Nuclear Instruments and Methods in Physics Research B39 (1989) 567-572

North-Holland, Amsterdam

561

~R~NSI~C T~NSFORMATIONS IN 304 STAINLESS STEEL AFTER IMPLA~A~ON WITH HELIUM, HYDROGEN AND DEUTE~UM

E. JOHNSON I) L GR.tiBAEK I)* A. JOHANSEN I), L. SARHOLT-KRISTENSEN ‘), P. BQRGESEN ‘), iM.U. SCHERZER , . 3, N HAYASHI 4, and I. SAKAMOTO 4,

‘) Physics Laboratory, H.C. 0rsted Institute, Uniuersitetsparken 5, DK-2IOO Copenhagen 0, Denmark ‘/ Department of Materials Science and Engineering, Bard Hall, Ithnca, NY 14853, USA ii Max-P&vzck-lnstitut ftir Plasmaphysik, Euratom Association, D-8046 Garching bei Miinchen, FRG 41 E/ectrotechnicaI Laboratory, Sakurama, Ibaruki 305, Japan

Using conversion electron Mijssbauer spectroscopy (CEMS) and glancing angle X-ray diffraction, martensitic transformations have been studied in type 304 austenitic stainless steels implanted with 8 keV helium, hydrogen and deuterium. Furthermore, using CEMS in the energy selective mode (DCEMS), the distribution of martensite in the implantation zone has been analysed as a

function of depth. Transformation of the implanted layer occurs after implantation with lo*’ m-* He ’ ions while 100 times higher

fluence is required for the implanted layer to transform after hydrogen or deuterium implantations. This difference is due to the

ability of helium to form high pressure gas bubbles, while implanted hydrogen is continuously lost by back diffusion to the surface.

The helium bubbles, which are confined under pressures as high as 60 GPa, will induce extremely high stress levels in the implanted

layer, by which the martensitic transformation is directly induced. The fact that a much higher fluence of hydrogen or deuterium is

required to induce the transformation, shows that radiation damage plays only a minor role. In this case, the martensitic

transformation first occurs when the implanted layer resembles the state of a cathodically charges surface.

1. Introduction

Martensitic transformations can be induced in austenitic stainless steels in a variety of processes, by cooling below the martensite start temperature M,, by heavy cold-work such as rolling or drawing or by charg- ing with high concentrations of hydrogen [l]. In connec- tion with applications of stainless steels in radiation environments, it is of particular interest to notice that martensitic transformations can also be induced by ion implantations [2-41. The primary contribution to the driving force for ion implantation induced martensitic transformation is due to accumulation of high stress levels in the implantation zone, whilst primary radiation damage and changes in composition only give minor contributions [4]. The highest transformation efficiency is therefore achieved after implantation with noble gases which form microscopic inclusions where the gas is confined under pressures sufficiently high (- 2 GPa) to keep the heavier inert gas elements solid at room tem- perature [5]. Alloying only plays a significant role under conditions (such as implantations with Fe) where the composition of the implanted layer is changed to the extent that it becomes a high martensitic stainless steel

WI.

* Present address: Physics Department, Rise, National Laboratory, DK-4000 Roslcile, Denmark.

0168-583X/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

It is in this context, of particular interest to look at the transformation characteristics which may be achieved after implantation with the light noble gas helium or with hydrogen. Earlier studies on 304 type stainless steels have shown that martensitic transforma- tions can be induced after implantation with helium (7-91. More recent, although still only preliminary stud- ies have shown that hydrogen, despite the fact that it is known to be a martensite stabilising element, is much less efficient in formation of martensite after implanta- tion than helium [lo].

It is the purpose of this paper to present the results of an investigation of the phase distribution in hydro- gen and helium implanted 304 type austenitic stainless steels. The analysis was carried out using Miissbauer spectroscopy in the conversion electron mode (CEMS), which with decreasing efficiency probes the surfaces down to depths - 300 nm [Ill. Depth selectivity was achieved in special cases by energy analysis of the emitted conversion electrons (DCEMS) ]12]. The results were supplemented by X-ray diffraction analysis which under conditions of glancing incidence and asymmetric Bragg geometry enhances the surface sensitivity suffi- ciently to monitor the phase distributions in the im- planted layers [10,13].

