Frequency dependence of the high temperature fatigue properties of He-implanted stainless steel

Journal of Nuclear Mat&ah 116 (1983) 136 140 North-Holland Publishing Company

I36

FREQUENCY DEPENDENTS OF THE HIGH ~MPE~~RE He-IMPLA~D STAINLESS STEEL I.S. BATRA *, H. ULLMAIER Instiiut fi(r Festkiirperforschung Euratom - KFA

FATIGUE PROPERTIES OF

and K. SONNENBERG

Kernforschungsanlage Jiilich, Postfach 1913, D - 5170 Jiiiich, Fed. Rep. Germany; Assoziation

Received 7 February 1983; accepted 25 March 1983

We have measured the frequency dependence of the number of cycles to failure, Nr, of type 316 stainless steel specimens fatigued at 600°C with a total strain range of 1.2 %. Whereas for the helium-free reference material Nr is high and decreases only shghtfy with decreasing frequency, there is a sharp drop in Nr below a critical frequency, v~, for specimens containing 800 ppm He. This behaviour and the value of Y== 3 Hz is compatible with a theoreticai model by Trinkaus which predicts a change from transgranular to intergranular failure below a critical frequency. This conclusion is substantiated by SEM observations of fracture surfaces and grain boundary bubble structures.

The influence of irradiation on the fatigue behaviour of structural materials in fission reactors received only limited attention since these energy sources essentially operate in a stationary mode. Furthermore, experiments on stainless steel [l] and TZM f2] showed only minor

changes for displacement damage levels up to several 10 dpa. However, the situation will be quite different in the case of fusion devices where almost all conceptual reactor designs employ cyclic operation and where extremely high helium production rates due to (n, a) reactions are excepted in addition to atomic displacements. Similar loads will occur in window materials of spallation sources with even accelerated rates [3]. Whereas the reductions in fatigue life found in reactor irradiated stainless steel specimens (4,5] at temperatures below 5OO’C are not attributed to helium embrittlement, there is clear evidence from a-implanted specimens that, similar to the creep rupture behaviour, helium has a drastic influence on the fatigue life at high temperatures [6,7]. It was found that the temperature above which embrittlement effects are observed, strongly depends on the fatigue frequency. These preliminary results support theoretical predictions [S] of a temperature dependent critical frequency below which grain

* Guest scientist from BARC Bombay, India.

0022-3115/83/0000-0000/$03.00

0 1983 North-Holland

boundary cavities can grow even under fully reversed cyclic stresses and thus lead to premature intergranular failure. Since the critical frequencies he in a range which is relevant for tokamak fusion reactors we started a systematic experimental study of these effects and in the following first results for solution annealed type 316 stainless steel at 600°C are reported.

2. Experimental Helium was introduced into the specimens by n-particle implantation. Because of the restricted range of the available 28 MeV a-particles (- 125 pm in stainless steel), foil samples in the reverse bending mode must be employed. The test apparatus connected to a beam line of the CV 28 cyclotron in Jiilich is schematically shown in fig. 1. The sample is bent cyclically by rocking the knife edges around the center of the sample by an angle cp. The total strain amplitude, AC, is determined as a function of + by measuring the divergence of a Iaser beam reflected from the bent specimen surface. Since fracture always occurs in the middle of the sample where it cross section is reduced, only a small area around this region must be implanted. For a homogeneous deposition of helium, the a-particle energy is degraded by wedge type Al-foils mounted at the rim of a wheel which rotates with about 10 revolutions per second during irradiation.

IS. Batra et al. / Fatigue properties

'3;

137

of He -implanted stainless steel

parameter was the fatigue frequency which was varied between 0.02 and 15 Hz corresponding to average strain rates between 5 x 10e4 and 4 x 10-l s-‘. All tests were performed in an inert atmosphere. The results are given in fig. 2. For the reference specimens the number of cycles to failure is high and decreases only slightly with decreasing frequency. All reference specimens failed by transgranular fracture (fig.

/SPECIMEN

PYROMETER

RE

2BMeV a-BEAM

Fig. 1. Schematic view of fatigue test system installed at the a-beam line of the CV 28 cyclotron in KFA Jtilich. The apparatus is suitable both for in-beam and for post-irradiation tests. In the present experiments the latter mode was employed.

The temperature of the sample is measured by an infrared pyrometer and maintained by helium gas cooling and ohmic and a-beam heating, respectively. The cooling gas is purified by a liquid N,-cooled molecular sieve providing an inert environment (equivalent to vacuum with an O,-pressure of 10Y6 mbar). A more detailed description of the test system is given in ref. [6]. The sample material was commercial type 316 stainless steel from which 100 pm thick foils were ,prepared. After cutting the samples by spark erosion they were solution annealed at 105O’C in vacuum for two hours resulting in an average grain size of 15 pm. After helium implantation and fatigue testing until failure, the fracture surfaces were investigated by SEM. For a few specimens, the fatigue tests were stopped after a certain number of cycles in order to follow the evolution of the bubble structure (see section 3.2). For this purpose the specimens were sectioned and polished in planes perpendicular to the bending axis.

