Quantitative Metallography

ELECTRON MICROSCOPY Quantitative Metallography Electron Backscatter Diffraction in a FEGSEM A scanning electron microscope equipped with an electron backscat- ter diffraction system enables the quantitative analyses of the micro- structure of crystalline materials, even phase analysis at individual grains is possible. This provides a bet- ter comprehension of the mechanical properties of materials and facilitates the development of materials with special predefined properties. Introduction between these individual d=10 mm) of this alloy were granularity) as the final pol- Most materials are polycrys- grains [1]. With a field emis- annealed at a temperature of ishing step. talline and especially their sion scanning electron micro- 1220° C for 60 seconds, sub- The application of EBSD for mechanical properties are scope (FEGSEM), it is possible sequently compressed with a phase identification is dem- strongly influenced by char- to carry out quantitative ana- Gleeble 3800 testing system onstrated at a cross section of acteristics like the average lyses of grains/subgrains as at a temperature of 1120° C a commercially available du- grain size, the grain size dis- small as ~100 nm, of course to different strains (strain plex steel 1.4462 (no further tribution, textures and the depending on the type of ma- rate 0.1 s-1), and immediately treatments). The final polish- crystal structure. To tune the terial. quenched to room tempera- ing step was also performed properties of metals and al- ture. Transversal cross sec- with colloidal silica. loys with respect to these tions at a quarter of the height All the measurements and Materials, Instrumentation parameters, recrystallisation of the specimens were cut out analyses were performed on a of the respective materials at The nickel base alloy 80 A for the microstructural inves- Zeiss Gemini 982 DSM specified temperatures, strain was used to exhibit the capa- tigation. In order to obtain (primary electron energy: rates and strains is simu- bility of the EBSD technique EBSD patterns with high 20 keV; probe current: 2.8 nA), lated. for the determination of the quality, the specimens were equipped with an EDAX-TSL Traditionally, the size distri- recrystallised fraction. Cylin- polished with an alkaline col- system (SIT camera, OIM 4 bution of the grains and the drical specimens (h=12 mm, loidal silica solution (0.04 µm software, see figure 1). Grain recrystallised fraction were boundaries were character- determined with optical mi- ised by a disorientation larger croscopy, whereas the crys- than five between neighbour- tallographic texture was ing measurement points. measured with x-ray or neu- tron diffraction. The advan- Recrystallised Fraction tage of these techniques is the possibility to measure large Because the original and re- volumes of a specimen within crystallised grains differ in a rather short time. In the op- their deformation, size and posite, electron backscatter orientation distribution, the diffraction (EBSD) is a rela- mechanical properties of a tively time-consuming analy- material also depend on the sis method, but has the great fraction of recrystallised advantage that data (e.g. grains. The discrimination grain size, orientation, disori- between the deformed and entation between grains, recrystallised grains with phase information) are gained EBSD can be performed by from single grains, which can several methods, but the be used to elucidate direct Fig. 1: Image of the SEM sample chamber with the phosphor screen for grain orientation spread was neighbourhood relationships EBSD analyses inserted. proven to be most successful 38 • G.I.T. Imaging & Microscopy 1/2006  ELECTRON MICROSCOPY for this nickel alloy [2]. A typi- angle (~ 0.5°) and thus makes cal example of the grain ori- exact calculations of lattice entation spread of a partly re- plane spacing rather difficult. crystallised specimen (strain In the case of ferrite and 0.5) and a fully recrystallised austenite, a cubic body cen- specimen can be found in fig- tred and face centred crystal ure 2. The comparison of both system exists. As a conse- distributions clearly shows quence, these two phases can that the first peak of the grain be separated by EBSD. In fig- orientation spread belongs ure 4a the phase map of the predominantly to the re- investigated duplex steel is crystallised fraction. The de- shown. Since the phase of formed and recrystallised every measured point is ob- fraction can be separated by tained directly from the asso- setting a threshold of the ciated EBSD pattern and grain orientation spread at a stored in the data set, the re- value of 1.5° as marked in spective fractions can be de- figure 2. The evolution of the termined without setting any resultant structures is illus- threshold as it is necessary, Fig. 2: Grain orientation spread of a partly recrystallised specimen (blue, trated in figure 3 and is for instance, when using light strain 0.5) and a fully recrystallised specimen (red) with the range for the determination of the recrystallised fraction market. represented there as inverse microscopy. The ferrite frac- pole figure map (IPF). From tion was determined to an this figure it can be seen that amount of 54 % for the inves- the recrystallisation predomi- tigated duplex steel. Addition- nantly starts at the edges of ally, the orientation of the grains and mainly takes place grains of both phases can be at ‘normal’ grain boundaries displayed separately (see IPF and not at twin boundaries. of fig. 4b and c). From these Additionally, the recrystal- figures it is obvious that the lised grains form closed net- austenite phase contains a lot works very soon, which result of twinned grains, which is a in the well-known necklace general phenomenon for face structure. Since the IPF shows centred cubic materials. a statistical distribution of the Nearly no twinning can be ob- orientation of the individual served in the ferrite grains. recrystallised grains, defi- nitely no texture is present. Summary Fig. 3: Inverse pole figure maps of a) the deformed and b) the recrystallised Additionally, figure 3 depicts grains of nickel alloy specimens treated with different strains. the much smaller grain size The investigations clearly of the recrystallised grains demonstrate the capability of compared to the original EBSD in conjunction with a ones. Furthermore, a nearly FEGSEM for the quantitative linear correlation between analysis of the structure of the strain of the specimen crystalline materials. The op- and the amount of recrystal- portunity to gain direct neigh- lised grains was found. bourhood relationships be- tween individual grains Fig. 4: a) phase map (blue ferrite, red austenite), b) IPF of the ferrite phase enables a better understand- and c) IPF of the austenite phase of the duplex steel Ferrite – Austenite ing of the influence of the An important parameter for manufacturing process on the References [2] Mitsche, S., Poelt, P., Sommitsch, C., Walter, M., the properties of a duplex microstructure and in addi- Microsc. Microanal. 9:3, 344-345 (2003) [1] Randle, V., Engler, O.: Introduction to Texture steel is its ferrite fraction. tion on the macroscopic prop- [3] Nowell, M. M., Wright, S. I., Journal of Microscopy Analysis Macrotexture, Microtexture and Orienta- With EBSD it is also possible erties of the materials. This tion Mapping, Gordon and Breach Science 213:3 296-305 (2004) to determine the phase infor- knowledge is the prerequisite Publisher, The Netherlands, 2000 mation of a material down to for the development of new a micrometer scale (e.g. [3]). materials. Contact: Dr. Christof Sommitsch Dr. Stefan Mitsche, Dr. Peter Pölt University of Leoben An important prerequisite for Graz University of Technology Christian Doppler Laboratory of a successful differentiation Research Institute for Electron Materials Modelling and Simulation between two phases is that Microscopy, Graz, Austria Chair of Metal Forming they differ either significantly Tel.: +43 316 873 8346 Leoben, Austria in their lattice parameters or Fax: +43 316 811 596 Tel.: +43 3842 402 5605 in their crystal structure. The [email protected] Fax: +43 3842 402 7702 small wavelength of the elec- www.felmi-zfe.at [email protected] trons entails a small Bragg www.unileoben.ac.at G.I.T. Imaging & Microscopy 1/2006 • 39