Temper embrittlement and surface segregation, an AES and ILS study

Oool-6160 79.0401-0567$& .M’O

TEMPER

EMBRITTLEMENT AND SURFACE AN AES AND ILS STUDY WOLFGANG

Programa

SEGREGATION,

LOSCH

COPPE/Universidade Federal do Rio de Janeiro. de Engenharia Metalurgica e de Materiais, Rio de Janeiro. Brasil. C.P.1191-ZC-00 (Rrceiwd

28 Frhruury

1978; in rwisrdforrn

3 July

1978)

Abstract-The process of temper embrittlement provoked by impurity (P, S) segregation in a high alloy Cr. Ni steel is studied by means of Auger electron spectroscopy (AES) and ionization loss spectroscopy (ILS). In some cases the studies are similarly realized on vacuum fracture surfaces and on free surfaces after the same heat treatment. Identical results are observed. This supports the assumption that boundary and free surface segregation are equivalent. Chemical shifts observed on free surfaces by ILS in the 3s and 3p levels of all metallic components indicate the strongest interaction between Ni and P after a heat treatment which also caused a maximum of intergranular fracture. AES fine structure, interpreted as interatomic transitions, also indicate a strong Ni-P interaction. Reduction of boundary cohesion, the origin for the embrittlement, is discussed on the basis of weak 3d-3p valence orbital overlapping as expected from metal-impurity interactions. R~sum~Nous avons etudie par spectroscopic Auger (AES) et par spectroscopic de pertes par ionisation (ILS) la fragilisation au revenu produite par la segregation des impure6 (P. S) dans un acier au chrome at au nickel fortement allie. Dans certains cas. nous avons effect& parallelement des etudes sur des surfaces de rupture sous vide et sur des surfaces libres apres le meme traitement thermique. On obtient alors des resultats identiques, ce qui confirme l’equivalence des segregations aux joints et a la surface libre. Les deplacements des niveaux 3s et 3p de tous les composants mitalliques. observes par 1LS sur les surfaces libres, montrent que l’interaction la plus forte entre Ni et P est obtenue pour le traitement thermique qui produit la rupture intergranulaire maximale. La structure fine par AES, que l’on interprete par des transitions interatomiques, indique egalement une forte interaction Ni-P. Nous discutons l’origine de la fragilisation, c’est a dire la diminution de la cohesion des joints, en nous basant sur le faible recouvrement des orbitales de valence 3d-3p. attendu avec les interactions metal-impure6 Zusammenfassung-Der Prozess der Temperversprodung, hervorgerufen durch die Segregation von Verunreinigungen (P, S) in einem hochlegierten Cr-Ni-Stahl wurde mittels Augerelektronen(AES) und Ionisationsverlustspektroskopie (ILS) untersucht. In einigen Fallen wurden parallel Vakuumbruchfliichen und freie Oberflachen nach derselben Warmebehandlung untersucht. Identische Ergebnisse zeigen. da13 die Segregation an Korngrenzen und an der freien Oberflache als gleichwertig angenommen werden konnen. Chemische Schiebungen, die mit ILS an freien Oberflachen in den 3s- und 3p-Niveaus aller metallischer Komponenten beobachtet wurden. deuten hin auf eine stiirkste Wechselwirkung zwischen Ni und P nach der Wlrmebehandlung, welche such ein Maximum im intergranularen Bruch bedingte. AES Feinstrukturen, die als interatomare Uberglnge interpretiert werden, zeigen such eine starke Ni-PWechselwirkung an. Die Verminderung des Korngrenzzusammenhaltes, die Ursache fir die Versprii dung, wird auf der Grundlage einer schwachen 3d-3p-Valenzorbitaliberlappung. wie sie von den Metall-Verunreinigung-Wechselwirkungen her erwartet wird, diskutiert.

1. lNTRODl_JCTION

on embrittlement. Actually, very little can be said about the physical basis for the reduction of cohesion along boundaries by inpurity elements [3]. One interpretation was given on the basis of atomic volume considerations [7]. This mechanical model tries to explain lowering of cohesion by the increasing separation of metal atoms at the boundaries with the implantation of impurities of larger atomic size. Even this model can explain some facts of embrittlement [7] ; other experimental results seem to indicate a more physical
The phenomenon of temper embrittlement of steels, known for a long time in metallurgy [l, 21, is still not well understood [3]. After many efforts and studies applying classical means [2] a big step forward in understanding was achieved by application of such modern analytical methods as Auger electron spectroscopy (AES) [4-63. By this method one was able to prove the process of segregation of impurities as Sb, P, S and As to the grain boundaries. This demonstrated that the accumulation of impurities at the boundaries is responsible for the phenomenon of embrittlement. However, the knowledge of the dynamic processes of segregation still does not explain the function of these impurities at the boundaries and their influence 567

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/ 0

Fig.

