Stabilization of metastable γ-co: Combustion synthesis and rapid processing

Abstract

This study reports a novel method for stabilizing the metastable γ-Co phase, which has a face-centered cubic structure. This phase is known for its challenging synthesis due to its tendency to transform into the stable hexagonal-closed packed (ε-Co) allotrope. Combustion synthesis and a rapid pressure-less sintering approach overcome the traditional barriers to γ-Co preparation. The combustion process involves exothermic reactions in a solution of cobalt nitrate hexahydrate and hexamethylenetetramine, generating enough heat to increase the synthesis temperature above the γ↔ε phase transition temperature (∼690 K). Rapid cooling of the solid product by gases is critical to preventing the reverse transition to ε-Co, thus stabilizing the γ-Co phase with a minimal amount of the stable phase. Thermal analysis has demonstrated that the decomposition of cobalt nitrates hexahydrate leads to the formation of cobalt oxides. These oxides are then reduced to Co by methane (CH4) and hydrazine (N2H4), released during hexamethylenetetramine decomposition. Kinetic data analysis shows that the rate-limiting step in forming Co is the CH4 (or N2H4)-mediated reduction of CoO. When subjected to rapid pressure-less sintering at 1275 K, the combustion synthesized materials consolidate into compact samples with a relative density of ∼90 % and micrometer-size grains. High-resolution electron microscopy investigation shows increased lattice dislocation due to the ε→γ transition at high-temperature processing. Prepared γ-Co exhibits a higher nanohardness and Young's modulus than ε-Co prepared by slow melt solidification and rolling methods.