Low cycle fatigue behaviour of an aluminium alloy with small shearable precipitates: Effect of surface coating

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Abstract

This work shows that the origin of premature Low Cycle Fatigue (LCF) fracture of an underaged Al–Li–Mg–Cu 8090 alloy is a consequence of severe strain localisation mode in the planar slip bands. It is known that an aggressive gaseous environment (even air) can be a major component in the damage process by fatigue. LCF tests are conducted in air and after surface protection of samples by gold coating (deposited under vacuum). The results obtained on coated samples exhibit the extension of fatigue life (Nf) to a value of 200% Nf. This result is discussed in terms of strain irreversibility in slip bands, emergence of these bands at the surface and the surface defect/gaseous environment reaction.

Introduction

It has been shown that one of major mechanisms of microcrack initiation is the irreversibility of slip. The first fundamental studies concerning this subject were principally conducted on materials having persistent slip bands (PSBs) which differ from planar slip bands by internal structure, Magnin et al. [1]. The functioning mode of PSBs has been reported by a model of Essmann et al. [2] in which the irreversibility of strain is particularly attributed to the annihilation of screw segments of dislocations and the climbing of edge segments created by cycling. Annihilation of edge dislocations by climbing generates point defects as vacancies, which induce volume-increasing. This increase of volume is linked to the observation of several extrusions and intrusions (surface defects). Irreversibility due to screw dislocations appears as steps at emergence of slip bands at the surface. After a sufficient cumulative strain, extrusions and intrusions constitute the local surface morphology. Other work by Bao-Tong Ma and Laird [3] indicates that for a protrusion generating an excess of matter observed outwards, there corresponds a deficit of matter towards the interior. These diverse irregularities of the surface lead, when the stress concentration is sufficiently high, to crack initiation.

On the other hand, the fatigue resistance of metals can be profoundly affected by environmental reactions that affect crack initiation and/or propagation at room temperature, Duquette [4]. Later studies have exhibited an extension of fatigue life of different materials in an inert environment. These results, summarised by Witmer, Farrington and Laird [5], confirm that both chemical and mechanical damage mechanisms play an important role. The experimental researches conducted on copper single crystals have been the origin of our understanding of cyclic deformation mechanisms in an aggressive environment. It seems that at the beginning of cycling in a vacuum the persistent slip band morphology, the extrusion and step formation are little different from those in air. However, the fatigue life is extended in a vacuum because the microcracks do not grow and can even be rewelded, Witmer et al. [5]. As cracks do not grow, the cumulative strains in the persistent slip bands increase and induce secondary hardening, Kuhlmann-Wilsdorf and Laird [6]. The consequence is the appearance of a local stress level sufficient to nucleate new active PSBs. This process will continue until the entire crystal is filled with PSBs. Hence, the structure of deformation becomes more homogeneous resulting in an extension of fatigue life.

Transmission Electron Microscopy (TEM) observations on copper cycled in a vacuum show that PSBs accumulate dislocations and can be converted to a cell structure. Once this transformation occurs, the local flow stress increases into the neighbouring matrix in which new active PSBs nucleate. The old PSBs become completely passive. Witmer et al. [5] partially justify these results. They demonstrate that the PSBs expand on the whole crystal but the old PSBs do not become completely inactive: they continue to deform but with a reduced local strain amplitude.

The Scanning Electron Microscopy (SEM) observations show the formation of thin regularly spaced extrusions and intrusions at the edges of old PSBs. The more homogeneous and reversible structure deformation obtained slows down the growth of the microcracks.

Polycrystal LCF behaviour has also been studied by several authors who observed almost the same mechanism, Verkin and Grinberg [7]. Some results have been summarised by Duquette [4] as follows: a partial vacuum of only 10−3 torr resulted in appreciable increases in fatigue lives for carbon steels, brasses and copper, whereas copper–nickel and chromium steels showed little improvement. Experiments on copper and 70–30 brass in air led to some reduction of fatigue life.

Experiments on lead and Armco iron also showed marked changes in fatigue behaviour. The same materials tested in a vacuum exhibit extended fatigue lives. The effect of atmospheric oxygen on the fatigue lives of copper, aluminium and gold has also been investigated; oxygen reduces fatigue life in copper and aluminium but has no effect on gold. Other works show that slip bands cyclically produced in air became regions of high oxygen concentration because of strain-induced vacancy generation. Dissolved oxygen, hence, serves to prevent initial rewelding of nascent microcracks on slip bands.

Atmospheric water vapour has also been cited to be important to the crack nucleation process in age-hardened aluminium alloys, as the dissociation of water vapour leads to hydrogen-embrittlement of the alloy and causes premature cracking.

A recent investigation on Al–Li–Cu–Mg alloys reveals that the underaged state is more susceptible to hydrogen embrittlement than the peakaged state. The hydrogen-dislocation interaction and hybrid cracking mechanisms introduced by embrittlement are indicated, Thakur et al. [8].

The aim of the present work is to study the low cycle fatigue behaviour of an underaged aluminium alloy (8090) characterised by planar slip bands in air. The effect of surface protection by gold coating deposited in a vacuum is also investigated.

Section snippets

Experimental procedure and material

The LCF tests were conducted at ambient temperature. Controlled plastic strain amplitude with a reversed triangular waveform (plastic strain range 0.01%≤Δεp/2≤0.5%) and a frequency of 0.1 Hz were used.

The amplitude of plastic strain is kept constant by periodic manual adjustments. The tests were carried out on a Mayes servohydraulic machine using cylindrical specimens (nominal diameter 6 mm and gauge length 12 mm) cut from the core of the bar and parallel to the rolling direction.

During the LCF

LCF behaviour in air (without coating).

The hardening curves at different applied plastic strains (Fig. 2) show that the material is suddenly broken after the hardening period and the softening period is completely absent. This finding suggests that fracture is premature. Generally, for age-hardened alloys, the softening period that corresponds to the shearing of coherent particles follows cyclic hardening.

Fig. 3 presents a cyclic stress–strain curve. Since saturation does not occur (Fig. 2), we chose to define the cyclic

Discussion

The review by Khireddine et al. [11] summarises different works on irreversibility of the strain in fatigue and confirms that slip irreversibility is an important factor inducing crack-initiation. This study also presents the cyclic behaviour of the 8090 peakaged alloy characterised by a mean diameter of δ′ precipitates estimated as 200 Å. The cumulative shear strain within bands has been evaluated. The results obtained, combined with estimation of the volume fraction of slip bands by TEM,

Conclusion

The underaged Al–Li–Cu–Mg polycrystal alloy is characterised by a severe localised and irreversible deformation induced by intensive shearing of small coherent precipitates δ′. Consequently, it shows a very short life duration in air. However, it is possible to extend its fatigue life by a surface coating, to a value of 200% Nf. An improved value of fatigue life might probably be reached in an inert environment.

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