2

4

/ / 6

TEMPER EMBRITTLEMENT

io

/

t [mini 6

IO

12

I4

16

1.

Variation of Auger line intensity with time after fracture, due to adsorption from the residual gas.

embrittlement should consider electronic level modification as consequence of the chemical composition changes provoked by impurity segregation. At this point serious experimental problems enter the discussion. Grain boundary segregation studies have to be carried out in situ on ultra-high vacuum fracture specimen. A certain time is necessary to investigate the electronic state of these surfaces. But these freshly created surfaces are extremely reactive and modify within minutes their surface conditions by interaction with the residual gas. Photospectroscopy [lo] is the best method for the study of single electronic levels and their modifications by chemical interaction. The accuracy of photospectroscopy has two disadvantages which make the application of this method somewhat doubtful in grain boundary studies: (a) the time to run a spectrum is large compared to the time of retaining a surface unchanged from chemical reactions; (b) the lateral resolution is poor in relation to the surface area obtained after vacuum fracture. Consequently the pioneering attempt of Coad et aI. [11] has given somewhat weak results in the determination of electronic level modifications caused by the impurity segregation. AES does not suffer from the above disadvantages of photospectroscopy. An Auger spectrum may be recorded in 1 min and the lateral resolution of the electron beam is in the order of pm, being thus much smaller than the fracture surface. But conventional AES is not a powerful tool in studies where chemical reactions are in focus. This is because of the participation of three electronic states in a single Auger transition [lo]. An observed chemical shift AE,,, of an XYZ Auger line is the sum of the partial shifts of three levels, AE,, = AE, + AE, + AE,, and it is impossible to determine the AEi without further knowledge. However, on the basis of two experimental observations, AES may also become a more informative method in studies of chemical interactions: (a) Auger spectra show a lot of fine structure which can be interpreted in terms of chemical interactions [ 12-141;

AND SURFACE SEGREGATION

(b) from physical principles there is no difference between grain boundary and free surface segregation. Chemical reactions may therefore be studied on free surfaces. Experimental observations support this assumption of segregation analogy [ 12, 15-J. In the present paper we report on boundary and free surface segregation studies on a high-alloy steel. The fine structure of the Auger transitions is analysed as a function of the applied heat treatment. Ionization loss spectroscopy (ILS) is applied in order to determine single chemical shifts of the 2p, 3p and 3s electronic levels. The results strongly support the above assumption that consideration of the electronic structure may help to interpret unsolved problems in temper embrittlement. 2 EXPERIMENTAL A high alloy austenitic steel, type 304, was chosen for the present studies for two reasons: most of the actual work on temper embrittlement is concentrated on low alloy steels and, second, on performing Auger fine structure and ILS studies, a certain concentration of the metallic elements is necessary in order to obtain sufficient line intensity. The chemical bulk composition of the steel is given in the following table : Element

Cr Ni Mn MO Cu

C

P

S

Sb

Weight %

17.0 8.5 0.6 0.34 0.1 0.08 0.04 0.02
Rod-shaped samples of 4mm dia. were machined, notched and then submitted to a heat treatment which led to intergranular fracture. As the aim was to compare with free surface segregation, the following simple process was chosen: 2 h austenitization at 1100°C followed by water quenching, embrittling for 2 h at 600, 650, 700 and 750°C and again water quenched. The samples were mounted in an ultrahigh vacuum system with residual pressure of about p = 5 x lo- lo torr. Up to five samples could be mounted in a carousel holder. One was positioned into the fracture device and cooled down to about - 13o”C, controlled by a FeConstantan thermocouple. Immediately after fracture the pressure increased to about lo-’ torr for approx. 2 min due to the strong fracture impact. This caused enhanced adsorption on the clean fracture surface. We tried to control this unavoidable perturbation by recording the surface composition as a function of time after fracture (t = 0). Figure 1 shows the time variation of different Auger peaks. It is seen that the existence of oxygen is due completely to adsorption after fracture. Because of the increase in adsorption layer (O,C, S) a reduction of the Fe, Cr and Ni lines is observed. The minimum time necessary after fracture to position the sample, focus the beam and to run the first spectrum was 4min. When the first spectrum was taken we noticed some perturbation by oxygen.