The further interest in these experiments is due also to the possible applications of austenitic stainless steels in the vicinity of the hot plasma in fusion reactors. The

VI. METALS

implantations were therefore all done at an energy of 8 keV (or 7.3 keV for deuterium), which is a compromise between the very low energies characteristic for the radiation environment seen by the first wall surfaces (0.1-0.2 keV), and an energy high enough to ensure a sufficient thickness of the transformed layer for CEMS analysis.

2. Experimental

10 X 15 mm samples of 304 austenitic stainless steel for implantations were cut from a 25 pm thick foil from Goodfellows Ltd. This is the same material which has earlier been used for a series of permeation measure- ments of 7.3 keV implanted deuterium ions [14]. The foil sheet which was produced by cold-rolling was slightly ferromagnetic, and using transmission Moss- bauer spectroscopy (TMS) the as-rolled foils were found to contain 20-30 vol.% ci’ phase. In order to obtain fully austenitic foils for implantation, the samples were solution annealed and austenitised for 20 min at 1050 * C and subsequently waterquenched. This treatment re- moved the (Y’ phase as verified by TMS. Surface oxida- tion of the samples during annealing was prevented by clamping the foils between two slabs of stainless steel welded together to form an airtight pocket.

implantations were carried out in a light-ion high flux low energy accelerator in Garching [15]. The beam current was - 0.06 mA on an implanted area 8 mm in diameter and fluences were in the range 102’-3 x 1O23 m-*. The temperature of the samples during implanta- tion was estimated to be J 200 o C, and the pressure in the target chamber, which is evacuated with turbomo- lecular pumps, was maintained below lo-’ Pa

304 STAINLESS STEEL

I I I I

z RANGE

DEPTH [nm]

I I

DAMAGE . . . . . . &He

- ‘H

___ 2D

0 50 100 150

.DEPTH [nm]

Fig. 1. Range and damage distributions for 8 keV H+, Df and He+ implantations in 304 stainless steel (Fe : 18Cr : 12Ni)

calculated by the TRIM program.

Table 1 Range and damage parameters for 8 keV H+, D+ and He+ implanted into 304 stainless steel (Fe: 180: 12Ni) calculated

using the TRIM code

H+ D+ He+

Mean projected range (nm) 61 69 40 Average range straggling (nm) 49 66 57

Average damage depth (nm) 40 44 22

Average damage straggling (nm) 49 66 46 No. of vacancies/incident ion 4 13 55

The implant profiles and damage distributions for 8 keV He+, Ht and D+ implanted into an Fe-18Cr-12Ni steel (close in composition to the 304 steels) have been calculated using the TRIM code. They are shown in fig. 1 and the mean projected ranges and values for the straggling are given in table 1.

CEMS spectra were recorded at room temperature using a dilute solution of “Co in rhodium as source and

122.0 (a)

116.3 .-’ 1

I? c

G

110.5 1

104.3

-8.0 -4.0 0.0 4.0 8.0 mmls

119.Or (b)

mmls

116.Or CC) I

mm&

Fig. 2. CEMS spectra from 304 stainless steel implanted with 8 keV He+ ions. (a) 1.9~10’~ mm*, (b) 4.4x10*’ m-* and (c)

9.4X10” m-*.

E. Johnson et al. / Martensitic transformations in 304 SS 569

14O.Or (a) MSK-304

114.Or (c) MSK-355

I 8keV H’

110.3 L 3=10z3m-*

1065

102.8

9 9.0 :i - 8.0 -4.0 00 4.0 8.0

mm15 mm/s

MSK- 318 134.0 (b)

F

h

8keV H+

125.31. 2.1=10z3 mm2

- 8.0 - 4.0 0.0 4.0 8.0

114.0 (d)

110.3

106.5 :

MSK-347

7.3keV D’

2.5*1023m-2

mm15 mm Is

Fig. 3. CEMS spectra from 304 stainless steel implanted with 8 keV H+ ions, (a) 1 X 1O23 rne2, (b) 2.1 X 1o23 me2. (c) 3 X 102’ mm2 and 7.3 keV Df ions(d) 2.5 X1O23 mm’.

a gas flow proportional counter as detector. Depth selective spectra were recorded by monitoring the elec- trons in three energy intervals: lo-13 keV (O-20 nm depth), 6-9 keV (20-60 nm depth) and 2-5 keV (> 60 nm depth [12]. X-ray diffraction under conditions of glancing incidence (2O) was carried out using a 2 kW Rikagu X-ray generator. The diffractometer is a Rikagu BAD-B system equipped with graphite monochromator and a pulse-motor driven goniometer.

