Formation of metastable phases of Ni-C

Mechanical alloying (MA) of nickel and graphite powders was performed in the composition range Ni_1-xC_x (x = 0.10 to 0.90) by use of a conventional ball mill. The structure of me-chanically alloyed samples was examined by X-ray diffraction, transmission electron micros-copy (TEM), scanning electron microscopy (SEM), and differential scanning calorimetry. A remarkable supersaturation of carbon in face-centered cubic (fcc) nickel phase was observed. A metastable phase Ni₃C was formed by a prolonged MA treatment. For the purpose of com-parative study, MA of cobalt and graphite powders was also performed in composition Co_1-xC_x (x = 0.10, 0.15, and 0.30). The supersaturation of carbon in fcc cobalt and formation of a metastable carbide Co₃C were confirmed.     

Calorimetric study and description of the composition dependence of the mixing enthalpy of liquid Cu—Fe—Co aloys

The partial enthalpy of cobalt in ternary liquid Cu-Fe-Co alloys is studied at a temperature of 1873 K along sections characterized by ratios x _Cu/x _Fe = 3, 1, and 1/3 in the composition range x _Co = 0–0.55. The experiments have been carried out on a high-temperature isoperibolic calorimeter. The composition dependences of the partial mixing enthalpy of the cobalt and the integral mixing enthalpy of Cu-Fe-Co melts are described using the Muggianu-Redlich-Kister equation over the entire concentration triangle. The contributions of a ternary interaction to the partial mixing enthalpy of cobalt and the integral mixing enthalpy of Cu-Fe-Co melts are calculated.     

Thermodynamics of Liquid Alloys and Metastable Phase Transformations in the Copper - Titanium System

We have used solution calorimetry at temperatures of 1573 K and 1873 K over broad concentration ranges to study the mixing enthalpy of Cu - Ti liquid alloys. The molar mixing enthalpies of the system are significant negative values. We have established the temperature dependence of the molar mixing enthalpies of the system: there is an increase in their exothermicity as the temperature is lowered. The significant negative mixing enthalpies of the system allow us to conclude that the chemical bonds are localized in the studied melts and consequently associates form. We tested this conclusion within ideal associated solution theory, which describes well the results obtained with a set of CuTi and CuTi₂ associates. Using the model obtained, we have calculated the excess thermodynamic functions of mixing (enthalpy, Gibbs free energy, heat capacity) for the liquid alloys. We estimated the Gibbs energies of fcc, bcc, and hcp solutions in the system by the CALPHAD method, using data from the initial sections of the phase diagrams and from the corresponding thermodynamic data. We have calculated the metastable phase equilibria between the limiting solid solutions and the liquid or supercooled liquid phase. It was shown that for the supercooled liquid and the amorphous phase, a broad concentration range of relative thermodynamic stability can be obtained. The concentration range of amorphization of Cu - Ti melts corresponds to the position of the metastable liquidus line and the T₀ line at temperatures close to the temperature range of amorphous solidification. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 67–80, May–June, 2005.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part I. Theoretical analysis of nucleation behavior

The selection of the primary solidifying phase in undercooled stainless steel melts is theoretically analyzed in terms of nucleation theory. Nucleation phenomena are considered using different models for the solid-liquid interface energy. The classical nucleation theory for sharp interfaces and an improved modification, the diffuse interface theory, are applied. The influence of deviations of the nucleus composition from the overall alloy composition is also revealed. A preferred nucleation of the metastable bcc phase in fcc equilibrium solidification-type alloys is predicted. The critical undercooling of metastable crystallization as a function of alloy composition is calculated for an isoplethal section at 69 at. pct Fe of Fe₆₉Cr_31-x Ni _x alloys. The results are summarized in a phase selection diagram predicting the primary solidification mode as a function of undercooling and melt composition.     

Solubility of Li<Subscript>2</Subscript>O in an LiCl–KCl Melt

The phase diagrams of the binary LiCl–Li₂O system and the quasi-binary [LiCl–KCl]–Li₂O systems containing 10 and 20 mol % KCl have been built using thermal analysis of cooling curves and isothermal saturation curves. The solubility of Li₂O in LiCl–KCl melts is determined in the temperature range 500–800°C. The solubility of Li₂O in the melts LiCl, LiCl–KCl (10 mol %) and LiCl–KCl (20 mol %) decreases as the KCl content increases;at 650°C, it is 11.5, 7.7, and 3.9 mol %, respectively.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part II. Containerless solidification experiments

The solidification behavior of undercooled Fe-Cr-Ni melts of different compositions is investigated with respect to the competitive formation of δ-bcc (ferrite) and γ-fcc phase (austenite). Containerless solidification experiments, electromagnetic levitation melting and drop tube experiments of atomized particles, show that δ (bcc) solidification is preferred in the highly undercooled melt even at compositions where δ is metastable. Time-resolved detection of the recalescence events during crystallization at different undercooling levels enable the determination of a critical undercooling for the transition to metastable bcc phase solidifcation in equilibrium fcc-type alloys. Measurements of the growth velocities of stable and metastable phases, as functions of melt undercooling prior to solidification, reveal that phase selection is controlled by nucleation. Phase selection diagrams for solidification processes as functions of alloy composition and melt undercooling are derived from two types of experiments: X-ray phase analysis of quenched samples and in situ observations of the recalescence events of undercooled melts. The experimental results fit well with the theoretical predictions of the metastable phase diagram and the improved nucleation theory presented in an earlier article. In particular, the tendency of metastable δ phase formation in a wide composition range is confirmed.     

