C-Ni and Al-C-Ni phase diagrams and thermodynamic properties of C in the alloys from 1550 °C to 2300 °C

The C-Ni and Al-C-Ni phase diagrams were determined by chemical analysis of alloys saturated with carbon within sealed graphite crucibles. The solubility of carbon in nickel over the temperature range 1550 °C to 2300 °C is given by log (at. pct C)=2.0376−1874.68/T, where T is temperature in kelvin. Isothermal sections for the ternary system were determined at intervals of 150 °C over the range of temperatures investigated. The univariant points on the 1700 °C, 1850 °C, and 2000 °C isotherms were determined by metallographic examination of rapidly cooled alloys to be about 67.2Al-1.1C-31.7Ni, 70.3Al-2.3C-27.4Ni, and 82.5Al-7.0C-10.5Ni, respectively, where all concentrations are in atomic percent. Graphite, Al₄C₃ (decomposition temperature 2156 °C), and AlNi (decomposition temperature 1638 °C) were the only solid phases observed within the temperature range investigated. The excess partial Gibbs energy for dissolved C, , in liquid Al-C-Ni solutions in equilibrium with C, as calculated from the experimental solubilities and thermodynamic data on Al-Ni, is

Effect of the phase composition on the properties of a complex magnesium alloy at temperatures from –70 to +300°C

The phase composition and the structural evolution of an Mg–Zn–Zr alloy containing REM are studied in comparison with the commercial MA14 alloy. It is found that the phases formed with REM exhibit high thermal stability. An equiaxial and fine-grained structure and a favorable morphology of the phases provide a high level of the properties of the new alloy with respect to the properties of the commercial MA14 alloy over the wide temperature range from –70 to +300°C.     

Experimental investigations and thermodynamic calculations of the structural and phase composition in the Ti–Si–C system

Thermodynamic calculations of the structural-phase equilibrium in the Ti–Si–C system at 1100–1400°C are performed using the CALPHAD method. Calculated phase diagrams of this system are presented. It is established that 100% of the Ti₃SiC₂ phase is formed with the stoichiometric component ratio. With the deviation of the carbon or silicon content, titanium carbide, titanium disilicide, or silicon carbide appear in the system. The temperature almost does not affect the phase composition in the studied temperature range. The calculated data are compared with the experimental determination of the phase composition of the samples of the mentioned system after the spark-plasma sintering of the mechanically activated powder composition. In practice, the process temperature and duration of high-temperature holding substantially affect the phase composition of the final product, which is associated with the limited rate of solid-phase reactions during the synthesis of compounds. The samples have a grain size of 1–5 μm and hardness of 4–15 GPa, depending the phase composition.     

Effect of silicon on the rate of de-sulphurization of liquid Fe-S alloys by argon-hydrogen at 1550°C

Abstract

The passivation of copper anodes due to precipitation of copper sulfate on the anode surface was investigated as a function of electrolyte composition and temperature, and of anode composition. The slime layer present on the anode surface was shown to be the primary factor in causing passivation by inhibiting the diffusion of copper ions. Factors such as temperature, free acid level, Ni²⁺ and Cu²⁺ ion levels were also important in so far as they affected the mass transfer characteristics of Cu²⁺ ions and the solubility of copper sulfate.

Résumé

La passivation des anodes de cuivre due à la précipitation du sulfate de cuivre sur la surface de l'anode a été étudiée en fonction de la composition et la température de l'électrolyte et aussi en fonction de la composition de l'anode. Nous avons montré que la couche d'impuretés présente sur la surface de l'anode est la cause première de la passivation car elle empêche la diffusion des ions cuivre. D'autres facteurs tels que la température, le niveau d'acide libre, le niveau d'ions Ni²⁺ et Cu² sont également importants en ce qu'ils affectent les caractéristiques de transport de matière des ions Cu²⁺ et la solubilitédu sulfate de cuivre.     