3. Results and analysis

Fig. 2 shows a series of CEMS spectra from solution annealed and austenitised 304 stainless steel samples implanted with 8 keV He+ ions to fluences in the range 1021-1022 m- *. The spectra consist of two separate

Table 2 The amount of martensite (a’) and austenitic (y) in vol.58 measured by CEMS in 304 stainless steel after implantation with 8 keV H+, Df (7.3 keV) or He+ ions

Ion Fluence (m * ) a’ (vol.%) y (vol.%)

He+ 1.9x102’ 38 62 He+ 4.4 x lo*’ 51 43 He+ 9.4x102’ 70 30 H+ 1.ox1o23 0 100 H+ 2.1 x 1o23 33 61 H+ 3.0x1023 41 59 D+ 2.5 x 1O23 47 53

contributions. The large central peak originates from the austenite (y) phase. In all the spectra from the implanted samples this peak is superimposed with a six-line spectrum originating from the ferromagnetic martensite (~1’) phase induced during the implantations.

The relative area under the two sets of spectral components is a direct measure for the volume fraction of the two phases present in the layer probed by the conversion electrons. This has been determined by fit- ting the spectra to Lorentzian line shapes, and the results are given in table 2. The amount of material transformed in the implanted layer is substantial. It is considerably higher than what can be achieved by severe

t 304 stainless steel foils l

,100 0 0

1

5,1023

Fig. 4. The amount of martensite (cx’) phase in vol.% as function of fluence after implantation of 304 stainless steel

with H+ , D+ and He+ ions. The figure also includes data

from the DCEMS analysis. H) lo-13 keV (O-20 nm depth),

M) 6-9 keV (20-60 nm depth) and L) 2-5 keV ( > 60 nm depth).

VI. METALS

570 E. Johnson et al. / Martensitic transformations in 304 SS

Table 3 The relative amounts of martensite ((Y’) and austenite (v) phases and values for the internal magnetic field H as function of depth from the DCEMS spectra for 304 austenitic stainless steel implanted with 8 keV He+ ions.

Fluence

(mm*)

4.4x 102’ 4.4x102’ 4.4x102’ 9.4x102’ 9.4x102’ 9.4x 102’

Depth ’ (nm) pLvo1.W)

O-20 67 20-60 45

> 60 30 O-20 79

20-40 62 > 60 39

;“ol.%) (HT)

33 28.5 + 0.2 55 28.5 0.2 + 70 28.1 f0.3 21 29.2 f 0.2 38 28.6 + 0.1 61 29.1+ 0.3

cold-work [16], and it is comparable to what can be achieved after implantation with heavier inert gases such as argon or krypton [4]. This is in full agreement with the earlier results of Hayashi et al. [7-91.

A similar series of spectra from samples implanted with 8 keV H+ or D+ are given in fig. 3. It is significant that while martensite is already formed after implanta- tion with helium to a fluence of lo*’ m-*, a fluence of more than 1O23 m-* IS required to induce martensite after implantation with hydrogen or deuterium.

All these results have been summarised in fig. 4 which shows the amount of martensite induced after the various implantations. The high efficiency of martensite formation after helium implantations is striking. On the other hand, the ability of hydrogen and deuterium to form martensite differs only a little. In order to see whether already existing martensite would influence the implantation induced transformations, a few unan- nealed samples were implanted in the as-rolled state. The CEMS results of this experiment are also included in fig. 4, and it shows that at the high fluences where the amount of martensite begins to saturate, the effect is not significant.

In order to study in more detail the distribution in depth of the implantation induced martensite, some of

28 (deg) Fig. 5. X-ray diffraction spectra taken at an angle of incidence of 2O under asymmetric Bragg conditions from 304 stainless

steel after implantation with 8 keV H+ or He+ ions.

the helium implanted samples were analysed as func- tions of depth with DCEMS [10,12]. The phase distribu- tions and the values of the hyperfine field for the various depth intervals are given in table 3, and the results are incorporated in fig. 4.

