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5. In situ nanoindentation

(1) In situ nanoindentation of twinned Al films

(2) LC lock in nc Ni

(3) Slip transmission on ITB migration in nt Cu

(4) Detwinning mechanisms in nanotwinned Cu

(5) Dislocation climb in nanolayer Al/Nb

In situ nanoindentation study on sustainable plasticity and work hardening in nanotwinned aluminum
D. Bufford, Y. Liu, J. Wang, H. Wang, and X. Zhang, Nature Comm, 2014.
  • Incoherent twin boundaries (ITBs) enable large work hardening
  • ITBs migrate during indentation
  • MD simulations reveal drastic differences in plastic deformation between nt Cu and nt Al  

 Direct observation of Lomer-Cottrell Locks during strain hardening in nanocrystalline nickel by in situ TEM
Joon Hwan Lee, Troy B. Holland, Amiya K. Mukherjee, Xinghang Zhang, Haiyan Wang, Scientific Reports 3, Article number: 1061
  • In situnanoindentation experiment shows solid evidence for significant work hardening in nc Ni based on sequential loading-unloading cycles. 
  • During work hardening, the dislocation density along the TB increases, and the yield strength increases gradually by 40%.
  • Frequent formation of L-C locks was identified in grain interior and along twin boundaries. L-C locks are effective barriers to dislocations and lead to work hardening. 
  • Several mechanisms of interaction between L-C locks and twin boundaries were identified. These studies provide important insight to the understanding of plasticity in nc metals.
Influence of slip transmission on the migration of incoherent twin boundaries in epitaxial nanotwinned Cu
N. Li, J. Wang, J.Y. Huang, A. Misra, X. Zhang
  • Using in situ nanoindentation, we studied the migration mechanisms of Σ3{1 1 2} incoherent twin boundary (ITB) in epitaxial nanotwinned Cu films. 
  • ITBs migrate by propagation of steps or disconnections with step heights of three or multiples of three {1 1 1} interplanar distances, and the migration may lead to the detwinning of nanotwins. 
  • The transmission of a glide dislocation across an ITB is shown to form a sessile dislocation in the ITB and locally pin the boundary at the site of the slip transmission.

Detwinning mechanisms for growth twins in face-centered cubic metals
J. Wang , N. Li, O. Anderoglu, X. Zhang, A. Misra, J.Y. Huang, J.P. Hirth
  • The detwinning process is accomplished via the collective glide of multiple twinning dislocations that form an ITB
  • Detwinning can easily occur for thin twins, and the driving force is mainly attributed to a variation of the excess energy of a coherent twin boundary
  • Shear stresses enable ITBs to migrate easily, causing the motion of coherent twin boundaries; and
  • The migration velocity depends on stacking fault energy. 
  • The results imply that detwinning becomes the dominant deformation mechanism for growth twins of the order of a few nanometers thick.
In situ TEM observations of room temperature dislocation climb at interfaces in nanolayered Al/Nb composites
Nan Li, J. Wang, J.Y. Huang, A. Misra, X. Zhang
  • Using in situ nanoindentation, we investigated dislocation–interface interactions in Al/Nb multilayers. 
  • Preferential storage of dislocations at interfaces, as opposed to within layers, was observed. 
  • Recovery of dislocations was observed to occur through climb in the interfaces. 
  • The rapid climb of dislocations is ascribed to high vacancy diffusivity and vacancy concentration in the interfaces. 
  • The vacancy formation energy at interfaces, 0.12 eV, as estimated from the experimentally measured climb rates, was found to be significantly lower than in the bulk.