Strength and Ductility in Electrodeposited Nanocrystalline Nickel

Abstract

Electrodeposited nanocrystalline (nc) Ni having an average grain size of 20 nm was annealed at 443 K for different holding times. An examination of the microstructure following annealing showed three important features. First, all annealed samples exhibited abnormal grain growth, which was manifested by the presence of large grains that were surrounded by regions of small grains (bimodal grain distributions). Second, annealing twins existed in the large grains of the samples that showed a bimodal grain distribution. Third, by estimating the density of annealing twin, it was found that annealing nc-Ni at 443 K resulted in a maximum twin density after 5h. Following annealing treatment, specimens with different volume fractions of twins were tested under uniaxial tension at 393 K and a strain rate of 10-4 s-1. The results showed that both strength and ductility in nc-Ni attained maximum values after annealing for 5h. The role of both bimodal grain distributions and annealing twins in enhancing ductility and strength was discussed.

Deformation Mechanisms in Nanocrystalline Materials

“…[14] does not include a term related to thermal activation. The absence of this term is not consistent with the experimental results obtained on nc materials, [3,5,[16][17][18][19][20][21][22][23][24][25] which clearly show that the deformation behavior of nc material is not athermal and that strain rates measured during the deformation depend on temperature according to the following expression:…”

“…[20] for the purpose of plotting e f pct against the tensile strain rate at 393 K. This plot is shown in Figure 12. Included in the plot are the ductility data obtained for nc Ni (40 nm) at 393 K. [25] It is clear that the agreement between prediction and experiment is good in terms of the trend. However, the values of the predicted ductility are higher than those experimentally reported.…”

“…12-Percentage elongation to failure as a function of the strain rate for nc Ni (40 nm) tested at 393 K. Data were taken from Ref. 25. Included in the figure is the prediction of the model of dislocation-accommodated boundary sliding.…”

“…In particular, these investigations focused on nc Ni and nc Cu. As a result of these investigations, there exist several sets of data for these two metals [3,5,[16][17][18][19][20][21][22][23][24][25] that are identified in Table II. All data are characterized in terms of shear stresses s and shear strain rates _ c: To convert tensile data into shear data, the following relations were used:…”

“…In order to provide a close comparison between the values of ductility predicted from Eq. [20] and experimental data measured [25] for nc Ni at 393 K, three steps were carried out. First, the values of stresses at the imposed strain rates were calculated from Eq.…”