Softening due to disordered grain boundaries in nanocrystalline Co

Abstract

Nanocrystalline Co consisting of fcc and hcp phases was processed by electrodeposition, and its mechanical properties were investigated by hardness tests. In addition, high-resolution transmission electron microscopy observations and molecular dynamics (MD) simulations were performed to investigate the grain boundary structure and dislocation nucleation from the grain boundaries. A large amount of disorders existed at the grain boundaries and stacking faults were formed from the grain boundaries in the as-deposited Co specimen. The as-deposited specimen showed a lower hardness than did the annealed specimen, although the grain size of the former was smaller than that of the latter. The activation volume of the as-deposited specimen (=1.5b(3)) was lower than that of the annealed specimen (=50b(3)), thus indicating that nucleation of dislocations from grain boundaries is more active in the as-deposited specimen than in the annealed specimens. The MD simulations showed that dislocation nucleation was closely related to a change in the defect structures at the boundary. Therefore, it is suggested that a significant amount of defects enhance changes in the defect structures at the boundary, resulting in softening of the as-deposited specimen.

Structure and Mechanical Properties of Electrodeposited Nanocrystalline Ni-Fe Alloys

“…The analysis of the structure of ECD alloy in the range 50-60% Fe shows that lower values of indentation hardness and modulus are invariably associated to the appearance of the α phase. Actually, in alloys with very small crystal size (∼10 nm for alloy in the 50-60% Fe range), low hardness and modulus may be expected as a result of high density of defects, particularly at the grain boundaries, therefore due to highly disordered grain boundary regions, as reported for nanocrystalline Ni-P 79 and Co. 80 Interestingly, comparable values of hardness and elastic modulus, from indentation measurements performed at similar strain rate, were reported for Ni-Fe 49% deposits obtained under conditions similar to those of the present work. 81 The overall variation of the indentation modulus vs. composition (Fig.…”