GRAIN BOUNDARY ANALYSIS IN Ni-C BY MEANS OF ATOM-PROBE FIELD-ION MICROSCOPY

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1 GRAIN BOUNDARY ANALYSIS IN Ni-C BY MEANS OF ATOM-PROBE FIELD-ION MICROSCOPY L. Alvensleben To cite this version: L. Alvensleben. GRAIN BOUNDARY ANALYSIS IN Ni-C BY MEANS OF ATOM-PROBE FIELD-ION MICROSCOPY. Journal de Physique Colloques, 1988, 49 (C6), pp.c6-335-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 C6, suppl6ment au noll, Tome 49, novembre 1988 GRAIN BOUNDARY ANALYSIS IN Ni-C BY MEANS OF ATOM-PROBE FIELD-ION MICROSCOPY L.v. ALVENSLEBEN Institut fiir Metallphysik, Universitdt Gbttingen, Hospitalstrasse 3-5, Gbttingen and Sonderforschungsbereich 126, Gbttingen/Clausthal, F.R.G. Abstract - Grain boundary analysis with the help of FIM/AP needs an aimed tip-preparation technique unless the grain size of the material is very small. Therefore a dedicated FIM-tip holder for the TEM (Philips EM 400) was designed to analyze and optimize the distance from grain boundary to tip apex and to determine the orientation of the grain boundary with the tip. In a Ni-lat% C alloy, the carbon atoms image "brighter" than the surrounding Ni-atoms, as shown by a drop of the detector current. A segregation of carbon at grain boundaries was recorded with the atomprobe. I - INTRODUCTION The chemical composition of interfaces influences macroscopic characteristics such as mechanical properties of all materials in very important ways. In metallic alloys, the brittleness and the dynamics of recrystallization are often effected by a segregation of nonmetallic impurities at grain boundaries. The aim of this work is to improve the feasibility of field-ion microscopy (FIM in combination with a time of fli ht mass spectrometer, called atom probe (AP), to investigate grain boun d ary segregation of carbon in nick3. I1 - SPECIMEN PREPARATION Nickel wires (99.99 at% Ni, diameter 0.5 mm) have been carburized in a gas flow furnace in a C? H2 atmosphere at 1500 K. By resistivity measurements at 1500 K, the integral carbon content was deterrmned to be 1.15 at% C. During the following furnace cooling a segregation of carbon atoms to grain boundaries was expected. A detailed description of this kind of apparatus is given by Grabke /I/. First experiments have been performed with this material, where equilibrium segregation was expected. Subsequent wire drawing was performed (diameter 0.1 mm) and a second recrystallization procedure was performed (1173 K, 2 hrs; 623 K, 1 hr; quenched in water). Normal electropolishing procedures have been applied to produce suitable tips for field-ion microscopy (10% perchloric acid in ethylene glycol monobutyl ether, 20 VD~). I11 - LOCALIZATION OF THE GRAIN BOUNDARY Unless the grain size is so small that the analysis of grain boundaries is possible by random FIM/AP experiments, i.e., a grain size smaller than 150 to 200 nm, an aimed tip preparation technique must be performed. In our material the average grain size is on the order of several microns, so we had to apply other methods to localize the grain boundary "far" away from the apex of the tip. Therefore, we modified a double tilt holder of a transmission electron microscope (Phillips EM 400) in such a way that it is possible to fix a FIM tip holder reproducibly and easily. The right part of the outer shell of the double tilt holder is completely detachable (fig. la). The double tilt mechanism has been replaced by a rip unit (fig. lb) which can hold either a tip holder for ribbons or other shaped materials (fig. lc, CU-BL$ or copper tubes into which the specimen is squeezed (fig. Id, outer diameter 1.5 mm, inner diameter 0.2 mm). Additional to the normal x movement possibilities of the goniometer of the TEM (about 3 rnm the transfer mechanism for the double tilt unit provides a displacement of about 5 mm which makes it possi b le to adjust "all" lengths of tips quickly and easy. In the low magnification (lm) mode of the TEM (magnification x), the objective lens is fed with a low fixed current to image the crossover of the electrons in the plane of the objective aperture. The diffraction lens itself functions as an objective lens to focus the image. For our specimen, only a sharp shadow image of the tip is obtained. In the lm mode of the TEM the current of the objective lenses is too small to change the direction of the magnetic field of the specimen. In the region where no shape anisotropy is expected, the dominant crystal anisotropy determines the direction of the magnetic field. Due to the change of the so called "soft" directions (for Ni [Ill]) at grain boundaries the orientation of the magnetic field is also changing. The electrons of the beam of the TEM are interacting with the magnetic field of the specimen. This can be seen in heavy under or overfocus, which leads to violent distortions of the shadow image of the specimen. Because of these distortions, grain boundaries far away from the apex of the tip can easily be made visible (fig. 2). Article published online by EDP Sciences and available at

