TEM characterization of a nitrogen implanted austenitic stainless steel

TEM characterization of a nitrogen implanted austenitic stainless steel

Applied 288 Surface Science 25 (1986) 288-304 North-Holland, Amsterdam TEM CHARACTERIZATION OF A NITROGEN IMPLANTED AUSTENITIC STAINLESS STEEL S. ...

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Applied

288

Surface

Science 25 (1986) 288-304 North-Holland, Amsterdam

TEM CHARACTERIZATION OF A NITROGEN IMPLANTED AUSTENITIC STAINLESS STEEL S. FAYEULLE Laboratom

and D. TREHEUX

de Mkllurgie,

UA No. 447, Ecole Centrale de Lyon, F-69131 Eculi) Cedex, France

and C. ESNOUF GEMPPM, Received

UA No. 341, INSA 10 September

Lyon, F-69622 Vdleurbanne,

7985; accepted

for publication

France

5 November

1985

Pure austenitic stainless-steel samples (18% Cr. 10% Ni) were implanted at room temperature with nitrogen ions at an energy of 40 keV with fluences from 70” to 6X10” ions cmm2. Microstructures obtained after implantation were studied by transmission electron microscopy and selected-area diffraction. The observations show the formation of E martensite (hexagonal), of n’ martensite (tetragonal) and the appearance of nitrides (Fe, Cr, Ni),N, _ ~ hexagonal or orthorhombic.

1. Introduction The effectiveness of nitrogen implantation in improving the tribological properties of austenitic stainless steel 304 (18% Cr, 10% Ni) has been established by several studies over many years [l-7]. However, mechanisms responsible for this improvement are not well known but some hypotheses have been proposed. Yost et al. [3] have detected chromium nitrides CrN (I$ = 4 X 10” ions cmp2, E = 50 keV) and explained the decrease of wear by the formation of a hardened superficial layer. Singer [6] suggested that N implantation might prevent the undesirable hardening which occurred during wear of the 304 surface [S]. He thought that N would prevent the 304 steel surface from transforming and becoming brittle strain-induced martensite. Effectively, transmission electron microscopy (TEM) investigations carried out by Vardiman et al. [9] showed that nearly all the martensite of a polished 304 steel surface was converted to austenite. Thus, nitrogen implantation in 304 stainless steel would cause the formation of a supersaturated solid solution (up to 24% at N) (without appearance of nitrides) which is very stable against the martensitic transformation responsible for wear. 0169-4332/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

S. Fayeulle et al. / TEM on austenitic steel

289

Finally, Oliver et al. [7] underlined the importance during wear of the oxide layer without being able to say if the oxide is formed due to a hardness-induced decrease in wear rate or if implantation promoted formation of the oxide layer. Because of the diversity of these hypotheses, a microstructural study of implanted 304 steel appears to be essential. Such a study had already been partially achieved by several groups but here too, results are rather different [3,9-141. With low fluences (5 X lOi ions cm p2, 40 keV) Whitton et al. [lo] showed that N was located in the octahedral interstitial position. For higher fluences, iron and chromium nitrides have been detected by Auger spectroscopy by Singer et al. [ll]. By TEM, others groups have observed essentially the formation of CrN [3,12]. Finally, a study using secondary ion mass spectroscopy (SIMS) has shown that both CrN and Cr,N nitrides were formed in preference to iron nitrides. Owing to the use of several techniques (TEM, glancing X-ray diffraction), Whitton et al. [14] have recently underlined the importance of the transformation austenite/martensite. Nitrides, Cr,N and probably some Fe,N, are formed only when austenite is fully transformed to martensite. The martensitic transformation y(fcc) + Lu’(bcc) has already been observed in helium, phosphorus, antimony and tin implanted 304 austenitic stainless steel [15-191 and we have studied it during nitrogen implantation [20]. This paper gives complete results of our investigation of nitrogen implantation in 304 steel. Particularly, evolutions of microstructures versus fluences are studied and a model for the formation of different phases is proposed. 2. Experimental

procedure

The highly purified austenitic steel had the following composition (in wt%): Cr 18; Ni 10; C 1.5 X 10m5; 0 10-5; N 10m5; S 2.5 X 10-5: Fe balance. The samples were cold rolled to 0.2 mm, then punched from the foil (3 mm discs). vacuum annealed at 1370 K and quenched in water. The side to be implanted was then electrolytically polished. Implantations were made using the isotope separator of the Institut de Physique Nucltaire de Lyon. The ion fluences were lo”, 2 x 10” and 6 x 10” ions cmm2 at 40 keV. The samples were implanted at room temperature (25°C) maintained by water circulation and liquid nitrogen cold traps near the specimen reduced contamination to a minimum. After implantation, the samples were thinned through the non-implanted side, the implanted one being protected by a translucent varnish coating. Some implanted samples were polished to selected depths before being thinned from behind to allow examinations a few nanometers under the surface. The electrolyte was 20% perchloric acid in 80% acetic acid used at 5°C

S. Fayeulle et (11./ TEM on awtenitic

290

steel

and 30 V. Examination was carried out with a 200 CX JEOL microscope some EDX analyses were made with a 400 STEM Philips microscope.

