T. Ya. Velikanova

Relationships governing phase equilibria in refractory carbide-bearing systems of transition metals

Equilibrium diagrams of the carbide systems were examined and constructed of binary and ternary systems which made it possible to propose several generalizations predicting the phase transitions in unexamined, including multicomponent, systems. It was found in most cases that the phase equilibria in the ternary systems of the metal groups IV-VII was determined by the structure of the NaCl type. The limiting content of solid solution carbon of the transition metal cubic carbides can be evaluated using the limiting density of the valency electrons in the carbide phases of the NaCl type.

Phase equilibria in the Cr-Ni-C system and their use for developing physicochemical principles for design of hard alloys based on chromium carbide

The Cr—Ni—C phase diagram at the melting point was plotted by a combination of procedures (metallography, x-ray, microprobe, differential thermal analysis, Pirani—Alterthum method, etc.). A general feature of this system is the existence of equilibria between the nickel-based phase and all the other phases. The temperature of the quasibinary (Ni)+(Cr7C3) eutectic was determined to be 1324±6°C. Based on both the phase diagram of the Cr—Ni—C system and the bending strength and Rockwell hardness of the alloys, the optimal composition of the initial carbide ingredient for production of hard alloys based on Cr3C2 with nickel—phosphorus binder was estimated as 13.0–13.3 at.%, substoichiometric with respect to Cr3C2.

Phase diagram of the Cr-Mo-C system 1. Phase equilibria in the area of crystallization of alloys of the Mo-Mo2C-Cr7C3-C partial system

This article presents the results of an investigation of alloys of the Cr-Mo-C system at temperatures of existence of solid phases with the liquid one and in the subsolidus area, which made it possible to construct the high temperature area of the phase diagram of the Mo-Mo/sub 2/C-Cr/sub 7/C/sub 3/-Cr partial system. The phase transformations were investigated by differential thermal analysis on an instrument with a W-VR-20 string thermocouple in high-purity helium. The specimens were placed in a crucible of Y/sub 2/O/sub 3/, HfO/sub 2/, or Sc/sub 2/O/sub 3/. The calibration was done using Al, Au, Pd, Pt, and Rh as reference points (auxiliary ones carbonyl iron and + Mo/sub 2/C eutectic). To decrease and take into consideration the error in temperature measurement of the nonvariant equilibria the tests were made several times. The error in reproduction of the temperature scale was determined with the use of regression analysis.

The Cr−Re phase diagram

The phase diagram for the Cr−Re system has been derived from published data in conjunction with measurements on alloys cast and annealed at subsolidus temperatures (XRD, metallography, microprobe analysis, and measurement of the temperature for the onset of melting by the Pirani—Alterthum method). The phase equilibria at solidus temperatures have been determined from studies on alloys annealed at those temperatures. Phase homogeneity regions on the solidus have been identified: for (Cr) up to 43 at.% Re, for the σ phase −50–72 at.%Re, and for (Re) up to 17 at.%Cr; temperatures have also been determined for the invariant equilibria L+σ ⇆ (Cr) at 2150±15°C and L+(Re)⇆σ at 2335±15°C.

Phase equilibria in the ternary systems formed by molybdenum and tungsten with the groups IV and V transition metals and carbon

Conclusions1.The constitution diagrams of the ternary systems (Mo, W)-(Ti, Zr, Hf, V, Nb, Ta)-C may be divided, according to the type of their singular complex, into two groups reflecting the difference in thermodynamic stability between the cubic carbides of the Groups IV and V metals.2.In all the systems examined, the intersolubility of the cubic carbides is unlimited.3.The solubility break observed in some Me2C series (in spite of the fact that the necessary crystallochemical conditions for the formation of continuous series of solid solutions are fulfilled) is due to the high thermodynamic stability of the higher carbides of the Group V metals.4.Additions of the Groups IV and V transition metals stabilize the high-temperature modifications of molybdenum and tungsten carbides.

Solid state transformations and phase equilibria in the molybdenum-carbon system

The Mo-C alloys were examined in a large number of investigations which were analyzed in detail in [i]. These data show that the Mo-C system contains a solid solution based on molybdenum (~), Mo2C carbide which has a hexagonal close-packed lattice (6) at high temperatures, and at low temperatures a rhombically distorted lattice, two high-temperature carbides with hexagonal (N-MosC2, above 1650~ and cubic of the NaCI type (6, above 1960~ lattices, and a low-temperature hexagonal carbide with the type WC structure (y, below 1200~ The variants of the equilibrium diagram of the Mo-C system published in various sources differ in the temperature and concentration boundaries of the polymorphous modifications of Mo2C carbide, the nature of the order-disorder transition in this carbide, by the mode of formation of N-Mo3C2, and by equilibria in which ~-phase takes part. The amount and type of all the modifications of Mo2C have not been accurately determined [i].

Structure of alloys and the phase equilibrium diagram of the Hf-Mo-C system VI. Isothermal section of the Hf-Mo-C system at 1400°C

Conclusions1.The isothermal section of the Hf-Mo-C ternary system at 1400°C has been constructed.2.The features distinguishing the phase equilibria at 1400°C from those in the 1700°C section are the decomposition of the binary carbide η-MoC1−x, its stabilization in the ternary system, resulting in the appearance of (γ + C) and (γ + η + C) fields, and a substantial widening of the (λ2 + β2 + 6) and (β2 + δ + α) fields, caused by the (β2+ 6) field.3.The extent of the homogeneity field of the phase based on the cubic carbides (Mo, Hf)C1−x differs little, as regards both hafnium and molybdenum contents, from that at 1700°C.

Structure of alloys and the phase equilibrium diagram of the system Hf-Mo-C

Conclusions1.The isothermal section of the ternary system Mo-Hf-C at 1700°C has been constructed.2.The phase equilibria in the section at 1700°C differ from those on the solidus surface in the following respects: The homogeneity range of solid solutions of the cubic (Mo, Hf)C1−x carbides is interrupted; the section has (η+γ), (η+δ), (η+C), (η+δ+γ), and (η+δ+C) fields; the lattice of the Laves phase is of a different type (cubic, of the MgCu2 type, and not hexagonal, of the MgNi2 type, as on the solidus surface); the fields of the three-phase equilibria are wider.3.At the temperature under consideration hafnium carbide dissolves up to about 90 mole % of the molybdenum carbide “MoC0.80”.

Thermodynamic parameters of scandium-iridium compounds and ScIr2

EMF measurement for galvanic cells has been used in the range 813–1023 K to determine the Gibbs energies, enthalpies, and entropies of formation for the phases ScIr3 within its region of homogeneous existence and also ScIr2.

The (Cr) + (NbC) quasibinary eutectic in the Cr - Nb - C system

An experimental investigation of the Cr - Nb - C alloys has shown that in the (Cr) + (NbC) two-phase region there are the fold with maximal solidus temperature and the saddle point (Cr79.5Nb12.2C8.3) on the liquidus surface, relating to Lc ⇔ (Cr) + (NbC) invariant equilibrium at ≥1640°C.