3 C6-336 JOURNAL DE PHYSIQUE Fig. 1 - Dedicated TEM double tilt holder for FIM tips. The right part of the outer shell of the double tilt holder is completely detachable (a). The double tilt mechanism has been replaced b a grip unit (b) which can hold either a tip holder for ribbons or other shaped materials &) or copper tubes into which the apecimen is squeezed (d). Fig. 2 - Visibility of a grain boundary in the low magnification mode of the TEM. ( a = underfocus IDiffr=0.8 A, b = focus IDiffr=1.08.A, c = overfocus IDiff,=1.28 A). Selective grain boundary etching leads to a local roughness in the surface ( d). Depending on the type of the grain boundary, selective etching may occur, which is another means of identifying grain boundaries in regions which are not transparent to the electron beam (fig. 2 d). In the diffraction mode, Kikuchi lines are visible up to a thickness of the specimen of about 500 nm. The advantage of the wires, we are normally using in our experiments, is that a Kikuchi diagram is almost always available, because the thickness at the edge of the specimen is small enough to produce these lines; one only has to choose a small enough spot size with a fully focused beam. An instantaneous change of the Kikuchi line diagram, which can even be seen at low intensities, is always related to the occurrence of a boundary. The Kikuchi diagrams in front of and behind a grain boundary are shown in fig. 3. Analyzing these dia rams carefully one can very precisely determine the orientation of the two grains, i.e., the tilt axis (<310>7 and the tilt angle (30 "). This information is important for the interpretation of the segregation behaviour, which heavily depends on the relation between the two grains, i.e., what kind of grain boundary it is. Calculating the 24 equivalent relationships of the 30 " <310> orientation we choose the " <loo> as

4 Fig. 3 - Kikuchi diagrams of the grain far from the apex (a) and close to the apex (b). description with the lowest indices of the tilt axis /2 d. Comparing this orientation relationship with those of the coincidence site orientations of Warrington an Bufalini /3/ one can identify the O <loo> as a C 17 ( O <loo>) coincidence grain boundary. If the specimen is thin enough that normal electron microscopical methods, such as bright or dark field imaging, can be applied, a complete characterization of the position of the grain boundary compared to the tip axis is possible. Fig. 4a shows a micrograph of a grain boundary nearly perpendicular to the tip axis, whereas in fig. 4b an inclined faceted grain boundary is visible. Fig. 4 - TEM micrographs of a nearly perpendicular (a) and of an inclined faceted (b) grain boundary.

