ACOUSTIC EMISSION DETECTION OF ENVIRONMENTALLY ASSISTED CRACKING IN ZIRCALOY-4 ALLOY

E. Munch a,b, L. Duisabeau a, M. Fregonese b, L. Fournier a

aCEA Saclay, DEN/DMN/SEMI/LCMI, Bât. 625P, 91191 Gif-Sur-Yvette Cedex,

E-mail: lionel.fournier@cea.fr bINSA Lyon, LPCI, Bât. 401, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex

ABSTRACT

The environmentally assisted cracking (EAC) behavior of Zircaloy-4 alloy in iodized

methanol solution at room temperature was investigated. Both constant load tests at 150 MPa and constant elongation rate tensile (CERT) tests at 10-5 s-1 in air and in iodized methanol solution were carried out and monitored by acoustic emission (AE). Constant load tests performed for around 45000 s led to significant pure intergranular cracking (IGC) and almost no AE activity. In contrast, CERT tests performed in iodized methanol solution led to IGC for a few tens of microns and then to transgranular/quasi-cleavage cracking. CERT tests performed in iodized methanol solution were accompanied by a significant AE activity in the plastic domain in comparison to CERT tests performed in air. The occurrence of two different EAC phenomena in iodized methanol, namely stress-assisted intergranular dissolution and stress corrosion cracking (SCC), as well as the AE signature of these two EAC phenomena are discussed.

INTRODUCTION

Iodine-induced stress corrosion cracking (I-SCC) is known as the cause of pellet-cladding

interactions (PCI) failure of zirconium alloys cladding in light water reactors [1-2]. I-SCC under PCI conditions results from the synergic effect of (i) the hoop tensile stress and strain imposed on the cladding by fuel thermal expansions during power transients and (ii) corrosion by iodine released from the UO2 fuel as a fission product. A wide range of techniques, including power ramps in materials testing reactor, is used in studying PCI failures. At the laboratory scale, comparative internal pressurization tests in inert and gaseous iodine environment at 350°C are usually performed to determine the susceptibility of claddings to I-SCC. Both power ramps and internal pressurization tests lead to I-SCC cracks propagating in an intergranular way for a few tens of microns and then in a mainly transgranular way. One way of modelling I-SCC is to consider three successive phases, i.e. (i) initiation, (ii) propagation of a subcritical crack under KI/I-SCC, and (iii) propagation of a critical crack once KI/I-SCC is reached [3]. Critical crack growth rate is usually determined experimentally by internal pressurization tests on pre-cracked claddings [4]. In contrast, experimental data on crack initiation and subcritical crack propagation are difficult to gather. The aim of this work is therefore to determine the interest of the acoustic emission (AE) technique for the detection of I-SCC initiation and the quantification of both subcritical and critical propagation stages.

AE refers to the generation of transient elastic waves due to the rapid release of energy from localized sources within a material. Such discrete energy releases are likely to occur during plastic deformation and crack growth of materials. Several recent studies [5-10] have demonstrated the interest of the AE technique for the detection of SCC in various material-environment systems. The results of a preliminary experimental approach aimed at evaluating the capability of AE for the detection of I-SCC in zirconium alloys are presented in this paper. Since mechanical tests performed in iodized methanol solution at room temperature were

shown in the literature to relatively well emulate the I-SCC phenomenon occurring in gaseous iodine environment at 350°C [11-13], this preliminary study was carried-out in iodized methanol solution at room temperature.

EXPERIMENTAL PROCEDURE

The material used in this study was a commercial grade Zircaloy-4 alloy supplied by CEZUS in the form of a 6 mm thick sheet. The chemical composition of this material is given in Table 1. This alloy was re-crystallized at 730°C for 3 minutes, leading to a mean grain size around 7 µm. The texture of the material was analysed by X-ray diffraction with a D500 Brucker apparatus. The Ψ angle, i.e. the deviation angle between the c-axis orientation maxima and the R direction in the RT plane was determined to be 35°.

Smooth tensile test specimens with a gage length of 21 mm, width of 2 mm, and thickness

of 1.5 mm were machined out of the Zircaloy-4 alloy sheet in the T direction. All samples were fabricated by electric discharge machining. Prior to mechanical testing, specimens were mechanically wet-polished using SiC papers (grit 320-1200) and rinsed in successive acetone and methanol ultrasonic baths. Two types of mechanical tests were performed in this study. Constant elongation rate tensile (CERT) tests were conducted at 10-5 s-1 in both air and in various iodized methanol solutions ranging from 5×10-6 to 10-4 gram of iodine per gram of methanol (g/g) at room temperature. Additional mechanical tests were carried-out at a constant load of 150 MPa in 10-5 g/g iodized methanol solution for around 45000 s. Following to exposure in iodized methanol solution, specimens were rinsed in methanol and strained up to fracture at 10-2 s-1 in air. All mechanical tests were performed on a Zwick Z020 electromechanical traction-compression machine. After mechanical testing, the fracture surface and the side surfaces of all specimens were carefully examined in a JEOL 5004 scanning electron microscope.

