Activation volume and density of mobile dislocations in plastically deforming nanocrystalline Ni

Abstract

We report the analysis of repeated transients to monitor the coupled evolution of dislocation velocity and mobile dislocation density in plastically deforming nanocrystalline Ni. The stress relaxation series allowed the determination of the physical activation volume, indicating a rate-controlling mechanism different from that in coarse-grained Ni. The mobile dislocation exhaustion observed is correlated with the unusually high apparent work-hardening rate during the early stage of straining.

Less is more

“…Now, what is the diff erence if these polycrystals have tiny grains? Th e lattice dislocations no longer matter as much, because the strength becomes dominated largely by how dislocations originate from and interact with GBs, as revealed by previous computer simulations 8 and experiments 9 . Th e stored dislocations are closely spaced, shown in Fig.…”

“…Even when the metal incorporates only a trace amount of impurities during its processing, the segregation to the GBs could be suffi cient to suppress the dislocations from emerging, or to pin them down as they propagate 3,8 . Th e dislocation-GB interactions are known to be thermally activated processes 8,9,13 .…”

Time, stress, and temperature-dependent deformation in nanostructured copper: Stress relaxation tests and simulations

“…On and near GBs, wide-awake of dislocation sources might be existed to allow dislocations to nucleate readily. However, these dislocation sources are not regenerative and might be exhausted instantly upon straining (Dalla Torre et al, 2005;Koch, 2003;Wang et al, 2005), which leads to the absence of workhardening and thus the limited ductility in ng metals. Fig.…”

“…The von Mises shear stress conjugated activation volume, called the apparent activation volume (Zhu and Li, 2010;Zhu et al, 2007), is widely adopted (Dao et al, 2007;Lu et al, 2005;Wang et al, 2005Wang et al, , 2006 and also used in the previous work (Yang et al, 2016) for the convenience in comparison. In materials community, resolved shear stress τ r is usually employed and the value of τ r is linked to the uniaxial tensile stress by Taylor factor M, i.e., τ σ = M / r .…”

Deformation twinning mechanisms in nanocrystalline Ni

“…8 For the small plastic strains ͑ϳ3%͒ sustainable in tension, 8 the initial portion of the stress-strain curve involved apparent bending, in a process possibly using up sources of dislocations that are pre-existing or relatively easy to escape, and GB segments that are relatively easy to shear/slide. 17 Twinning was apparently not yet competitive with other possible means that can produce the plastic strain. Unfortunately further tensile straining was not possible beyond the point where the ͑necking͒ instability set in; 8 otherwise twinning could have come in after a certain level of stress ͑concentra-tions͒ is reached, as is the case in our large-strain rolling/ milling experiments.…”