The role of environmental exposure in the fatigue behaviour of an aluminium alloy

Corrosion Science, Vol. 39, No. 12, pp. 2117-2141, 1997 0 1997 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 001&938X/97 $17.00+0.00

PII: soo1~938x(97)ooo9s4

THE ROLE OF ENVIRONMENTAL EXPOSURE IN THE FATIGUE BEHAVIOUR OF AN ALUMINIUM ALLOY

J. RUIZ and M. ELICES

Departamento de Ciencia de Materiales, Universidad Polit&cnica de Madrid, Madrid, Spain ETSI de Caminos, Canales, y Puertos, Ciudad Universitaria, s.n. 28040-Madrid, Madrid, Spain

Abstract-This paper reports measurements of fatigue crack velocities for an aluminium alloy 7017-T651 in high purity gaseous environments with different values of water vapour pressure and frequency. Also a detailed fractographical analysis by SEM is presented and the fracture surface morphology is correlated with the testing parameters and with the crack stress intensity level. Within a range of environmental exposure (defined as the product between the pressure and the time available for surface reaction during one fatigue cycle) crack propagation rates depend linearly on the exposure. The experimental fatigue crack propagation data are satisfactorily explained in terms of a corrosion-fatigue model for gaseous environments. 0 1997 Elsevier Science Ltd

Keywords: A. aluminium, B. SEM, C. corrosion fatigue, C. hydrogen embrittlement

INTRODUCTION

It is well known that cyclic loads in the presence of aggressive environments cause a deterioration of mechanical properties and a reduction in the service life of materials. Fatigue crack propagation rates in aggressive media increase noticeably over those in an inert high vacuum atmosphere. One of the most significant drawbacks of this phenomenon, which is known as corrosion-fatigue, is its unspecific character: for a given material under cyclic load, almost any environment is likely to provoke an acceleration in crack growth rates. Aqueous environments, particularly chloride solutions, produce the most detrimental effects in aluminium alloys, but some supposedly inert environments, such as atmospheric air, can substantially enhance crack propagation rates with respect to the results obtained in high vacuum.

The effect of aqueous environments on the mechanical behaviour of aluminium alloys under both monotonic load (stress corrosion cracking), and cyclic load (corrosion-fatigue), has been the subject of extensive research, see for example Refs l-4. However, environmental fatigue of aluminium alloys in gaseous environments is less well documented,5-7 probably due to experimental difficulties. In order to maintain and monitor a high purity atmosphere surrounding the sample, one needs a sophisticated setup with a vacuum chamber and a series of gauges to measure the pressure and the composition of the residual gas, to control the testing environment in a precise and reproducible way.8P9

Experimental results have shown that the chemical species responsible for embrittlement in gaseous atmospheres when the material is subjected to cyclic loads, both in aluminium alloys and in steels and intermetallic compounds, is the water vapour contained in the

Manuscript received 3 May 1997.

2117

2118 J. Ruiz and M. Ekes

environment.6’7”0”’ When modelling this behaviour, two approaches are used; some

researchers emphasize the influence of water vapour on the plastic flow processes at the

crack tip,‘*,13 whereas others pay attention’to the physico-mechanical aspects of the process,

specifically to the surface reactions between water vapour and aluminium in the crack tip

region6,’ The latter have developed a semi-empirical model which postulates a linear

relation between crack growth rate and environmental exposure (defined as the quotient between the external gas pressure and the testing frequency or, alternatively, the product between the pressure and the time available for surface reaction during one fatigue cycle).14 The preliminary results obtained in two different aluminium alloys in atmospheres with changing water vapour pressure at a constant testing frequency seem to confirm this

hypothesis,6*7 but a detailed investigation of the joint influence of water vapour pressure and

frequency on crack propagation rate in aluminium alloys is needed to asses the validity of the model and to confirm the proposed embrittlement mechanism; this is the subject of this

paper. This contribution is the second part of a previous paperI recording tests at a single

frequency of 5 Hz. In this part original results of fatigue tests performed on 7017-T651 aluminium alloy in high purity gaseous atmospheres, changing both the environmental water vapour pressure and the frequency -- with the aim of checking the environmental exposure hypothesis - are presented and analyzed. After testing the samples, a detailed fractographical analysis by SEM examined the information contained on the fracture surfaces. The objective was to identify in a systematic way the characteristic features of the fatigue fracture for different values of water vapour pressure and frequency so as to

correlate the fracture surface morphology with the environmental exposure, i.e. the macroscopic parameter ~-- crack propagation rate - with the microscopic mechanisms. It is hoped that the micrographs shown will be valuable for modelling environmental crack growth. The crack growth data were analysed as a function of environmental exposure by

using theoretical models developed for other aluminium alloys,6~7~‘4 based on the hypothesis of hydrogen embrittlement. Experimental results and modelling show good agreement over

a range of exposures, supporting the proposed explanation and the postulated embrittlement mechanism.