5 C6-338 JOURNAL DE PHYSIQUE I1 - BACKPOLISHING PROCEDURE After the distance between grain boundary and apex has been determined, a controlled backpolishing procedure has to be performed to make the grain boundary accessible for FIM/AP analysis. With the help of a pulse generator which supplies rectangular pulses of variable width and frequency, well determined amount of material can be electropolished, until the distance between grain boundary and apex is smaller -than 100 nm. Intermediate recording of the distance with the TEM is indispensable. Fig. 5 shows such a polishing sequence. A more detailed description of this procedure is given by Karlsson /4/. Fig. 5 - Sequence of a controlled backpolishing procedure to make the grain boundary accessible for FIM/AP experiments. (20 VDC; a -+ b: 10 pulses of 20 rnsec; b + c: 10 pulses of 20 msec; c + d: 3 pulses of 20 msec; d + e: 1 pulse of 20 msec. IV - FIELD-ION MICROSCOPY AND ATOM PROBING FIM and AP have been carried out after a controlled backpolishing procedure has been performed. Fig. 6 shows a typical field ion micrograph. The grain boundary is perpendicular to the tip axis. The more "brightly" imaging atoms have been identified with the AP to be single carbon atoms. Therefore we manipulated the tip in such a way that a "bright" atom was lying under the projection of the probehole; the ion current at the detector was relatively high, which can either be recorded by an electrometer or by simply observing the image at the detector. An AP experiment was performed until a drop of the detector curi~nt occurred. When an atom was recorded in coincidence with the drop of the detector current, it was assumed that this atom was the "bright" atom. A final check wether the atom really had been field evaporated was also carried out. For 88 "bright" atoms, 41 coincidental atoms have been recorded, 32 of them have been identified as carbon atoms. The high amount of non-coincidental events is due to the limited detection eeciency of the channel plates, which is about 60%. Fig. 7 shows two continuously recorded concentration profiles across a grain boundary. The asymmetric shape of the carbon concentration profile in fig. 7a may be due to a diffusion induced grain boundary motion (DIGM), which was also found in this alloy by Parthasarathy and Shewmon /5/. The profile in fig 7b is probably related to a stable grain boundary where equilibrium segregation of carbon occurred. In both cases the orientation of the grain boundary was not perpendicular to the tip axis; this means that the width of the grain boundary has to be corrected by the inclination of the interface. For the calculation of the carbon content no doubly-charged carbon dimers were assumed.

6 Fig. 6 - Field ion micrograph of a Ni-1 at% C alloy with a grain boundary darkring) nearly PerPf tndicular to the tip-axis. The "bright" atoms have been identifie iras carbon atoms throu ~gh a drop in the detector current (13 kv, mbar Ne, 80 K). 40 Grainboundary Number of Atomic Layers Number of Atomic Layers Fig. 7 - Carbon concentration profiles across a grain boundary. To be noted is the asymmetric shape (fig. 7a) which may be due to a diffusion induced grain boundary movement prior to the analysis. Fig. 7b shows a symmetric profile which could be related to a stable grain boundary. It is to be noted that the direction of analysis is not perpendicular to the grain boundary.

7 JOURNAL DE PHYSIQUE V - CONCLUSIONS Various transmission electron microscopic methods have been described to localize grain boundaries in FIMtips near the apex of a Ni-lat% C,alloy. With the help of a controlled backpolishing technique of the tip, grain boundary analysis with field-ion microscopy in combination with atom probing is possible. Segregation of carbon to the grain boundaries has been shown. ACKNOWLEDGEMENTS I would like to thank Prof. P. Haasen, Prof. R. Wagner and the Gottingen FIM group for stimulating discussions, Dr. G. Horz, MPI Stuttgart for the provision of the material, Dr. P. Wilbrandt for the good working conditions at the TEM, and Beate Aschpalt for her indispensable help. REFERENCES 1. H.J. Grabke, Ber. d. Bzcnsengeseitschafl69/5, 409 (1965). 2. U. Klement, private communication. 3. D. H. Warrington and P. Bufalini, Scripta Metall. 5, 771 (1971). 4. L. Karlsson, H. NordCn, Acta Metall. 36, T.A. Parthasarathy, P.G. Shewmon, Scripta I,eta , 943 (1983).