The AE activity was recorded during the entire duration of all mechanical tests. A

piezoelectric AE sensor R50 was attached to the upper pull-rod approximately 10 cm away from the specimen. A 40 dB pre-amplification was applied to the AE signal before its acquisition by the EuroPhysical Acoustics Mistras signal recording and processing unit. The pass band filter was set to 10 Khz - 2000 Khz. A threshold detection level of 30 dB was used. Temporal parameters for identification of the hits, i.e. peak detection time (PDT), hit definition time (HDT) and hit lockout time (HLT) were set at 300 µs, 600 µs and 1000 µs, respectively.

Table 1. Chemical composition (in wt.%), grain size and texture of the re-crystallized Zircaloy-4 used in this study.

Mean grain size (µm) TextureΨ angle (°)

Sn Fe Cr O Zr

1.34 0.22 0.11 0.17 bal. 7 35

Chemical composition (wt.%)

RESULTS

The environmentally assisted cracking behavior of Zircaloy-4 in iodized methanol solution was investigated by means of both (i) CERT tests and (ii) constant load tests. The AE activity was recorded during both types of tests. (i) Results of CERT testing in both inert environment and corrosive environments are summarized in Table 2. The elongation to failure, percentage of both IG and TG cracking on fracture surface, the ratio of total crack length to elongation to failure, as well as the total AE hits number until rupture were tabulated for each specimen. The results of duplicate tests performed in air show a good repeatability. Specimens strained in iodized methanol solution exhibit a significant loss in ductility in comparison to specimen strained in air. The importance of the loss in ductility increases with an increase in the iodine concentration of the iodized methanol solution. The stress-strain curves corresponding to the CERT tests summarized in Table 2 are plotted in Figure 1. It should be noted here that the specimen strained in the 10-4 g/g iodized methanol solution fails with very little plastic deformation. All specimens strained in iodized methanol solution exhibit both IGC and quasi-cleavage on their fracture surface. In all cases, the percentage of IG cracking is significantly lower than the percentage of quasi-cleavage. This observation is consistent with literature data [11] reporting a rapid transition from IG to TG stress corrosion cracking in iodized methanol solution for stress intensity factor as low as 11 MPa√m. A high concentration in iodine, around 10-4 g/g, leads to a significant increase in the percentage of both IG and TG cracking on the fracture surface in comparison to concentration in iodine around 10-5 g/g. SEM micrographs of the fracture surface of the specimen strained in 5×10-6 g/g showing IG propagation for a few tens of microns followed by TG / quasi-cleavage propagation are displayed in Figure 2. Additional SEM micrographs showing multiple IGC on one of the side-surfaces of the specimen strained in 5×10-6 g/g are displayed in Figure 3.

AE data related to CERT tests are also summarized in Table 2. Both the total AE hits number until final rupture and the AE hits number after 5% elongation are tabulated. The total AE hits number is shown to be higher for the CERT test performed in air than for the two CERT tests performed in iodized methanol solution. However, the AE hits number recorded after 5% elongation is shown to be significantly lower for the CERT test performed in air than for the two CERT tests performed in iodized methanol solution. The evolution of the stress is plotted together with the evolutions of the cumulated AE hits number and cumulated AE energy as a function of the elongation for the specimen strained in air in Figures 4.(a) and 4.(b), respectively. For comparison purpose, the evolution of the stress is also plotted together with the evolutions of the cumulated AE hits number and cumulated AE energy as a function of the elongation for the specimen strained in the 1.2×10-5 g/g iodized methanol solution in Figures 4.(c) and 4.(d), respectively. In good agreement with literature data available for other materials [14], the AE activity during CERT testing in air is found to be significant up to 5% of elongation and then extremely low with further elongation. In contrast, AE activity during CERT testing in iodized methanol solution is shown to be significant both in terms of hits number and hits energy after 5% of elongation and up to fracture.

Table 2. Summary of CERT tests performed with AE detection at 10-5 s-1 in air and in iodized methanol solution with concentrations ranging from 5×10-6 g/g to 10-4 g/g.