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569

problems in ILS and AES fine structure measurements cannot be the subject of this paper and therefore will not be reported here. The Auger data for conventional AES experiments were: primary energy E, = 2 keV, beam current i, = 20pA and modulation voltage V’, = SV. The ILS measurements were performed with E, = 350 eV, i, = 20 PA and VP,,= 0.5 V.

loo.

80. 60.

40. 20.

T [*Cl

I

\

I

_r

ino

800

3. RESULTS

\ 930

Fig. 2. Degree of intergranular fracture obtained after 2 h treatment at different temperatures. All experiments were performed in a commercial Varian AES system with a single-pass cylindrical mirror analyser (CMA) and an integral electron gun (O-3 keV). Sputtering was applied with a scanning sputter gun (G3 keV). The exact sample position in the x, y-plane was controlled by a scanning sample positioner. Correct z-position was obtained by adjusting the zero crossing of the elastic peak. For the free surface segregation studies a thin foil of -0.3 mm thickness from the same material was formed by cold rolling. After mechanical polishing the sample was mounted in the vacuum system and annealing was done by direct passing a current of i, = &8 A. ILS measurements have been performed on this sample. It was not possible to record ILS spectra on brittle fracture surfaces because of the time necessary to run a complete ILS spectrum. Within this time (- 10 min) the fracture surface showed too much oxygen, which changed the energetic levels of the elements completely. Only in the case of a ductile fracture, where no strong fracture impact was realized, could we apply ILS. In order to obtain a good energy resolution a small modulation voltage of V,, = 0.5 V was applied. All experimental details and 30

The degree of intergranular fracture obtained as a function of the applied heat treatment, i.e. after 2 h embrittlement at various temperatures, is shown in Fig. 2. The best result of nearly 100% intergranular fracture is observed at 650°C. Scanning electron micrographs of the fracture surfaces have been published elsewhere [16]. Figure 2 may be interpreted as a vertical section of the embrittle temperature+mbrittle time graphs used frequently in embrittlement studies when the degree of fracture is considered the third dimension [17]. The impurity segregation on free surfaces was studied analogically as a function of 2 h annealing in the temperature range 4%800°C. The segregation values of P and S are plotted as a function of temperature in Fig. 3. The phosphorus and sulfur concentrations C,, C,, given in at.%, are calculated in a first approximation using the method of sensitivity factors [lS]. This method gives the atomic surface concentration of the elements determined from the line intensities of the Auger spectra. The calculated concentrations are, of course, averaged values over a layer thickness of about 2nm, corresponding to the escape depth of the Auger electrons. In segregation studies we expect a strong inhomogeneity in this surface film with enrichment of the segregant in the top atomic layers (see Section 3.2). It is seen from Fig. 3

-s i

500 Fig. 3. Free surface

3.1 Fracture and segregation characteristics

600

700

800

segregation of P and S impurities after 2 h annealing at different After each experiment the surface was sputter cleaned.

temperatures,

570

LDSCH:

TEMPER

EMBRITTLEMENT

AND

SURFACE

that P and S have two different temperature ranges where preferenfial segregation is observed [19]. The onset of sulfur segregation is 650°C.

s =0.5 Ni

t

It has been shown that the main impurity component found at the brittle fracture surface was phosphorus with a little sulfur [IZ]. This result is in qualitative agreement with the free surface segregation composition after the same heat treatment (650°C) as seen from Fig. 3. By application of argon ion sputtering it could be shown that the P content of the brittle fracture surface is concentrated in a very thin film of about 0.551.0nm thickness at the grain boundary. The decrease of the AES phosphorus line with distance from the boundary is seen in Fig. 4. It is in good agreement with the results of other investigations [17]. Consequently, in order to study chemical effects in this surface film, one has to use means which give information of a depth of _ 0.5 nm only. The escape depth of electrons depends strongly on their energy [lo]. The three metallic components of the alloy in this study have Auger transitions in the low energy (- 50 eV) and in the high energy (>2OOeV) range. High energy electrons have an escape depth of 1.5-2.5 nm, thus giving information of depths which belong to the grain rather than to the boundary film. The low energy transitions, however, show two advantages. First, the electrons have an escape depth of s 0.5 nm, thus detecting exclusively the boundary film. Second, they are all of the M2,sVV type, i.e. valence band transitions are involved which are sensitive to chemical reactions. We have therefore analysed these Auger lines on brittle and ductile fracture and on free surfaces. The results are seen in Fig. 5. No remarkable shape and position change with heat treatment was observed in the Cr and Fe lines. They are therefore represented for most conditions

dtnml -

Fig. 4. Auger line intensity variation of segregated phosphorus as function of material removed from grain boundary fracture surface by argon ion sputtering.