EXPERIMENTAL METHOD

Material Medium-strength commercial Al-Zn-Mg alloy 7017 (Al 5.O%Zn 2.4%Mg) in the T651

heat-treated condition (peak aged) was employed in this investigation. The detailed chemical composition (measured by atomic absorption) is given in Table 1. It was supplied as a 30 mm thick rolling plate. Due to the fabrication process, the microstructure is highly anisotropic, as shown in Fig. 1, with the grains elongated in the rolling direction and flattened in the other two directions.

Table I. Chemical composition of 7017-T65 1 aluminium alloy

Zn Mg Fe Si Mn Cr CU Zr Ti Ni Al

5.01 2.44 0.23 0.1 I 0.29 0.17 0.12 0.13 0.05 0.01 bal

Fatigue behaviour of an Al alloy 2119

d W

Fig. 1. Orientation of the samples relative to the rolling direction and representative microstructure of 7017-T651 aluminium alloy.

2120 J. Ruiz and M. Elites

Table 2. Mechanical properties of 7017-T651 aluminium alloy (L-T orientation)

E F-34 00 2 Wpa) OM (Mpa)

65.8 415 465 13.7 34.6

Test procedure

Fatigue tests were performed in a servohydraulic machine in load control with constant load amplitude, following the recommendations of the ASTM E647 standard, Test Method

for Measurement of Fatigue Crack Growth Rates. Standard compact tension specimens

(CTS) were machined from the plate in the L-T orientation - the notch plane

perpendicular to the rolling direction - with a thickness of B= 5 mm and a nominal

width of W= 50 mm (Fig. 1). The mechanical properties of the material in this orientation are shown in Table 2 (each value represents the average of three tests). The experimental

procedure involved precracking the samples in air at a frequency of 10 Hz and stress ratio R = 0.1, with a sinusoidal waveform. A manual load shedding technique was used to reduce

the stress intensity factor amplitude, AK, during pre-cracking, from AK= 9.5 MPaJm, to the initial value for the fatigue tests (AK= 6.5 MPaJm). The last two steps were conducted with the specimen in the testing environment to avoid transient effects in the fatigue crack

growth rates. Fatigue experiments were carried out with the same waveform and stress ratio as were used during precracking at the different frequencies: 1, 5 and 10 Hz.

For the tests at 5 and 10 Hz, crack length was measured on one specimen side with a travelling microscope (accuracy of 0.01 mm.). After breaking the sample, the curvature of the crack front was examined and corrected when necessary according to the recommendations of the standard. For the tests at 1 Hz, crack length was calculated from compliance measurements using a clip gauge extensometer (5 mm gauge length) located at the crack mouth. The extensometer was specifically modified and re-designed for high-

vacuum use because the use of a commercial extensometer without special precautions can modify the material behaviour due to its influence on the environmental composition when testing in high purity gaseous atmospheres.’ Crack growth rates were calculated by the

secant method. The stress intensity factor and the relationship between the crack length and the specimen compliance were computed from values given by Saxena and Hudak.16

Testing environments The experiments were performed at room temperature inside an ultra-high vacuum

chamber specially designed for mechanical testing in high purity gaseous atmospheres,

which is fitted to a universal testing machine. This experimental setup is described elsewhere,‘,’ and was proved to be suitable for experiments in the most demanding conditions of purity and control of the testing atmosphere surrounding the sample. In addition to the tests in high vacuum (10-j Pa), used as the inert reference data, experiments were carried out in high-purity water vapour at different pressures (ranging from 0.2 to

300 Pa) and in atmospheric air (50% relative humidity). Water vapour was obtained by means of a vacuum distillation process from de-ionized liquid water.’

