Features of obtaining composite powders and alloys in the  CrSi2–Ti (Ta)Si2 systems in a nanodisperse state under mechanosynthesis of mechanical  activation 

Iryna V. Uvarova ,  Irina V. Kud ,  Lydmila I. Yeremenko ,  Lydmila Lykhodid ,  Dmitro Ziatkevich 

Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Krzizanovsky, Kyiv 38(044), Ukraine

Abstract

Refractory materials are traditionally synthesized via solid phase reactions between elements at high temperature. The achievement of a nanosized state for ceramics in this way is, however, practically impossible because of high temperatures of diffusion interaction, which causes coalescence and aggregation of fine refractory particles. Under intense milling, formation of new phases is possible:

-         at room temperature thanks to activation of one or several chemical reactions, formation of nuclei in the course of mechanical treatment or growth from amorphous state, decomposition and phase transitions (mechanosynthesis);

-         at reduced temperatures of subsequent synthesis thanks to prior activation of initial components (mechanical activation).

In both cases, synthesis includes milling. It is milling that provides nanoscale of initial powders and formation of high concentration of interfaces, at which numerous defects arise.

In this work, mechanosynthesis was studied depending on the process intensity, which was controlled by varying the duration of milling (60 and 90 min) in an impulse regime and the mass balls–to–powder ratio (BPR) of 20:1 and 30:1. Mechanical activation of the initial mixtures was performed under softer conditions (lower milling intensity, BPR 10:1, process duration 60 and 90 min), which leads to smaller particle sizes and to accumulation of structural defects, but this degree of activation does not provide running of the chemical reaction with formation of  refractory compounds.

Numerous researches devoted to synthesis, structure and physicochemical properties of complex compounds on the basis of transition metal silicides make it possible to conclude that the level of physicochemical properties of solid solutions is higher as compared to that of individual silicides. When used, solid solutions provide high reproducibility of operation parameters thanks to wide regions of their existence. Materials based on solid solutions of higher silicide phases possess most stable properties.

 To develop protective coatings operating at high temperatures, the CrSi2–(Ti)TaSi2 system was selected, in which two solid solutions, (Cr,Ti(Ta)Si2  and (Ti(Ta),Cr)Si2, exist in a wide homogeneity region. The solid solutions are formed by a reaction diffusion mechanism through subsequent formation of individual silicides from lower to higher ones followed by their interaction (homogenization). At the latter stage diffusion processes prevail.

The application of mechanosynthesis to these systems failed: a solid solution was not obtained. By the XRD data, the products of solid phase interaction under mechanosynthesis were dispersed composites composed of both higher and lower silicide phases. Mechanism of their formation is reaction diffusion characterized by prevailing of the rate of chemical reaction over that of diffusion processes. In this case, it is impossible to produce solid solutions of silicides because the process of their formation, that is, mutual solution of disilicides requires long–term homogenization owing to diffusion. The use of low temperature synthesis of prior activated initial components permits one to produce a nanosized powder of solid solution with high enough reproducibility of the element composition. Composite systems prepared via mechanosynthesis are characterized by a particle size of about 50 nm, whereas solid solutions are characterized by agglomerates of about 300 nm.

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Presentation: Invited oral at E-MRS Fall Meeting 2009, Symposium I, by Iryna V. Uvarova
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-07 11:22
Revised:   2009-06-07 00:44
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