b)

dEN(E)

1

3.2 AES fine structure

SEGREGATION

Fe

c)

dE

k)

30

40

50

60

70

Fig. 5. Auger fine structure of the Cr. Fe. Ni-MI,3VV transitions. (a) Typical Cr and Fe lines from free and fracture surfaces; (b) ductile fracture surface; (c) free surface sputter cleaned; (d) brittle fracture surface. 65o’C: (e) free surfaces: 6OO”C, 2 h; (f) 65O”C, 2 h; (g) 700°C. 2 h; (h) 75O”C, 2 h; (i) free surface 650°C. 2 h + little oxygen; (k) free surface 750°C. 2 h.

in Fig. 5a. The Ni transition from clean surfaces appears as a doublet with peaks at 57 and 61 eV. An identical shape of this doublet is observed from ductile fracture and from sputter cleaned surfaces (Figs. 5b and 5~). The origin of this doublet, due to Cu segregation (Cu is present in our specimen, see Section 2), can be excluded. Cu shows Auger MVV lines at low energies: 58 and 60eV and high energy KLL lines at 776, 849 and 920eV, where the intensity of the KLL lines is much stronger [lo]. The high energy transitions of Cu have never been observed in our studies. Furthermore, the low energy Cu lines are separated by AE = 2eV, whereas the observed energy difference was AE = 4eV. The fine structure of a brittle fracture surface is seen in Fig. 5d. It appears as a triplet with energy peaks at 57, 58 and 61 eV. Free surface fine structure is shown in Figs. 5e, f, g and h after 2 h annealing at 600, 650, 700 and 750°C respectively. It seems evident that strong P segregation (600-700°C) creates the peak at 58 eV. This line disappears at higher temperature (T > 700°C). As discussed earlier [12] the Ni triplet from the brittle fracture surface can be attributed to the existence of oxygen. This is shown in Fig. 5i. A free surface Ni fine structure is seen after treatment of 2 h at 650°C plus a little oxygen adsorption of the same Auger line intensity as observed at the brittle fracture surface. The structures from Fig. 5d and 5i are identical. When high temperature annealing (T > 700°C) was applied to the free surface the Cr line changed to that shown in Fig. 5k. A shoulder appeared at about 32.5 eV. All the discussed fine structures depend also on the time of annealing, which indicates time-dependent chemical processes. But this was not studied systematically so no result will be presented here.

LOSCH:

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those of the 3s and 3p orbitals of the metallic components Fe, Cr and Ni and the 2p levels of the impurities P and S. The results are shown in Fig. 7. Clean surface values have been determined from sputtered free surfaces and from a ductile fracture surface. The agreement is good. Looking for the energy variations as a function of annealing temperature (each for 2 h) one notes that nearly all levels show a shift to higher bonding energies. With a maximum shift at that temperature, namely 65O”C, which also resulted in a maximum intergranular fracture. Related to the three metals the biggest shift is observed in the Ni-3p level, with AE = + 1.2 eV. The Cr-3p level is the only one which shifts to lower bonding energies with a maximum of BE = - 1.2 eV at 750°C. The 2p levels of P and S have been measured twice at 650 and 750°C and show an average of about E,(2p) = 164.4 and E,(2p) = 131.1 eV. 1.

100

I

110

I

s

120

I1

.

I30

I

140

.

I

150

.

I

160

Fig. 6. Auger fine structure of the L1.3M2.3M2.3 transitions of P and S from free surfaces. (a) With excess P segregation; (b) and (c) after different argon ion sputter application; (d) sulfur structure without sputtering. The fine structure observed on brittle fracture surfaces corresponds to that of (b) [12].

