Twins can be easily introduced into nanocrystalline materials during their preparations and deformations. Twin boundary (TB) is an especial high angle grain boundary and will lead to particular deformation behaviors because of its lower interface energy than that of general grain boundary. Recently, the effect of TBs on advancing material’s tensile strength has been studied widely and intensively by many investigation groups. However, the study of increasing plasticity and tensile ductility is still absent, especially for that the dislocations are whether or not been emitted from TBs and that twin boundary migration is whether or not occurred during deformation. In addition, the investigation on deformation mechanisms in nanocrystalline (nc) materials is only limited to molecular dynamics simulations, the experimental evidences on deformation mechanisms is in need. Therefore, in this dissertation, the deformation mechanisms on nanoscale growth twin Cu and nc Ni have been investigated by means of in situ transmission electron microscopy (TEM) straining technique associated with high resolution TEM and diffraction contrast principle. The main results are as follows:
The strength of nanoscale growth twin Cu is enhanced because the movement dislocations are blocked by TBs when it was strained along different directions. At the same time, the situations of twin ends and twin steps are also helpful to Shockley dislocation emissions, which do not necessarily move on slipping plane with the maximum Schmid factor. The results are attributed to deposited Shockley dislocation types in twin ends and twin steps. The dislocation emissions and their propagations will apparently increased material’s plasticity and tensile ductility.
The dislocation reactions of extended dislocation crossing through TBs were discussed, i.e., Dγ + γA→ Dγ + γγ′ + γ′A→ DA → BD′ + Cδ → Bγ′ + γ′D′ + Cδ. At the same time, it is also indicated that three criteria of lattice dislocations crossing through GBs is fit for TBs.
Three stages of dislocation emissions from TBs were discussed: The first stage is the steps formation with sessile Frank dislocation; The second stage is that incoherent TBs with steps act as dislocation sources; The third stage is crack tip dislocations join the dislocation emission processes. In addition, it is also revealed that the existence of a sessile Frank partial step and its obstruction for glissile Shockley dislocations in TBs is good for TBs to serve as dislocation sources. The dislocation burgers vector was determined by serials of dual beam diffraction contrast images.
The first direct evidence of twin boundary migration via Shockley partial dislocation slipping in TBs was provided via TEM, such migration is the dominant deformation mechanism in the initial stage of plastic straining. It is also revealed that Shockley dislocations move in slip plane with lower Schmid factor (TB plane). This behavior is discussed in comparison with molecular dynamics simulations (high stress). The unique characteristics of the sample microstructure with high density TB/GB intersections are helful to twin boundary migration from Shockley emissions.
Nano-beam electron diffraction and series of dark field images techniques were used to investigate the deformation mechanisms of nc Ni (average grain size 20 nm) in response to in situ tensile deformation under TEM. The experiments exhibit the complete processes of individual grain rotation and neighboring grain rotation/growth, and propose a physical model of deformation-induced grain rotation and grain growth. This provides real time and compelling evidences for GB-mediated deformation mechanism in nc materials. At the same time, these results were confirmed further by ex situ TEM observation and XRD experiments on deformed sample, eliminating two dimensions effect of in situ tensile sample.