The composition of the atmosphere surrounding the sample is analyzed with a quadrupolar mass spectrometer placed in a differentially-pumped secondary chamber joined to the main vacuum chamber. This device provides useful qualitative information


about the composition of the environment surrounding the sample. Several examples of the strict environmental control achieved are reported elsewhere.’ Regarding the high purity water vapour atmospheres used in the experiments, the spectra recorded during the fatigue tests are very clean, with an almost negligible residual content of nitrogen.

Fractography Fracture surface morphology was analyzed with a scanning electron microscope using

secondary and backscattered electrons to form the image. Secondary electron images show the topographical aspect of the fracture surface, whereas the backscattered electron images show the existence of phases with different chemical composition. Chemical composition of the different phases was qualitatively analyzed with energy dispersive X-ray microanalysis (EDX).

EXPERIMENTAL RESULTS

Environmental-fatigue tests The crack growth rate per cycle, da/dN, in a high vacuum atmosphere (px lop5 Pa) is

presented in Fig. 2 as a function of the stress intensity factor amplitude, AK, for the three frequencies investigated: 1,5 and 10 Hz, respectively. The most remarkable feature is that in

AK (MPadm)

Fig. 2. da/dN vs AK data for 7017-T651 aluminium alloy tested in a high vacuum atmosphere @x10-5Pa)for1,5and10Hz.


Water vapour exposure 106 0.2 Pa-s Q 0

0

i

6 7 6 9 10 20 25 AK (MPadm)

Fig. 3. da/dN vs AKdata for 7017-T651 aluminium alloy tested in high purity water vapour at an

environmental exposure of 0.2 Pa.s (1 Hz 0.2 Pa, 5 Hz 1 Pa and 10 HZ 2 Pa).

the range of AK tested (from 7 to 20 MPaJm), no noticeable effect of frequency is observed on crack propagation rates; the data at the three frequencies almost coincide in the same

curve. In high vacuum, the spectra recorded with the quadrupolar mass spectrometer exhibit the typical composition of a clean, unbaked system, where the most abundant species is the water vapour from the previous contact of the system surfaces with

atmospheric air.” So, the water vapour pressure can be estimated as approximately

10W5 Pa. Figures 3 and 4 show crack growth rates in high-purity water vapour atmospheres at

different pressures and different testing frequencies. In order to facilitate the analysis, experimental results corresponding to the same environmental exposure are represented in each graph. Figure 3 shows crack propagation rates at an environmental exposure of 0.2 Pas - that is, 0.2 Pa at 1 Hz, 1 Pa at 5 Hz and 2 Pa at 10 Hz. A comparison between Figs 2 and 3 shows that crack growth data in water vapour at an exposure of 0.2 Pa-s are very similar to those obtained in high vacuum. As suggested previously, there appears to be a threshold value of the environmental exposure under which no noticeable embrittlement occurs.15 Fig. 3 shows that crack propagation rates are very similar in the three testing conditions despite the differences in water vapour pressure and frequency. For this level of environmental exposure, the effects of water vapour pressure and frequency are then


6 7 8 9 10 AK (MPadm)

20 25

Fig. 4. da/dN vs AK data for 7017-T651 aluminium alloy tested in high purity water vapour at an environmental exposure of 1 Pa.s (1 Hz 1 Pa, 5 Hz 5 Pa and IO Hz 10 Pa).

interchangeable (an increase in pressure has the same influence on crack propagation rate as a decrease in frequency) and the experimental evidence confirms the postulated dependence of crack propagation rates on environmental exposure.6’7

When the exposure increases to 1 Pas, that is 1 Pa at 1 Hz, 5 Pa at 5 Hz and 10 Pa at 10 Hz, a saturation in the embrittlement process is reached.15 As seen in Fig. 4, the most striking point is the similitude in behaviour observed for the three testing conditions. As before (at an exposure of 0.2 Pas), environmental exposure is the parameter controlling crack propagation rates. Besides this, the crack velocity at 1 Pas is higher than at 0.2 Pas; for AK= 7 MPa,/m, where the sharpest differences in crack velocity were found, the crack propagation rate at 1 Pas is twice that at 0.2 Pas, and the fractographic results described later indicate a clear difference in the crack growth micromechanisms between the two cases.