Reaction of Tungsten with Melts in the Cu - Co System

We have studied the solubility at 1200°C of tungsten in Cu - Co melts and the growth kinetics of a W₆Co₇ layer at the tungsten — melt interface. We have established the composition of the melt in the three-phase equilibrium tungsten — W₆Co₇ — melt: 0.0195 Co, 4.8 · 10⁻⁵ W, the rest is Cu (in atomic fractions). In the studied composition range for the melt, the solubility of tungsten is described well by the expression: lgX_W = −7.117 + 25.7 · X_Co ^1/2 − 41.06 · X_Co, where X_W and X_Co are the atomic fractions of the corresponding elements in the melt. We have determined the correlation between the growth rate for a layer of tungsten-containing phase at the tungsten — melt interface and the thermodynamic characteristics of the melt. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 81–86, May–June, 2005.     

Probing the Relationship of Long-Range Order in Nanodomain FeCo Alloys with Ternary Additions Using Neutron Diffraction

The effects of tailoring FeCo alloys with addition of ternary elements Pt, Pd, Mn, Ir, and Re have been investigated. In the composition range of 30 to 70 pct Co, FeCo alloys undergo a continuous order-disorder phase transition at a maximum temperature of 730 °C at the equiatomic composition. The effects of temperature and composition on the degree of long-range order in six alloys (Fe₇₀Co₃₀, Fe₆₇Co₃₀Pt₃, Fe₆₇Co₃₀Pd₃, Fe₆₇Co₃₀Mn₃, Fe₆₇Co₃₀Ir₃, and Fe₆₇Co₃₀Re₃; at. pct) were compared using neutron diffraction. The transition goes along with growing of antiphase domains during the heating process.     

Nonequilibrium solidification of Fe-P alloys

The specific features of solidification of Fe-P alloys in the concentration range 5?C25 at % P have been studied by differential thermal analysis and X-ray diffraction. The equilibrium ??-Fe + Fe₃P eutectic is shown to form immediately from melt only for compositions with 17?C19 at % P when a melt is superheated below temperatures of 1130?C1160°C, at which the type of composition short-range ordering changes (Fe₃P ?? Fe₂P) under insignificant (??50°C) supercooling conditions. A nonequilibrium ??-Fe + Fe₂P eutectic forms during cooling of the alloy at a large (??200°C) supercooling.     

Viscosity of niobium-containing oxide-fluoride melts

Influence of Nb₂O₅ on viscosity of the CaF₂-based and CaF₂-30% Al₂O₃-based melts was investigated using a vibration viscosimetry method in the range 1523–1873 K. It is found that viscosity of the melts rise and the crystallization range shifts to lower temperatures as the Nb₂O₅ concentration increases from 0 to 25 wt %. The activation energy of viscous flow increases from 34 to 110 kJ mol^?1 and from 56 to 148 kJ mol^?1 for the CaF₂-based and CaF₂-30% Al₂O₃-based melts, respectively. The obtained data indicate the complexing behavior of niobium in the oxide-fluoride melts.     

Kinematic viscosity of Mg-Al melts in the magnesium-rich portion

Kinematic viscosity ν of Mg-Al melts containing up to 14 at % second component is measured by the method of damped torsional vibrations of a crucible with a melt. The measurements are performed in the temperature t range between liquidus and 900°C upon heating of a sample after its melting and upon subsequent cooling. Kinematic viscosity isotherms and the concentration dependences of the activation energy of viscous flow are constructed.     

Microstructure of an Si-Cr-Co alloy for magnetron sputtering targets

The influence of the cooling rate on the microstructure of an Si-54% Cr-6% Co cast alloy for magnetron sputtering targets is discussed. The alloy contains highly brittle phases CrSi, CrSi₂ and Cr₃Co₅Si₂. It is experimentally established that the required quality of a cast target of the considered composition cannot be achieved at high cooling rates of the alloy. Slow cooling can be achieved using a heated casting mold by adjusting the alloy cooling rate. It has been demonstrated that the alloy has the most uniform structure and the lowest porosity at cooling rates in the range 0.5–5°C/s.     

Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10 and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystallization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.     

