Experimental investigation of the thermodynamics of the Fe-Ti-C austenite and the solubility of titanium carbide

The thermodynamics of Fe-Ti-C austenite has been investigated in the temperature range of 1273 to 1473 K using a dynamic gas equilibrium technique. Hydrogen-methane mixtures have been used for fixing carbon potentials, and the carbon contents have been determined as dynamic weight changesvia a sensitive Cahn microbalance. The effect of titanium-carbon interaction in austenite has been observed (1) as a minimum in the carbide solubility curve, (2) as increases in carbon content due to titanium additions at constant carbon activity, and (3) as the variation of solubility limit of the carbide with carbon content at high carbon contents. The results on the isoactivity measurements in the ternary Fe-Ti-C austenite have been analyzed using the modified Wagner formalism. The ternary interaction parameter ε _C ^Ti been quantitatively related to the solubility minimum and the relative increase in carbon content at constant carbon activity. The variation of the solubility limit of the carbide with carbon content has been described using an additional term to the classical solubility relationships. This additional term is related to the self-and cross-interaction of carbon. Using the solubility relations, the dissolution free energy of body-centered cubic (bcc) Ti in face-centered cubic (fcc) Fe has also been determined. Very good agreement between the thermodynamic calculations and the experimental results was found. K. BALASUBRAMANIAN, Formerly with McMaster University,     

Nitridation kinetics of niobium in the temperature range of 873 to 1273 K

The kinetics of Nb (Cb) nitridation and ε-NbN growth obey a parabolic relationship and their temperature dependence can be expressed ask _p = 5.19 × 10⁻⁷ exp (−125,500/RT) and (k _{p ξ} = 1.15 × 10⁻⁴ exp (−61,500/RT), respectively, with the activation energies in joules/mole. The nitrogen diffusion coefficient in niobium, obtained from microhardness traverses, is given byD = 1.02 × 10⁻⁵ exp (−77,000/RT). A diffusion model accounting for the partition of nitrogen between ε-NbN and Nb is proposed. The total nitrogen uptake calculated from the model is compared to that obtained experimentally.     

Investigation of (Cr,Co)7C3-fcc-graphite equilibrium in the temperature interval 1373 to 1473 K

In the current study, the phase equilibria between fcc, graphite and M₇C₃ (M = Cr,Co) have been studied at 1373 °K, 1423 °K, and 1473 °K. The solubility of Co in the M₇C₃ phase and the solubility of Cr and C in the fcc phase have been determined by the high-temperature equilibration and quenching technique. Appropriate mixtures of Cr₇C₃ + Co or M₇C₃ + Co + C were equilibrated and subsequently quenched in liquid nitrogen. The quenched samples were characterized by X-ray diffraction and by metallographic examination. The studies were carried out on the samples to determine the homogeneity of the sample as well as the phases and their composition. From the results, the compositional regions of the three-phase triangle M₇C₃ + fcc + graphite could be accurately determined. The results show that the Co solubility in the Cr₇C₃ in the experimental temperature interval is higher than previous investigations performed at higher temperatures.     

Calculation of the interfacial energy of B1-type carbides and nitrides with austenite

The interfacial energy of NaCl (B1)-type carbides and nitrides that have a cube-on-cube orientation relationship with austenite was calculated using a discrete lattice-plane (DLP), nearest-neighbor broken-bond (NNBB) method. The required bond energies were evaluated from Miedema’s semiempirical model for alloys and available thermodynamic data. For Ti, V, Zr, and Nb carbides and nitrides, a significant chemical interfacial energy arises from the large difference in the concentration of nonmetallic atoms between austenite and the compound phase. In contrast to binary fcc/fcc cases, where (111)-type interfaces have the smallest interfacial energy, (100)- and (111)-type interfaces have been found to have the smallest and the largest energy, respectively, of all orientations. The structural component of the interfacial energy arising out of lattice misfit is likely to be larger than the chemical component for these compounds. The orientation dependence of the total interfacial energy and the associated Wulff construction indicate that, due to the retention of the strong anisotropy of the chemical interfacial energy, the equilibrium shape of these B1 compounds is a cube, possibly with facets at corners and edges over a wide temperature range.     

Isothermal sections of diagrams of phase equlibria of the (terbium,dysprosium, holmium)-molybdenum-boron systems at 1273°K

Translated from Poroshkovaya Metallurgiya, No. 9(309), pp. 70–73, September, 1988.     

Thermodynamic investigation of the austenite and the delta ferrite in the system Fe-Cr-Mn-N

The nitrogen solubility was measured in a wide range of temperatures including both the austenitic and the ferritic phase of iron-chromium-manganese alloys. The mass contents of the samples were 6.0 to 20.5 % Cr and 5.7 to 16.6 % Mn. Parameters describing the austenite and the delta ferrite in the system Fe-Cr-Mn-N were calculated analyzing all available experimental data. This evaluation yielded an improvement of the known parameters describing the austenitic phase by means of the regular solution model. At the first time, accurate thermodynamic parameters describing the delta phase both in the systems Fe-Mn-N and Fe-Cr-Mn-N could be deduced. By means of the phase diagram program package PD-pp the austenite-delta ferrite phase boundaries of the investigated alloys were calculated. They were compared with experimentally determined temperatures and nitrogen concentrations in the two-phase region. There is a good correspondence between theory and experiment. Using published data describing the melt a calculation of phase diagrams in the high temperature region can be performed.