Using X-ray diffraction under conditions of glancing incidence, it is possible to increase the sensitivity of the technique sufficiently for it to be used on thin surface layers [13]. This makes it possible not only to identify new phases, but also to obtain precise information on their lattice parameters. Fig. 5 shows a series of X-ray powder diffraction patterns taken from He+ and H+ implanted samples. Qualitatively, the X-ray diffraction data agree with the CEMS results. From the diffraction patterns the lattice parameter of the implantation in- duced martensitic ((Y’) phase has been obtained to be a, = 0.286 nm.

4. Discussion

Hayashi et al. [7-91 have shown that implantation of a 304 stainless steel with 40 keV helium ions to high fluences induces a martensitic transformation in the implanted layer. Using depth selective (DCEMS) analy- sis they were also able to determine the amount of austenite and martensite as function of depth, and they found less than 50 vol.% retained austenite at the maxi- mum of the implant profile [9]. This transformation pattern follows closely that seen after implantation of stainless steels with heavy ions [2-41, where the highest transformation efficiency is seen after implantation with inert gases [4]. Implanted noble gas atoms are known to form microscopic inclusions, which are confined under a pressure sufficiently high (= 2 GPa) to keep them solid at room temperature [5,17-191. Such highly pres- surized inclusion will induce stress levels in the con- fining matrix largely exceeding the yield stress limit. Subsequently, the implanted surface layer will undergo a stress induced martensitic transformation. It is in this respect noteworthy that the critical fluences for onset of the martensitic transformations, coincide with the fluences at which blistering - another deformation re- lated process - is seen to occur under similar implanta- tion conditions [20,21].

Alternatively, it has recently been proposed that the transformation might instead be induced by preferential sputtering of nickel and chromium, leading to a subse- quent reduction in austenite stability of the implanted surface [22]. If this was the case, however, the threshold fluence for initiation of the martensitic transformation would scale with the sputtering yield. The sputtering yield of 8 keV He is about 7 times that of 7.3 keV D and about 4 times that of 40 keV He [23]. The transfor- mation to martensite occurs at fluences which scale about 1: 10 : 100 for 8 keV He, 40 keV He [7-91 and 7.3

E. Johmon et al. / Martensitic transformations in 304 SS 571

keV D, respectively, and hence this does not support the preferential sputtering mechanism. Furthermore, the layer thickness removed by sputtering with the threshold deuterium fluence of 1 x 1O23 rn-’ corresponds to the thickness from which enhanced sputtering of Cr and Ni is observed. At higher fluences the ratio of the sputtered particles agrees with the bulk composition 1241.

Combining the data from the helium and hydrogen implantations in the present work, the results therefore fully favour the presumption that the y -+ e’ transfor- mation is martensitic and that is in induced by the high stress levels built up in the matrix during implantation. The TRIM calculations (fig. 1, table 1) show that the range and damage distributions of the various ions are nearly equal, and that helium at the most creates a damage density which is 5-10 times higher than that created by hydrogen or deuterium. The critical fluence for martensite formation after implantation with hydro- gen or deuterium is, however, at least 100 times larger than the corresponding helium fluence (fig. 4). This cannot be accounted for by the much smaller dif- ferences in primary radiation damage features. The high efficiency for martensitic transformations in connection with helium implantations must therefore be related to the formation of helium bubbles confined under very high pressures [19]. The martensitic transformations oc- cur at fluences where dense distributions of bubbles are formed [25]. In this respect there is hence no difference between the martensitic transformations induced after helium implantations and after implantations with the heavier inert gas ions. They are all stress-induced and associated with formation of dense dist~butions of highly pressured inclusions which establish high stress levels in the implantation zone. Contributions to the driving forces for the martensitic transformations from various radiation effects such as primary damage, sputtering, radiation enhanced diffusion or segregation, which would be comparable for helium, hydrogen and deuterium implantations, can therefore only play a minor role.

