Peter Ettmayer

The molybdenum-nitrogen phase diagram

Abstract In high and low pressure experiments in the Mo-N system the solidus line (α + L) and the composition and temperature of the eutectic (L = α-Mo + γ-Mo 2 N) have been determined. Mo dissolves 1.08 at.% N at the eutectic temperature of 1860°C and at the equilibrium pressure of 670 atm (6.7 × 10 7 Pa). The eutectic composition is 19 at.% N and the corresponding N content of γ-Mo 2 N is 27 at.% N. The solubility of N 2 in Mo(1) and the liquidus line (α + L) L have been calculated on the basis of existing data. For γ-Mo 2 N a melting temperature of 2000°C has been estimated. An Mo-N phase diagram is presented and the phases are discussed in detail. Equations for the solubility of N 2 in solid and liquid Mo, the solid solubility limit and the dissociation and plateau pressures are given together with the Gibbs free energy of the corresponding reactions. The special behaviour of the metal-gas system Mo-N is additionally treated in a p - c diagram.

Investigations of phase equilibria in the TiN and TiMoN systems

As a part of a programme to develop hard materials with refractory binder metals the TiN and TiMoN systems were investigated. In order to reassess the TiN system the following were prepared: arc-melted TiNTi mixtures; sintered TiNTi mixtures; diffusion couples. Special attention was paid to the area around 33 at.% N, where two new high temperature phases were found: η−Ti3N2−x and ζ−Ti4N3−x, both crystallizing in the space group R3m and being isostructural to ϵ−Hf3N2 and ζ−Hf4N3 respectively. The stability range and the formation of ϵ−Ti2N were investigated by metallography and high temperature X-ray diffraction. ϵ−Ti2N decomposes at 1332±13 K. At this temperature it contains only 31.5 at.% N. The results of the metallographic study indicate that the formation of ϵ−Ti2N from pure δ−TiN1−x is retarded. In contrast, the formation of ϵ−Ti2N is favoured by the presence of α−Ti(N). In the case of the TiMoN system, arc-melted TiTiNMo mixtures were equilibrated at 1423 K and investigated by means of X-ray diffraction, metallography and electron probe microanalysis. The TiMoN system is characterized by the coexistence of δ−TiN1−x with β−TixMo1−x, from Ti0.70Mo0.30 up to nearly pure molybdenum. The solubility of nitrogen in β−TixMo1−x is low (less than 1 at.%), as also is that of molybdenum in α−Ti(N) and δ−TiN1−x. The composition of the β phase in equilibrium with α−Ti(N) + ζ−Ti4N3−x and ζ−Ti4N3−x + δ−TiN1−x respectively was evaluated to be near Ti0.70Mo0.30. The width of the two-phase field ζ−Ti4N3−x + β−TixMo1−x is very narrow. The alloy state of molybdenum as a binder phase is strongly dependent on the titanium activity in δ−TiN1−x. If δ−TiN1−x loses nitrogen during sintering, titanium diffuses into molybdenum. Since the titanium activity is a function of the nitrogen partial pressure, the latter should be kept as high as possible if a low alloy status of molybdenum is desirable.

Physical and mechanical properties of cubic δ-VN1−x

Abstract Compact samples of cubic δ-VN1-x were prepared by the reaction of vanadium metal with pure nitrogen in the pressure range 10 kPa ⩽ p(N2) ⩽ 4 MPa and at temperatures T from 1353 to 1923 K. The samples had compositions between VN0.765 and VN0.996 and lattice parameters between 0.4097(4) and 0.4135(3) nm. In this range the lattice parameter is a linear function of the nitrogen-to-metal ratio. The pressure isotherm at 1723 K as a function of composition was established. Near-stoichiometric VN1−x can only be prepared under elevated nitrogen pressures. The microhardness HV0.1 of VN1−x was measured. Hardness decreases significantly with increasing nitrogen content. Within the composition range investigated the vanadium sublattice is fully occupied at substoichiometric compositions, whereas the occupancy decreases just perceptibly as stoichiometry is approached. The occupancy of the nitrogen sites is a linear function of the [N] [V] ratio.

Zur Entmischung von kubischen Mehrstoffcarbiden

ZusammenfassungDie Mischungslücke in den Systemen: TiC−{ZrC, HfC} und VC-{NbC, TaC} wird im Bereich von 1200° C bis zur Temperatur des kritischen Punktes experimentell ermittelt und mit Hilfe der freien Exzeß-Enthalpie von der Form: ΔGE=x(1−x) {(a0+a1T)+(2x−1)(b0+b1T)} berechnet.AbstractThe miscibility gap within the systems: TiC−{ZrC, HfC} and VC—{NbC, TaC} has been experimentally determined in the region from 1200°C up to the temperature of the critical point. A calculation of the binodal curve has been carried out by means of an excess free enthalpy of the form: ΔGE=x(1−x){(a0+a1T)+(2x−1)(b0+b1T)}.

