Papers by Author: Walter Lengauer

Abstract: A common and straightforward method for the standardisation in electron-probe microanalysis (EPMA) is the use of homogeneous reference materials prepared by various techniques such as by melting, sintering, high-temperature annealing and hot-pressing. The reference materials have to be analysed by independent methods accurately in order to define their “true” composition. For some compounds the preparation techniques are difficult because of their specific thermo-chemical properties (e.g. low diffusivities, high equilibrium nitrogen pressure, incongruent melting). In addition, many compounds show large homogeneity ranges with an a priori existing uncertainty in composition, contrary to what is generally preferred: to use compounds with a narrow homogeneity range (“line compounds”). For the latter, diffusional preparation techniques can be applied to yield diffusion layers instead of massive samples for standardisation. However, also single-phase samples with narrow homogeneity ranges can be prepared by diffusion, depending on the phase equilibria in the corresponding system. The presentation summarises efforts that have been made in order to prepare various reference materials for carbon and nitrogen standardisation of EPMA by various techniques. The boundary conditions such as phase stabilities, phase compositions and diffusion kinetics, which are important for their preparation to obtain well-defined reference samples are discussed. These samples were applied to various WDS/EPMA-based studies of phase diagrams and diffusion kinetics by means of Cameca SX 50 and SX 100 microprobes.

Abstract: A general trend in the field of hardmetals is to achieve a refinement of the microstructures, usually by using sub-micron powders as raw materials. In this study, an alternative route to produce fine structures within the fcc hard phase (W,Ti)(C,N) is investigated: nitrogen indiffusion into (W,Ti)C leads to precipitation of tungsten-rich phases. The mechanism of precipitation (lamella- and labyrinth-like structures with a size of 100-400nm) is thought do be discontinuous segregation on the one hand and spinodal decomposition on the other hand. Hot-pressed (W,Ti)(C,N) samples of different compositions were annealed at different temperatures and C activities in high-pressure N2 atmosphere. The composition and resulting structures of the precipitates were correlated with composition of the (W,Ti)(C,N) phase as well as with annealing conditions. An outlook of a possible application of the observed phenomena to powder particles is given to achieve micron-sized particles of this hard phase with nanometer-sized structures as a raw material for fine-grained hardmetals.