Molecular dynamics study of the milling-induced allotropic transformation in cobalt
KO Hara and E Yamasue and H Okumura and KN Ishihara, PHILOSOPHICAL MAGAZINE, 92, 2117-2129 (2012).
The milling-induced allotropic transformation in cobalt from the stable hcp phase into the fcc phase has been studied by molecular dynamics simulation. Uniaxial compression of nano-sized spheres composed of the hcp Co phase is simulated as the elementary process of milling. The c axis is tilted from the compression direction at 0 degrees, 30 degrees, 60 degrees and 90 degrees to investigate the orientation dependency. The Honeycutt-Andersen index analysis and powder X-ray diffraction pattern calculation by the Debye formula show that the transformation into the fcc phase occurs when the tilt angle is 0-60 degrees. The basal slip is essential for the transformation when the tilt angle is 60 degrees, while the c axis compression-induced transformation mode is observed for the 0 degrees tilt angle, which involves lattice contraction along the c axis and lattice dilation in the basal plane. Both modes are observed for the 30 degrees tilt angle. Analysis of the pressure induced during compression reveals that the pressure necessary to initiate the latter transformation mode is too high to attain during conventional ball milling. The basal slip mode is proposed as the probable transformation mode for the milling-induced allotropic transformation in Co.
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