Crack propagation rates in humid air (50% relative humidity) are depicted in Fig. 5 at the three frequencies studied: 1,5 and 10 Hz. A small effect of frequency is noticed, so crack velocity is slightly increased when testing frequency is decreased. The differences diminish, however, as the crack-driving force is increased, and the crack growth rates are roughly the same in the high AK region. Water vapour pressure in humid air can be calculated from the product of the saturation pressure (approximately 2600 Pa at room temperature) and the relative humidity, which gives a value of nearly 1300 Pa in this case.

2124 J. Ruiz and M. Ekes

Freauency

. 1Hz

0 5H.z

0 10Hz

7 8 9 10 AK (MPadm)

20 25

Fig. 5. da/dN vs AK data for 7017.T65 I aluminium alloy tested in atmospheric air (50% Rh) for 1, 5 and 10 Hz.

Fractographic analyses

As the macroscopic crack growth rates were controlled by the environmental exposure, a detailed fractographic study was performed to analyze the influence of this parameter on the fracture surface appearance. It was intended to demonstrate the correlation between the transitions observed in crack propagation rates and the changes in the fracture micromechanisms. As the fractographic results at a frequency of 5 Hz have been

presented elsewhere,15 only the microfractographs corresponding to 1 and 10 Hz are shown. In all the micrographs, cracks propagate from bottom to top. It is important to note that the microscopic features shown in the figures were observed uniformly over the entire fracture surfaces, so no essential dissimilarities were encountered between near surface and inner regions of the specimen. This suggests that the same micromechanisms are operative over the whole fracture surface.

High vacuum (p z 10W5 Pa) For the lower values of AK investigated (AKx 7 MPaJm), the fracture surface in high

vacuum exhibits an evenly patterned appearance which is not affected by the testing frequency. Figures 6 and 7 show the morphology of the fracture surfaces in high vacuum at the frequencies of 1 and 10 Hz. As observed at 5 Hz, the most remarkable features are the structures perpendicularly oriented to the crack advance direction, which are periodically


Fig. 6. Microfractographs in high vacuum at 1O-5 Pa (AK%7 MPdm and v= 1 Hz): (a) typical

appearance of the fracture surface, (b) higher magnification.

repeated and superimposed with a puff pastry-like appearance. l5 This morphology is typical of the fatigue process in inert environments in aluminium alloys, and these features have been termed coarse striations by other authors.‘* From Figs 6b and 7b, striation spacing can be estimated as approximately 2 urn, very similar to the spacing at a frequency of 5 Hz in the same experimental conditions,” which far exceeds crack propagation rate for this value of

J. Ruiz and M. Elites

AK : (around 0.01 pm per cycle from Fig. 2). Therefore, if the coarse striations are related to dis Crete propagation events, as the pictures suggest, in this case crack advance does not

prc )ceed cycle by cycle, but is an intermittent process.‘*

the

Fig. 7. Microfractographs in high vacuum at IO ’ Pa (AK-7 MPaV,m and v= IO Hz): (a) typical

appearance of the fracture surface. (h) higher magnification.

In addition to the coarse striations, some regions are observed with markings parallel to crack advance direction, as shown by the arrows in Figs 6 and 7a. These features were


also found at the frequency of 5 Hz and their existence was attributed to the irregular crack growth process. They were thought to be associated with consecutive positions of the crack front in grains unfavourably oriented to the average crack advance direction, because the spacing between successive markings is very similar to the coarse striations spacing.15

As with high vacuum at a frequency of 5 Hz, deep holes, 20 to 30 urn diameter, are sometimes encountered on the fracture surfaces. When the image from the backscattered electrons (BEI) was analyzed, a number of particles with a bright contrast were found inside these holes. Energy dispersive X-ray (EDX) analyses showed that these particles were rich in iron, manganese, silicon and aluminium. These impurities often form agglomerates which are supposed to act as stress concentrators ahead of the crack tip. An explanation was proposed for the presence of these holes, based on the accumulation of plastic deformation around the impurity particles when the crack front encounters one of them in its path.15

As the value of AK increases, the fracture surface morphology changes slightly. Coarse striations remain as the most outstanding feature, but their aspect is more diffuse and they are completely covered with small dimples, as seen in Fig. 8. The average diameter of the dimples is below 1 urn at AK% 11.5 MPa,/m and some small particles are often encountered inside them, as shown by the arrows in Fig. 8. Their size suggests that they might be associated with the dispersoids in the aluminium alloy matrix.”

