In situ TEM observations of reverse dislocation motion upon unloading in tensile-deformed UFG aluminium

Abstract

Loading–unloading cycles have been performed on ultrafine-grained (UFG) aluminium inside a transmission electron microscope (TEM). The interaction of dislocations with grain boundaries, which is supposed to be at the origin of the inelastic behaviour of this class of materials, differs according to the main character of the dislocation segments involved in pile-ups. Pile-ups are formed by spiral sources and lead to the incorporation of dislocations into grain boundaries (GBs) during loading. Upon unloading, partial re-emission of dislocations from GBs can be observed. Stress and strain measurements performed during these in situ TEM loading–unloading experiments are in agreement with the rather large inelastic reverse strains observed during unloading in loading–unloading tests on bulk macroscopic UFG aluminium specimens.

Introduction

Bulk ultrafine-grained (UFG) metals produced by equal channel angular pressing (ECAP) have attracted much attention during the last decade because of their extraordinary strength properties (e.g. [1], [2], [3]). As compared to conventional grain (CG) size materials, UFG materials exhibit a much higher flow stress, which can be described by the well-known Hall–Petch law. The origin of this higher strength undoubtedly lies in their higher content of grain boundaries (GBs), which are very efficient obstacles to dislocation motion. Another intriguing property of UFG metals, which is probably related to the first one, is their unusual large inelastic reverse deformation upon unloading, as has been shown some time ago for both UFG copper [4], [5] and UFG aluminium [6], [7]. In contrast to reference CG materials, which show almost completely elastic unloading behaviour under similar conditions, the UFG metals studied exhibited an inelastic back strain which can be as large as 5 × 10⁻⁴ in the case of UFG copper and more than 2 × 10⁻⁴ in the case of UFG aluminium. The example shown in Fig. 1 refers to UFG aluminium.

Since the grain size of UFG materials is, typically, a few hundred nanometers, i.e. comparable to the observable foil thickness in transmission electron microscopy (TEM), the ratio of boundary surface to free surface is higher than in CG materials (although generally still smaller than unity). Surface effects should accordingly be reduced, and the inelastic behaviour be at least partly reproduced in in situ TEM experiments. Such experiments have been carried out recently on nanocrystalline Al and Au [8]. In such materials, the very high inelastic back strain has been explained by a heterogeneous distribution of grain size leading to a heterogeneous deformation [3]. However, in the UFG samples discussed here, preliminary experiments indicated that the inelastic back flow should result from another origin, probably related to dislocation–GB interaction and the related deformation-induced internal stresses [9]. Dislocation interaction with low-angle boundaries has been studied in situ a long time ago, in crept Al [10], [11]. However, dislocation interactions with high-angle GBs are expected to be different from those with low-angle subgrain boundaries consisting of lattice dislocation networks.

Dislocation–GB interactions have been the subject of a large amount of theoretical and experimental studies, which will not be reported here, but which can be found in the review paper of Priester [12]. In situ straining experiments were performed on a Ge bicrystal by Michaud et al. [13] to observe the insertion of dislocations into GB. In situ annealing experiments were also carried out by Poulat et al. [14] and Couzinié et al. [15] in order to observe the kinetics of relaxation of lattice dislocations inserted in GBs of Ni and Cu. Recently, Chassagne et al. [16] combined in situ TEM and molecular dynamics to study dislocations– twin interactions. However, only particular GBs with low Σ values were studied in these experiments and the elastic interaction forces between dislocations and GB were not determined.

In order to observe the real interaction processes between mobile dislocations and random GBs, more detailed loading–unloading in situ TEM experiments than reported earlier [9] were performed on a UFG aluminium sample produced by ECAP. The results described below are complementary to those already published in our preliminary article [9].