Solidification dynamics and microstructure evolution in nanocrystalline cobalt

Abstract

Solidification and microstructures of fast quenched cobalt (from 1950K to 600K) are studied using molecular dynamics simulations. Two types of microstructures, a nanocrystalline type consisting of multiple grains and a lamellar type are obtained. The lamellar type microstructure shows a dominant layering direction with fcc phases separated by stacking faults, coherent twin boundaries or hcp phases. The nanocrystalline type is dominant at fast quenching rates. Both microstructures are highly twinned. Twins can originate either from ab initio faults of several morphologies in the nuclei or from stacking accidents in 〈111〉 directions during growth. It is supported that fivefold twins form by a successive growth of the fcc subunits, but not by an equilateral layer-by-layer growth from a five-folded symmetric core that exists at the very beginning of nucleation. Annealing of the as-quenched samples above the phase transformation temperature leads to gradual growth of the fcc phase and local reversible fcc-hcp transformation facilitated by existing Shockley dislocations. Under uniaxial tensile loading, the nanocrystalline samples show an early initial yielding, followed by continuous work hardening, reaching an ultimate tensile strength of about 3000 MPa. The lamellar samples, in contrast, show a much more intermittent plastic flow. The strongest sample has a nanotwinned structure resembling TEM images of vapor deposited films of fcc metals. The ultimate tensile strength is over 5000 MPa, assisted by coherent twin boundaries serving as strong obstacles to dislocation motion.