The Auger lines of P and S also present fine structure as shown from free surfaces and a brittle fracture surface in Fig. 6. When strong P segregation occurs on free surfaces the P line appears as a single transition (Fig. 6a). Sputter application removes some P and a doublet with a new peak appears, moved by about 3eV to lower energies from the main peak (Fig. 6b and c). The P fine structure appears relatively stronger the more P is removed (Fig. 6a-c). This indicates a chemical reaction of the lower layers only of the multilayer segregation P-film. The same P fine structure is observed at a brittle fracture surface [12]. Sulfur fine structure observed after 750°C annealing at a free surface, is shown in Fig. 6d. No attempt was made to study this on the small S line from a brittle fracture surface.

4. DISCUSSION 4.1 Grain boundary

and frrr

surface segregation

The basis for the interpretation of the present results is the assumption that chemical reactions at the grain boundaries, due to heat treatment and impurity segregation, can be studied on free surfaces after the same heat treatment. This assumption is crucial for the present work, but there is experimental

3.3 ILS rneasurenwnts From the previous section we have two examples of identical Auger structure from different surfaces. One is from ductile fracture and sputter-cleaned free surfaces and the other is from brittle fracture and free surfaces after the same heat treatment. Assuming that the same Auger fine structure represents the same chemical state, we analysed by means of ILS the variations of electronic levels as a function of applied heat treatment on free surfaces in order to penetrate deeper inside the structure. Assuming further that surface chemical states can generally be considered to reflect the grain boundary chemical states after the same treatment, we may then come to a conclusion about chemical effects on the grain boundaries from free surface results. The investigated energies are

Fig. 7. ILS values of the 3s and 3p bonding energies of Cr, Fe and Ni from free surfaces as a function of 2 h annealing at different temperatures (P and S segregation); and 3s. 3p bonding energies from a clean ductile fracture surface and sputter cleaned free surfaces.

572

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support for it in two cases where comparison was possible (see Section 3.2). Furthermore, the activation energy for certain chemical reactions between impurities I and metallic components M should be the same for grain boundaries and free surfaces, i.e. when a certain composition of I and M is obtained at the interfaces the reaction will occur at the same temperature. We suppose that an excess of impurities I-which is, of course, observed on free surfaces-will not modify this model. The excess impurities will not participate in the reaction. This assumption is supported by Fig 6a-c which shows two chemical states of the impurity P. The segregated phosphorus participates on a chemical reaction only with those atoms which are in the lower layers, i.e. which are in contact with the metallic substrate. Following this hypothesis we notice evidence that the embrittling process is not only a dynamic process of impurity segregation but also a consequence of chemical reactions at the interface. This means embrittlement is a complex process which needs activation of impurity segregation and activation of chemical reactions, both depending on time and temperature. This dependence is best presented in the above-mentioned embrittling temperatureeembrittling time graphs[llJ which may be described by functions F = J”(T t). In the present work a partial function, F, = S(T, t,), tt = 2 h = const., is studied. This may be seen from Figs. 2 and 7, where a maxims of _ 10% intergranuIar fracture is obtained for 650°C and a maximum of chemical shifts is observed for the same temperature. This means that for a 2 h treatment at 650°C a compound is formed at the boundaries which shows a minimum of cohesion in relation to other heat treatments of 2 h at T # 650°C with compounds of stronger cohesion, thus not permitting a complete intergranular fracture. 4.2 Ckemicaf reactions Little can be said about the composition of this boundary compound at the present time. Shifts of bonding energies, as observed by our ILS experiments and shown in Fig. 7, are generally caused by charge transfer between neighbored atoms, and shifts to stronger bondings means electron loss [20]. This is valid for core levels, a concept which can hardly be applied in all the present results. However, the shifts observed in the Ni (3~) Cr (3s) and Fe (3s) levels seem to indicate such an electron transfer from the metals M to the impurities I. This means that at 650°C all three components (Ni, Cr, Fe) show interaction with P, the strongest probably existing between Ni and P. In the light of thermodynamical calculations this is not a surprising result. As shown by Guttmann [21] in the theory of chemically induced segregation and illustrated by Seah [22] there are strong interactions, especially between Cr, Ni and P, in an Fe base ternary solution. These preferential reactions, in Guttmann’s theory determined by the enthalpies AGi, depend on temperature [21]. In our case this means that at T = 650°C the reaction Ni-P is stron-

