Understanding the competition between brittleness and plasticity in refractory ceramics is of fundamental importance for screening and design of hard materials with enhanced resistance to fracture from room to elevated temperature.

Ab initio and classical molecular dynamics (AIMD & CMD) simulations are used to investigate fracture mechanisms in defect-free, as well as notched B1 Ti_1-xAl_xN (0≤x≤0.75) supercells subject to tensile and shear deformation as a function of temperature. The interatomic potential employed in CMD – thoroughly validated for several structural, mechanical, and thermodynamic properties of Ti-Al-N systems – accurately reproduces the results of AIMD simulations obtained for small (1100 atoms) supercells. Hence, the results of relatively large (≈10⁵ atoms) CMD simulations of notched crystals subject to mode-I tension allow gaining a comprehensive understanding of the competition between unstable crack growth vs plasticity mechanisms at crack tips in Ti-Al-N systems.

The talk also briefly introduces an AIMD database (24 investigated systems) of B1-structure ceramic properties calculated for 300≤T≤1200K. The database includes both raw ab initio data – ≈10⁹ phase-space configurations with associated energies, forces, total stresses, and magnetic moments – as well as mechanical properties including elastic constants, tensile and shear strengths, moduli of tensile toughness, Schmid vs non-Schmid lattice-slip mechanisms, and strain-mediated lattice transformation pathways. Taking Ti-Al-N systems as representative case, it is illustrated how indicators (determined from the ideal properties of single-crystal ceramics) can reliably predict statistical trends in mechanical performance evaluated for systems that contain native structural flaws.

Sangiovanni, Inherent toughness and fracture mechanisms of refractory transition-metal nitrides via density-functional molecular dynamics, Acta Materialia (2018).

Sangiovanni et al, Strength, transformation toughening, and fracture dynamics of B1 Ti-Al-N alloys, Physical Review Materials (2020).

Mei et al, Adaptive hard and tough mechanical response in single-crystal B1 VNx ceramics via control ofanion vacancies, Acta Materialia (2020).

Almyras et al, Semi-Empirical Force-Field Model for the Ti-Al-N System, Materials (2019).

Sangiovanni et al, Enhancing plasticityin high-entropy refractory ceramics via tailoring valence electron concentration, Materials &Design (2021).

Sangiovanni et al, Temperature-dependent elastic properties of binary and multicomponent high-entropyrefractory carbides, Materials & Design (2021).

The mechanical strength and fracture toughness of thin films can be tuned by microstructure. Superlattices, for example, have shown an increase in both hardness and fracture toughness at small layer thicknesses for a variety of materials. The introduced interfaces in these systems govern the mechanical response of the coating. However, their key role during mechanical loading is not yet understood. In this work, classical molecular dynamics simulations were performed to study the behavior of rocksalt cubic-structured AlN(001)/TiN(001) superlattices under mechanical loading in the form of uniaxial tension and indentation.

The tensile loading simulations aimed at revealing mechanisms behind plasticity and crack growth. Cells with layer thicknesses between 1.25 and 10 nm were put under tensile loading in different crystallographic directions. Depending on the load direction different mechanisms for plastic deformation are activated resulting in anisotropic behavior. Tensile loading perpendicular to the (001)-interface shows only minor plasticity accompanied by the nucleation of only a few dislocations and fracture parallel to the layers near the interface. Strain applied along the [100] and [110] directions on the other hand reveals a significant increase in toughness due to B1-to-B3 or B1-to-B4 phase transformations in AlN and later the development of shear bands. Under these load scenarios, networks of dislocations form that can, for small layer thicknesses, span over the interfaces. The findings were supported by ab initio molecular dynamics and nanoindentation and transmission electron microscopy experiments. From the joint results, we could conclude that the layer thickness of superlattices can impede the formation of cracks.

The indentation simulations focused on the intermixing of the alternating layers in superlattices under the load of the indenter. We were able to reveal that with ongoing deformation, a single phase starts to form near the indenter, degrading the coherent interface. The findings were supported by nanoindentation and high-resolution transmission electron microscopy experiments.

These cutting-edge simulations provided novel insights into the deformation mechanisms and processes of thin bi-layer superlattices at the atomistic level, hence complementing information available through high-end sophisticated experiments.