3. Experimental

and

results

3. I. Non-implanted

samples

Examination of the FeeCr-Ni alloy in the unimplanted condition revealed the fee structure (a = 0.357 nm) with few defects (dislocations and twins). No precipitation is seen in grain boundaries (fig. 1). 3.2. Implantation

of lOI

ions cm- ’

3.2.1. Direct observation of the implanted surface The implanted layer is highly defected with a high density of dislocations (fig. 2). According to the electron beam incidence, Bragg fringes are seen (fig. 3) showing that the surface relief is very irregular. Nevertheless, the fringes are relatively oriented and so are the surface undulations. The study of diffraction patterns (fig. 4) allows the identification (in addition to y austenite) of (Y’ martensite (bee, a = 0.285 nm, c = 0.304 nm), of orthorhombic nitride ({-Fe,N type, a = 0.552 nm, b = 0.483 nm, c = 0.442 nm) and of hexagonal nitride ((Cr, Fe),N, _ ~ type, a = 0.480 nm, c = 0.446 nm). The spots of the nitrides are rather weak: there are small amounts of these compounds. 3.2.2. Observation in the implanted layer (60 nm below the initial surface) Many of very small plate precipitates are detected (fig. 5) and the diffraction patterns (fig. 6) show the y matrix (fee), the (Y’martensite (bee) and a

Fig. 1. Grain

boundary

m a non-implanted

sample

291

Fig. 2. 10 ” N cm~’

Fig. 3. 10 ” N cm

implantation:

’ implantation:

dlslocatvxs.

Bragg fringes.

0 . 0

O oZo**

0

212

l

0

..Ti2

l

0020

O

o

. 0

Fig. 4. Electron diffraction pattern: (0) Dashed circle: double diffraction on a’.

LX’martensite,

z = (513); (0)

e nitride;

(0)

3 nitride.

Fig. 5. F martensite

(bright field (BF) and dark field (DF)

micrographs)

third hexagonal phase with parameters u = 0.254 nm and c = 0.408 nm. This phase is the E martensite. The orientation relationships between the three phases are those currently observed in that kind of steel [21,22]: (110),, II (OOOl), II(ill), and (ill),, II (2iiO), II (lOi), (these are the well known Kurdjumov-Sachs orientation relationships). On the diffraction patterns, the existence of small plate precipitates gives streaks perpendicular to the precipitate direction on the matrix spots (fig. 6). 3.3. Implantation

of 2 X I 0i7 ions cm ~ 2

3.3.1. Direct observation of the implanted surface The aspect of the surface is now very much modified and different contrasts are seen according to the electron beam incidence (fig. 7). Particularly, oriented precipitates are detected (fig. 8). Diffraction patterns (fig. 9) show that the layer is made up only of orthorhombic or hexagonal nitrides (the

S. Fuyeulle Ed(II. / TEM on crustenitic steel

‘100E

293

-b 0 0 0 0

ioio . oil i .

0 0

x

.

l-j01 0. 222v

/

0

22ocY"

0

0

Fig. 6. Electron diffraction patterns. Upper: (0) L = (1213). Lower: (0) y austenite, z = (122); (0) z = (0001).

y austenite, a’ martensite.

z = (123); (0) E martensite. z = (001); (0) E martensite,

oriented precipitates) and of (Y’martensite. The EDX analysis of this layer in different points (diameter of the analysis spot is 60 A) indicates that iron, chromium and nickel are always’simultaneously present (fig. 10). Thus, the compounds (nitrides and martensite) are mixed (Fe, Cr, Ni). The surface fringes can sometimes be very directional (parallel to the direction of precipitates) (fig. 11). In this case, they look very much like contrasts observed in aluminium-implanted iron [22] where these fringes result from a modulation of the chemical composition. In our case, the contrast in fig. 11 is neither moire fringes which are finer (i = 30 A) (see for example fig. 7, 0 = - 55”, or fig. 9 where very close-set spots give moire fringes with i = 10d = 30 A) nor modulations since the interference is changed when the electron wave is changed: the

Fig. (O=

variations 7. 2X 10 ” N cm ’ implantation: -55”. -500. -4O”, -30”. -10”. -5”. +15”.

of contrast with +300. +40”).

electron

beam

incidence

Fig. 8. Oriented precipitates after a 2~ IO” N cm ’ implantation: large view (BF) anddetalls (BF and DF) (rotation angle between images of C;= 25000 and G=100000 on the 200 CX JEOL microscope = 125”).

