A. Vinogradov

Fatigue properties of 5056 Al-Mg alloy produced by equal-channel angular pressing

Abstract The fatigue behaviour of the fine-grain 5056 Al-alloy processed by equal-channel angular pressing (ECAP) is explored. This material exhibits a slightly enhanced fatigue life at low stress amplitudes. However, no improvement in the fatigue limit is observed. Fatigue performance is discussed in terms of fatigue life, crack nucleation and propagation. Structural changes during fatigue are investigated by transmission electron microscopy. It is shown that the fine structure achieved during processing is unstable and tends to relax with cycling, resulting in local recovery of the pre-deformed material. Structure relaxation during fatigue is supposed to provoke notable cyclic softening which is particularly pronounced at higher applied stresses. It is found that the crack growth rate is greater in the fine-grain ECAP material than in its coarse-grain counterpart. The latter is attributed to the roughness-induced crack closure and crack deflections which is more significant in conventional alloy. The improvement of fatigue properties at low-cyclic regime is believed to be due to a higher resistance to crack nucleation in the fine-grained material having a larger yield stress value.

Cyclic response of ultrafine-grained copper at constant plastic strain amplitude

The investigation in the fatigue behaviour of ultrafine-grained copper produced by severe plastic deformation was performed in fully reversed tension/compression under constant plastic strain amplitudes. It is shown that UFG copper possesses unusual fatigue properties which are determined by both the small grain size and the specific non-equilibrium state of GBs. The experimintal results can be summarised as follows: - UFG materials show a clearly pronounced saturation stage; - no cyclic softening is observed during cyclic deformation; - in comparison with conventional polycrystals, UFG matarials reveal a high saturation stress and significant Bauschinger effect; - fatigue properties depend on internal structure of UFG materials which is strongly influenced by a thermal treatment. The saturation stress decreases and the Bauschinger energy parameter increases after short (3 min) annealing at 473 K as a result of recovery of non-equilibrium GBs.

Acoustic emission in ultra-fine grained copper

During the past decade, the ultra-fine grained (UFG) materials (both nanostructured and submicrocrystalline) have attracted much of attention largely due to their superior mechanical properties such as a high strength with significant plasticity (or even superplasticity) at relatively low temperatures [1]. Despite considerable efforts, the mechanisms of plastic deformation of these advanced materials remain unclear [2–5]. Strain (stress) localization is an important factor which controls degradation of mechanical properties in a wide range of materials. Modern high strength materials such as ceramics are rather brittle and sensitive to the stress concentration. Hence, the investigation of strain (stress) localization in UFG materials is of utmost importance for characterization of their mechanical stability. An acoustic emission (AE) technique provides real-time information on the structural rearrangements in very small volumes of solids. For this reason, in the present work we employ AE to explore possible plastic instabilities in UFG copper subjected to monotonic tensile loading. AE at the onset of plastic deformation of metallic materials is usually associated with the motion of dislocations. AE parameters are usually connected to dislocation velocity, mean free pass, etc. A strong effect of grain size on AE has long been recognized [6–15]. The AE experimental results which are available to date reflect a pretty complicated role of grain boundaries in plastic deformation. Comprehensive reviews concerning the grain size and grain boundary effect on AE are given in refs. [11–13]. To our knowledge, no reports are available on dislocation AE detected in pure polycrystalline metals with the grain size smaller than 10 mm. From the aforementioned data on AE in the crystalline solids it is hard to expect measurable AE during plastic deformation of pure nanocrystals. Furthermore, adopting the criterion for AE detectability proposed by Wadley and Mehrabian [12] naV

On the role of free surface in acoustic emission

0.035 m s (a is the radius to which a dislocation loop can expand before arrest at a pinning point, V is the radial velocity andn is a number of dislocations involved into a co-operative motion) we obtain that for a 5 10 m nV should be greater than 3.53 10 m/s. This is unreasonable value to expect, particularly in view of the estimates made in ref. [4] where it was shown that only one dislocation passes in average through each grain of 200 nm per second upon loading with a strain rate of 10 23 s. Although these estimates are rather rough, it seems that there is neither experimental nor theoretical basis for dislocation AE in nanocrystals. However, there are at least two strong points in a favour of possible AE in UFG polycrystals. (1) The idea of AE measurements in these materials was stimulated by former observations of shear bands and fine slip lines during cyclic and tensile experiments [4, 16, 17]. It has been shown that plastic flow in UFG Cu is not limited to the intergranular deformation and extends over a large number of grains. Localization of plastic deformation in the form of shear bands implies that the co-operative rearrangements in the ensembles of defects occurs, facilitating AE detection. (2) Several authors have argued Pergamon Scripta Materialia, Vol. 39, No. 6, pp. 797–805, 1998 Elsevier Science Ltd Copyright

