Phase relationships in the Fe-Cr-Ni system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the Fe-Cr-Ni system as part of a larger study of the Fe-Cr-Ni-C system. The investigation consisted of measurements and modeling of tie-lines and the liquidus surfaces of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria and the peritectic surface involving all three phases in the iron-rich corner of the Gibbs triangle bounded by 0 to 25 wt pct Cr and 0 to 25 wt pct Ni (bal Fe). The temperature ranged from the melting point of iron (1811 K) to about 1750 K. Compositions for tie-lines were obtained from liquid-solid equilibrium couples and temperatures for the surfaces were obtained by differential thermal analysis. Parameters for modeling the system were then selected in the subregular solution model to minimize the square of the difference between experimental and calculated tie-lines. With one ternary parameter employed for each phase, calculations by the model are in excellent agreement with the tie-line and liquidus measurements and in fair agreement with the temperatures for the peritectic surfaceL + δ/L + δ + γ. The usefulness of the model is demonstrated by calculation of the solidification paths of selected alloys in the composition field investigated for the limiting cases of (a) complete equilibrium followed by the alloy system, and (b) no solid diffusion (i.e., segregation) with equilibrium maintained at the solidifying front and complete mixing in the liquid phase.     

Phase relationships in the Fe-Cr-C system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the Fe-Cr-C system. The investigation consisted of measurements of tie-lines and the liquidus surface of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria in the Gibbs triangle, bounded by 0 to 1.4 wt pct C and 0 to 25 wt pct Cr (bal. Fe). The peritectic surface of the three-phase equilibrium was also measured. The temperature ranged from 1811 to about 1750 K. The tie-lines were obtained from liquid-solid equilibrium couples, and the liquidus and peritectic surfaces, by differential thermal analysis (DTA). A statistical procedure was applied to determine from the experimental results the parameters required for a thermodynamic model of the system. Calculations by the model are in good agreement with the experimental results. As a consequence the model can be used to interpolate and extrapolate properties and compositions of phases in equilibrium in the system within the composition and temperature field investigated. D.M. KUNDRAT, formerly Research Fellow at Massachusetts Institute of Technology M. CHOCHOL, formerly Research Assistant, Massachusetts Institute of Technology     

Phase relationships in the iron-rich Fe-Cr-Ni-C system at solidification temperatures

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich comer of the Fe-Cr-Ni-C system as part of a larger study of the Fe-Cr-Mn-Ni-C system. The investigation consisted of measurements of tie-lines for the liquid-delta (bcc) and the liquid-gamma (fcc) equilibria in the iron-rich corner of the Gibbs tetrahedron bounded by 0 to 25 wt Pct Cr, 0 to 25 wt Pct Ni, and 1.2 wt Pct C (bal. Fe). The temperature ranged from 1811 to 1750 K. Compositions for the tie-lines were obtained from liquid-solid equilibrium couples and the temperatures of the equilibrium, by differential thermal analysis (DTA). A mathematical procedure was employed on the experimental data to obtain parameters for a thermodynamic model of the alloy system. This involved minimization of an error function. The details of this analysis are discussed fully in this paper. Calculations by the model employing the “best-set” parameters are in good agreement with the experimental results. The usefulness of the model is demonstrated by calculation of the three-phase equilibrium in the quaternary system as a function of temperature. Formerly Research Fellow, Massachusetts Institute of Technology, is Senior Research Engineer, Armco Inc., Middletown, OH 45043     

Phase relationships in the Al-Ta system

Phase relationships have been investigated in the Al-Ta system in the region between Al and AlTa₂. Al-Ta alloys with compositions between 20 and 60 at. pct Ta were prepared using different processing routes. Following appropriate heat treatments, the alloys were characterized microstructurally, by quantitative electron probe microanalyses, X-ray diffraction, and differential thermal analysis. Based on our observations, a revised Al-Ta phase diagram is proposed. Significant revisions include the verification of Al₃Ta as a line compound, the absence of a eutectic transformation in the region between Al₃Ta and AlTa₂, and the conclusive identification of a phase designated as AlTa near the equiatomic composition.     