In the upper AKregion (AK> 15 MPa,/m) extensive plastic deformation took place and the fracture surface lost its regular appearance. The increase in crack tip plastic zone size produces deep holes (around 30 urn diameter) frequently associated with impurity particles of the same composition as those described above. In addition, fracture surfaces are characterized by tear ridges and continuous changes in the crack propagation plane. As a consequence of the higher AK level, mechanical effects (especially the superposition of static modes of fracture on the cyclic ones) are dominant in this region, and the amount of plastic deformation involved in the process may hide any other micrographical feature typical of the environment.

Water vapour (p/v = 0.2 Pas) Figures 9 and 10 show the typical morphology of the fracture surfaces at an

environmental exposure of 0.2 Pas. Although water vapour pressure changes one order of magnitude (from 0.2 Pa to 2 Pa), the fractographic appearance in the two testing conditions is very similar, in agreement with the crack propagation rate results (see Fig. 3). Besides this, the microscopic aspect closely resembles that observed in a high vacuum atmosphere for the same value of AK (see Figs 6 and 7). For AK% 7 MPa,/m, coarse striations perpendicular to the crack advance direction cover most of the fracture surfaces. In addition to this characteristic, typical of the fatigue process in inert environments, there appear some distinct features in the steeply inclined regions. They are easily recognizable by their brightness, and consist of shallow dimples and round particles (average diameter of 1 urn) identical to those found for the same value of exposure at a frequency of 5 Hz. Energy dispersive X-ray (EDX) analyses showed that these regions are rich in oxygen, and it was postulated that they might be corrosion products from the fretting of the fracture surfaces upon unloading in a fretting-fatigue process.”

When the value of AK increases, the fractographic appearance changes. The larger amount of plastic deformation involved in the process becomes apparent. Deep holes, from 20 to 30 urn average diameter, associated with impurity particles, appear on the fracture surfaces, along with steeply inclined zones with a bright look whose origin was explained

2128 J. Ruin and M. Elites

abo ve, and frequent changes in the crack propagation plane. Coarse striations are : still

visit Ae but they are sometimes poorly defined and at high magnification are seen t .o be COW :red with shallow dimples like those found in high vacuum (see Fig. 11 and compare with

Fig. 8). The typical diameter of the dimples is below 1 urn, and inside some of them z imall

part :icles are found (see the arrows in Fig. 11). The size of the particles encountered OI n the

Fig. 8. Microfractographs in high vacuum at IO ~-’ Pa showing small dimples in the fracture surface (AK- Il.5 MPajm): (a) v= 1 Hz,(b) Y= 10 Hz.


3g. 9. Microfractographs in water vapour at an environmental exposure of 0.2 AKa7 MPaJm, 0.2 Pa and Y= 1 Hz): (a) typical appearance of the fracture surface, (b) hi

magnification.

Pas

surface and the fractographic morphology closely resemble the results in high vacuum, so it is assumed that the dimples are also related to the dispersoids in the aluminium alloy matrix in this case.

The similarity between the fractography in high vacuum and in a high purity water

.I. Ruiz and M. Elites

vapo the s crack

ur atmosphere at an environmental exposure of 0.2 Pas is remarkable. It suggests t #ame fracture micromechanisms operate in both cases. Moreover, the macroscc c propagation rates are almost identical over the range of AK investigated (camp 2 and 3). As at the frequency of 5 Hz, fractographic observations corroborate

,ured da/dN data. Figs mea?

Fig. IO. Microfractographs in water vapour at an environmental exposure of 0.2 Pa+

(AK-7 MPa,im. 2 Pa and LF 10 HI): (a) typical appearance of the fracture surface, (b) higher magnification.

hat

bare the

ch ch Wi


Fig. 11. Microfractographs in water vapour at an environmental exposure of 0.2 Pa

(AKx 11.5 MPdm): (a) 1 Hz, 0.2 Pa, (b) 10 Hz, 2 Pa.