With respect to hydrogen (or deuterium) implanted into stainless steels, it is known to be spontaneously lost again by back-diffusion to the surface 1261, such that only a very low ~n~ntration of hydrogen atoms, trapped on defects, can be retained in the implanted layer (14), where hydrogen gas bubbles are unabte to form. It is therefore much more difficult during implan- tations with hydrogen than with helium to build up stress levels that can sustain martensitic transforma- tions. From conventional metallurgy, hydrogen induced in stainless steels by cathodic charging is, however, known to cause substantial formation of both c and (Y’ martensite [27,28]. This is usually ascribed to the fact that high concentrations of hydrogen lowers the stack- ing fault energy of the austenitic (y) phase, and thus increases the ability of the hydrogen charged surface to

form stacking faults [29]. Considering the formation of dense arrays of stacking faults to be a precursor of the y + e transformation, hydrogen induced martensitic transformations are frequently seen to follow a y -+ c -+ (Y’ path. A martensitic transformation induced after hydrogen (or deuterium) implantations is hence first anticipated to take place when the ~crost~cture of the implantation zone begins to resemble cathodically charged stainless steels. Rozenak and Eliezer [28] have measured hydrogen concentrations in cathodically charged 316 stainless steels to be = 100-300 wt.ppm, corresponding to = 1 at.%. A comparable hydrogen (or deuterium) concentration is achieved after an implant fluence as low as = 1 x 10” mm2 [26]. The critical fluence for onset of the martensitic transformation after hydrogen implantation is more than 1000 times higher. This shows that retention of hydrogen in the implanted layer - and the accompanying lowering of the stacking fault energy - is not in itself a sufficient condition for formation of martensite; buildup of a highly defective damage layer is necessarily required too. As such, the y + IX’ transformation induced by hydrogen implanta- tion appears to have some similarity with conventional y+c-+iX’ martensitic transformations in stainless steels, which have been cathodically charged with hy- drogen.

Bearing in mind that the CEMS technique with decreasing efficiency is sensitive down to a depth - 300 nm, while the thickness of the He+ implanted layer is only - 60 nm (fig. I), the amount of martensite in the implantation zone obtained from the CEMS spectra (table 2) might be underestimated. This problem can be elucidated from the DCEMS results which are incorpo- rated in table 3 and fig. 4. The amount of martensite is largest in the implanted layer itself, where values of 60-80 vol.% martensite can be reached. This is much more than can be induced in conventional stress in- duced surface transformations by plastic deformations [16], and it seems to reflect the highly stressed state of the implanted layer. Beyond the implantation zone (de- pth > 60 nm) the amount of martensite is considerably lower (30-40 vol.%), depicting the gradual decrease of the high stress levef of the implanted layer at larger depth. This is different from what can be seen after implantation with heavy inert gases, where the marten- sitic transformation zone extends well beyond the TRIM range [30].

The lattice parameter of the implantation induced IX’ phase is 0.286 nm which agrees very well with the value found after cathodic hydrogen charging [28] and after phosphorus implantation [31]. It shows that the nearest neighbour distance of the (Y’ phase is reduced = 2% compared with the y phase, supporting the presump- tion that the implantation induced martensitic transfor- mation is indeed comparable to the conventional martensitic transformations [l]. This is further substan-

VI. METALS

tiated from the values of the hyperfine magnetic field which are close to those seen in austenitic stainless steels after conventional transformations [16] and after implantations with heavy ions [32].

5. Conclusion

The fact that the fluence required to induce marten- site after hydrogen implantation of austenitic stainless steel is at least 100 times larger than the corresponding helium fluence, shows that the influence of primary radiation damage on the martensitic transformation is insignificant. The easy formation of martensite after helium implantations is due to the ability of helium to form gas bubbles confined under extremely high pres- sures, and as such helium implantations resemble im- plantations with the heavier inert gases. The stress- induced nature of the implantation induced martensitic transfo~ations is also reflected in the fact that the critical fluences for onset of the transformations are comparable with the fluence at which blistering occurs. Martensitic transformations after implantations with hydrogen, which is largely lost by back-diffusion through the implanted surface, are on the other hand, first induced when the implanted layer resembles the state of a cathodically charged surface. A small but distinct difference in the amount of martensite formed after

hydrogen and deuterium implantations (fig. 4) may indicate a small contribution from primary radiation damage to the driving forces for the martensitic trans- formation, which can otherwise only be observed under low stress conditions.

This work is supported by The Danish Natural Sci- ence Research Council. Assistance with heat treatment of the stainless steel foils by M. Keller, The Technical University, is gratefully acknowledged.

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