Autumn School on Materials Science and Electron Microscopy 2007

"Microscopy - advanced tools for tomorrow's materials" Berlin, October 8th - October 11th, 2007

Orientation mapping in TEM applied to examination of gradient structure FeCr25Co8 alloy subjected to severe plastic deformation

Magdalena Bieda1, Anna Korneva*1, Krzysztof Sztwiertnia1, Galija Korznikowa2

1 Polish Academy of Sciences, Institute of Metallurgy and Materials Science,

Reymonta 25, 30-059 Krakow, Poland 2 Russian Academy of Sciences, Institute of Problems of Metal Superplasticity,

Khalturina, 39, Ufa 450001, Russia

The structural evolution of the hard magnetic alloy FeCr25Co8, were examined after severe plastic deformation [1]. Deformation was realized in two separate stages: upsetting and then torsion [2] applied to the lower sample part and was carried out at temperatures that corresponded to the α phase appearance. In the sample sections parallel to the upsetting direction a gradient microstructure (Fig. 1), with the smallest grains layer in the highest deformation zone, was formed. The gradient microstructure of the material results in a gradual change of the material properties. The possible application of such a material is for magnets, which should have good magnetic properties within their volume while maintaining good mechanical properties on the surface.

Deformation microstructure was studied mainly by means of the Orientation Mapping (OM) technique in the transmission and scanning electron microscope (TEM and SEM). Microscopic observations and orientation maps measured by TEM were combined with Analytical TEM investigations for the bottom parts of samples. Orientation maps and phase maps were obtained by collecting and indexing of the set of the convergent beam electron diffraction (CBED) in the transmission electron microscope (TEM) from the particular parts of thin foils. This technique, with spatial resolution about 10 nm and angular resolution about 0.1 °, complements EBSD analysis and allows investigating of fine grained and multiphase materials [2, 3]. The method enables more detailed study of grain interphases. Information about grains distribution, misorientation between grains of one or different phases are essential in characterization of materials properties.

The analysis of the lower sample parts indicates that the microstructure is dominated primarily by the α and σ phases. The process of formation of σ phase grains is observed (Fig.2). The dependencies between the grains of α and σ phases are calculated. High deformation results in the precipitation of the σ phase and an increased hardness of the material. The optimal temperature range for obtaining submicron-grained microstructures in the high deformation zone is 800 °C. The maximal grain size reduction and increasing of mechanical properties are observed near the σ phase appearance. References: [1] A.V. Korznikov: Structure and Mechanical Properties of Metalls and Alloys Subjected to Severe Plastic Deformation, Thesis, IPMS Ufa, Russia (2000) [2] A.V.Korneva, G.F.Korznikova, M.Bieda, K.Sztwiertnia: Arch. Metall. Mater. 51 (2006) 69 [3] J.J. Fundenberger, A. Morawiec, E. Bouzy & J.S. Lecomte Ultramicroscopy 96, (2003), 127 ∗ On leave from the Institute of Problems of Metal Superplasticity, Ufa, Russia

Fig. 1 a) Gradient microstructure over the entire cross-section SEM of a sample deformed at 750°C; b) the upper sample part; c) the middle sample part; d) the lower sample part.

Fig. 2 The σ phase precipitation by a) XEDS measurements; b) CBED diffraction measurements - phase map; c) bright field TEM image from bottom part of the sample