A Computational Study of Structural and Mechanical Properties of Silicon Carbide Alloyed with Lithium or Sodium for Aerospace Applications

Abstract

Aluminium (Al) and its alloys have historically been preferred materials in the aerospace sector owing to its exceptional strength-to-weight ratio, corrosion resistance, and ductility. These attributes contribute to enhanced fuel efficiency and ensure the structural integrity and mechanical reliability of modern aircraft. However, despite these advantages, the high ductility and malleability of Al and its alloys make them prone to dents and scratches, which can limit their performance in certain aerospace applications. This limitation has spurred the search for alternative materials that can address the weaknesses of Al and its alloys. Enhancing the mechanical strength of Al and its alloys is possible through processes like cold working and alloying. However, these methods often come at the cost of reduced corrosion resistance, which is a critical property for aerospace materials. Therefore, there is a pressing need to explore alternative materials that offer superior mechanical properties without sacrificing other essential characteristics. Compared to Al alloys, silicon carbide (SiC) has a higher Young’s modulus and superior wear resistance. Silicon carbide (SiC) has better wear resistance and a greater Young’s modulus than Al alloys. SiC is intrinsically brittle and has an even higher density than Al and its alloys, leading to it being unsuitable for aeronautical uses where both strength and low weight are vital, regardless of these benefits. This study focused on improving the structural and mechanical properties of SiC by alloying it with lithium (Li) or sodium (Na), using ab initio calculations. The primary aim was to modify the properties of SiC through alloying to create materials that better meet the demands of aerospace applications. The specific objectives were: to model SiC alloys with varying concentrations of Li or Na, to investigate how these concentrations affect the structural properties of the alloys, and to assess the impact of Li or Na concentration on the mechanical properties of the SiC-Li or SiC-Na alloys. The research began with modeling the cubic SiC structure, by creating a 2 × 2 × 2 supercell containing 64 atoms. Burai software, known for its user-friendly graphical interface with Quantum ESPRESSO, was used for this purpose, facilitating the creation of input files and crystal structure visualization. Alloying was achieved by substituting up to 25 % of the silicon atoms with Li or Na atoms while retaining the cubic structure of the alloys. Ab initio calculations were then performed using density functional theory (DFT) with the Perdew-Burke-Ernzerhof functional for solids (PBESOL), implemented in the Quantum ESPRESSO code. Ultrasoft pseudopotentials were employed in self-consistent field (SCF) calculations. The results showed that the SiC-Li and SiC-Na alloys, particularly SiC-6Li, SiC-8Li (2784, 2656 kg/m³, respectively), and the SiC-6Na and SiC-8Na (2889, and 2788 kg/m³, respectively), exhibited lower densities, higher ductility, and superior mechanical strength (8.74 and 2.46, and 6.99 and 0.74 GPa, respectively) compared to Al and its alloys. The alloys with lower compositions of Li or Na (SiC-2Li and SiC-2Na), are the strongest mechanically (strength very close to pure SiC though a bit more ductile), therefore can be used on the plane skins to curb surface erosion. These findings suggest that these alloys could be used to manufacture lighter, yet structurally and mechanically stronger materials, making them highly suitable for aerospace applications. To create even more optimal materials for aerospace applications, future studies should concentrate on empirically confirming these findings and investigating the simultaneous alloying of SiC with both Li and Na.