EXPERIMENTAL EVIDENCE OF SULFUR EFFECT ON THE PLANE AND ON THE EXTRINSIC DISLOCATIONS OF A Σ = 3 GRAIN BOUNDARY IN NICKEL

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1 EXPERIMENTAL EVIDENCE OF SULFUR EFFECT ON THE PLANE AND ON THE EXTRINSIC DISLOCATIONS OF A Σ = 3 GRAIN BOUNDARY IN NICKEL D. Bouchet, B. Aufray, L. Priester To cite this version: D. Bouchet, B. Aufray, L. Priester. EXPERIMENTAL EVIDENCE OF SULFUR EFFECT ON THE PLANE AND ON THE EXTRINSIC DISLOCATIONS OF A Σ = 3 GRAIN BOUNDARY IN NICKEL. Journal de Physique Colloques, 1988, 49 (C5), pp.c5-417-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1988 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C5, supplément au n"10. Tome 49, octobre 1988 C5-*17 EXPERIMENTAL EVIDENCE OF SULFUR EFFECT ON THE PLANE AND ON THE EXTRINSIC DISLOCATIONS OF A Z = 3 GRAIN BOUNDARY IN NICKEL D. BOUCHET, B. AUFRAY* and L. PRIESTER Laboratoire de Métallurgie Structurale, CNRS-UR 1107, Université Paris-Sud, F Orsay Cedex, France 'Laboratoire de Métallurgie, CNRS-UR 443, Université Saint-Jérôme, F Marseilles Cedex 4, France Résumé - Les évolutions de la dêsorientation, du plan du joint et des caractéristiques des dislocations extrinsèques d'un même joint de grains S. = 3 de nickel sont analysées en fonction du niveau de ségrégation du soufre dans ce joint. Des teneurs variables en soufre sont obtenues par sulfuration et désulfuration d'une même lame mince de nickel. Abstract - The evolutions of the misorientation, of the grain boundary plane and of the extrinsic dislocations ( EGBD's ) characteristics of the same «" =3 grain boundary in nickel are analyzed in function of the level of sulfur segregation in this boundary. The variation of the sulfur content is obtained by sulfidation and desulfidation of the same nickel thin foil. I - INTRODUCTION It is now widely recognized that the grain boundary chemistry, more than the grain boundary crystallography, may control most intergranular properties. For a long time, indirect experiments have been done to reveal the influence of segregation on the grain boundary behavior. More recently, fundamental studies using computational methods have been developed to understand the influence of segregation on the grain boundary structure. But until now,few attempts were made to experimentally investigate the correlation between segregation and grain boundary geometrical parameters. The main purpose of this work is to analyze the effects of variation of the sulfur content on some characteristics of a same Z. =3 grain boundary in nickel. The modifications of the [Disorientation, of the grain boundary plane and of the EGBD's lines and Burgers vectors are determined. II - PRELIMINARY INVESTIGATION The study of the influence of sulfur on grain boundaries is made, by an usual. practice, on thini foils with different sulfur contents ( 0 ppm S, 8 ppm S,16 ppm. S) which have undergone three different thermal treatments. Table I shows i - the average segregation levels of the grain boundaries in the specimens [ij - the segregation levels of the studied 2=3 grain boundaries estimated by mean of the microattack technique fl,2j The aspects of the three = 3 grain boundaries containing increasing sulfur segregation are shown on figure 1. The crystallographical analysis by Transmission Electron Microscopy ( TEM ) of the grain boundary and dislocations characteristics (table I ) points out that : - the mean relative interplanar spacing (d/a) of the grain boundary plane,which is proportional to its atomic density,increases as the segregation level decreases, in agreement with our previous results [2,3]. Article published online by EDP Sciences and available at

3 C JOURNAL DE PHYSIQUE - EGBD's are more numerous if the segregation level is higher.simultaneously, they are more stable i.e; incorporated matrix dislocations preserve their Burgers vectors. We must notice that the deviation to the exact coi'ncidence misorientation A%is not a function of 1:he segregation level. As a matter of fact, we have observed a z=3 Af3.3?'grain boundary without segregation in the pure nickel thin foil( 800" 3 h 1. 'I Specimens Pure nickel 800 C - 3 h quenching TABLE I : Sulfur segregation levels and crystallographical characteristics of three = 3 grain boundaries in nickel with increasing sulfur contents ( the Burgers vectors are determined by the MARUKAWA met hod [4] 1. Average Sulfur segregation level in grain boundaries No Segregation level in the studied Z=3 boundaries No L\B 1 O dm a EGBD ' s b~~~ Ni t 8ppm S 625OC- 3 days quenching Important Low 2 " b~~~ bmatri x Ni t 16 ppm S 625 C- 7 days Important quenching Medi um 9" bmatrix.in order to confirm and to precise the previous effects of sul.fur on the grain boundaries in nickel, it should be necessary to proceed on the same 213 grain boundary. This is the purpose of the following step of the study. I11 - EVOLUTION OF THE SAME = 3 GRAIN BOUNDARY WITH VARIATIONS OF SULFUR CONTENT I EXPERIMENTAL PROCEDURE The originality of our experimantal procedure consists in the fact that TEM observations and thermal treatments are performed on the same thin foil. More precisely, it takes place as follows : - First, the characteristics of a selected Z =3 grain boundary in a nickel thin foil and those of its extrinsic dislocations are analyzed by TEM. - Then,the foil is -annealed under hydrogen flow at 500 C during 10 hours in order to take out sulfur. Next, the previous =3 grain boundary is analyzed again - After that,! j~lfur introduction in the thin foil is performed by identical thermal treatment ( 500 C - 10 h ) under a H / H S atmosphere. - Finally, the possible modifications of thz gra;fn boundary parameters and of its EGBD's are investigated again. The elementary sulfur is brought from the vapour phase into thin foil by a diffusion process. A H / H2S mixture, the composition of which is known and remains constant, circulates %bove the thin foil during annealing. By this process, the sulfur chemical potential in the vapour phase can be selected so that no 3D- nickel sulfide may form in the thin foil. The potential range is determined from the stability of Ni3 S2 sulfide. Although the sulfur solubility in nickel at 500 C is not known, we