The phase equilibria in W-Fe-Co-Ni system alloys. II. Isotherms of the solidus and the phase composition of alloys containing 10 and 20% (Fe + Co + Ni) at temperatures above 1200°C

Conclusions The isostructural intermediate -phases Fe₇W₅ and Co₇W₆ in the W—Fe—Co system form a continuous series of -solid solutions. In the 1640–1630°C range the L + (Fe₇W₆) peritectic equilibrium in this system changes to a similar L + (Co₇W₆) equilibrium, where is the tungsten-base boundary solution.In the W-Fe-Co-Ni polythermal tetrahedron in the 1470–1460°C range conversion of the L +(Fe₇W₆)+, peritectic equilibrium into the similar L + (Co₇W₆) + , where is the nickel-, -ironcobalt-base boundary solution, occurs.Upon completion of crystallization at 1400°C, the W-Fe-Co system alloys with 10–20% (Fe + Co) have a + phase composition, while the W-Fe-Co-Ni system alloys with 10–20% (Fe + Co + Ni) accordingly have + , + + or + . At temperatures below 1215°C in alloys rich in iron, FeW may be formed instead of -phase and therefore the alloys may have an + FeW, + + FeW, + + + FeW and + + FeW phase composition.Translated from Poroshkovaya Metallurgiya, No. 5(281), pp. 86–89, May, 1986.     

Segregation and order in binary substitutional alloys—II. Ground state phase stability diagrams

A general method to determine phase diagrams and local order from the electronic theory of alloys is presented; we report a first numerical application of this method to the study of binary substitutional transition metal alloys using a canonical tight binding model for the “d” band structure. The relevant physical quantities (chemical, elastic…energies), which are required in such calculations are discussed in detail. Thus the results about the concentration dependence of the pair interactions and the energy of the disordered state are discussed. The stability of the homogeneous ordered structures, against a mixing of two phases is investigated and the general trends about segregation, miscibility and order are then reported. Finally the ground state phase stability diagrams of a series of systems corresponding to physical situations are presented.     

Kinetics of the amorphous to icosahedral phase transformation in AlCuV alloys

Single phase icosahedral samples are obtained by annealing melt-spun Al₇₅Cu₁₅V₁₀ amorphous alloys. The kinetics of this amorphous to icosahedral phase transformation were measured isothermally and nonisotheramally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a “micro-quasicrystal” model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2–2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m² ⩽ α ⩽ 0.015 J/m², demonstrating that the short range order must be similar on both sides of the interface.     

Ordering and phase separation in the ternary Ag-Cu-Zn β-phase alloys

The microstructure of the ternary Ag_xCu_55−xZn₄₅ and Ag_xCu_53−xZn₄₇ β-phase alloys was studied by means of electron microscopy. The range of temperature and composition where the mottled and modulated structures appeared was investigated, and the composition dependence of the spinodal decomposition temperature T_s was also estimated. The composition dependence of T_s had a maximum at about Ag 22 at.%. The critical temperature of the CsCl(B2)-type superstructure T_c decreased with increase in Ag content, but was constant in a range of composition where the phase separation was observed. The precipitation of α-phase (f.c.c.) was observed even in the quenched specimens, that suggests the phase separation of β-phase at a high temperature. The composition dependence of T_c and T_s, was calculated theoretically. The occurrence of the ordering and the spinodal decomposition in the ternary Ag-Cu-Zn β-phase alloys is discussed briefly in terms of the difference of ordering energy that is negative in Ag-Zn and Cu-Zn pairs and positive in Ag-Cu pair.     

Section of the isothermal tetrahedron of the Al-Cu-Mg-Zr phase diagram at 490°C

The phase equilibria in the Al-Cu-Mg-Zr system at 490°C have been studied for Al-rich alloys with 0.3% Zr and from 0 to 10% Cu or Mg. The (Al) solid solution is found to be in equilibrium with only binary θ(CuAl₂) and ZrAl₃ and ternary S (CuMgAl₂) phases of the ternary Al-Cu-Mg system. The section of the isothermal tetrahedron of the Al-Cu-Mg-Zr phase diagram at 490°C, which corresponds to 0.3% Zr and up to 10% Cu or Mg, is constructed.     

Carbon Diffusion Measurement in Austenite in the Temperature Range 500 °C to 900 °C