AND SURFACE

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gest. As all three metals (Fe, Cr, Ni) are quite reactive elements there is no reason to expect one interaction only for a specific element M. We may compare further our ILS measured binding energy of phosphorus Er(2p) = 131.1 eV with photospectroscopic results of similar compounds. Peiavin et al. [24] have measured E, = 128 eV and E, = 129.3 eV for the binary compounds CrP and MnP respectively. This seems to indicate increasing E, values with the atomic number z of the 3d transition elements. We may thus expect a slightly larger E, for compounds involving Fe and Ni, in agreement with our experimental value. 4.3 Interpretation

of Auger /ine structure

Looking now to the observed Auger fine structure we will try a first interpretation of this data in order to obtain further information. On the basis of the recently discovered interatomic Auger transitions 1,231 most of the peaks may be identified using possible valence band cross-transitions between the components M and I of the alloy. For this object we will use published results of band structures of the involved elements which represent the electronic density of states [24]. The metallic elements Cr, Fe and Ni have d-band structures, published by Eastman [25], Broden et al. [26] and Nguyen [27]. The maxima of the density of states are located at about 2.3, 0.95 and 0.3 eV below the Fermi level for Cr, Fe and Ni respectively. For S adsorbed on Fe a resonance was observed at E(M2. a) = 4.2 eV [26]. Nothing is known about P(3p) levels adsorbed on Fe. We therefore determine the P(3p) binding energy from our AES and ILS measurements according to E(M,,,) = &%,,) - EAES) = 3.3 eV. By appropriate combinations between the energetic levels M1, ,(3p) and Mq, ,(3d) (= d-band) of the metals and of the impurities L,,,(2p) and Mz,,(3p) we can calculate energies which satisfy the ex~rimentally observed fine structure. Important are the Ni (57 and 58 eV), Cr (32.5 eV) and P (116.7 eV) lines which are not observed in the corresponding pure elements. As binding energies are related to the Fermi level we have to add the workfunction 4” of the analyser to the observed values. Assuming 4A = 5eV we obtain the following results: the Ni (57 + 5 eV) = Ni (62eV) peak (without phosphorus) can be explained by a Ni (M& Cr (M4, s, M1,5) = 67.8 - 4.6 = 63.2 eV cross-transition. For the Ni (58 + 5 eV) = Ni (63 eV) peak (with phosphorus) good agreement is obtained by a Ni (M,,,) P (M2,J, M,*,) = 69 - 6.6 = 62.4 eV interatomic transition. A transition Cr (M2,J P (Mz,3, Mzs3) = 44.3 - 6.6 = 37.7 eV satisfies well the observed Cr (32.5 + 5 eV) = Cr (37.5 eV) peak. Finally the P (116.7 + 5 eV) = P (121.7 eV) Iine may be combined from a P (L&, Fe (I&, 5, M4,s) = 131.0 - 8.6 = 122.4 eV or from a P (L& S (MU, M2,J = 131.0 - 8.4 = 122.6eV transition.

LOSCH:

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We have been quite successful in our interpretations by utilizing known band structures, but the method has to be considered with care. Nothing is known about cross-transition probabilities and finalstate perturbations. We further assumed that d-band structure is not affected by alloying. Probably Auger spectra cannot be necessarily related to ground-state density of states but rather to a density of final states [28]. However, as the conclusions are in agreement with the normal Auger observations and the ILS measurements, our interpretation was justified. Furthermore, in interatomic transitions the argument of a two-hole perturbation [28] is not valid, as in a single Auger cross-transition two different atoms are involved. We thus believe that observation of interatomic cross-transitions may become a powerful concept in metallurgical studies. Although these results are not satisfactory and more research is necessary, the concept of interatomic transition offers a big advantage for metallurgical purposes: it indicates next neighbors of the probe atom, i.e. interatomic transitions occur between next neighbors and their identification may thus help to determine chemical interaction. We can therefore conclude that as well as ILS, AES fine structure experiments give evidence for a strong NipP interaction after 650°C treatment, which probably changes to a more pronounced Cr-P interaction after 750°C heating. Further. if the above-mentioned P-S interaction is right, we notice after a 2-h embrittlement at the boundaries

an

increased

number

of atomic

interac-

M-l and I-I (M = metal atom. 1 = impurity). In the case of M-1 as well as in the case of I-I interactions (as in P-S bondings). a reduction of cohesion is to be expected. Qualitative considerations based on electronic orbital overlapping, which is a measure for the bond strength [9], show that overlapping is reduced in the case of an M-I interaction. An example of M-I orbital overlapping is shown in Fig. 8 for the 3d and 3p orbitals in the z y-plane. The ILS measurements show shifts, and thus chemical reactions, for all three metallic components tions

of the

type

VALENCE

ORBITALS

3d

TRANSITION

METAL

t’

