N are of considerable technological importance due to the spinodally induced

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