Mapování pnutí pomocí SLEEM

Strain mapping by scanning low energy electron microscopy

abstract

en

Various techniques exist which are capable of studying the material microstructure, the scanning electron microscopy (SEM), (scanning) transmission microscopy ((S)TEM) and focused ion beam (FIB) microscopy being perhaps the most known. A specific way to visualizing the microstructure of polycrystalline materials at high spatial resolution, to achieve a high contrast between grains in polycrystals and very fast data acquisition, is to use the cathode lens (CL) mode in SEM. The CL mode in the SEM enables us to observe specimens at arbitrary landing energies of the primary electrons and to detect slow but not only slow, high angle scattered electrons that carry mainly crystallographic contrast based on the electron channelling, mostly in the Mott scattering angular range. The material under investigation was a commercial purity copper prepared by equal channel angular pressing (ECAP) method using 8 passes, route Bc, namely in the as-pressed state and after annealing (in argon atmosphere, 180 C, 6 minutes). Ultra-fine grained materials (UFG) prepared by severe plastic deformation are characterized by a very high density of (sub)grain boundaries. Grain boundary dislocations and their assemblies can be regarded as the source of elastic stress fields. These stresses result in significant distortions and dilatations of the crystal lattice near the grain boundaries. It is known that UFG materials are often rather non-stable and their grain growth occurs already at temperatures near 0.4 T (T as the melting temperature) or even lower. Annealing of these materials leads first to partial annihilation of defects at grain boundaries and inside grains and relaxation of internal elastic stresses, later to migration of non-equilibrium grain boundaries and abnormal grain growth.

Existují různé techniky ke studiu mikrostruktury materiálů, mezi nimiž jsou patrně nejznámější rastrovací elektronová mikroskopie (SEM), (rastrovací) transmisní elektronová mikroskopie ((S)TEM) a mikroskopie fokusovaným iontovým svazkem (FIB). Specifickou cestou k zobrazení mikrostruktury polykrystalických materiálů s vysokým prostorovým rozlišením, při dosažení vysokého kontrastu mezi zrny a vysoké rychlosti akvizice je využití módu katodové čočky (CL) v SEM. CL mód v SEM nám umožňuje pozorovat vzorky při libovolné dopadové energii primárních elektronů a detekovat (nejen) pomalé, do velkého úhlu rozptýlené, elektrony, které nesou hlavně krystalografický kontrast, založený na kanálování elektronů, povětšinou v úhlovém rozsahu Mottova rozptylu. Zkoumaným materiálem byla měď komerční čistoty, vyrobená metodou bezkontrakčního protlačování (ECAP) osmi průchody metodou Bc, zejména ve stavu po tváření a po žíhání (v atmosféře argonu při 180 st. C/6 minut). Ultrajemnozrnné (UFG) materiály připravené rozsáhlou plastickou deformací jsou charakteristické velkou hustotou hranic (sub)zrn. Dislokace na hranicích zrn a jejich soustavy mohou být považovány za zdroje elastických napěťových polí. Tato napěťová pole způsobují výrazné distorze a dilatace krystalové mříže poblíž hranic zrn. Je známo, že UFG materiály jsou spíše nestabilní a růst zrn v nich nastává již při teplotách blízko 0,4T (kde T je teplota tání) nebo ještě nižších. Žíhání v těchto materiálech vede nejprve k částečné anihilaci defektů na hranicích a uvnitř zrn a k relaxaci vnitřních elastických pnutí, později pak k migraci nerovnovážných hranic zrn a k abnormálnímu růstu zrn.

Various techniques exist which are capable of studying the material microstructure, the scanning electron microscopy (SEM), (scanning) transmission microscopy ((S)TEM) and focused ion beam (FIB) microscopy being perhaps the most known. A specific way to visualizing the microstructure of polycrystalline materials at high spatial resolution, to achieve a high contrast between grains in polycrystals and very fast data acquisition, is to use the cathode lens (CL) mode in SEM. The CL mode in the SEM enables us to observe specimens at arbitrary landing energies of the primary electrons and to detect slow but not only slow, high angle scattered electrons that carry mainly crystallographic contrast based on the electron channelling, mostly in the Mott scattering angular range. The material under investigation was a commercial purity copper prepared by equal channel angular pressing (ECAP) method using 8 passes, route Bc, namely in the as-pressed state and after annealing (in argon atmosphere, 180 C, 6 minutes). Ultra-fine grained materials (UFG) prepared by severe plastic deformation are characterized by a very high density of (sub)grain boundaries. Grain boundary dislocations and their assemblies can be regarded as the source of elastic stress fields. These stresses result in significant distortions and dilatations of the crystal lattice near the grain boundaries. It is known that UFG materials are often rather non-stable and their grain growth occurs already at temperatures near 0.4 T (T as the melting temperature) or even lower. Annealing of these materials leads first to partial annihilation of defects at grain boundaries and inside grains and relaxation of internal elastic stresses, later to migration of non-equilibrium grain boundaries and abnormal grain growth.

rastrovací mikroskopie pomalými elektrony (SLEEM), krystalografický kontrast, mikroskopická pnutí

scanning low energy electron microscopy (SLEEM), contrast of crystal orientation, microscopic strain

28.06.2010

VUTIUM Brno

Brno

978-80-214-4112-5

6th International Conference on Materials Structure & Micromechanics of Fracture

6

177–177

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