3). The helium containing samples show a quite different behaviour: whereas at high frequencies N, approaches the values of the reference material, there is a drop to very low N, values at frequencies around a few Hz. This deterioration of the fatigue life is also reflected in the fracture mode which changes via a mixed fracture in the transition region (fig. 4a) to a completely intergranular rupture at low frequencies (fig. 4b). The Occurrence of intergranular brittle failure is well known from experiments on helium containing specimens under static tensile (= creep) loads and there is wide agreement that this high temperature embrittlement is caused by helium bubbles in the grain boundaries [9]. After implantation at high temperatures the nucleated bubbles are in equilibrium, i.e. the internal pressure pHe is balanced by the surface tension 27/r,, ( y surface free energy, r,, equilibrium radius). If a tensile stress (I normal to the grain boundaries is applied, the bubbles will grow by accumulation of vacancies diffusing along the grain boundaries if CI exceeds a critical STRAIN

RATE

bee-'I -

1o-2

1o-3

10-l

I

I

REFERENCE

T =600°C

3. Results and discussion 3. I. Frequency dependence of fatigue Iife In a first set of experiments the dependence of the number of cycles to failure, N,, was determined as a function of fatigue frequency, v. Both helium free reference samples and samples containing 800 appm He were investigated. In the latter the helium has been introduced at 600°C prior to fatigue testing. All specimens were fatigued at the same temperature of 6OO’C with a total strain range of 1.2 %. The only variable

I 0.01

I

I

I

0.1 FREQUENCY

I 1

I

1

1

10

v(Hz) -

Fig. 2. Number of cycles to fracture, Nr, as a function of fatigue frequency, Y, for solution annealed 316 stainless steel specimens fatigued at 600°C with a total strain range of 1.2 %. (0 helium free reference, 0 800 ppm helium implanted at 6OOT).

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I.S. Batra et al. / Fatigue properties of He -implanted

stainless steel

Fig. 3. Typical transgranular fracture surface of reference specimens fatigued at 6OO’C with a total strain range of 1.2 S.

value given by [ 10,111 oC= 0.76 y/ra.

(1)

bubbles then perforate the grain boundaries until eventually intergranular cracks are initiated. If faster than other failure mechanisms, this grain boundary weakening will determine the rupture time of the specimen [ 121. At first sight it may appear surprising that an embrittlement by the above mechanism occurs also under cyclic loads since a growth of bubbles during the tensile phase should be compensated by sintering during the compressive phases. However, a net growth can occur under certain circumstances since the thermal vacancy concentration changes non-linearly by an external stress [ 131 leading to a net volume growth rate of [8] The growing

Here Sz is the atomic volume, kT the thermal energy, Ds,S the self diffusion coefficient in grain boundaries and ~a the local stress amplitude. Trinkaus [8] was able to show that eq. (2) yields a sufficiently high positive net growth rate for cyclic loads if (1) the bubble radius r is above a critical value given by rC = 4y/30, (= 2 nm in our case) and (2) the fatigue frequency is below a temperature dependent critical value V, given by

[s-II

Fig. 4. selected mixed totally

Fracture surface of two helium containing specimens from the fatigue tests on fig. 2 in order to show (a) the fracture in the transition region (2.5 Hz) and (b) the intergranular fracture at low frequencies (0.1 Hz).

For v c v, the radius increase during the tensile phase of one cycle (“breathing amplitude”) becomes large compared to the mean radius. Although eq. (3) does not explicitely contain any helium parameters, the presence of helium is essential for the mechanism to operate since (a) in the relevant temperature range cavities can

I.S. Batra et al. / Fatigue properties of He

nucleate only in the presence of gas and (b) the gas pressure acting like a hard core is needed for preventing the breathing cavities from complete sintering at the lower turning point. If stabilized in this way and if v < vC, some gram boundary bubbles with r z r, at positions of high local stress ffs (e.g. at triple points or precipitates) will start to grow and finally lead to the initiation and/or propagation of grain boundary cracks. Therefore, around v = v, the failure mode should change from transgranuiar to inter-granular and N, should drop. This is in qualitative agreement with the results of figs. 2 and 4. A quantitative prediction of the critical frequency by eq. (3) is difficult since the value of the local stress amplitude u0 is not known. Going the reverse way and inserting the experimental value of v, = 3 Hz (fig. 3) and the known parameters 51= 1.16 X 10ez9 m3, kT= 1.2 X 10V20Nm, GsbS = 1O-22 m3 s-r [14] and y = 1 Nm-‘, one obtains u, = 800 MPa. This value is not unreasonable since the unrelaxed local stresses at grain boundary heterogeneities are certainly much higher than the ap-

-implantedstainless steel

139

plied stress and can even exceed the macroscopic strength of the material considerably. 3.2. Asymmetric

bubble sizes

In order to back up the above considerations by microscopic evidence we attempted to detect the large breathing of cavities expected at frequencies v < vc by another set of experiments: after helium implantation at 750°C * the specimens were fatigue tested at low frequencies at the same temperature. However, instead of determining the number of cycles. to failure, the test was stopped at maximum strain after a number of cycles N < N,. Simultaneous to the stopping of the rocking knife edges (fig. l), the specimen was quenched to room temperature in a time which was short compared to the cycle period. In this way the state of the bubble popula* The higher temperature was chosen because at 6OO“C the bubble sizes in the initial stages of the tests are too small to be resolved by SEM.

