Ultrafine AIN and Cr2N Composite ParticlesPrepared by Reaction between Nitrogen Plasma and Molten Al-Cr Alloys

Ultrafine ceramic composite particles with a matchstick-like shape consisting of hexagonal AIN and Cr₂N phases were found to form at approximately 100% fraction by the reaction between nitrogen plasma and molten Al₅₀Cr₅₀liquid. The AIN phase has a hexagonal prism shape with a transverse size of about 50 nm and a length of about 200 nm while the Cr₂N phase has hexagon-pyramids with an edge size of about 100 nm and connects with the AIN hexagonal prism. The two phases have a close crystal orientation relationship of [001]_A1N//[001]_Cr2Nand [110]_A1N//[100]_Cr2Nwhich has low misfit strains of about 11%. The ultrafine composite AlN + Cr₂N particle is presumed to form through the following process; (i) the formation of Al-Cr liquid containing large amounts of dessociated nitrogen (N) and hydrogen (H) by the plasma reaction, (ii) the formation of ultrafine supercooled Al-Cr-N liquid particles by condensation of alloy vapor, (iii) the faceted growth of AIN phase along the preferential direction of [001], and (iv) the solidification of the remaining liquid to Cr₂N in the close orientation relationship with AIN phase. In the coexistent state of AIN solid and Cr-Al-N liquid, the vapor condensation occurs preferentially at the liquid region which lies on the AIN phase, leading to the preferential growth of AIN phase. The vapor-liquid-solid (VLS) growth mechanism is thought to be the origin for the formation of the matchstick-like composite particles with radially elongated and tree-like morphologies.