P. Ettmayer

Ti(C,N) cermets — Metallurgy and properties

Abstract An overview of the metallurgical reactions during the vacuum sintering process of powder mixtures for the manufacture of cermets is presented. The relatively complex phase reactions in the multi-component system Ti/Mo/W/Ta/Nb/C,N-Co/Ni are discussed. The liquid binder phase reacts with titanium carbonitride by preferentially dissolving titanium carbide leaving titanium nitride undissolved. The compositions and the amounts of the gas species set free during the sintering process were monitored and led —together with differential thermal analysis — to a better understanding of the mechanisms that govern the sintering behaviour. The properties and the microstructure of cermets depend on the nature and the alloy status of the prematerials. The composition of the prematerials with respect to the carbon-nitrogen ratio, the stoichiometry of the hard phase and the amount and composition of the binder phase have a decisive influence on the properties and the cutting performances of the final products. Optimization of the properties with respect to the desired performance is possible. Examples of the cermet cutting performance in various applications are discussed.

Solid state properties of group IVb carbonitrides

Technically relevant solid state properties of the hot-pressed group IVb carbonitrides Ti(CxN1−x)≈1.00, Ti(CxN1−x)0.82, Zr(CxN1−x)≈1.00 and Hf(CxN1−x)≈1.00, such as microhardnesses, electrical conductivities, heat conductivities (from specific heats and temperature diffusivities) and optical reflectances (visible region), were measured as functions of the [C]([C] + [N]) ratio. Generally the electrical conductivities, heat conductivities and molar heat values increase with increasing nitrogen content whereas the microhardnesses decrease. In samples of the series Ti(CxN1−x)0.82 a discontinuity in the Cp vs. T curves could be detected at 700–820 °C depending on the [C]([C] + [N]) ratio, indicative of a phase transition. The colour of the IVb carbonitrides changes from yellow through violet to grey with increasing [C]([C] + [N]) ratio. The extension of the coloured region is greater the higher the atomic number of the metal. A variety of these properties of IVb carbonitrides were measured for the first time. A literature review is presented.

Critical review on the elastic properties of transition metal carbides, nitrides and carbonitrides

Presently available data on the elastic properties of transition metal carbides, nitrides and carbonitrides are critically reviewed. It is shown that a lot of information is available on binary systems, but the scatter of the data is quite large, due to different sample preparation methods, different measurement and evaluation methods and insufficient chemical and microstructural characterization of the materials studied. Furthermore, experimental data were used to compare the evaluation methods for Youngs modulus as a function of the sample porosity. Only few data are available for ternary and quaternary systems. For the nitrides the elastic constants were determined almost entirely at room temperature only, although the compounds are frequently used at much higher temperatures. Information about the temperature dependency of the carbides is scarce. Therefore, it is obvious that for the design and tailoring of composites additional research work is necessary.

Lattice parameters and thermal expansion of Ti(CxN1−x), Zr(CxN1−x), Hf(CxN1−x) and TiN1−x from 298 to 1473 K as investigated by high-temperature X-ray diffraction

Abstract The thermal expansion values of the fee phases Ti(CxN1−x), Zr(CxN1−x) and Hf(CxN1−x with [C+N]/[Me] ∼1 and TiN1−x with [N]/[Ti] = 1.0 to 0.67 were determined from lattice parameter measurements made with high-temperature X-ray diffraction (HTXRD) in the temperature range of 298–1473 K. Stoichiometric titanium carbonitrides, zirconium carbonitrides and hafnium carbonitrides obey the Vegard rule up to 1473 K quite well, i.e. at any temperature up to 1473 K the lattice parameter behaves linearly between TiNTiC, ZrNZrC and HfNHfC, respectively. The lattice parameter of titanium nitride is also a linear function of the nitrogen/metal ratio in the temperature range investigated.

Fundamentals of liquid phase sintering for modern cermets and functionally graded cemented carbonitrides (FGCC)

Abstract Metallurgical reactions and microstructure developments during sintering of modern cermets and functionally graded cemented carbonitrides (FGCC) were investigated by modern analytical methods such as mass spectrometer (MS), differential thermal analysis (DTA), differential scanning calorimeter (DSC), dilatometer (DIL), microscopy and analytical electronic microscopy with energy dispersive spectrometer (EDS). The complex phase reactions and phase equilibria in the multi-component system Ti/Mo/W/Ta/Nb/C,N-Co/Ni were studied. The melting behavior models in the systems of TiC–WC/MoC–Ni/Co, TiC–TiN–WC–Co and TiCN–TaC–WC–Co have been established. By an in-depth understanding of the mechanisms that govern the sintering processing and metallurgical reactions, new cermets and different types of FGCC with desired microstructures and properties were developed.

