Microstructure of severely deformed metals from X-ray peak profile analysis

J. Gubicza^a, N. H. Nam^a and V. V. Stolyarov^b

^aDepartment of Solid State Physics, Eötvös University, Budapest, Hungary

^bInstitute of Physics of Perspective Materials, Ufa State Aviation Technical University, Ufa, Russia

E-mail: gubicza@ludens.elte.hu

Keywords: Crystallite size; Dislocation structure; severely deformed metals

Severe plastic deformation (SPD) is an effective tool for producing bulk ultrafine grained (submicron grain sized or nanostructured) metals. One of the most common SPD methods is equal channel angular pressing (ECAP) – a technique that results in a homogeneous sub-micron grain structure of the workpiece [1]. The ultrafine grained materials produced by ECAP have an attractive combination of high strength and good ductility due to their low contamination and unique structures. For understanding the mechanical behavior of materials produced by ECAP, it is necessary to characterize their microstructure. In this work the microstructure of cubic TiNi and hexagonal Mg(Al) alloy produced by ECAP is studied by X-ray diffraction peak profile analysis. The high resolution X-ray diffraction experiments are performed using a special double-crystal diffractometer (Nonius FR591) with rotating Cu anode [2]. The peak profiles are evaluated by the Multiple Whole Profile (MWP) fitting procedure described in detail in Ref. [3]. In this method, the Fourier coefficients of the experimental profiles are fitted by the theoretical Fourier transforms calculated on the basis of a model of the microstructure [3]. The crystallite size distribution and some characteristic parameters of the dislocation structure (e.g. density and arrangement of dislocations) are obtained from the fitting. Additionally, the procedure enables the determination of the prevailing dislocation slip systems in the sample [4]. The eleven dislocation slip systems in a hexagonal Mg alloy can be classified into three groups based on their Burgers vectors: <a> type, <c> type and <c+a> type [5]. X-ray diffraction peak profile analysis reveals the abundance of <a>-type dislocations besides the <c>- and <c+a>-type dislocations in the as-received Mg alloy. During high temperature ECA pressing (at 270 °C) the fraction of <c+a>-type dislocations increases. The correlation between the microstructure and the room and high temperature mechanical behavior is also studied and discussed.

This work was supported by the Hungarian Scientific Research Fund, OTKA, Grant Nos. F-047057, T-046990 and T-042714.

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