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with a maximum for the Ni-P reaction in agreement with the AES fine structure interpretations. The compound, formed at the boundaries after embrittlement. therefore seems to be of the type (Fe,Cr)Ni-P. At elevated temperature (T > 700°C) a Cr-P interaction is observed [Ni (58eV) disappears and Cr (32.5) appears]. After this treatment the compound seems to be of the form (Fe,Ni)Cr-P. This means that although there is a large amount of sulfur segregated at this temperature the phosphorus reaction seems to be stronger. More quantitative and structural measurements of the grain boundary films are necessary in order to estimate the origins of reduction of cohesion. We undertook this work as a contribution towards a more detailed investigation of embrittlement based on physical-chemical considerations. REFERENCES Proc. Inst. Autornh. Enyny II. 307 I. H. Brearly, (1916-17). 2. J. H. Hollomon. Trans. A.S.M. 36, 473 (1946). 3. C. J. McMahon Jr., Mawr. Sci. Engny 25. 233 (1976). 4. L. A. Harris, J. appl. Phys. 39, 1419 (1968). 5. P. W. Palmberg and H. L. Marcus. Trans. At?,. Sot. Sec. Metall. 62, 776 (1969). 6. D. F. Stein, A. Joshi and R. P. Laforce. Truns. Am. sot. Metall. 62, 776 (1969). I. M. P. Seah, Proc. R. Sot. (A)349, 535 (1976). Jr., Metall. 8. H. Ohtani, H. C. Feng and J. C. McMahon Trans. 7A, 1123 (1976). 9. H. B. Gray. Electrons and Chemicul Bond. Benjamen. New York (1964). 10. G. Err1 and J. Kippers. Low Energy Elecfrons and Surface Chemistry. Verlag Chemie, Weinheim (1974). 11. J. P. Coad, J. C. Riviere, M. Guttmann and P. R. Krahe, Acta metaIl. 25, 161 (1977). 12. W. Losch and I. Andreoni. Scripra mrrall. 12, 277 (1978). 13. R. Weissmann. R. Koschatzky. W. Schnellhammer and K. Miiller, Appl. Phys. 13, 43 (1977). 14. M. H. Frommer, Sur$ Sci. 64, 251 (1977). 15. C. Lea and M. P. Seah, Scripra tnrtull. 9, 583 (1975). 16. I. Andreoni and W. Losch. XXX111 Congress0 Anual da AssociaCBo Brasileira de Metais, Rio de Janeiro, July (1978). 17 M. P. Seah. Sur$ Sci. 53, 168 (1975). 18 P. W. Palmberg, J. uac. sci. T&no/. 13, 214 (1976). 19 J. Rousselet and W. Losch, XXX111 Congress0 Anual

da Associa@o Brasileira de Metais. Rio de Janeiro. July (1978). 20. S. H. Hercules and D. M. Hercules, in Characterization of Solid Surfaces (edited by Kane and Larrabee). Plenum Press, New York (1974). 21 M. Guttmann, Surf. Sci. 53, 213 (1975); Metal Sci, 337 Ott (1976); Metal/. Trans. 8A, 1383 (1977). 22. M. P. Seah, Acta metall. 25, 345 (1977). 23. M. Salmerbn, A. M. A. M. Bar6 and J. M. Roj6, Phys. Reu. B13, 4348 (1976). 24. M. Pelavin, D. Hendrickson, J. Hollander and W. Jolly, J. phys. Chem. 74, 1116 (1970). 25. D. E. Eastman, in Hectron Specrroscopy (edited by D. A. Shirley). North Holland Amsterdam (1972). 26. G. Broden, G. Gafner and H. P. Bonzel. Appl. Phys. 13. 333 (1977). 2-l. T. T. Anh Nguyen and R. C. Cinti. SurJ Sci. 68, 566

z t

AND

3P

IMPURITY

Fig. 8. Qualitative overlapping of the valence orbitals case of a metal-impurity (M-l) mteractlon.

in

(1977). 28. C. J. Powell.

Phys. Rev. Lrrf. 30, 1179 (1973).