Abstract
The review of the theoretical models, which describes mechanisms of deformation twinning in nanocrystalline and ultrafine-grained materials, is presented. Realization of special mechanisms of nanoscale deformation twin generation at locally distorted grain boundaries (GBs) in nanocrystalline and ultrafine-grained materials is observed. In particular, the micromechanisms of deformation twin formation occur through (1) the consequent emission of partial dislocations from GBs; (2) the cooperative emission of partial dislocations from GBs; and (3) the generation of multiplane nanoscale shear at GBs. The energy and stress characteristics of the deformation nanotwin generation at GBs in nanocrystalline and ultrafine-grained materials are calculated and analyzed. Competition between the twin generation mechanisms in nanocrystalline and ultrafine-grained materials is discussed.
Highlights
With a large volume fraction occupied by grain boundaries (GBs) which act as effective obstacles for lattice dislocation slip, the conventional lattice dislocation slip is hampered in nanostructured materials
Theoretical description of deformation twin generation mechanisms is based on results of the following theoretical papers [14–17]. According to these papers [14–17], generation of nanotwins occurs at locally distorted GB segments (GB segments being rich in GB dislocations) which were produced due to either events of consequent trapping of extrinsic lattice dislocations by GB and their splitting transformations into a wall of climbing GB dislocations (Figure 1a–d) or GB deformation processes involving slip and climb of GB dislocations (Figure 1e–h)
We revealed dependences of the energy change ΔWN on the Burgers vector magnitude s of the noncrystallographic dislocations for various sizes of the nanotwin
Summary
The study of the plastic behavior of nanostructured solids is one of the most important and rapidly developing directions in the mechanics of deformed solid and in the physics of condensed state. Theoretical description of deformation twin generation mechanisms is based on results of the following theoretical papers [14–17] According to these papers [14–17], generation of nanotwins occurs at locally distorted GB segments (GB segments being rich in GB dislocations) which were produced due to either events of consequent trapping of extrinsic lattice dislocations by GB and their splitting transformations into a wall of climbing GB dislocations (Figure 1a–d) or GB deformation processes involving slip and climb of GB dislocations (Figure 1e–h). Consequent generation of such dipoles of partial dislocations joined by stacking faults is capable of forming a nanoscale twin (Figure 2b–d) Such consequent events of partial dislocation emission from GBs were examined in several theoretical works (see, e.g., [16, 17]), which, did not concern formation of locally nonequilibrium GB structures considered here as initial ones for nanotwin generation. This value is very high, but it can be reached in shock load tests of nanocrystalline materials
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