‘ater vapour (p/v = 1 Pas) When the exposure reaches 1 Pas, the fracture surface morphology suffers a s rudden

Lange. The features associated with the fatigue process in inert environments, which laracterized the fracture surface in high vacuum and in water vapour at 0.2 Pa-s, vanish ithout trace. Instead, a completely different pattern emerges with a microscopically brittle

.s

.I. Ruiz and M. Ekes

t-i

CA

g. 12. Microfractographs in water vapour at an environmental exposure of I Pa+

X27 MPaJm, I Pa and I’- 1 Hz): (a) typical appearance of the fracture surface, (b) higher

magnification.

aspect. As shown in Figs 12 and 13, coarse striations are replaced by flat facets frequently covered with fine fatigue striations, called brittle striations in the literature,” perpendicular to the crack growth direction. For AK% 7 MPa,,/m, striation spacing (as measured in Figs 12b and 13b) ranges from 0.2 to 0.3 urn, whereas crack growth rate is around 0.04 mm per

cycle for this value of AK. So it seems that in this case crack growth is not a contir 1uous

procc :ss either, but is an intermittent one. The plastic deformation steadily accumula .tes in front of the crack tip until a brittle crack extension occurs.” As with the frequency of 5 Hz, from the values computed for the striation spacing and the crack growth rate it c an be estim lated that for AK% 7 MPa,/m in high vacuum at least 200 cycles would be necessa ry for


Fig. 13. Microfractographs in water vapour at an environmental exposure of 1 Pa.s (AKx7 MPadm, 10 Pa and v= 10 Hz): (a) typical appearance of the fracture surface, (b) higher

magnification.

2134 .I. Ruiz and M. Ekes

every discrete advance of the crack, whereas in water vapour at an environmental exposure of 1 Pas, this number would be reduced to 10.15

In addition to these characteristics, some features resembling the fracture surface morphology in water vapour at an exposure of 0.2 Pas are observed in the micrographs. Bright zones identical to those reported above (consisting of shallow dimples and round particles) are found in the steeply inclined regions, so they are thought to be corrosion products from the fretting of the fracture surfaces upon unloading in a fretting-fatigue process.

When AK is increased, the fracture surface morphology does not change abruptly. The flat facets with a microscopically brittle aspect remain as the most outstanding characteristic. Some of them are covered with brittle striations (see Fig. 14a), whereas others present ductile striations (see Fig. 14b), which are more difficult to resolve by S.E.M. because their spacing is smaller. Ductile striation spacing is approximately 0.2 urn (see Fig. 14b), which agrees very well with the crack propagation rate for this value of AK (around 0.18 urn per cycle). This suggests that, unlike the preceding cases, crack advance in these conditions proceeds cycle by cycle.

Air (50% relative humidity) Despite the small differences found in crack growth rates between the results at 1 and

10 Hz (see Fig. 5) the fracture surface morphology is similar as is seen in Fig. 15. As at the frequency of 5 Hz, for AK% 7 MPa,,/m the surface is composed of small facets with a brittle microscopic appearance which are periodically repeated. l5 Other investigators have found a similar morphology in a 7075-T65 1 aluminium alloy tested in air at AK z 6 MPaJm, and have termed these features periodic structures. i8 Although the flat facets exhibit a plain aspect with no remarkable features, sometimes fine fatigue striations are detected, but they are very difficult to resolve at this value of AK. A fracture surface with a fragmentary aspect is produced, which seems to indicate a complex and irregular propagation process.

As AK is increased, the fracture surface becomes more irregular. The increase in plastic zone size at the crack tip produces deep holes associated with impurity particles and frequent changes in the local crack plane. However, the most outstanding features are the fine fatigue striations which cover a great portion of the fracture surfaces. From Fig. 16, striation spacing can be estimated as around 0.3 urn for a frequency of 10 Hz and 0.6 mm for 1 Hz, whereas the measured crack propagation rate is approximately 0.7 urn/cycle at the same value of AK (AK% 15.5 MPaJm), which suggests a continuous crack growth process in this case.

DISCUSSION

A close relation has been found between three aspects of the environmental-fatigue problem, namely fatigue crack growth rate (macroscopic parameter), fractographic morphology (microscopic parameter) and environmental exposure (environmental parameter). Next, the macroscopic and microscopic aspects of the investigation are discussed separately, and a model is proposed to explain the observed behaviour.