The V-VO phase system

A phase diagram is proposed for the V-VO system based on melting point determinations, differential thermal analyses, metallographic observations, and X-ray parametric measurements. A eutectic reaction occurs at 1640°C and 29 at. pct O. The intermediate phases V₉O and V₂O form peritectoidally at 510° and 1185°C, respectively, while V₄O forms by a peritectic reaction at 1665°C. The VO phase melts congruently at 1790°C. The terminal solubility of oxygen in vanadium increases from 3.2 at. pct at room temperature to a maximum of 17.0 at. pct at the peritectic temperature. There is also extensive solid solubility associated with each of the intermediate phases. Two martensite-like phases form in alloys in the composition range 6 to 9 at. pct O upon quenching from above the 510°C peritectoid horizontal.     

Metastable phases in the Tl-Sn alloy system

The structures of metastable Tl-Sn alloys prepared by rapid quenching (splat cooling) to —190°C have been investigated. Over a large part of the total composition range single phase alloys were obtained. Four new metastable phases with relatively simple, elementlike structures were found: α₁ (tetragonal), ω (hexagonal), γ (hexagonal), and γ₁ (not determined in detail); α₁ represents the rarely found transition between the A1 and A2 structure types. Crystalchemically, the new phases are in agreement with previously established general rules for the occurrence of stable and metastable B-metal phases.     

Structure and properties of the slags of continuous converting of copper nickel-containing mattes and concentrates: IV. Influence of the slag temperature and composition on the slag melt viscosity

The viscosities of the slags that form during the continuous converting of copper mattes and concentrates into blister copper are determined. The compositions of SiO₂-CaO-Al₂O₃-FeO _x -Cu₂O-NiO slag melts are varied over a wide range of (Fe + Ni) concentrations (25.5–40.2 wt %) and ratios SiO₂/CaO (0.25–3.9) and (Fe)/(Cu + Ni) (0.4–1.6). The slag viscosity is measured by a vibrational method in the temperature range 1050–1450°C. The viscosity of a homogeneous melt is shown to be 0.5–2.0 P, and it increases sharply below the primary solidification temperature.     

Nonequilibrium Solidification Behavior of Co-Si Alloys Near the First Eutectic Point

Adopting a fluxing purification and cyclic superheating technique, Co-10 wt pct Si and Co-15 wt pct Si alloys had been undercooled to realize rapid solidification in this work. It was investigated that the solidification modes and microstructures of Co-Si alloys were deeply influenced by the undercooling of the melts. Both alloys solidified with a near-equilibrium mode in a low undercooling range; the peritectic reaction occurred between the primary phase and the remnant liquids, and it was followed by the eutectic reaction and eutectoid transformation. With the increase of undercooling, both alloys solidified with a nonequilibrium mode, and the peritectic reaction was restrained. As was analyzed, a metastable Co₃Si phase was found in Co-10 wt pct Si alloy when a critical undercooling was achieved.     

Thermodynamics of oxygen solutions in Fe-Mn melts

Oxygen solutions in Fe-Mn melts are analyzed thermodynamically. The composition of the oxide phase is determined, and the equilibrium oxygen concentrations in Fe-Mn melts are calculated over a wide composition range. The oxide phase mainly contains MnO: even at a molar fraction of manganese of 0.02 in the melt, the molar fraction of manganese oxide in the slag is more than 0.9. This is due to a much higher oxygen affinity of manganese as compared to iron; that is, manganese additives to iron considerably decrease the oxygen solubility. When the manganse content in the melt is 19.32%, the oxygen solubility curve has a minimum corresponding to an oxygen concentration of 5.136 × 10^?3%. However, a further increase in the managanese content results in an increase in the oxygen concentration in the melt. In liquid manganese, the oxygen saturation concentration at 1873 K is 0.0472%. The interaction parameter e _o(Mn) ^o (?0.207) and the activity coefficient γ _o(Mn) ^o (1.131 × 10^?4) have been calculated for the first time.     

Influence of the structure of the AlSi20 foundry alloy on the microstructure and viscosity of the Al-6% Si model silumin in solid and liquid states

The influence of the structure of starting charge alloys Al-20 wt % Si on their structure after single remelting and crystallization with an identical cooling rate of 20 K/s is investigated. It is established that the structural transformation from the starting charge alloys stably retains, being translated in the “solid-liquid-solid” system. The viscosimetry investigation of the melts obtained from the coarse-crystalline and the fine-crystalline charge alloys in the heating mode to 1350°C and the subsequent cooling confirmed the conclusion that these melts are translators of structural information. It is established that the branching temperature (hysteresis) of viscosity polytherms (t _h) in the heating and cooling modes for the melt made from the coarse-crystalline charge alloy is 1100°C, while that for the fine-crystalline charge alloy is 1000°C. It is noted that, in the second case, the viscosity in the cooling mode below t _h is elevated, while the polythermal branch ascends more steeply. These results indicate the favorability of using special methods of processing the charge metals with the goal of laying positive structural information into them. The use of this approach is most reasonable when obtaining aluminum-based foundry alloys.