Thermodynamic calculation of phase diagrams in the iron-nitrogen system

On the basis of recent measurements of the equilibria between N₂ and NH₃ containing gas atmospheres, respectively, and of the nitrogen solved in the α, γ and ε iron phases a thermodynamic analysis of the Fe—N system has been carried out. The Gibbs free energy of the γ, ε transformation of pure iron was derived. The composition Fe_4.3N was calculated concerning the γ’ phase above 593°C. The phase boundaries were stated as temperature functions of the N₂ equilibrium pressures. The resulting phase diagram shows the stability regions of the α, γ, γ’ and ε phases and an α/γ/ε triple point which corresponds to that of the pure iron.     

On the iron-carbon phase diagram

Experimental results that are obtained by electron microscopy, X-ray diffraction, and carbide analysis and indicate the precipitation of carbon atoms clusters in a hypereutectoid steel during its annealing above the eutectoid temperature are presented. These results are compared to the reported data in order to construct a new Fe-C phase diagram, where cementite forms below the eutectoid temperature due to the tendency of the Fe-C system toward ordering and carbon unbound to iron precipitates above this temperature in the form of clusters or graphite particles due to the tendency of this system toward phase separation.     

On the Mn-MnS phase diagram

Using a DTA technique the melting point of pure MnS has been determined to 1655 ±5°C. The monotectic temperature in the Mn-MnS system was found to be 1570 ±5°C. With these new data a thermodynamic analysis of the Mn-MnS system was carried out applying a previously developed thermodynamic model.     

Calculation of the titanium-aluminum phase diagram

The Ti-Al phase diagram has been calculated by optimization of Gibbs energies with respect to phase diagram and thermochemical data.T ₀ curves, the locus of compositions and temperatures where the Gibbs energies of the liquid and one of the solid phases are equal, have been calculated over the entire composition range. In order to assure physically reasonable extrapolations of theT ₀ curves of the ordered phases far from their equilibrium stability ranges, the Bragg-Williams approximation was used to provide start values for the empirical optimizations. This approximation led to good convergence of the optimizations, and only small deviations from the Bragg-Williams Gibbs energies were needed to obtain excellent agreement with experimental data.     

Thermodynamics and phase diagram of the Fe-C system

A critical review of published data provides a fairly accurate knowledge of the thermodynamic properties of all of the phases of the system Fe-C that are stable or metastable at atmospheric pressure. Selected data are shown as tables and equations. A proposed phase diagram differs only slightly from others recently published but has the following features. Peritectic compositions and the α-γ equilibrium are shown to agree with measured values of the activity of iron in the solid and liquid solutions and the thermodynamic properties of pure iron. Of all the reported carbides of iron only two may be studied under equilibrium conditions. The solubilities of cementite and of χ-carbide in α-Fe are deduced from measured equilibria. Both are metastable at all temperatures with respect to graphite and its saturated solution in iron. The χ-carbide becomes more stable than cementite below about 230° Certain published data on ε-carbide permit an estimate of its free energy as a precipitate during the aging process.     

The Dy-Zn phase diagram

The dysprosium-zinc phase diagram has been investigated over its entire composition range by using differential thermal analysis, (DTA) metallographic analysis, X-ray powder diffraction, and electron probe microanalysis (EPMA). Seven intermetallic phases have been found and their structures confirmed. DyZn, DyZn₂, Dy₁₃Zn₅₈, and Dy₂Zn₁₇ melt congruently at 1095 °C, 1050 °C, 930 °C, and 930 °C, respectively. DyZn₃, Dy₃Zn₁₁, and DyZn₁₂ form through peritectic reactions at 895 °C, about 900 °C and 685 °C, respectively. Four eutectic reactions occur at 850 °C and 30.0 at pct Zn (between (Dy) and DyZn), 990 °C and 60.0 at pct Zn (between DyZn and DyZn₂), 885 °C and 76.0 at pct Zn (between DyZn₃ and Dy₃Zn₁₁), and 875 °C and 85.0 at pct Zn (involving Dy₁₃Zn₅₈ and Dy₂Zn₁₇). The Dy-rich end presents a catatectic equilibrium; a degenerate invariant effect has been found in the Zn-rich region. The phase equilibria of the Dy-Zn alloys are discussed and compared with those of the other known RE-Zn systems (RE=rare earth metal) in view of the regular change in the relative stabilities of the phases across the lanthanide series     

