Dresden 2003 – scientific programme

Parts | Days | Selection | Search | Downloads | Help

M: Metallphysik

M 41: Postersitzung

M 41.4: Poster

Thursday, March 27, 2003, 14:30–16:30, HSZ

Precipitation Kinetics of Transition Metal Diboride-Based Materials — •E. R. Fotsing¹, H. Schmidt¹, G. Borchardt¹, C. Schmalzried², and R. Telle² — ¹Inst. f. Metallurgie, TU Clausthal — ²Inst. f. Gesteinshüttenkunde, RWTH Aachen

The quasiternary system TiB₂−WB₂−CrB₂ is a model system for the development of in-situ reinforced boride materials. A tailored de-mixing of a supersaturated host matrix may lead under certain conditions to the formation of tungsten rich, platelet-shaped precipitates embedded in a (TiWCr)B₂ matrix which are well suited to improve the toughness of the materials. In this study we systematically investigated the precipitation kinetics to understand and to quantitatively describe the changes in microstructure, occurring during de-mixing and grain growth. Ceramics of a certain composition (Ti_1−x−yW_xCr_y)B₂ were produced by high pressure reaction sintering of high-energy milled powders of TiB₂, WB₂, and CrB₂ at 1800^o C for 30 min at 60 MPa and subsequent homogenisation at 2000^o C for 8 h at ambient pressure under Ar atmosphere. Nucleation and growth of precipitates of the structure W₂B₅ and β-WB was investigated as a function of time and composition in the temperature range between 1450^o C and 1700^o C by X-ray diffractometry, scanning electron microscopy, and energy dispersive X-ray analysis. The fraction of precipitated phase was determined and numerically fitted, using models based on Johnson-Mehl-Avrami-Kolmogorov (JMAK), from which rate constants and activation enthalpies were determined. The results indicate the presence of a diffusion limited precipitation process which is very sensitive to the Cr content. Possible growth geometries are discussed.