Elsevier

Acta Materialia

Volume 56, Issue 17, October 2008, Pages 4836-4845
Acta Materialia

Microstructural changes of nanocrystalline nickel during cold rolling

https://doi.org/10.1016/j.actamat.2008.05.040Get rights and content

Abstract

The microstructural changes of electrodeposited nanocrystalline Ni with an initial grain size of about ∼30–40 nm during cold rolling up to 76% thickness reduction have been studied using X-ray diffraction and transmission electron microscopy. In response to the cold deformation processing we observed significant changes in the scale and morphology of grains, defect content, as well as of crystallographic texture. Our experimental findings for nanocrystalline Ni are directly compared to the behavior of coarse-grained Ni. The role of the grain scale reduction to the nanometer regime is discussed with respect to the microstructural changes.

Introduction

Grain size reduction in polycrystalline aggregates of metals offers the prospect of enhancement of many mechanical properties, especially of the yield stress, which typically increases with decreasing average grain size. Conventionally the empirically derived Hall–Petch law [1] describes this relationship between the average grain size and the yield stress of a polycrystalline metal. Using various methods of synthesis and processing, it is possible to reduce the average grain size of coarse-grained (CG, with average grain size >1 μm) face-centered cubic (fcc) metals down to the size regime of nanocrystalline (NC, with average grain size <100 nm) and ultrafine-grained (UFG, with average grain size between 0.1 and 1 μm) polycrystalline aggregates [2]. The great theoretical potential to improve the yield stress along with this recent capability to produce bulk scale samples of metals with nanocrystalline microstructures has triggered a lot of interest [2].

In this study, we examined the microstructural responses of nanocrystalline (NC) Ni to plastic deformation processing by cold rolling to large strains. Cold rolling was chosen as the deformation processing route because microstructural changes during cold rolling of CG Ni have been documented well [3], [4], [5], allowing comparison with our findings for NC Ni. Most previous studies of deformation of NC Ni and other NC metals have typically focused on tensile or compressive testing [6], [7], [8]. To date, only one experimental study of NC Pd presented data on cold-rolling deformation and the effects on microstructure [9]. Pd is considered to be a medium to low stacking fault energy fcc metal, while Ni is considered a high stacking fault energy fcc metal, which in CG form exhibits different texture development during cold deformation processing [10]. Cold rolling to large strains differs from the uniaxial loading routes of pure compression and pure tension regarding the applied stress state, the maximum strain [6] and, in some cases, also the strain rate [4]. Hence, the results of this study of the microstructural response of NC Ni to cold rolling are expected to present new insights not previously reported.

Prior to cold rolling, the average grain size of the NC Ni material used here was 37 nm. For NC Ni significant deviations from the conventional Hall–Petch behavior of the yield stress, i.e. the monotonic increase of yield stress for decreasing grain size [1], are expected for grain sizes below 10–20 nm [11]. Furthermore, the Hall–Petch relationship [1] has been confirmed experimentally to remain applicable for NC Ni with average grain sizes larger than 20 nm [2], [6]. We chose NC Ni with this average grain size of 37 nm and deformation conditions, i.e. room temperature, strain rate ∼10−1 s−1, purposely such that dislocation-mediated processes are known to dominate the accommodation of plastic deformation [2], [11], [12]. Contributions of grain boundary diffusion processes to plastic strain accommodation are known to be very limited [2]. This allowed us to compare the similarities and differences of dislocation-mediated plasticity and microstructural evolution in NC and CG Ni during cold rolling.

For example, the dislocation density has been found to be uncommonly low in NC metals, in contrast to their CG metals counterparts [13], [14]. Most grains in NC metals are free of pre-existing dislocations. Conventional dislocation source operation mechanisms, e.g. the activity of the regenerative Frank Read source [15], are not generally available in NC metals. Grain boundaries have been suggested to act as dislocation sources and sinks instead during dislocation-facilitated plasticity in NC metals [16], [17].

Furthermore, simple models based on continuum elasticity and dislocation theory have been used to conclude that, at the stresses associated with yielding, most NC metals can only support a single dislocation at a given time in a single grain [18], [19]. Only single slip systems are active simultaneously in a single grain during plastic deformation of an NC metal [17], [20]. Thus, dynamic dislocation–dislocation interactions, typically responsible for work hardening in CG fcc metals, are not expected to occur in the grain interiors in NC metals. However, interactions of the glide dislocations with the grain boundaries instead of intersection of and reactions with other active glide dislocations are predicted to play a major role. Dislocation–grain boundary interactions can result in the changes in grain boundary character, macroscopic texture and grain shape necessary to accommodate the externally applied stress. Meyers et al. [21] describe simple models visualizing such dislocation–grain boundary interactions.

The effects of these two major differences in the dislocation mechanism associated with NC metals relative to their CG counterparts on work hardening and global texture evolution during plastic deformation have not been studied extensively. We use X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments to document the microstructural changes in NC Ni during cold rolling to a reduction in thickness of up to 75% of the initial thickness. We report on changes in grain scale and shape, crystallographic macrotexture (XRD) and also local or microtexture (TEM) in the NC Ni in response to the cold-rolling deformation. Our experimental results are discussed with respect to the microstructural changes that have been established for CG Ni during cold rolling, invoking arguments based on dislocation theory.

Section snippets

Experimental

Electrodeposited NC Ni sheets of dimensions 25.4 × 25.4 × 0.3 mm, with a nominal grain size of ∼30 nm (stated by the vendor), were acquired from Goodfellow. The composition of this commercially available material regarding impurity content has been reported previously [22]. The NC Ni was plastically deformed by cold rolling at room temperature. The NC Ni sheets were cold rolled in subsequent passes in identical orientation to the final reduction in thickness with approximately 5–10% reduction in

Results

Fig. 1 compares the θ/2θ XRD scans of the undeformed and the cold rolling deformed NC Ni. For each of the scans we show the normalized intensity vs. scattering angle, 2Θ, which is the intensity ratio I(2Θ)/Imax, where Imax is the maximum intensity measured for the strongest peak, here the 200 peak, I200 (Fig. 1). The intensity of the 200 peak is enhanced relative to that of the 111 peak for the undeformed NC Ni (top scan, Fig. 1). With increasing deformation during cold rolling, the normalized

XRD/texture analysis

In Fig. 1, Fig. 2 we present results of the XRD and texture analysis of the undeformed and cold-rolling-deformed NC Ni. The enhanced intensity of the 200 peak in the undeformed NC Ni as compared to a powder XRD pattern from an elemental untextured fcc metal (top scan, Fig. 1) is consistent with the presence of a cube texture. The increase of the 220 peak along with a concomitant decrease of the 200 and 111 intensities indicate a change in texture upon plastic deformation. We determined grain

Conclusions

  • NC Ni has been plastically deformed to a large deformation strain by cold rolling to thickness reductions up to 76%.

  • The cold rolling of the initially NC Ni induced a change from a cube texture in the undeformed state to a texture composed mainly of <111>{112} Copper- and <634>123} S-fiber components, similar to the cold-rolling texture reported for CG Ni.

  • Deformation twinning does not play a significant role during cold rolling of the initially NC Ni.

  • The average grain size in the initially NC Ni

Acknowledgements

The material presented in this article received partial support from the National Science Foundation under Grants DMR-0094213 and CMS-0140317 to the University of Pittsburgh. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Electron microscopy was facilitated through the Materials Micro-Characterization Laboratory, Department of Mechanical Engineering and

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