Environment [I 2 ] Elongation to failure % IGC on % TGC on Ratio of total crack length Total AE AE hits number

(g/g) (%) fracture surface fracture surface on side surface (µm) over hits number after 5% elongationelongation to failure (%)

Air - 28 - - - 1280 152Air - 28.6 - - - No data No data

Iodised Methanol 5×10-6 17.5 5.3 32.6 800 770 438Iodised Methanol 1.2×10-5 14.6 4 23.3 1000 500 312Iodised Methanol 1×10-4 3.1 23 77 2000 No data No data

0

100

200

300

400

500

0 5 10 15 20 25 30

5x10-6 g/g

1.2x10-5 g/g

10-4 g/g

Inert

Inert

Stre

ss (M

Pa)

Elongation (%)

Figure 1. Stress-elongation curves corresponding to CERT tests performed at 10-5 s-1 in air and in iodized methanol solution of different concentrations.

Figure 2. SEM micrographs showing the fracture surface of the Zircaloy-4 specimen strained at 10-5 s-1 in 5×10-6 g/g iodized methanol solution. (a) Macroscopic aspect. (b) Detail of (a) showing intergranular cracking for a few tens of microns and then transgranular / quasi-

cleavage cracking.

(a) (b)

Figure 3. SEM micrographs showing one of the side-surface of the Zircaloy-4 specimen strained at 10-5 s-1 in 5×10-6 g/g iodized methanol solution. (a) Macroscopic aspect.

(b) Detail of (a) showing multiple intergranular cracking.

0

100

200

300

400

500

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30

Stress

Hits

Cum

ulated hits number

Elongation (%)

Stre

ss (M

Pa)

0

100

200

300

400

500

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

Stress

Energy

Cum

ulated Energy (arbitrary unit)

Elongation (%)

Stre

ss (M

Pa)

0

100

200

300

400

500

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30

Stress

Hits

Stre

ss (M

Pa)

Cum

ulated hits number

Elongation (%)

0

100

200

300

400

500

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

Stress

Energy

Stre

ss (M

Pa)

Cum

ulated Energy (arbitrary unit)

Elongation (%)

Figure 4. Evolutions of the stress and (a) cumulated AE hits number and (b) cumulated energy as a function of the elongation for Zircaloy-4 strained in air. Evolutions of the stress and (c) cumulated AE hits number and (d) cumulated energy as a function of the elongation

for Zircaloy-4 strained in 1.2×10-5 g/g iodized methanol solution.

(b) (a)

(a) (b)

(c) (d)

(ii) Results of constant load testing in iodized methanol solution are summarized in Table 3. The AE hits number recorded during exposure to iodized methanol and AE activity, as well as the mode of failure and the percentage of IGC observed on the fracture surface are tabulated for each specimen. Both specimens exposed for around 45000 s at 150 MPa in 10-5 g/g iodized methanol solution exhibit very low AE activity. Interestingly, both specimens exhibit significant amounts of IGC on their fracture surface after straining in air and no quasi-cleavage. SEM micrographs of the fracture surface of the sample exposed for 45000 s in iodized methanol solution are given in Figure 5. Brittle failure is shown to be only IG with a sharp transition from IG to transgranular ductile failure. As shown in Figure 5.(d), SEM examination of the side surface of these specimens revealed the presence of numerous IG cracks.

Table 3. Summary of constant load tests performed with AE detection at 150 MPa

in 10-5 g/g iodized methanol solution for around 45000 s. After exposure to corrosive environment, specimens were fractured at 10-2 s-1 in air.

Environment [I 2 ] Applied Stress Test duration AE hits number AE activity Mode of failure % IG cracking(g/g) (MPa) (s) (hits/s) on fracture surface

Iodised Methanol 1×10-5 150 45000 24 5.3×10-4 IG + TG Ductile 5.7Iodised Methanol 1×10-5 150 43500 14 3.2×10-4 IG + TG Ductile 3.9

DISCUSSION

In good agreement with literature data [11-13], Zircaloy-4 alloy was found in this study to be highly susceptible to EAC in iodized methanol solution. As described in the results section, constant load and CERT tests carried-out in iodized methanol solution lead to different cracking modes and different AE activities, suggesting the manifestation of two different EAC mechanisms. The following discussion will be therefore focused on the different EAC mechanisms that occur in iodized methanol solution and their AE signature.

As already mentioned above, constant load tests performed at 150 MPa, i.e. approximately

at one third of the yield stress of Zircaloy-4 alloy, leads to pure IGC with no significant AE activity. The occurrence of IG cracking as such low stress and the absence of AE signals suggest that stress-assisted intergranular dissolution rather than I-SCC is responsible for IGC. Stolarz and Beloucif [13] also attributed severe IG attack to stress-assisted intergranular dissolution after a few cycles during low cycle fatigue tests performed in iodized methanol solution. Farina and Duffò [11] recently suggested as well that IG cracking is due to intergranular attack and that the transition from intergranular attack to transgranular SCC occurs when a critical value of the stress intensity factor around 11 MPa√m is reached.