Carbon diffusion in austenite plays a critical role in phase transformation in steel. However, it can only be estimated in the fully austenitic range and has then to be extrapolated to the temperature range of the phase transformation. Therefore, published data are limited to temperatures above 750 °C. In this study, new experiments are carried out to determine the carbon diffusion coefficient in austenite at temperatures as low as 500 °C. Carburization experiments are performed in the austenitic range for a Fe-1.5 pct Mn 0.13 pct C and a Fe-31 pct Ni alloy (wt pct). Composition profile measurements, which are done using glow discharge optical emission spectrometry (GDOES), show that the surface composition is not constant with time. A methodology has been developed to assess the diffusion coefficient of carbon in austenite combining the measured carbon profiles and a numerical method to compute the diffusion profile taking into account the time evolution of the boundary condition. This method is first validated on the Fe-C-Mn steel. Carburization experiments are carried out on a Fe-31 pct Ni alloy at 900 °C, 800 °C, 700 °C, 600 °C, and 500 °C. The carbon diffusion coefficient is assessed using the method described above and fitted with the following expression (T in Kelvin): \( D = 1.23\cdot10^{{ - 6}} \cdot e^{{ - \frac{{15,050}} {{T{\left( {\text{K}} \right)}}}}} ({\text{m}}^{{\text{2}}} {\text{/s}}) \). The new expression is compared with previous experimental results measured for comparable nickel content at higher temperatures, and it shows a reasonable agreement. The model proposed by Ågren for carbon diffusion has been modified to take into account the thermodynamic contribution of nickel. This model also shows good agreement with the present experimental results, even if it was fitted to experiments performed at higher temperatures.     

The effect of heat treatment variables on the phase transformations at 1420°c in ti-48a1 and Ti-48al-2Mn-2Nb alloys

The effect of heat treatment variables such as initial microstructure, isothermal reaction time and cooling rate on the phase transformations occurring at 1420°C in Ti-48A1 and Ti-48Al-2Mn-2Nb alloys was studied. The main effect of the initial microstructure, which comprised either lamellae of α₂and γ or equiaxed γ grains, was to alter the kinetics, through a change in the chemical driving force of the phase transformations. Therefore, the equiaxed γ grains transformed to α much faster than the lamellar structure and in the initially lamellar structure, growth of α resulted in the delineation of the initial dendritic structure formed during solidification. The effect of the rate of cooling from the heat treatment temperature on the final morphology of these alloys was drastic and resulted in a change in morphology from lamellar grains obtained on furnace cooling to a feathery and mottled morphology obtained on water quenching. TEM analysis of water quenched Ti-48Al-2Mn-2Nb revealed complex morphologies including a structure which consisted of equiaxed γ grains and residual α₂and abutting colonies of γ and α₂. Based on the TEM results, the early stages of formation of γ from α were studied and mechanisms of nucleation and growth discussed. The relative importance and the coexistence of massive and martensitic transformation products is also discussed.     

Lengthening kinetics of (01\bar 10) γ-TiH precipitates in α-Ti in the temperature range of 25 °c to 80 °c

Formation of y hydrides in α titanium involves a hcp → fct lattice transformation with hydrogen as an interstitial diffusing species. Since the crystallography of this reaction resembles that of a martensitic transformation and the hydrides form at temperatures where both titanium and hydrogen atoms may have negligible mobility, many investigators have concluded that γ hydrides form by a martensitic mechanism. In this study, the lengthening kinetics of γ hydrides were determined using conventional transmission electron microscopy (TEM). The length of the hydrides was measured as a function of time, temperature, and hydrogen concentration in samples isothermally aged in the range of 25 °C to 80 °C. These measurements were compared with the Zener-Hillert model of volume-diffusion-controlled precipitate growth. The experimental results indicate that y hydride growth is diffusionally controlled at 25 °C and possibly interfacially controlled at aging temperatures of 50 °C and higher. These and other aspects of the transformation are discussed.     

Calculation of the equilibrium phase diagrams and the spinodally decomposed structures of the FeCuNi system

Thermodynamic stability of ternary systems with miscibility gaps is discussed and applied to the f.c.c. phase of FeCuNi alloys. The stability of the system with respect to infinitesimal composition fluctuation is described. The direction most likely for initial composition fluctuation is defined as the one with the largest negative curvature on the Gibbs energy surface. Along this direction, zero-time wavelength at initial stage of spinodally decomposed alloy structure may be calculated. The thermodynamic values of the f.c.c. FeCuNi alloys are obtained from limited amount of thermochemical and phase equilibrium data. It is found that the ternary interaction parameters for both the f.c.c. and liquid phases of FeCuNi may be approximated from the binary interaction parameters of the constituent binaries.     