High vacuum experiments have shown that crack propagation rates in 7017-T651 aluminium alloy are insensitive to the testing frequency (between 1 and 10 Hz). As seen in Fig. 2, the data corresponding to the three frequencies investigated are almost identical in the range of AK studied (from 7 to 20 MPaJm). This behaviour differs from that observed

in frc ca

a high-strength steel tested in vacuum at a pressure of approximately 10W3 Pa and at :quencies ranging from 0.2 to 35 Hz.” As explained elsewhere, the dissimilar behaviour n be attributed to the different testing conditions in the two cases2’

The fatigue tests performed in high purity water vapour confirm the dependence of crack opagation rates on environmental exposure. As shown in Figs 3 and 4, within a certain

tnge of exposure values the effects of pressure and frequency are interchangeable; an pr ra


Fig. 14. Microfractographs in water vapour at an environmental exposure of 1 Pa.s (AK= 11.5 MPaJm): (a) 1 Hz, 1 Pa, (b) 10 Hz, 10 Pa.

J. Ruiz and M. Ekes

Fig. IS Microfractographs in atmospheric air (50% Rh) (AK-7 MPaJm): (a) I’- 1

(b) I”- 10 HT.

increase in water vapour pressure has the same effect on crack velocity as a de frequency. Besides this, the crack propagation rates increase with the envirc exposure and those in high-purity water vapour are limited by the data in high (lower bound) and humid air (upper bound).

crease snmen L vacua

in tal .im

If a semi-empirical model for corrosion-fatigue in gaseous environments” is a, pplied to

anall /ze the crack propagation data, the results shown in Fig. 17 are obtained (SC Aid lines repre sent the model predictions).20 Theoretical curves are matched to the expe :rimental resul ts and show remarkable agreement. Two different regions can be observed in crack grow ,th response: the lower and the upper pressure regions. In the lower pressui re region there : seems to exist a threshold level of environmental exposure under which no nl oticeable


Fig. 16. Microfractographs in atmospheric air (50% Rh) (AKz 15.5 MPa,/m): (a) V= 1

(b) V= 10 Hz. Hz


AK=155 MPadm

.s

AK = 11.5 MPadm

Vacuum AK = 7.0 MPadm

Vacuum

n

.

0 d

A

Frequency

A 1 Hz

. 5 Hz

0 10 Hz

4

Fig. 17. Influence of the environmental exposure (pressure divided by frequency) on the fatigue

crack growth rate in 7017-T651 aluminium alloy (solid lines represent the predictions of the model).

embrittlement is produced, as crack growth rates coincide with the high vacuum data. In 7017-T651 aluminium alloy, the threshold exposure is approximately 0.2 Pa+ As the exposure is increased, crack growth rates also increase until a saturation point is reached, above which the crack velocity remains almost constant within a certain range of values. In the material studied, the saturation exposure is around 1 Pa+ and a linear dependence of fatigue crack growth on exposure is observed until the saturation level is reached for all the frequencies tested. In the upper pressure region (exposures higher than 10 Pa-s), further

increases of crack velocity are detected. However, crack growth response no longer depends on environmental exposure, and the results are generally more sensitive to water vapour pressure than to frequency. ” The good agreement between the predictions of the model and the measured crack growth rates confirm the proposed explanation of the embrittlement process, which is based on the hypothesis of hydrogen embrittlement.

The microscopic fracture mode is very sensitive to the transitions observed in crack

propagation rate, from the threshold region (exposure of 0.2 Pa.s) to the saturation region (exposure of 1 Pa.s). For the threshold value of the exposure (0.2 Pa.s), fracture surface morphology in water vapour is very similar to that observed in high vacuum. Both exhibit the characteristics of the fatigue process in inert environments: superimposed coarse striations perpendicular to the crack advance direction with a puff-pastry like aspect (see


Figs 6,7,9 and lo), and shallow dimples (average diameter lower than 1 pm) which cover a great portion of the surfaces for intermediate values of AK (see Figs 8 and 11). For AKx 7 MPaJm, coarse striation spacing can be estimated as approximately 2 pm, much higher than the crack velocity (around 0.01 pm per cycle for this value of AK). If it is assumed that these features are associated with consecutive positions of the crack front, crack advance must be an intermittent process in this case and at least 200 cycles would be needed for a brittle crack extension to take place.12 However, the size of the small dimples found on the fracture surface suggests they might be associated with dispersoids in the aluminium alloy matrix.

