Abstract
Pop-in statistics from nanoindentation with spherical indenters are used to determine the stress required to activate dislocation sources in twin boundaries (TBs) in copper and its alloys. The TB source activation stress is smaller than that needed for bulk single crystals, irrespective of the indenter size, dislocation density and stacking fault energy. Because an array of pre-existing Frank partial dislocations is present at a TB, we propose that dislocation emission from the TB occurs by the Frank partials splitting into Shockley partials moving along the TB plane and perfect lattice dislocations, both of which are mobile. The proposed mechanism is supported by recent high resolution transmission electron microscopy images in deformed nanotwinned (NT) metals and may help to explain some of the superior properties of nanotwinned metals (e.g. high strength and good ductility), as well as the process of detwinning by the collective formation and motion of Shockley partial dislocations along TBs.Graphic abstract
Highlights
Grain boundaries and their structures play a vital role in the mechanical properties of polycrystalline materials
It is known that Frank partial dislocations can act as dislocation sources in face-centered cubic single crystals [26,27,28,29] according to the dislocation reaction: a0 3
We note that other Frank partial dissociation reactions that result in glissile lattice dislocations and mobile Shockley partial dislocations have been observed but such Frank partial dissociation reactions are not discussed here [26, 27, 29]
Summary
Grain boundaries and their structures play a vital role in the mechanical properties of polycrystalline materials. It is well accepted that grain boundaries obstruct dislocation motion and enhance the yield strength of most metals and alloys [1, 2]. The important role of grain boundaries in dislocation nucleation and multiplication has been known for some time. There is experimental evidence that dislocations nucleate in the vicinity of grain boundaries, as observed with transmission electron microscopy (TEM) [4]. Many experimental studies have focused on coherent Σ3 twin boundaries (TBs), which are of particular interest because bulk materials with nanotwinned (NT) structures composed of coherent TBs with a spacing of less than 100 nm [5] often show unique mechanical properties [6,7,8]. NT materials frequently offer high strength and good ductility – two properties that are typically mutually exclusive
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