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

TL;DR

This study uses ab initio molecular dynamics to explore the mechanical strength, toughness, and fracture behavior of rocksalt-structure Ti1-xAlxN alloys, revealing how Al content influences their deformation and phase transformation mechanisms.

Contribution

It provides new insights into the transformation toughening and fracture dynamics of Ti1-xAlxN alloys, highlighting the role of local structural transformations in enhancing toughness.

Findings

TiN exhibits higher strength but brittle fracture.

Ti0.5Al0.5N and Ti0.25Al0.75N are more fracture-resistant.

Transformation mechanisms contribute to toughness enhancement.

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.