Phase relationships in the neodymium-magnesium alloy system

The Nd-Mg system was studied using differential thermal analysis (DTA), X-ray examination, metallography, and microprobe analysis. The following intermetallic compounds were found to exist and their crystal structures confirmed or determined: NdMg (cubic, cP2 CsCl type, melting point 800 °C), NdMg₂ (cubic, cF24 MgCu₂ type, peritectic formation ∼755 °C), NdMg₃ (cubic, cF16 BiF₃ type, melting point 780 °C), and Nd₅Mg₄₁ (tetragonal, tI92 Ce₅Mg₄₁ type, decomposes peritectically at 560 °C). The NdMg₂ phase undergoes a eutectoidal decomposition at 660 °C. Three eutectic equilibria were observed to occur at 42.5 at. pct Mg and 775 °C, 64.5 at. pct Mg and 750 °C, and 92.5 at. pct Mg and 545 °C, respectively. In the Nd-rich alloys, previously determined data^[15] concerning the Mg solubility in α-Nd (8.2 at. pct Mg, ≈550 °C) were accepted. The Mg solubility in β-Nd was evaluated as 34 at. pct Mg at 775 °C. The β-Nd phase was observed to decompose eutectoidally at 17 at. pct Mg and 545 °C. Moreover, in the Mgrich alloys, a metastable NdMg₁₂ phase (tetragonal, tI26 ThMn₁₂ type) was observed in samples quenched from the liquid. The general properties of the Nd-Mg phases are compared with those of the R-Mg compounds and briefly discussed.     

Phase equilibria in the ternary system Sc−Cr−C at subsolidus temperatures

Phase equilibria in the ternary system Sc−Cr−C were investigated by metallography, differential thermal analysis, x-ray diffraction, and electron probe microanalysis. A projection of the solidus surface was constructed for the first time. The nature of phase equilibria in the system is defined by the presence of two thermodynamically stable phases based on the compounds Sc₂CrC₃ (whose existence was confirmed) and ScC_1−x. The melting point of the alloys increases with increasing carbon concentration. Compositions in the 〈Cr〉+〈ScC_1−x〉+〈Sc〉 range have a minimum melting temperature equal to 1018±2°C, and the maximum melting temperature in the system, 1660±2°C, is found in alloys containing 〈Cr₃C₂〉+〈Sc₂CrC₃〉+C. Institute for Materials Science Problems. Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4, pp. 18–26, March–April, 1997.     

Phase transformations and phase equilibria in the Fe-N system at temperatures below 573 K

The phase transformations of homogeneous Fe-N alloys of nitrogen contents from 10 to 26 at. pct were investigated by means of X-ray diffraction analysis upon aging in the temperature range from 373 to 473 K. It was found that precipitation of α″-Fe₁₆N₂ below 443 K does not only occur upon aging of supersaturated α (ferrite) and α′ (martensite), but also upon transformation of γ′-Fe₄N_1-z and ɛ-Fe₂N_1-x (<20 at. pct N). No α″ was observed to develop upon aging of γ(austenite). Therefore, it is proposed that γ′ is a stable phase at temperatures down to (at least) 373 K. Phase formation upon annealing at low temperatures is apparently governed by the (difficult) nucleation and (slow) growth of new Fe-N phases: α″ forms as a precursor for α because of slow nitrogen diffusion, and nitrogen-enriched ɛ develops as a precursor for γ′ because of a nucleation barrier.     

Phase equilibria in the nickel corner of the Mo-Ni-B system at temperatures close to melting

The Mo-Ni-B alloys prepared by arc-melting are examined in as-cast and annealed states using x-ray diffraction, scanning electron microscopy with electron probe microanalysis, and differential thermal analysis. The temperatures of invariant equilibria are refined. The projections of liquidus and solidus surfaces are plotted for the Ni-rich region.     

Phase equilibria in the solidification range of alloys of the Ti-Ru-Ir system

Conclusions The processes taking place during solidification of the alloys of the Ti-Ru-Ir ternary system have been examined for the first time. The results show that the system contains three nonvariant quaternary equilibria with the liquid of the incongruent type: L + + at 2220°C, L + + at 1920°C, and L + + at 1465°C.In the region with the composition 0–50% Ti, the processes take place in the direction to the Ti-Ru side of the concentration triangle where they are also completed at the temperature of the binary eutectic L '+ equal to 1855°C. In the region with the composition 50–100% Ti the processes are completed at 1460°C at the nonvariant point corresponding to the eutectic reaction L + of the Ti-Ir binary system.Translated from Poroshkovaya Metallurgiya, No. 11(299), pp. 72–77, November, 1987.     