Reactive diffusion and phase equilibria in the V–C, Nb–C, Ta–C and Ta–N systems

Abstract The reactive formation of vanadium, niobium and tantalum carbides as well as tantalum nitrides was investigated by means of diffusion couples. For the preparation of the carbides the compact metals were reacted with graphite powder and the formation of the phases MeC 1− x and Me 2 C could be observed. The slow growth rate of the Me 4 C 3 phases made it necessary to provide restricted diffusion geometry (wedge-type or plane-sheet samples) or, preferentially, diffusion couples of the type Me 2 C/graphite. The phases ζ -V 4 C 3− x , ζ -Nb 4 C 3− x and ζ -Ta 4 C 3− x could be observed as distinct diffusion bands below 1900, 1575 and 2170°C, respectively. The compositions of the phase boundaries as a function of temperature were measured by EPMA. The composition of ζ -V 4 C 3− x is 37.9 at.%C (with a narrow homogeneity range of ζ -Nb 4 C 3− x is 40.1–40.7 at.%C and that of ζ -Ta 4 C 3− x is 38.2–39.0 at.%C (standard deviations ca . ±0.2 at.%C). For the β -Me 2 C phases the carbon content was found to be greater than the values given in recent assessments, whereas the homogeneity regions measured for the δ -MeC 1− x phases are consistent with established data. In the Ta–N system the e -TaN and the δ -TaN 1− x phases usually form very narrow phase bands. Here again wedge-type diffusion samples were applied in order to enlarge the thickness of the phase bands. δ -TaN 1− x undergoes a eutectoid decomposition at 1821±19°C into e -TaN and β -Ta 2 N. The formation of e -TaN occurs at 1888±15°C at 25 bar N 2 , its homogeneity range at 1873°C is smaller than 0.4 at.%N. Re-investigated portions of the phase diagrams of all four systems are given.

Effects of vacancy ordering on structure and properties of vanadium carbide

Abstract The microhardnesses, electrical conductivities, specific heats at constant pressure ( C p ), lattice parameters and microstructures of ordered and disordered vanadium carbides were investigated. It was found that ordered vanadium carbide has a higher microhardness and electrical conductivity than a disordered carbide of the same composition. In a thermodynamic study the C p and temperatures of the order→disorder transformations were measured by using differential scanning calorimetry (DSC). The values of latent heats of the disorder-order reactions δ-VC 0.87 →V 8 C 7 and δ-VC 0.83 →V 6 C 5 were 2354 J/gfw and 2114 J/gfw, respectively. The measured temperatures corresponding to the onset of the transformations were 1095°C for V 8 C 7 and 1163°C for V 6 C 5 . Ordered domains could be made visible in microstructural investigations, which showed that ordering in compact vanadium carbide specimens originates at the grain boundaries of the disordered compound.

Das kubische Tantalmononitrid (B 1-Typ) und seine Mischbarkeit mit den isotypen Übergangsmetallnitriden und-carbiden

The position of the recently detected cubic δ-TaN1−x phase (B1 type) with respect to the other tantalum nitride phases in the system Ta−N is discussed and a tentative phase diagram for the partial system Ta−N is proposed. δ-TaN1−x, a high temperature phase, is completely miscible with all isotypic mononitrides and monocarbides of the 4 a and 5 a group metals. Lattice parameters of the solid solutions are given.

Phase transformations in non-stoichiometric vanadium carbide

X-ray diffraction, differential scanning calorimetry and reflected light microscopy studies are made of disorder-order phase transformations in the region of homogeneity of a non-stoichiometric cubic vanadium carbide, (0.66 < y < 0.88). It is found that according to the composition of the carbide, both an ordered phase with monoclinic or trigonal symmetry and a cubic ordered phase may form in this carbide at a temperature below 1450 K. Consideration is given to the effect that non-stoichiometry and the ordering of structural vacancies have on the heat capacity of the carbide. Temperatures and heats of reversible equilibrium disorder-order transitions are determined. The ordering transformations and are shown to be first-order phase transitions. An equilibrium phase diagram of the V-C system is constructed which allows for the formation of ordered phases in a non-stoichiometric vanadium carbide. The order parameter functional method is used for calculation of phase equilibria in the region of ordering of non-stoichiometric cubic carbide.

Phase equilibria and multiphase reaction diffusion in the Cr-C and Cr-N systems

The phase formation in the Cr-C and Cr-N systems was investigated using reaction diffusion couples. The carbides were prepared by reaction of chromium metal with graphite powder in the range 1143 to 1413 °C in argon atmosphere; the nitride samples by reaction of the metal with N2 (≤31 bar) in the range 1155 to 1420 °C. While the carbide samples showed the three chromium carbide phases in form of dense diffusion layers between 1100 and 1400 °C, porosity occurred at temperatures above 1400 °C. The composition of the phase bands was measured by the means of electron probe microanalysis. For the Cr23C6 phase, a slightly higher C composition was found than given in the literature. In Cr-N diffusion couples both the δCrN1−x and βCr2N formed phase bands at T≥1150 °C. Because decomposition processes occurred upon cooling, quenching experiments were carried out in the range 1370 to 1420 °C at 31 bar N2 to stabilize the phases. The EPMA investigations of the homogeneity ranges yielded a large increase of the homogeneity range for δCrN1−x with increasing temperature. The nonmetal diffusion coefficients in all phases of both systems were calculated from layer growth and/or from concentration profiles. In δCrN1−x the N diffusivity was found to be strongly dependent on the composition. The Vickers microhardnesses of the various phases were obtained by measuring the diffusion layers.