S. Fayeulle et al. / TEM on austenitlc steel

295

041 04i

0

o

Oil

0

l

.

x

5OO4&a' 002 o

200 . 0

Fig. 9. Electron diffraction patterns. Upper: (0) { orthorhombic nitride; (0) martensite a’; dashed circle: double diffraction. Lower nitride; (-) double diffraction on 5.

nitride. Lower left: (0) right: (0) [ nitride: (0)

{ F

296

S. Fayeulle et al. / TEM on austenitlc steel

Fig. 10. EDX analysis: peaks of N. Cr. Fe, Ni (1 : N, 2 : Cr. 3 : Fe, 4: Ni, other ones are secondary peaks of Cr, Fe, Ni).

Fig. 12. Large view of underlayer

after a 2 X 10 ” N cm ~’ implantation

S. Fayeulle et al. / TEM on awtenitic steel

Fig. 13. Dissociated

Fig. 14. 6

X

10”

dislocation

N cm-’

seen under

implantation:

two incidences:

BF micrograph.

radius of curvature

297

= 125 nm.

298

S. Fayeulle et al. / TEM on austenitic steel

surface fringes are Bragg fringes. The contrast is caused by the surface relief and indicates that the strain relaxations of the implanted layer occur by parallel waves. 3.3.2. Observation in the implanted layer (I 00 nm below the initial surface) Examination of the deeper areas shows the appearance of many dislocations (fig. 12) and some are dissociated (fig. 13). On this figure, the curvature of the partial dislocation allows one to know approximatively the strain state of the alloy in the underlayer, i.e. in the area of accommodation between the implanted -layer and the bulk material. One finds u = 5 x 10m4 p which is a rather high strain (~1 is shear modulus). 3.4. Implantation

of 6 X IO” ions cm _ ’

The outermost layers are now more and more defective with many moire fringes (fig. 14) and precipitates clearly visible on a dark field micrograph (fig.

Fig. 15. 6x10

I7 N cm-’

implantation:

DF and BF micrographs

of nitrides.

S. FayeuNe et al. / TEM on austenitic steel

299

a

(notice that the precipitate Fig. 16. (a) Grain boundary after a 2 X 10” N cm- ’ implantation orientation depends on the initial matrix orientation). (b) Grain boundary after a 6~ 10” N cm-’ implantation.

300

S. Fayeulle et al. / TEM on austenitic steel

0

0

121

P

&O o&

x 1121 08

0.211 .

211

x

l

.

0 0

020

0

8

Fig. 17. Electron diffraction patterns. Upper: (0 ‘) Y austenite. i = (013); (0) { nitride; (0 ) a’ i =: (012); (0) [ nitride; martensite, z = (001). Lower: (0) y austenite, (0) a’ marten Isite, z = (531).

15). The aspect of the grain boundaries underlines also the importance of the damage caused by the ion bombardment (fig. 16). After a 2 X lOI ions cm-’ implantation, the grain boundaries are visible in which precipitates are detected. After an implantation of 6 X 10” ions, the grain boundaries are nearly invisible (fig. 16 has to be compared with fig. 1 of a boundary before implantation). The damaged aspect of the surface can also be analysed through the diffraction pattern: the spots are not as well defined as before and discontinuous rings are seen (fig. 17). The patterns can be identified with y austenite, LY’ martensite and orthorhombic nitrides.

S. Fayeulle et al. / TEM on austenitic steel

301

4. Discussion 4.1. Kind of phases formed By transmission electron microscopy, it can be established that nitrogen implantation in austenitic stainless 304 steel causes essentially two phenomena: _ formation of martensites E and cu’; - formation of nitrided phases. The EDX analysis shows that the three elements, iron, chromium and nickel are present in the different phases. This result is consistent with the fact that nitrogen implantation causes many atomic displacements (about one hundred for each atom of the matrix during a 10” N cm-’ implantation) and consequently that it is difficult to understand the formation of pure iron or pure chromium nitrides. The nitrided phases are thus, for all fluences, M,N orthorhombic or hexagonal with M = (Fe, Cr, Ni). Carbon may be present in small quantities in these phases (thus of M,(N, C) type) as it has been established by some groups using TEM [24,25] or CEMS [26,27]. This carbon results essentially from a pollution from the residual vacuum. The weak differences in the parameters do not allow us to discriminate between them. Neither MN nitrides nor &-Fe z+,N (this one being observed very easily on pure iron or on low carbon steel implanted in the same conditions [20]) have been detected in the Fe-Cr-Ni alloy. Heterogeneities of composition of the compounds surely exist. For example, located accumulations of carbon can be achieved (since carbon is not directly implanted and thus remains in the outermost layers) which cause chromium accumulations because of the high affinity of these two elements. This can be seen on the diffraction patterns because of some variations of parameters of a few hundredth of A, consistent with the ASTM data (Fe,N, Cr,N, Cr,(C, N), (Cr, Fe),N,_,). Finally, the lattice can be more or less distorted according to strains and either an hexagonal phase or an orthorhombic one is seen, the latter allowing an anisotropy of the strain relaxation which explains the particular appearance of the Bragg fringes. 4.2. Mechanism

