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 (CH) and hydrazine (NH), released during hexamethylenetetramine decomposition. Kinetic data analysis shows that the rate-limiting step in forming Co is the CH (or NH)-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.

中文翻译：

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亚稳态γ-co的稳定：燃烧合成和快速加工

这项研究报道了一种稳定亚稳态γ-Co相的新方法，该相具有面心立方结构。该相以其具有挑战性的合成而闻名，因为它倾向于转变为稳定的六方密堆积 (ε-Co) 同素异形体。燃烧合成和快速无压烧结方法克服了γ-Co制备的传统障碍。燃烧过程涉及六水合硝酸钴和六亚甲基四胺溶液中的放热反应，产生足够的热量以将合成温度提高到γ↔ε相变温度（∼690 K）以上。通过气体快速冷却固体产物对于防止逆转变为 ε-Co 至关重要，从而以最少量的稳定相稳定 γ-Co 相。热分析表明，六水硝酸钴的分解导致氧化钴的形成。然后这些氧化物被六亚甲基四胺分解过程中释放的甲烷 (CH) 和肼 (NH) 还原为 Co。动力学数据分析表明，形成 Co 的限速步骤是 CH（或 NH）介导的 CoO 还原。当在 1275 K 下进行快速无压烧结时，燃烧合成材料固结成相对密度约为 90% 的致密样品和微米尺寸的晶粒。高分辨率电子显微镜研究表明，高温加工时由于 ε→γ 转变，晶格位错增加。与慢熔凝固和轧制方法制备的ε-Co相比，制备的γ-Co表现出更高的纳米硬度和杨氏模量。