4 Figure 1 : Transmission El.ddtron micrographs of Z = 3 grain boundaries of nickel with different sulfur segregation levels ( see table I 1 a 1 no segregation : all dislocations are dissociated b 1 low segregation : few dislocations are dissociated c 1 medium segregation : no dislocation is dissociated

5 C5-420 JOURNAL DE PHYSIQUE can deduce the solubility between 1 and 3 ppm, from high temperature values 153. biore detai 1s dbout this suffidation process are given in referencesc61 andl RESULTS The modifications of the = 3 grain boundary features after sulfur has been taken out then after sulfidation are shown on figure 2. The equilibrium parameters ( misorientation and plane ) and the defects characteristics of this grain boundary for each segregation state are reported on table I1. Finally, a stereographic plot of the grain boundary plane and dislocations lines related to the three steps of the thermal treatment is given on figure 3. TABLE I1 : Evolution of the misorientation, the intetplanar spacing and the Burgers vectors of EGBD's in a same = 3 grain boundary of nickel in function of its sulfur content. A thorough analysis of the results allows us to distinguish between the temperature effects and the influence of the sulfur content : (il Influence of temperature - The graici boundary misorientation evoluates towards the exact co'incidence one - The grain boundary plane becomes more vertical.. moves towards its macroscopic equilibrium position ( Fig. 3) - The EGBD's density decreases ( ii I Influence of desulfidation - The atomic planar density of the grain boundary increases - Some EGBD's are dissociated ( iii I Influence of suzfidation - The atomic planar density decreasles again t Q reach a very low value (d/a), = 0,105 - None of the EGBD's are dissociated Note that,in case of sulfur segregation, the different dislocation sets display the same Burgers vector 1/2 [llok when, without sulfur segregation, different Burgers vectors ( matrix and DSC ) ate involved ( Table I1, Fig. 3.

6 $Figure 2 : Transmission Electron micrographs of a same E= 3 grain boundary of nickel with different seareaation - - levels ( see table I1 : a ) Id. 1 c b ) after desulfidation : some dislocations are dissociated c aft& desulfidation and sulfidation : begbdls = bmatrix

7 JOURNAL DE PHYSIQUE Figure 3 : Stereographic plot showing the ebolution of grain boundary plane and EGBD ' s of a = 3 grain boun- dary as a function of S content for the three steps ( A,B,C ) of the thermal treatment. -.. G.&.plane ( trace 1 : Dislocation line + : Burgers vector ---- : COO] poles tri hedron of grain I. Our results clearly reveal that srlfur may influence both the equilibrium and non equilibrium characteristics of the = 3 grain boundary in nickel : - The planar atomic density decreases with increasing sulfur content - The EGBD's dissociation is impeded by sulfur segregation. The sulfur effect on grali~n boundary plane involves a grain boundary structural transformation and indirectly su ports the recent observation of a change in dislocation structure of small angle bg twist boundaries in iron induced by gold segregation f8]. The sulfur effect on the EGBD's stabilization suggests that sulfur atoms fill the substitutional grain boundary sites ; this seems a necessary condition to hinder the accommodation processes, among them the dissociation. of EGBD's in grain boundaries 93. IV - CONCLUSION_ Using an original experimental procedure, we have pointed out that : - Changes in grain boundary misorientation, grain boundary plane inclination and EGBD's density are thermally activated. - Sulfur segregation influences the atomic grain boundary plane density and the stability of extrinsic dislocations. REFERENCES 1 - L. BEAUNIER, D.BOUCHET, C. COLLIEX, P.TREBBIA, C. VIGNAUD J. de PHYsique C4-46 ( D. BOUCHET, L. PRIESTER S6ripta Met. 20 ( D. BOUCHET, L. PRIESTER Scripta Met. 21 ( K. MARUKAWA, Y. MAXSUBARA Trans J. I.M. 20 ( N. BARBOUTH J. OUDAR C.R. Acad. Sci. paris t.269 ( 1969 ) 6 - B. AUFRAY, i. CABANE-BROUTY, J. CABANE Acta Metal ( A. CHARAI,, A. ROLLAND, B. AUFRAY these proceedin s 8 - K.E. SICKAFUS, S.L.SASS Acta Metall. 35 ( 1987 ) %9 9 - S. LARTIGUE, L. PRIESTER Acts Metal1. 31 ( 1983 ) 1809