Solid-gas reactions driven by mechanical alloying of niobium and tantalum in nitrogen

Solid-gas reactions of niobium and tantalum with molecular nitrogen driven by mechanical alloying (MA) have been investigated by X-ray diffraction, transmission electron microscopy, and differential thermal analysis. It was found that the phase transition followed a sequence of Nb₂N → Nb₃N₄ → NbN when Nb was milled with N₂. However, Ta₂N and an amorphous phase were formed when Ta was milled with N₂. The chemosorption of nitrogen onto the clean metal surfaces created by ball milling is believed to be the fundamental process governing solid-gas reactions, and the defects generated during MA can promote the diffusion of adsorbed nitrogen, and consequently the formation of metal nitrides. The difference in phase transition between the two systems is discussed.