The neodymium-gold phase diagram

The Nd-Au phase diagram was studied in the 0 to 100 at. pct Au composition range by differential thermal analysis (DTA), X-ray diffraction (XRD), optical microscopy (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Six intermetallic phases were identified, the crystallographic structures were determined or confirmed, and the melting behavior was determined, as follows: Nd₂Au, orthorhombic oP12-Co₂Si type, peritectic decomposition at 810 °C; NdAu, R.T. form, orthorhombic oP8-FeB type, H.T. forms, orthorhombic oC8-CrB type and, at a higher temperature, cubic cP2-CsCl type, melting point 1470 °C; Nd₃Au₄, trigonal hR42-Pu₃Pd₄ type, peritectic decomposition at 1250 °C; Nd₁₇Au₃₆, tetragonal tP106-Nd₁₇Au₃₆ type, melting point 1170 °C; Nd₁₄Au₅₁, hexagonal hP65-Gd₁₄Ag₅₁ type, melting point 1210 °C; and NdAu₆, monoclinic mC28-PrAu₆ type, peritectic decomposition at 875 °C. Four eutectic reactions were found, respectively, at 19.0 at. pct Au and 655 °C, at 63.0 at. pct Au and 1080 °C, at 72.0 at. pct Au and 1050 °C, and, finally, at 91.0 at. pct Au and 795 °C. A catatectic decomposition of the (βNd) phase, at 825 °C and ≈1 at. pct Au, was also found. The results are briefly discussed and compared to those for the other rare earth-gold (R-Au) systems. A short discussion of the general alloying behavior of the “coinage metals” (Cu, Ag, and Au) with the rare-earth metals is finally presented.     

The Nd-Zn phase diagram

Metallographie, thermal, and X-ray techniques were used to determine the phase relations in the Nd-Zn system. Eight compounds, three eutectics and a eutectoid were found. The compounds NdZn, NdZn₂, and Nd₂Znn melt congruently at 923°, 925°, and 981°C respectively. The compounds Nd₃Zn₁₁, NdZn_4.46, and Nd₃Zn₂₂ undergo peritectic decomposition at 870°, 902°, and 950°C respectively, while NdZn₃ undergoes peritectoid decomposition at 849°C. The eutectics occur at 12 wt pct Zn and 630°C, 38 wt pct Zn and 868°C, and 56 wt pct Zn and 854°C. The eutectoid occurs at 4 wt pct Zn and 622°C. The existence of a NdZn₁₂ phase of the SmZn₁₂ type structure has been confirmed. An allotropie transformation between the tetragonal NdZn₁₁ structure and the hexagonal NdZn₁₂ defect structure is proposed.     

Liquidus surface of the Mg-Sm-Tb phase diagram

Differential thermal, electron microprobe, and X-ray diffraction analyses and metallography are used to study Mg-Sm-Tb alloys containing up to 30% Sm or Tb. Polythermal sections and the solidification surface of the Mg-Sm-Tb phase diagram are constructed for the Mg-rich region. In the composition range under study, nonvariant transition-type equilibrium L + Mg₂₄Tb₅ = (Mg) + Mg₄₁Sm₅ is found to exist at a temperature of 539°C.     

A thermodynamic analysis of the iron-nitrogen system

An attempt is made to rationalize and bring into mutual agreement different types of information on the phase equilibria in the Fe-N system by applying simple thermodynamic models to the individual phases. The attempt was successful except for some information regarding the ∈ phase. As a result, equations and curves describing the various phase equilibria are recommended.     

Al-rich portion of the Al-Hf phase diagram

Electrical resistivity measurements, differential thermal analysis, optical microscopy, and the deposition of hafnium compound particles from a melt are used to study the Al-rich portion of the Al-Hf phase diagram. Prominence is given to the hafnium solubility in solid and liquid aluminum. The studies show a peritectic character of the invariant reaction during the solidification of alloys with sufficiently high hafnium contents and a slight difference between the peritectic and melting temperatures of pure aluminum. The hafnium solubility in solid and liquid aluminum is shown to increase with the temperature. The hafnium solubility in solid aluminum at the peritectic temperature (maximum solubility) is 1.00 wt % (0.153 at %); the hafnium solubility in liquid aluminum at this temperature is 0.43 wt % (0.065 at %).     

The phase diagram of the Ni-VC-NbC system

Conclusions The Ni-VC_0.87-NbC_0.9 system has a ternary eutectic in the solidification of which the equilibrium phases are an Ni-base solid solution, (V, Nb)C carbide containing 10% NbC_0.9 and 90% VC_0.87, and (V, Nb)C carbide containing 20% VC_0.87 and 80% NbC_0.9. The point of the four-phase nonvariant equilibrium is in the region of the alloy containing 3% NbC_0.9 and 6% VC_0.87 and the temperature of the equilibrium is 1300±15°C. The diagram of the phase equilibria (Fig. 3) of this system has the same form as for the Ni-TiC-ZrC and Ni-TiC-HfC systems [9, 10].Translated from Poroshkovaya Metallurgiya, No. 8(296), pp. 67–79, August, 1987.