Spontaneous Crystallization for Tailoring Polymorphic Nanoscale Nickel with Superior Hardness

MK Zakaryan and SM Estalaki and S Kharatyan and AM Matzner and AS Mukasyan and TF Luo and KV Manukyan, JOURNAL OF PHYSICAL CHEMISTRY C, 126, 12301-12312 (2022).

DOI: 10.1021/acs.jpcc.2c03612

Hexagonal close-packed (hcp) Ni exists only on a nanometric scale and readily converts into the stable face-centered cubic (fcc) polymorph. This work reports a spontaneous crystallization (SC) process in amorphous nanoparticles to tailor polymorphism in nanoscale Ni. The heat generated during the amorphous-crystalline transition enables SC to self-sustain. The combination of thermal analysis methods, X-ray photoelectron spectroscopy, electron micros-copy, and diffraction methods with molecular dynamics (MD) simulations allows us to reveal the mechanism of the SC. This process consists of multiple exothermic steps: nuclei formation and growth of hcp/fcc-Ni grains within the amorphous nanoparticles, neck formation between particles, and defect-stimulated recrystallization (grain growth). The use of SC in conjunction with a rapid (minutes) and low-temperature (780 K) processing method allows for obtaining compact (94 +/- 2% relative density) hcp/fcc-Ni nanomaterials. Rapid cooling during this process prevents the hcp -> fcc phase transition, making it possible to preserve the metastable hcp-Ni phase. hcp/fcc-Ni materials possess a unique nanostructure and superior hardness to single-phase fcc-Ni. Detailed high-resolution imaging results show that hcp-Ni can be found in round-shape (less than 10 nm) or ultrathin (0.5-5 nm) laminated grains. Ultrasmall features reduce the dislocation motion at grain boundaries and significantly enhance the hardness of these materials.

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