In contrast to constant load tests, CERT tests carried-out in iodized methanol solution lead

to IG cracking on a distance of a few tens of microns and then to transgranular cracking, i.e. SCC, as well as to a significant AE activity. These results demonstrate that AE is a suitable technique for the detection of I-SCC. Since stress-assisted intergranular dissolution occurs at stress as low as 150 MPa, it is very likely that IGC observed after CERT testing results from stress-assisted intergranular dissolution rather than SCC. The transition from IGC, i.e. stress-

induced intergranular dissolution, to I-SCC then probably occurs, like suggested by Farina and Duffò [11], when a critical stress intensity factor is reached at the tip of the interganular defect. From this observation it can be concluded that all the AE activity registered during CERT testing stems from transgranular stress corrosion crack propagation. At this point, detailed AE signals analysis is required to precisely characterize AE activity associated with I-SCC. Also, since CERT testing on smooth specimens leads to the initiation and propagation of multiple cracks, CERT testing on single notched tensile test specimens is necessary to possibly distinguish between AE signals associated with stress corrosion crack initiation and propagation.

Figure 5. SEM micrographs showing (a)-(c) the fracture surface and (d) multiple IG cracking

on the side surface of the Zircaloy-4 specimen submitted to constant load at 150 MPa for 45000 s in 10-5 g/g iodized methanol solution and then strained in air at 10-2 s-1.

(a) Macroscopic aspect. (b) Detail of (a) showing pure IG cracking. (c) Detail of (a) showing the transition from IG cracking to transgranular ductile cracking.

CONCLUSIONS

The EAC behavior of Zircaloy-4 alloy in iodized methanol solution was investigated by means of both constant load and CERT tests monitored by AE. The main results of this study are the following:

(i) Two different EAC phenomena, namely stress-induced intergranular dissolution and I-SCC occur in iodized methanol solution. Stress-assisted intergranular dissolution is

(a) (b)

(d) (c)

favoured by important iodine concentration and may occur at low stress level. I-SCC which is characterized by a transgranular/quasi-cleavage propagation mode is favoured by low iodine concentration and seems to occur when a critical value of the stress intensity factor is reached.

(ii) Stress-induced intergranular dissolution is not detectable by AE. In contrast, I-SCC is accompanied by a significant AE activity.

ACKNOWLEDGMENTS

The authors are grateful to J.F. Lecot for his help in conducting SCC tests and SEM analyses. We also thank J.J. Vermoyal from CEZUS for providing the material of this study and J.L. Bechade for X-ray analysis. Support was provided by CEA DSOE and DSNI research programs.

REFERENCES

1. B. Cox, Journal of Nuclear Materials, 172 (1990) 249-292. 2. P.S. Sidky, Journal of Nuclear Materials, 256 (1998) 1-17. 3. D. Le Boulch, L. Fournier and C. Sainte-Catherine, “Testing and modelling iodine-induced stress corrosion cracking in stress-relieved Zircaloy-4”, in: Proceedings of the international seminar on pellet-clad interactions in water reactor fuels, Paper N° 17, OECD/NEA, (2004). 4. I. Schuster, C. Lemaignan and J. Joseph, Journal of Nuclear Materials, 156 (1995) 343-349. 5. S.A. Nikulin, V.G. Khanzhin and A.B. Rojnov, “Application of an acoustic emission method for SCC testing of zirconium cladding tubes”, in: Corrosion 2002, Paper N° 02437, NACE, (2002). 6. B. Cox, Corrosion, 30 (1974), 6, 191-202. 7. D. Caron, T. Cassagne, J. Daret, H. Mazille and G. Santarini, “Progress in using acoustic emission for environmentally assisted cracking studies” in: Corrosion 2003, Paper N° 03537, NACE, (2003). 8. C. Kato and K. Kiuchi, Journal of Acoustic Emission, 19 (2001), 53-62. 9. R.H. Jones, M.A. Friesel and R. Pathania, Corrosion, 47 (1991), 2, 105-114. 10. K.Y. Sung, I.S. Kim and Y.S. Yoon, Scripta Materialia, 37 (1997), 8, 1255-1262. 11. S.B. Farina and G.S. Duffò, “Fracture mechanics study of the intergranular to transgranular transition in the stress corrosion cracking of zirconium alloys”, in: Corrosion 2004, Paper N° 04681, NACE, (2004). 12. S.B. Farina, G.S. Duffò and J.R. Galvele, “Stress corrosion cracking of zirconium and Zircaloy-4 I iodine containing solutions”, in: Corrosion 2002, Paper N° 02436, NACE, (2002). 13. J. Stolarz and A. Beloucif, “Corrosion fatigue of Zircaloy-4 in iodized methyl alcohol”, in: Proceedings of the second international conference on corrosion-deformation interactions, pp.117-126, EFC, Nice, France, (1996). 14. C.R. Heiple and S.H. Carpenter, Journal of Acoustic Emission, 6 (1987), 177-204.