10pm Fig. 5. SEM-micrographs of helium bubbles in the grain boundaries of a specimen after 20 bending cycles at 750°C with a frequency of 0.1 Hz.

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tion at the instant of maximum tensile stress in one half and maximum compressive stress in the other half of the specimen was frozen in. Finally the specimen was sectioned and polished in planes perpendicular to the bending axis and viewed in a scanning electron microscope (SEM). A typical example of the micrographs obtained in this way is given in fig. 5. It shows that (a) the bubble size increases with increasing distance from the center, i.e. with increasing bending stress; (b) the bubbles are larger on grain boundaries perpendicular to the stress direction as compared to grain boundaries parallel to it; (c) twin boundaries do not seem to be preferred sites for helium bubbles and finally (d) a distinct asymmetry of the bubble sizes exists in the tensile and compression region, respectively. The first two observations only confirm a fact which is already well known from creep tests, namely that the growth of cavities in grain boudaries is determined by the normal stresses acting on them. Most interesting for our case is the last finding since it is a first direct evidence that cyclic stresses can indeed cause large “breathing” of bubbles at high temperatures and low frequencies.

4. Conclusions The above described high temperature fatigue experiments on helium containing type 316 stainless steel show that helium can be cause a drastic reduction of the number of cycles to failure if the fatigue frequency is below a certain value. For 6OVC and a total strain range of 1.2 % this critical frequency is around a few Hz, i.e. much higher than the pulse frequencies of most of the anticipated fusion devices. The data suggest that this behaviour could be explained by a theoretical model [S] which - due to a fast growth rate - predicts a large “breathing” of grain boundary cavities if’ the stress frequency is slow enough. Because of their importance for the life time of structural materials in future fusion reactors, these effects should be studied fur&z, We therefore,plan experiments on the temperature and stress dependence of the critical frequency. Their results are needed to fur-

ther confirm the proposed failure mechanism and to map out the parameter ranges in which the material is vulnerable to helium embrittlement.

Ackowledgements

We are indebted to Dr. H. Trinkaus for many stimulating discussions and to W. S&rnitz for technical assistance in constructing the irradiation and test facility and for his help during the implantation and fatigue experiments. The work was performed within the lndoGerman exchange programme and one of us (I.S.B.) is grateful to the International Bureau in KFA Jtilich for financial support.

References

[ 1) D.J. Michel and G.E. Korth, Radiation Effects in Breeder

[2] [3]

[4] [5] [6] [7] [S] [9f

[IO]

[ 1If [l2] [13] [14]

Structural Materials, June 19-23, 1977, Scottsdale, Ariz., ANS and AIME, New York (1977), p. 117. H.H. Smith and D.J. Michel, J. Nucl. Mater. 66 (1977) 125. W. Lohmann, Proc. 4th Meeting on Advanced Neutron Sources, Tsukuba, Japan Oct. 20-24, 1981, KENS Rep. II, p. 323. M.L. Grossbeck and KC. Liu, ANS Trans. 34 (1980) 185. M.L. Grossbeck and K.C. Liu, J. Nucl. Mater. 103 & 104 (1981) 853. K. Sonnenberg, G. Antesberger and B. Brown, J. Nucl. Mater. 102 (1981) 333. K. Sonnenberg and H. Ullmaier, J. Nucl. Mater. 103 & 104 (1981) 859. H. Trinkaus, Scripta Met. IS (1981) 825. For recent reviews see: D. Harries, J. Nucf. Mater. 82 (1979) 2. or H. Schroeder, Proc. Intern. Symp. on Helium in Metals, Jtilich, Sept. 21-24, 1982, to be published in Rad. Effects. E.D. Hyam and G. Summer, Proc. Conf. Radiation Damage in solids, IAEA, Vienna 1962, 1, p. 233. R.S. Barnes, Nature 206 (1965) 1307. H. Trinkaus, Proc. TMS-AIME Fall Meeting, St. Louis, Oct. 24-28, 1982. J. Weertman, Met. Trans. 5 (1974) 1743. P. Guiraldency and P. Poye, Mem. Sci. Rev. Metal. 10 (1973) 7i5, and A.F. Smith, CEGB Rep. No. RD/B/N 2270 (19720.