The crystal structure of a new phase in the titanium-nitrogen system

Abstract During a systematic reinvestigation of the Ti-N system, a new high temperature phase, ζ-Ti4N3 − x, was observed. The crystal structure was determined from X-ray powder diffraction patterns. It is trigonal, space group D 3d 5 − R 3 m (No. 166), with a rhombohedral unit cell with ar = 0.98070 nm, and α = 17.47 °, or in the hexagonal setting, ah = 0.29795 nm, ch = 2.8965 nm and c h a h = 9.7214 . The titanium atoms occupy the 6(c) sites with z = 0.1255 and z = 0.2910. The structure consists of a close-packed titanium atom arrangement with 12 titanium layers per unit cell in the stacking sequence (hhcc)3. The nitrogen atoms are most probably randomly distributed among the 6(c) (z = 0.4150), 3(a) and 3(b) sites. ζ-Ti4N3 − x is considerably deficient in nitrogen and is isomorphous with ζ-Hf4N3, ζ-V4C3, ζ-Nb4C3 and ζ-Ta4C3.

Preparation and properties of compact cubic δ-NbN1−x

Compact δ-NbN1−x was prepared by heating niobium wire for several days in nitrogen at 4 MPa pressure and temperatures of 1 723 to 1 923 K. The samples obtained had compositions between NbN0.924 and NbN0.975±0.002 and were coarse-grained. The lattice parameter increases with the nitrogen content froma=0.43884 nm for NbN0.924 toa=0.43913 nm for NbN0.975. From the determination of the lattice parameters up to 1 073 K the coefficient of linear thermal expansion as a function of temperature was evaluated. The microhardness HV0.1 decreases from 1 300±80·107Nm−2 for NbN0.924 to 1080±60·107 Nm−2 for NbN0.975. The occupancies of both the niobium and the nitrogen sublattices were calculated using experimental density data. The occupancy of the niobium sublattice decreases linearly with increasing nitrogen content. An extrapolation gives 2.9±0.4% vacancies in both sublattices for stoichiometric δ-NbN.ZusammenfassungKompaktes δ-NbN1−x wurde durch mehrtägiges Erhitzen von Niobdraht in Stickstoff bei einem Druck von 4 MPa und Temperaturen von 1 273 bis 1 923 K hergestellt. Die dabei erhaltenen Proben hatten Zusammensetzungen von NbN0.924 bis NbN0.975±0.002 und zeigten ein grobkörniges Gefüge. Der Gitterparameter steigt mit dem Stickstoffgehalt vona=0.43884 nm für NbN0.924 bisa=0.43913 nm für NbN0.975 an. Von einer Bestimmung der Gitterparameter bis 1 073 K wurde der lineare thermische Ausdehnungskoeffizient erhalten. Die Mikrohärte HV0.1 sinkt von 1 300±80·107 Nm−2 für NbN0.924 auf 1 080±60·107 Nm−2 für NbN0.975 ab. Die Besetzung sowohl des Niob- als auch des Stickstoffteilgitters wurde unter Verwendung von experimentell gemessenen Dichten bestimmt. Die Besetzung des Niobteilgitters fällt mit zunehmendem Stickstoffanteil linear ab. Eine Extrapolation dieser Werte ergibt für stöchiometrisches δ-NbN einen Leerstellenanteil von 2.9±0.4% auf beiden Teilgittern.

Über Mononitride und stickstoffreichere Nitride der Seltenerdmetalle (Mitt. I)

All Rare Earth metals (with the exception of Pm) and Y were nitrided with ammonia and nitrogen. Ammonia and nitrogen of normal pressure yield pure and stoichiometric mononitrides. Nitrogen of higher pressure (30 at and 300 at) results in the formation of nitrides with higher nitrogen content. In the case of Ce, Pr, Nd compounds with aSEN2 stoichiometry (SE=Rare Earth Metal) and an X-ray powder pattern similar to that of the La2O3-type can be observed. Higher nitrides of the metals Tb, Dy, Ho, Tm, Lu give powder patterns of the cubic Mn2O3-type. The precise composition of these latter nitrides could not be determined as yet.ZusammenfassungAlle Seltenerdmetalle (mit Ausnahme von Pm) und Yttrium wurden mit Ammoniak und Stickstoff nitridiert. Ammoniak und Stickstoff von Normaldruck ergeben reine und stöchiometrische Mononitride. Höherer Stickstoffdruck (30–300 at) führt zur Bildung stickstoffreicherer Nitride. Im Fall von Ce, Pr, Nd werden DinitrideSEN2 (SE=Seltenerdmetall) beobachtet mit Pulverdiagrammen, die denen des La2O3-Typs ähnlich sind. Die höheren Nitride der Metalle Tb, Dy, Ho, Tm, Lu ergeben Pulverdiagramme vom Mn2O3-Typ. Die genaue Zusammensetzung dieser Nitride konnte noch nicht festgelegt werden.

Lattice parameters and thermal expansion of δ-VN1−x from 298–1000 K

AbstractThe thermal expansion of VN1−x was determined from measurements of the lattice parameters in the temperature range of 298–1000 K and in the composition range of VN0.707–VN0.996. Within the accuracy of the results the expansion of the lattice parameter with temperature is not dependent on the composition. The lattice parameter as a function of composition ([N]/[V]=0.707−0.996) and temperature (298–1000 K) is given by

ber die Mischbarkeit von UN mit LaN, CeN, PrN, NdN, SmN, GdN, DyN, und ErN@@@The miscibility of UN with LaN, CeN, PrN, NdN, SmN, GdN, DyN, and ErN

\begin{gathered} a([N]/[V],T) = 0.38872 + 0.02488([N]/[V]) - \hfill \\ - (1.083 \pm 0.021) \cdot 10^{ - 4} T^{1/2} + (6.2 \pm 0.1) \cdot 10^{ - 6} T. \hfill \\ \end{gathered}