When the exposure reaches saturation, 1 Pa.s, the fracture surface morphology changes completely. The microscopic typical features of the fatigue process in inert environments tend to vanish, and they are replaced by others characteristic of the fatigue process in aggressive environments. Flat facets with a microscopically brittle appearance cover most of the fracture surfaces. Often fine fatigue striations of the two types reported in the literature:19 ductile and brittle, can be observed on these facets. In general, ductile striations are more difficult to resolve than brittle striations, and their spacing is similar to the crack growth rate per cycle for given values of AK, which suggests that in this case crack advance is not intermittent but proceeds cycle by cycle.

From the experimental results obtained in the 7017-T651 aluminium alloy, it can be suspected that water vapour is the species responsible for the embrittlement process when the material is subjected to cyclic loads in gaseous environments. However, the specific embrittlement mechanism is elusive. In the model employed to analyze the experimental data it is assumed that embrittlement is due to the hydrogen produced by the reaction of water vapour with the fresh aluminium surfaces created by fatigue in the crack tip region, according to the following sequence:6’7’14

2Al+ 3 Hz0 + A1203 + 6H+ + 6e-

The proposed explanation only takes into account the physical and chemical aspects of the surface reaction between water vapour and aluminium. The environmental contribution to the crack propagation rate is assumed to be proportional to the amount of hydrogen produced by the surface reaction (1) during each fatigue cycle. However, other models suggest that the presence of water vapour influences the plastic deformation processes at the crack tip region and produces embrittlement by local-enhanced plasticity mechanisms.‘2~‘3~‘8*21

Experimental evidence found in other aluminium alloys seems to indicate that crack growth is an intermittent process, where the plastic deformation is steadily accumulated during each fatigue cycle until a burst of crack advance occurs.18 In high vacuum, the number of cycles needed for the crack to propagate is large, and consequently the plastic deformation accumulated in front of the crack tip can also be very large. In this situation, impurity particles and dispersoids could act as stress concentrators inducing the formation of holes and dimples such as those found on the fracture surfaces in inert environments at the higher values of AK. However, the presence of water vapour embrittles the material and as a consequence the number of cycles before a brittle crack extension is decreased. In this case, plastic deformation accumulation in front of the crack tip is reduced and the dimples associated to the dispersoids vanish. Instead, crack growth may occur by an alternate-shear mechanism in two symmetric slip planes,” which could explain the appearance of fine fatigue striations on the fracture surfaces.


CONCLUSIONS

Environmental-fatigue tests were performed on the 7017-T651 aluminium alloy in gaseous environments with variable water vapour contents and at different frequencies. From these results some conclusions may be drawn:

(1) Crack growth rates in high vacuum are insensitive to the testing frequency, between 1 and 10 Hz, in the range of AK between 7 and 20 MPaJm.

(2) Crack growth rates in high purity water vapour show a good correlation with environmental exposure between 0.1 and 10 Pas. It seems that there is a threshold exposure around 0.2 Pas and a saturation exposure about 1 Pas. In between, crack propagation rates depend linearly on the exposure.

(3) The experimental crack propagation data, in the aforementioned interval, are satisfactorily explained in terms of a corrosion-fatigue model for gaseous environments, which predicts a linear dependence of crack velocity with environmental exposure.

(4) Fractographic features in high vacuum and for exposures below the threshold value (0.2 Pas) are similar: coarse striations, perpendicular to the crack advance direction, indicate that the crack progress is not cycle by cycle but in regular bursts. These features become blurred and mixed with dimples as the stress intensity approaches the fracture toughness value.

(5) Fractographic features in high purity water vapour, and for exposures above the threshold value, exhibit a transition from coarse striations to brittle and ductile striations, and it seems that crack growth changes from and advance by bursts to a cycle by cycle advance as the exposure increases. Again as AK increases, the fracture surfaces become more irregular regardless of the exposure, and more dimples appear, as fracture due to void growth and coalescence dominates over fatigue fracture.

Acknowledgemenu-The authors are grateful to Dra. c’. Garcia-Cordovilla and Prof. E. Louis for providing the

aluminium alloy and to Prof. J. LLorca for useful discussion. Support of this research by the Universidad

Politecnica de Madrid under Contract A-9530 is gratefully acknowledged.

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The role of environmental exposure in the fatigue behaviour of an aluminium alloy