Phase relationships and thermodynamic properties in the Mn-Ni-C system

In the present work, phase relationships in selected phase regions of the Mn-Ni-C system have been investigated at 1073 and 1223 K by use of an equilibration technique. Alloys of Mn-Ni-C were prepared from pure Mn, Ni, and C powders by the powder metallurgy method. The phase identification of the heat-treated samples was carried out by scanning electron microscope (SEM) and transmission electron microscope (TEM). The main phase compositions of the alloys have been analyzed by X-ray diffraction (XRD). The experimental results show that the site fraction of Ni in the metallic sublattice of the carbides M₂₃C₆, M₇C₃, and M₅C₂ is quite low and the value is around 0.02 to 0.03. The thermodynamic activities of manganese in 16 different Mn-Ni-C alloys have been studied by solid-state galvanic cell technique with single-crystal CaF₂ as the solid electrolyte in the temperature range 940 to 1165 K. The results are discussed in light of the available thermochemical information.     

Phase stability relationships and glass formation in the system Cu-Ag-In

Details are presented of phase relationships in the ternary system Cu-Ag-ln. Isothermal sections have been determined at 505 and 676 °C, and a most likely projected surface of primary crystallization is proposed. The phase relationships in this system are dominated by an extensive γ-phase (a Hume-Rothery “electron phase”) which extends approximately along a line of 30 at. pct In between the respective binary phases. The stability of the γ-phase follows approximately constant electron concentration lines and appears to be also enhanced by size relationships in the ternary system. The glassforming ability (GFA) in the Cu-Ag-ln system is poor, being limited to only a narrow region in the vicinity of alloy Cu₅₁Ag₃₁In₁₈. This behavior appears to be directly related to the stability of the γ-phase. An attempt has been made to interpret the GFA in terms of the most likely behavior of the T₀ surfaces.     

Phase relationships and thermodynamic properties of the Pd-S system

Sulfur pressures in the Pd-S system have been measured for liquid mattes and PdS/liquid matte mixtures by a Knudsen effusion technique, and for Pd/matte, Pd4S/matte, and Pd/Pd4S mixtures by an emf method employing a solid oxide electrolyte. The phase diagram has also been determined by conventional DTA methods from 0 to 50 at. pct sulfur, using microprobe analysis to confirm the identification of equilibrated phases. From this information, the standard free energies of formation of solid and liquid Pd₄S, Pd₃S, and Pd₁₆S₇ and solid PdS have been derived. For the reaction of solid Pd with S₂ gas to form solid sulfide, the values are ΔG°_TPd₄S = -184400 + 84.10T, ΔG°_TPd₃S =-172430 + 83.68T, ΔG°_TPd_16/7S = -182340 + 105.66T, and ΔG°_TPd_16/7S = -145180 + 88.12T, all in joules. The results for Pd₄S differ considerably from the previously accepted values.     

Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

To establish correlations between microstructure and mechanical properties for the Ti-ll alloy, twelve different combinations of hot die forging and heat treatment, in the a + 8 and Β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566‡C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior. Formerly a National Research Council Associate, Air Force Materials Laboratory Formerly with AFML     

Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

To establish correlations between microstructure and mechanical properties for the Till alloy, twelve different combinations of hot die forging and heat treatment, in the α+β and β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566°C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior.     

Phase equilibria in the Ni-Al-Ti system at 1173 k

The phase equilibria at 1173 K have been determined in the Ni-AI-Ti system for Al contents less than 50 at. pct. The extent of theH (Ni₂AlTi) phase field has been established as well as the extent of solubility in the binary compounds γ (Ni₃Al), ν(Ni₃Ti), β₂(NiTi), NiTi₂, and ζ(AlTi₃). Substantial differences were found between the phase equilibria determined in this study and previous studies, in part due to the large solubility of Al in NiTi₂.