Combined HRTEM and EFTEM Study of Precipitates in Tungsten and Chromium-Containing TiB 2
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ABSTRACT The structure and chemical composition of two types of precipitates in the system TiB 2WB 2-CrB 2 were studied by means of high-resolution TEM and energy filtering TEM. Type I particles (W2B5 structure) are precipitated at the basal plane of the hexagonal matrix whereas type II precipitates are thin platelets lying parallel to the {IIOO} prism planes. Lattice imaging yields displacements of the metal positions with respect to the matrix. Information on the chemical composition at high lateral resolution is obtained from elemental maps of all chemical constituents using electron spectroscopic imaging (ESI). The type II precipitates show a decrease in the B and Ti concentration, whereas the tungsten concentration increases and the Cr is homogeneously distributed. The HRTEM results combined with the results of the elemental maps allow to develop a structural model based on the intergrowth of the P3-WB structure in the TiB2 -rich matrix. The two deficient boron layers in W0.5Ti0.5 B with a spacing of 0.38 nm can be used to examine the resolution limit of ESI. INTRODUCTION A typical and powerful domain of transmission electron microscopy is the study of precipitation, where the shape, crystal structure, orientation relationship and chemical composition of precipitates can be elucidated. When precipitates are very small, at least in one dimension, any information is extremely hard to achieve: Electron diffraction can not be used to obtain crystallographic information, and EDX or EELS may yield at best the overall chemical composition of the precipitate, however the positions of the elements in the crystal lattice remain unknown. The present study is concerned with the structural and chemical characterization of thin precipitates in a transition metal boride applying high resolution TEM (HRTEM) and high resolution energy filtering TEM (EFTEM). The systems investigated are materials based on titanium diboride, TiB 2, which are candidates for cutting tools due to their high hardness and oxidation resistance at high temperatures. To increase the limited toughness in situ strengthening of the TiB2 -rich matrix can be realized by precipitation of borides of other transition metals. The phase diagram in the Ti-W-B system offers the possibility of precipitation from a supersaturated mixed crystal [1,2]. The mixed crystal regime can be increased by additions of chromium boride. Among other phenomena such as decomposition two different types of plate-shaped precipitates in the TiB 2-rich matrix were studied in detail. The combination of HRTEM and EFTEM allows us to specify the structure and chemical composition of the particles. The highly resolved images of the distribution of all the elements is particularly valuable for understanding the segregation and precipitation tendencies of the chemical constituents. The
289 Mat. Res. Soc. Symp. Proc. Vol. 589 0 2001 Materials Research Society
orientation relationship and the precipitate shape will be discussed on the basis of lattice misfit between precipitate and matrix. EX
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