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A Novel “In-situ-tracking” Approach for Evaluating Microstructural Variations Using SEM, EDS and EBSD and Its Applications in Materials Science

C. Pan*,1,2, Y. Huang1 and Q. Fu2 1Department of Physics, Wuhan University, Wuhan, 430072 China 2Center for Electron Microscopy, Wuhan University, Wuhan, 430072 China In order to reveal the variations in microstructures, compositions and crystallographic textures exactly at one position or point during corrosion, high temperature treatment, etc., this paper introduces an “in-situ-tracking” observation approach by using a SEM, EDS and EBSD techniques. The principle of the approach is: 1) making marks at the sample surface as a sign for the following tracking; 2) observing the interested areas around the marks and recording the results as the first micrograph; 3) treating the sample, and at each time, tracking and examining the sample at the same place as it was at the first time and taking the micrographs in series. Though the present approach is not a real “in-situ” examination, however, if the process is proceeded properly, a similar phenomenon and purpose can be researched. Comparing to the regular process, when it was applied in researches on corrosion, diffusion and microstructural transformation, the present approach provides a possibility to reveal the exact variations in materials

Keywords “in-situ-tracking”; SEM; EDS; EBSD; microstructure

1. Introduction

Generally, the microstructure-property relationship of materials depends upon its suffered conditions, such as heat treatment, processing, machining and services. Therefore, to realize the variation mechanisms exactly in a material during processing is vital for its applications. For example, during corrosion, different microstructures exhibit different corrosion properties when a heterogeneous material is serviced in variant environments and periods, a continuous examination of the materials will be very useful to evaluate the corrosion resistant property. In addition, a recovery and recrystallization phenomenon generally happens in materials during elevated service or heat treatment. However, if the progress of the grain growth and grain boundary movement can be observed continuously, it will be important to predict the property changes and also the service life.

In general, when the above experiments and researches are carried out, the samples are cut in several pieces, and then treated and examined separately according to the experimental purpose and conditions.

* Corresponding author: e-mail: [email protected], Phone: ++86-27-62367023

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The last conclusions come from the comparison of these separated results and data, which in most case are acceptable and correct. However, if the materials are heterogeneous or the property is sensitive to the microstructural variation, such as welded joints and composite materials, etc., these data will be scattered and lack of comparability sometimes.

In order to accurately reveal the microstructural changes during various treatment, many “in-situ” experimental methods have been proposed. Generally it needs special and even very expensive apparatus, such as tensile and hot sample stages in a scanning electron microscope (SEM) for researching plastic deformation and phase transformation [1-3], and the environmental SEM for oxidation and corrosion of materials [4,5]. However, sometimes it is very difficult to find a proper method or technique to get the desired data due to the broad research projects, which limits our research scope and accuracy.

In the present paper, we introduce an “in-situ-tracking” observation approach by using scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD) techniques. It provides a possibility to examine the changes of microstructures, chemical compositions and crystalline orientations, etc. in series at one position or region during various treatments simply and economically. When it was applied in researches on microstructural transformation, corrosion and atom diffusion, the desired and novel results have been obtained..

2. Principle and process of the “in-situ-tracking” approach

2.1 Principle

The present “in-situ-tracking” approach is based upon the SEM functions on microstructural observation and its accessories EDS on chemical composition measurement and EBSD on crystallographic texture detection. A series results on the microstructure – property relationships are obtained via making signs on sample surface for determining the observing location and then following the tracks during various treatments, i.e. a “treating – observing – re-treatment – re-observing ……” process.

Due to the limit and influence of SEM sample stages on space, size, vacuum, and actual research requirements, etc., this “in-situ-tracking” approach is different from the real “in-situ” technique because it is a intermittent testing process, i.e., the examination and treatment are separated, and the sample treatments are proceeded outside SEM. However, if the sample is treated properly and the instrumental operation (SEM, EDS and EBSD) is handled very carefully, and the topographic marker is located strictly after repetitious sample treatments, the series and comparable photographs and data will meet the “in-situ” purpose.

A problem have been noted during the experiment, i.e., a contaminant film generally is formed on the sample surface after treatment, such as oxide film or corrosion film, etc., which may affect the afterward examination. Therefore, a slightly re-polishing using water and re-etching are required. If the process was proceeded properly, the influence can be minimized to a very low level. In addition, some special techniques must be used during sample treatment according to the purposes, for example, using vacuum or special atmosphere furnace for minimizing oxide film, and anti-cement agent when measuring carbon diffusion, etc.