Reactions of Fe-Cr and Ni-Cr alloys in CO/CO2 gases at 850 and 950 °C

A series of Fe-Cr and Ni-Cr solid solution alloys was reacted at 850 and 950 °C in CO/CO₂ gas mixtures in which FeO and NiO were unstable. The compctitive tendencies toward the carburization and oxidation of the chromium solute, as compared to a graphical thermodynamic "metastability" criterion, were tested experimentally. Relatively good agreement was found between predictions and experiments for the occurrence of Cr carburization beneath Cr₂O₃ internal oxides or external scales. The chromium contents required for the transition from internal oxidation of Cr to the formation of Cr₂O₃ external scales in CO/CO₂ gas mixtures were established for Fe-Cr and Ni-Cr alloys. The Cr₂O₃ external scales formed on Fe-Cr alloys were found to be relatively impervious to carbon penetration for short (12-hour) experiments. No carburization was observed in the Ni-Cr alloys, but the only alloys that were predicted to carburize were the ones that formed external scales. Formerly Graduate Student, The Ohio State University     

Application of physical–empirical models to calculate a fragment of the phase diagram and the physical properties of bcc Fe–Cr alloys: II. Calculation of phase boundaries,spinodal, and the temperature dependence of the heat capacity of an alloy

The contributions at a temperature of 500 and 600 K of the chemical, elastic, vibrational, magnetic, electronic, and configurational energies to the Gibbs energy of mixing of bcc alloys without regard for the contribution of a short-range order are calculated as functions of composition and temperature using physical–empirical models. The temperature dependences of the heat capacity of an alloy in both one- and twophase states are calculated. The heat capacity jumps calculated for alloys of various compositions can be used to estimate the equilibrium solubility boundaries of Fe–Cr alloys, which can hardly be found from experimental data because of the slow diffusion processes that occur when an equilibrium state is reached. The calculated solubility boundary of bcc solid solutions and the spinodal and the heat capacity of Fe–Cr alloys are compared with the experimental data and the calculation results obtained in other works. The agreement and discrepancy between these data are discussed.     

Grain coarsening in FeNiCr alloys and the influence of second phase particles

The effects of second phase particles, e.g. M₂₃C₆, MC and M(C, N) carbides on the grain growth phenomenon of FeNiCr alloys have been determined. Various theoretical models on grain coarsening have been compared. The grain size at all stages of grain coarsening was dependent on the undissolved carbide particle size (r), the volume fraction (f), and the nature of the carbides. The nature of M₂₃C₆ carbides varied, since Fe, Ni and Mo entered the M₂₃C₆ unit cell; and complex carbides such as (Cr₁₅Fe₄Ni₂Mo₂)C₆ were formed. Gladman's equation was verified for predicting the observed grain size values to a significant degree, and other models were less successful.     

Formation regularities of grain-boundary interlayers of the α-Ti phase in binary titanium alloys

The microstructure of polycrystalline alloys of titanium with chromium (2, 4, and 5.5 wt %), cobalt (2 and 4 wt %), and copper (2 and 3 wt %) is investigated. Series of prolonged isothermal annealing of these materials are performed in a temperature range from 600 to 850°C (in vacuum). Annealing temperatures fall in two-phase regions α(Ti,Me) + β(Ti,Me) of phase diagrams Ti–Cr, Ti–Co, and Ti–Cu. Temperature dependences of the fraction of grain boundaries β(Ti,Me)/β(Ti,Me) completely “wetted” by interlayers of the second solid phase α(Ti,Me) and average contact angle are measured. The results of microstructural investigations showed that the type and concentration of the second component in the alloy strongly affect the formation of equilibrium grain-boundary interlayers. A nonmonotonic temperature dependence of the fraction of grain boundaries completely wetted by interlayers of the second solid phase in the absence of ferromagnet–paramagnet phase transformations in the volume is revealed for the first time.     

On the f.c.c. → h.c.p. phase transformation in high manganese-iron alloys

Electron transmission microscopy was done on the Fe alloys with 29.8 and 37.2 at.% Mn. with the γ→ϵ transformation induced by all three driving forces: temperature change, cold deformation and high-pressure soaking (up to 9.0 GPa (90 kbar)). Contrary to the well established data, the presence of the ϵ phase after cooling was observed at Mn concentrations not less, than 37at.%. It was proved by direct observation that high-pressure soaking and cold deformation strongly affect phase equilibria in Fe-Mn alloys: this effect is dependent on the Mn concentration, decreasing with it. The familiar orientation relationships between γ and ϵ phases, namely (1̄11)_γ ∥(0001)_ϵ, and [101̄]_γ ∥[112̄0]_ge, are observed in all obtained structures independently of the kind of driving force applied to promote the γ→ϵ transformation. It is proposed to explain the unusual behaviour of Fe-Mn alloys (limited growth of the ϵ martensitic phase upon cooling in the vicinity of M^γ→ϵ_s. very small sizes of the ϵ particles) by a model which takes into account the antiferromagnetic ordering in both phases. It is shown that this explanation is consistent with experimental results.