Effect of triple junction on fatigue crack growth in copper and copper-3at.% aluminium tricrystals

The relationship between the bulk and surface related sources of acoustic emission (AE) is studied by analysing the effect of geometry and the surface on AE-level. The AE maximum at the onset of plastic deformation is found to be proportional to the surface area. It is concluded that AE during early stages of plastic deformation of both single and polycrystals is largely determined by the surface related defects.

Hysteresis loop shape of a cyclically-deformed copper tricrystal having two longitudinal grain boundaries

It has been well understood that many properties of polycrystals depend on both characteristics of grain boundaries (GBs) and triple junctions. Cu and Cu-Al belong to the most extensively studied group of metals in the field of fatigue in the last 20 years. However, triple junctions in these materials have been scarcely studied from the viewpoint of their mechanical properties and fatigue, in particular. The question of what effect the GB junction has on fatigue crack behavior in different materials remains unanswered. The bicrystals are widely used for examination of interfaces in materials, but they do not adequately represent the behavior of polycrystals. Tricrystals are believed to be most suitable for investigation of the structure and properties of triple junctions. The purpose of the present work is to examine the effect of triple junction on the fatigue crack behavior in Cu and Cu-Al tricrystals having adjoined {Sigma}3-{Sigma}3-{Sigma}9 boundaries of the same geometry.

Cyclic Stress-Strain Response of Pb-Sn and Zn-Al Eutectic Alloys Fine-Grained by Equal Channel Angular Pressing

The nature of the Bauschinger effect in metallic materials is explained mainly by two different approaches. One takes account of the internal stresses related to macroscopic residual stresses between the neighboring grain which shows different hysteresis loops. The other approach is based on the long-range elastic interactions between dislocations creating microscopic stress field (see the review (1) for details). Obviously, these two mechanisms operate simultaneously in a common polycrystalline material. Discrimination between these two mechanisms is not quite straightforward, and the magnitudes of individual contributions to the Bauschinger effect are still ambiguous. In order to separate these two contributions, it might be convenient to compare the results obtained on a bicrystal having a longitudinal grain boundary with those of constituent single crystals. However, a technical problem arises in experiment with bicrystals. If the flow stresses of constituent grains are different, the deviation in the axial stress gives rise to a certain bending moment leading to inhomogeneous stress and strain distribution in the specimen. In order to diminish this bending moment, we employed a tricrystal with two longitudinal parallel boundaries as shown schematically in Fig. 1. The grains at both ends of the specimens are of the same orientation and dimension. In the present study, the cyclic deformation tests were carried out on the tricrystal and its constituent single crystals. The Bauschinger effect during the cyclic deformation was compared between the tricrystal and the constituent single crystals by means of the Bauschinger energy parameter bE (2) and the Bauschinger stress parameter bs (3). The authors attempted to understand the Bauschinger effect in the cyclically-deformed tricrystal by mixing the corresponding pairs of hysteresis loops of the constituent grains.

On the Cyclic Behavior of Ultra-Fine Grained Copper Produced by Equi-Channel Angular Pressing

Superplastic deformation usually requires testing temperature above 0.5T m when an average grain size is less than 10 µm (T m is the melting point) [1]. This is because the superplastic deformation is connected closely with thermally-activated processes such as dislocation climb and grain-boundary diffusion. It has long been recognized that the grain size affects the temperature at which the superplastic deformation occurs. To achieve low temperature superplasticity, several Al-alloys have been fine-grained by the equal channel angular pressing (ECAP) technique [2]. The properties of the superplastic materials have primarily been studied in monotonie straining while the limited results have been reported on their cyclic behavior [3]. Hence, it seems reasonable to utilize the superplastic alloys fine-grained by ECAP to perform a precise fatigue experiment at room temperature. In the present work, we carried out cyclic tests at room temperature on the ECAP fabricated Pb-62%Sn and Zn-22%A1 eutectic alloys. The attention is paid particularly to the strain-rate dependence of the stress amplitude.