2.2 Experimental process In order to obtain a desired result, every step is important during the “in-situ-tracking” examination, the procedures and points for attention are as follows:

Step 1: original sample preparation, including machining, polishing and etching, etc. for revealing microstructures according to the research purpose.

Step 2: making signs or markers at the interested place on the prepared surface for SEM, EDS and EBSD analysis, as shown in Fig.1.

©FORMATEX 2007Modern Research and Educational Topics in Microscopy. A. Méndez-Vilas and J. Díaz (Eds.)

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Note: 1) the signs should be made deeply in case it will fade away or disappear gradually after multi-treatment; 2) the shape and position of the signs should be made specially and easy to be find or recognized during SEM observation.

Fig. 1 the sign and observation area for the “in-situ-tracking” examination

Step 3: putting the sample into SEM and selecting an interested region around the signs, and taking the

first results on microstructure, compositions, etc. Step 4: making the first sample treatment. Note: in order to minimize the influence of treatment on the observed surface to a minor level, a

slightly re-polishing for removing the addictive film and re-etching for revealing the microstructure are necessary. Generally, the polishing should be kept in several micron level in depth which not only can remove the film, but also not affect the “in-situ-tracking” result.

Step 5: again putting the treated sample into the SEM stage in the similar position and orientation as it was in the Step 3. Adjusting the stage to find the sign and place exactly and making the similar examination as it was in the former case. Getting the second experimental result.

Step 6: repeating the Step 4 and Step 5 for multi-tracking examinations, and at last obtaining a series results.

Experimentally, it is better and strongly recommended to have several set of results, in case some uncertain influences happened during the multi-tracking examination, and conclusively at least one set of perfect result and data are guaranteed.

3 Applications of the “in-situ-tracking” approach in materials research We used the present “in-situ-tracking” approach to study variations in materials during different conditional treatments, and many fresh data were achieved when comparing with regular methods. The experiments were carried out by using a field emission gun SEM (SIRION, FEI, the Netherlands) with accessories EDS and EBSD (GENESIS 7000, EDAX, USA).

3.1 “In-situ-tracking” evaluation during corrosion progress at fusion boundary of dissimilar steel welded joints in H2S containing solution [6,7]

Pipes for chemical, oil refining, and coal conversion processes, operating at elevated temperatures under high pressure and corrosion conditions, are normally manufactured from welded joint between Cr5Mo base metal and A302 (Cr23Ni13) electrode. Its service life is generally depended upon the property of the joint region. However, the corrosion process in the joints is complicated, especially, in wet sulfureted hydrogen environment. Recently, compared to the base metal and weld metal, only limited information is available in the literature regarding the corrosion process of the fusion boundary [8]. It has been noted that the fusion boundary in the dissimilar steel welded joint became a critical region to the failure of “hydrogen induced disbonding” [9]. In addition, the width of the fusion boundary varied in a large range from 2 µm to 50 µm from region to region in a whole joint due to welding instability. It was also found

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that different width of the boundary exhibited different properties in carbon diffusion and hydrogen-induced disbonding.

In the present work, we applied the “in-situ-tracking” observation method to track the corrosion property process around the fusion boundary in as welded condition and after post-weld heat treatment (PWHT) in H2S containing solution (NACE TM-01-77 standard) with different times, as shown in Fig.2 to Fig.5. The results revealed that the fusion boundary was the worst region for corrosion resistance when comparing with other zones, and the broad boundary exhibited a stronger resistance for “hydrogen induced disbonding” than the narrow one. Therefore, we proposed a novel conclusion, i.e., when considering high temperature service life and security in a corrosion medium, it is a feasible way to adjust welding conditions to form a broad fusion boundary during welding.

