Silicon-stabilized three-dimensional covalent networks in high entropy diborides

Area of Science:

• Materials Science
• Solid State Chemistry
• Crystallography

Background:

• High entropy ceramics (HECs) enable stabilization of unique material compositions.
• Traditional alloying rules, like Hume-Rothery rules, limit the incorporation of certain elements.

Purpose of the Study:

• To investigate the incorporation of silicon into an AlB2-type high entropy diboride.
• To explore the structural and mechanical consequences of stabilizing silicon in this HEC.

Main Methods:

• Arc melting was used to synthesize the Cr0.2Nb0.2Si0.2Ta0.2Ti0.2B2 ceramic.
• Phase purity and homogeneity were confirmed using powder X-ray diffraction (pXRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS).
• Mechanical properties were evaluated through Vickers hardness and nanoindentation tests.

Main Results:

• A phase-pure, chemically homogeneous high entropy diboride incorporating silicon was successfully synthesized.
• Silicon was found to occupy the metal sublattice, forming Si-B covalent bonds and creating a 3D network.
• Mechanical testing showed no change in bulk hardness but a moderate increase at low loads and a ~10% enhancement in Young's modulus.

Conclusions:

• High configurational entropy can stabilize main-group elements like silicon in transition metal diborides.
• The incorporation of silicon leads to new bonding arrangements and improved elastic properties.
• This work expands the design space for high entropy alloys and ceramics.