Fatigue damage of ultrafine-grain copper in very-high cycle fatigue region

doi:10.1016/j.msea.2011.06.001

Materials Science and Engineering: A

Volume 528, Issues 22–23, 25 August 2011, Pages 7036-7040

https://doi.org/10.1016/j.msea.2011.06.001 Get rights and content

Abstract

Ultrafine-grained copper produced by ECAP technique was tested in ultrasonic fatigue in the region of lives from 10⁸ to 2 × 10¹⁰ cycles. The fatigue strength of ultrafine-grained copper is by a factor of 2 higher than that of conventional-grain copper. The occurrence of surface fatigue slip markings is rare. No grain coarsening was observed either in bulk or in areas underneath the surface slip markings. Localized collective grain-boundary slip of near-by oriented grains is considered to be the main damage mechanism leading to fatigue microcrack initiation either directly in the roots of surface intrusions or underneath the surface slip markings.

Highlights

► Ultrasonic fatigue tests of ultrafine-grain copper produced by ECAP. ► Rare occurrence of surface slip markings. ► No grain coarsening either in bulk or in volumes adjacent to surface markings. ► Collective grain-boundary slip produces surface extrusions and intrusions. ► FIB micrograph of subsurface microcracks.

Introduction

Fatigue failure of materials cycled in the very-high cycle fatigue region (VHCF, number of cycles to failure exceeding 10⁸) has been in focus of interest of many laboratories for at least the last two decades. Besides a great number of S–N data determined especially on high-strength materials, also some insight into the mechanisms of initiation and propagation of fatigue cracks has been gained [1]. This concerns again especially high-strength materials containing inclusions, where the fatigue failure frequently occurs in the form of so-called “fish-eye” fracture. Fatigue damage mechanisms in ductile single-face materials subjected to VHCF have been studied to a lesser extent. Nevertheless, in the last five years several studies carried out on conventional-grain copper cycled in the VHCF region were published [2], [3], [4]. They clearly show that a substantial fatigue damage does develop at loading amplitudes well below the persistent slip bands (PSBs) threshold provided the number of cycles is large enough [4]. Particularly it was shown that the cyclic slip localization into the PSBs [5], [6] is not the only possible mechanism for the fatigue crack initiation in conventional-grain (CG) copper. More specifically, the main results of the quoted papers can be summarized as follows:

•
Well below the PSB threshold, marked cyclic slip localization occurred in intense slip bands. The underlying dislocation structures are not always of the regular ladder-like type, as it is in the case of conventional cycling at the stress amplitude corresponding to the PSB threshold.
•
The slip bands were shown to be persistent (re-appeared after electropolishing and subsequent cycling).
•
At the sites of emerging PSBs at the surface, extrusions and intrusions (microcracks) were formed.
•
The cyclic slip activity seems to take place in the whole volume of the cycled specimen.

Since the beginning of 1990s the fatigue properties of the ultra-fine grain (UFG) metals prepared by the equal-channel angular pressing (ECAP) technique have been studied on quite a large scale. But practically all these studies concern only the low-cycle (LCF) and high-cycle (HCF) fatigue regions. Besides several hundreds of primary articles also a few overview articles have been written [7], [8], [9]. Generally it can be stated that the UFG metals exhibit higher fatigue strength than their conventional-grain (CG) counterparts provided the cycling is stress-controlled. For strain-controlled cycling the situation is usually reversed. In comparison to CG metals, the UFG metals have usually a lower resistance against fatigue crack propagation. Further details of fatigue behaviour vary from material to material.

The data on the fatigue behaviour of UFG metals in the VHCF region are very scarce. In fact, some data are available only for UFG copper in our earlier paper [10]. The basic result is that the fatigue strength of UFG copper of purity 99.9 pct is by a factor of about 2 higher than that of CG copper. The mechanisms of the fatigue damage in UFG copper in VHCF region have not yet been studied.

The main aim of this paper is to experimentally study the mechanisms of the fatigue damage in UFG copper in VHCF region and to compare them with the analogous mechanisms for the CG copper as presented by the above quoted authors.

Section snippets

Experimental

Copper of 99.9% purity was processed by ECAP technique at the laboratory of Prof. R.Z. Valiev at Ufa State Aviation Technical University (Russia). The limiting content of impurities is given in Table 1. Cylindrical billets 20 mm in diameter and 120 mm in length were produced by route Bc by eight passes with 90° rotation after each extrusion. The deformation was carried out at room temperature. After last ECAP procedure rods of 16 mm in diameter and 100 mm length were turned from the billets.

Tensile

Results

S–N plot of fatigue life data of UFG copper is shown in Fig. 1 together with the relevant S-N data for CG copper (grain size 50–70 μm) taken from our earlier papers [11], [12]. For UFG copper, it can be seen that the short life data obtained at 10 Hz, the HCF data obtained at 123 Hz and the VHCF data obtained at 20 kHz smoothly concur. It indicates a zero or a very weak frequency influence on the fatigue life. All the tests of the UFG copper were carried out on specimens from one batch. The two

Discussion

The estimated values of the plastic strain amplitudes in the ultrasonically cycled specimens of UFG copper are very low; they lie in the interval from 8 × 10⁻⁹ to 5 × 10⁻⁷ (Fig. 3). Let us consider the case of macroscopic plastic strain amplitude (taken over the whole gauge length) equal to 10⁻⁸ corresponding to the lifetime of about 1.5 × 10¹⁰ cycles. There are two possibilities how to reach this value. (i) 100% of the loaded volume deforms cyclically plastically at the plastic strain amplitude of 10

Conclusions

The main results of the present paper can be summarized as follows.

1.
Fatigue process in ultrafine-grain copper ultrasonically cycled in the very-high cycle fatigue region is a highly localized phenomenon. Estimation based on extrapolation of Coffin–Manson dependence and on the microstructural observations confirms that only an extremely small part of the total loaded volume (of the order of 0.1%) deforms cyclically plastically, the overwhelming rest deforms cyclically elastically.
2.
No grain

Acknowledgement

This work was financially supported by the Czech Science Foundation under contract 108/10/2001. This support is gratefully acknowledged.

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Fatigue damage of ultrafine-grain copper in very-high cycle fatigue region

Abstract

Highlights

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

Acknowledgement

Int. J. Fatigue

Int. J. Fatigue

Int. J. Fatigue

Mater. Sci. Eng.

Int. J. Fatigue

Mater. Sci. Eng. A

Mater. Sci. Eng. A