Cyclic deformation of nanocrystalline and ultrafine-grained nickel

Abstract

The cyclic deformation behavior of ultrafine-grained (UFG) Ni samples synthesized by the electrodeposition method was studied. Different from those made by severely plastic deformation, the UFG samples used in this study are characterized by large-angle grain boundaries. Behaviors from nanocrystalline Ni and coarse-grained Ni samples were compared with that of ultrafine-grained Ni. With in situ neutron diffraction, unusual evolutions of residual lattice strains as well as cyclic hardening and softening behavior were demonstrated during the cyclic deformation. The microstructural changes investigated by TEM are discussed with respect to the unusual lattice strain and cyclic hardening/softening.

Compressive-tensile deformation of nanocrystalline nickel at high pressure and temperature conditions

“…Thus, the application of pressure can effectively hinder dislocation movement and increase the yield stresses. In contrast, our experimental data on nano Ni show that the yield strength at a confining pressure of 6 GPa is comparable to that at ambient pressure, 31 suggesting that the dislocationmediated processes are no longer activated during the deformation. Similar conclusions have also been reached from the reversible peak broadening observed in nanocrytalline Fe 36 and nanocrystalline Ni 21,23 and (100% in nano-Fe and 84%-100% in nano-Ni) during stress loading/unloading.…”

“…This value is comparable to the tensile yield stress of 0.82 $ 0.85 GPa reported for 20 nm Ni but it is 50% higher than that of coarse-grained Ni (0.55 GPa at ambient conditions). 31 The fact that the tensile yield strength is the same as compressive yield strength during the deformation indicates that the Bauschinger effect is absent in nanocrystalline Ni. For coarse grained metals, the yield strength typically decreases when the direction of plastic strain is reversed during the deformation.…”

Deformation Crossover: From Nano- to Mesoscale

“…The observed reversal of residual strains in LD is consistent with deformation twinning. It is also noteworthy that an unusual " II hkl and deformation twinning were observed in UFG Ni subjected to cyclic loading [25].…”

Mechanical Properties of Nanostructured Metals

“…During cyclic strain-controlled tests, NC Ni exhibits higher cyclic stresses than CG Ni, as well as cyclic softening [74][75][76]. Moser and colleagues [74] suggest that dislocation sources residing in grain boundaries are exhausted during the early hardening phase, allowing diffusional mechanisms to dominate at higher cycles.…”