Fig. 2 “In-situ-tracking” SEM observations of corrosion process around the as-welded fusion boundary (narrow). Before corrosion; (b) 300 min. corrosion; (c) 800 min. corrosion; (d) 1800 min. corrosion

Fig.3 “In-situ-tracking” SEM observations of corrosion process around the as-welded fusion boundary (broad). Before corrosion; (b) 300 min. corrosion; (c) 800 min. corrosion; (d) 1800 min. corrosion

Fig.4 “In-situ-tracking” SEM observations of corrosion process around the PWHT fusion boundary (narrow). Before corrosion; (b) 300 min. corrosion; (c) 800 min. corrosion; (d) 1800 min. corrosion


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Fig.5 “In-situ-tracking” SEM observations of corrosion process around the PWHT fusion boundary (broad). (a) Before corrosion; (b) 300 min. corrosion; (c) 800 min. corrosion; (d) 1800 min. corrosion

3.2 “In-situ-tracking” observation of carbon distribution in dissimilar steel welded joints [10]

Carbon migration (uphill diffusion) from the ferrite base metal across fusion boundary into the austenitic weld metal is another problem in a dissimilar steel welded joint during heat treatment or high temperature service. The driving force for this diffusion is due to the strong affinity of carbon with Chromium (Cr), and a carbon depleted soft zone is formed in the base metal adjacent the boundary and a carbon enriched hard zone is formed inside the boundary and weld metal. This carbon migration has been recognized to be a key role for damaging mechanical properties and reduce the elevated service life of the joint.

Presently, we used FEG-SEM + EDS technique to measure the carbon migration around the fusion boundary of a welded joint (0.40% carbon steel + Cr25Ni13 electrode) during post-weld heat treatments with the “in-situ-tracking” approach. The heart treatment process was 700 oC with one and three hours in an atmosphere furnace for minimizing oxide film, and anti-cement agent was used for preventing decarburization. The results show that: 1) the carbon migration was restricted mainly inside the fusion boundary and formed a carbon enriched peak, only few carbon diffused into the weld metal; 2) the carbon content increased with the heat period; 3) the migration rate in a broad boundary is faster than that in a narrow one, as shown in Fig.6.

Fig. 6 “In-situ-tracking” EDS measurements of carbon distribution around the fusion boundaries

a. narrow boundary (W＝5µm)；＝b. broad boundary (W 34µm)

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3.3 EBSD “in-situ-tracking” study of microstructure transformation in austenitic stainless steel during high temperature service [11]

Austenitic stainless steels have been widely used in power generation plants, petrochemical and refinery, spacecraft, due to its advanced properties in high temperature mechanical property, corrosion and oxidation resistance, etc. It possesses a single f.c.c austenitic crystal structure and exhibits a clear grain growth through “recovery and recrystallization” during high temperature service which therefore deteriorates materials property and affects the service life.

Here, we used FEG-SEM + EBSD technique to “in-situ track” the grain growth, boundary and orientation variation of the Cr25-Ni13 high alloyed austenitic stainless steel served at high temperature. The samples including: 1) original steel; 2) served after one period ( 650 oC for 17 hours and 1200 oC for 27 hours, total 150 hours in one period); 3) served for 30 days.

The experimental results showed that after treatments the fraction of low angle grain boundaries (LABG) became increased and flattened obviously, in addition to the grains coarsened, as shown in Fig.7 to Fig.10. Comparing to the regular high temperature service (below 900 oC), the present “recovery and recrystallization” process was accelerated due to the dislocation fast movement and intensive interaction during 1200 oC super high temperature service, however, the grain growth mechanism was still meet the well-accepted dislocation model of subgrain combination induced grain growth.

Fig. 7 “In-situ-tracking” SEM morphologies of the microstructural variations after services a. original steel; b. after 150 hours service; c. after 30 days service

Fig. 8 “In-situ-tracking” EBSD crystallographic textures after services(similar place as above) a. original steel; b. after 150 hours service; c. after 30 days service

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a. original steel with average size 28um; b. after 150 hours service with average size 64um; c. after 30 days service with average size 105um

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Fig. 10 Corresponding profiles for grain orientations

a. original steel; b. after 150 hours service; c. after 30 days service

4 Conclusion

Comparing with the regular methods and the strict “in-situ” techniques, the present “in-situ-tracking” approach provides a possibility to achieve the examination in series on the variations of microstructures, compositions and properties at a fixed place during treatments simply and economically. If we use this approach flexibly according to its principle, it can be applied to a wide research area in materials science in future.

Acknowledgements The support by A Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD) (No. 200233) is gratefully acknowledged.

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Behavior, People’s Communication Press, 2000 [9] M. D. Rowe, T. W. Nelson and J. C. Lippold, Welding Journal, 87, 2 (1999) [10] W. Huang, C. Pan and Q. Fu, Materials for Mechanical Engineering, 30, 4 (2006) [11] C. Chen, Y. Huang, Y. Wu and C. Pan, Chinese Materials Science Technology and Equipment, 3, 6 (2006)

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