of formation

of the different phases

The mechanisms operative during nitrogen implantation seem very similar to that during the deformation of 304 steel in the first steps [22]. The introduction of nitrogen causes the formation of stacking faults in the stainless steel. Their number becomes more and more important during the ion bombardment which creates an expansion of the lattice and consequently an increase of the strains. These stacking faults are the preferred sites for the formation of small plate precipitates of E martensite (transformation y -+ E by

302

S. Fayeulle et al. / TEM on ausienitic steel

octahedral

interstitial

lo15

x

1OOnm

distortion/

\

CI' martensite

A”

expansion

E nitride

“J

Y Lie

a’

10

2x10

a, Fig. 18. Mechanisms

of formation

17

of different

17

surface

phases. operative during nitrogen implantation.

shearing of the fee lattice). The (Y’ martensite appears then from the E martensite by a homogeneous transformation of the lattice without supplementary shearing. When the nitrogen quantity becomes significant this first mechanism is surely enforced by the direct transformation y -+ (Y’. The E martensite can also initiate the formation of hexagonal nitrides (e-type) through an expansion of the crystal lattice. Finally, the increase of the nitrogen concentration (about 35 at% at 2 x lOI ions cm-‘) plus the strains and the thermal effects due to the bombardment result in the formation of orthorhombic phases (l-type) which are stable as soon as 50 at% N is reached (in the nitrided phases). Beyond 2 x 10” ions cm-‘, the nitrogen concentration does not increase anymore [28]. Nevertheless, the outermost layers are still influenced by the ion beam. Probably an equilibrium formation/destruction (on a small scale) of the different phases is reached, giving the very defective aspect observed after a 6 X 10” ions cme2 implantation. The mechanisms can be summarized as presented in fig. 18 (versus fluence as seen in the text or versus depth in the material, i.e. nitrogen concentration (for + > lOI ions cm-‘)).

5. Conclusions . Most studies of the microstructures of implanted 304 stainless steel have put in evidence only chromium nitrides, except Singer et al. [ll] and Whitton et al. [14] who also detect Fe,N. However, these authors [14] point out that the

S. Fayeulle et al. / TEM on austenitic steel

303

diffraction patterns showing the formation of CrN in the study of Yost et al. [3] “indexed well” to (Cr, Fe),N,_,. Actually, our results show that iron, chromium and nickel are simultaneously present in the nitrides. This result seems more logical because of the damages of the matrix caused by the ion bombardment and of the numerous atomic movements during the implantation. Besides, mixed compounds (Fe, Cr),N,_, have also been detected in an iron-chromium alloy [24]. Nevertheless, we must point out that our alloy is highly purified. In an industrial alloy, carbon, though in very small quantity, maybe can make easier the formation of chromium carbonitrides. Moreover, CEMS and XPS analyses [27] have underlined the importance of temperature during implantation for the introduction of carbon in the samples. In our case, temperature was maintained at 25°C but this parameter is not often mentioned in the other studies. Finally, the kind of implanter (here an isotope separator) and the quality of vacuum can also modify the amount of carbon introduced in the steel. Concerning the tribological behaviour, nitrogen implantation can harden surface layers through the formation of martensite and of nitrides. But the influence of these compounds remains uncertain because mechanisms of wear of stainless steels are not understood. Oxidation is known to play an important role during dry friction. The influence of the martensitic transformation is not very clear. Hsu et al. [8] think that friction and wear behaviour of austenitic stainless steels depends on their stability with respect to the martensitic transformation. On the contrary, the work of Smith [29] would indicate that martensite has not an important influence, the martensite formation being only a by-product of material breakdown. Moreover, in order to compare friction results of austenitic stainless steels, the initial conditions must be exactly the same. Particularly, the surface layer damaged by polishing (and where there are some amounts of martensite) must be eliminated. Now, this very fine layer is the one concerned by implantation. The mistake would be to use TEM results obtained on austenite after electro-polishing for friction tests achieved on layers of different kinds due to mechanical polishing.

Acknowledgments The authors wish to thank M.A. Plantier for carrying out the implantations and Miss I. Berbezier for STEM analyses. This work was partially funded by Minis&e de la Recherche et de la Technologie.

304

S. Fayeulle Ed al. / TEM on austenitlc steel

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