Structural and mechanical properties of nitrogen-deficient cubic Cr-Mo-N and Cr-W-N systems

TL;DR

This study combines first-principles calculations and experiments to analyze nitrogen deficiency, elastic properties, and phase stability in cubic Cr-Mo-N and Cr-W-N systems, revealing how nitrogen vacancies influence their mechanical behavior.

Contribution

It provides a detailed understanding of nitrogen vacancies, elastic properties, and phase stability in cubic Cr-Mo-N and Cr-W-N systems, with a new chemical formula accounting for nitrogen deficiency.

Findings

Nitrogen vacancies scale linearly with transition metal content.

Solid solutions show positive mixing enthalpies indicating phase decomposition.

Elastic properties and lattice parameters agree well between experiments and calculations.

Abstract

The tendency for nitrogen deficiency in cubic Cr-Mo-N and Cr-W-N solid solutions is predicted by a comprehensive evaluation of the lattice spacing, mixing thermodynamics, and elastic properties using first-principles calculations and experimentally confirmed by means of X-ray diffraction. A major conclusion is that these systems exhibit significant amount of N vacancies whose amount scales linearly with the TM content, hence making the Cr1-xTMxN1-0.5x chemical formula more precise and informative to describe the chemical composition of cubic Cr-Mo-N and Cr-W-N solid solutions as compared with the conventionally used Cr1-xTMxN. The cubic Cr1-xMoxN1-0.5x and Cr1-xWxN1-0.5x solid solutions exhibit large positive mixing enthalpies towards isostructural phase decomposition into cubic B1-CrN and {\gamma}-Mo2N or {\gamma}-W2N, respectively. Their ductility increases with increasing Mo or W content and both systems exhibit significantly direction-dependent Young's moduli over the entire composition range, even when using the approach to study their polycrystalline behavior. The excellent agreement between experimentally obtained lattice parameters, Mo- and W-dependent nitrogen content, elastic properties and their calculated values for our model descriptions, Cr1-xMoxN1-0.5x and Cr1-xWxN1-0.5x, allows to understand these complex material systems. Based on our results, we can conclude that their content of nitrogen vacancies scales with half of the alloying content Mo or W.