Evaporation behavior of aluminum during the cold crucible induction skull melting of titanium aluminum alloys

Taking the Ti-Al binary alloy as an example, this article studied the evaporation behavior of Al during the cold crucible induction skull melting (ISM) process of titanium alloys. A formula was deduced to predict the activity of Al in a molten Ti-Al binary system. The calculated activity of Al negatively deviates from an ideal solution. A model was established to judge the evaporation controlling mode and, on this basis, several conclusions were obtained. (1) The evaporation controlling mode of Al in molten Ti-Al transfers from the evaporation reaction controlling mode to the double controlling mode (diffusion and evaporation reaction) with increasing melt temperature (T _ms) and/or Al content (x _Al) and/or decreasing pressure (P) in the melting chamber. (2) The expression P≤P _crit (P _crit≈0.44 P _e(Al)) is a criterion used to judge whether the evaporation is in the state of free evaporation. (3) The term P _impe (P _impe=(3.5 to 4) P _e(Al)) is a critical value which impedes the evaporation loss. Almost all of common used ternary additions could enhance the activity of Al in molten Ti-Al and, accordingly, aggravate the evaporation of Al, except for Zr. The enhancing sequence is Y, Ni, Nb, Mn, V, Fe, Cr, Mo, Cu, Si, W, Mg, B, and Sn. The Al evaporation mass-transfer losses, measured from the melting experiments of several titanium aluminum alloys, were in reasonable agreement with the calculated results.     

Kinetics of Zn and Pb evaporation from liquid iron

The evaporation kinetics of Zn and Pb from liquid iron at 1600°C were studied under argon pressure 0.1 to 50 bar in an induction furnace. It was found that the limiting step of evaporation under p_Ar < 1 bar is diffusion in the melt. Diffusion in the gas phase becomes rate-determining with increasing argon pressure. The phase boundary reaction has no effect on the evaporation of Zn and Pb. The evaporation of Zn can evidently be reduced by increasing argon pressure above 1 bar, while the evaporation of Pb is not significantly effected by changes of argon pressure.     

Vacuum distillation of copper matte to remove lead,arsenic, bismuth,and antimony

Vacuum-refining experiments were carried out on copper matte melts, containing 35 to 73 pct Cu, to measure the removal rates of lead, bismuth, arsenic, and antimony over the temperature range of 1373 to 1523 K under pressures in the range of 50 to 130 Pa. High rates of refining, controlled by mass transport in the liquid phase, were achieved for all impurities in melts containing up to 65 pct Cu and for chamber pressures less than 100 Pa. After 40 to 60 minutes of treatment, lead elimination was between 70 and 96 pct, bismuth elimination was between 88 and 98 pct, arsenic, elimination was between 60 and 93 pct, and antimony elimination was between 40 and 92 pct. The overall mass transfer coefficients for vacuum refining for the impurities considered fell in the range of 5×10⁻⁵ to 2×10⁻⁴ m s⁻¹. The values were insensitive to small changes in melt temperature but decreased with increasing pressure above 250 Pa. Also, the rates of refining were seen to be influenced by the sulfur and oxygen activity in the melt. The evaporation and subsequent elimination of the impurities were mathematically modeled by extending previously published models beyond consideration only of evaporation of monomers to consideration of the evaporation of monomers and dimer compounds. The model showed that due to the contributions to the refining by evaporation of each of the metallic, oxide, or sulfide vapor species, arsenic and antimony exhibited a maximum in refining rates atP _{o 2} andP _{s 2} potentials corresponding to matte grades of about 55 pct copper, whereas lead showed a minimum at about the same matte grade, and bismuth showed a continuously decreasing rate of refining with increasing matte grade. The model was also used to simulate the refining behavior of copper matte melts. An example of a commercial-scale operation is given.     

Prevention of macrodefects in squeeze casting of an Al-7 wt pct Si alloy

The squeeze casting of an Al-7 wt pct Si alloy was carried out in order to investigate the conditions for the formation and the prevention of macrosegregation. The effects of process parameters such as applied pressure, die temperature, pouring temperature, delay time, degassing, and inoculation on the formation of macrosegregation were investigated, in correlation with the evolution of macrostructure and shrinkage defects. Three critical applied pressures were defined, based on the experimental results for the squeeze-cast Al-7 wt pct Si. The first is the critical applied pressure under which shrinkage defects form (P _SC). The second is the critical applied pressure above which macrosegregates form (P _MS). The third is the critical applied pressure above which and under which minor segregation forms. (P _m and P _MS, respectively). With the concept of these three critical pressures, an experimental diagram describing the optimum process conditions was proposed for obtaining sound squeeze castings. It was concluded that sound castings without macrosegregation and shrinkage defects can only be obtained when the applied pressure is in the range where P _SC < P<P _m (<P _MS). Both degassing and inoculation treatments greatly enhanced the soundness of the castings. It was also found that the pouring temperature and the delay time should not exceed T _D-critical and t _D-critical, respectively, in order to achieve sound castings.     

Characteristics of AA6061/BN composite fabricated by pressureless infiltration technique

Spontaneous infiltration and strengthening behaviors were analyzed in terms of microstructures and tensile properties of AA6061/BN composite fabricated by pressureless infiltration technique in the presence of both Mg and nitrogen. The microstructure and properties were compared with control AA6061 without BN fabricated by the same method. The Mg₃N₂ formed by the reaction of Mg vapor and nitrogen gas, which coated the particles in the powder bed, is believed to induce spontaneous infiltration through greatly enhancing wetting by means of the reaction Mg₃N₂ + 2Al → 2AlN + 3Mg. This was identified by the finding of AlN particle layers on the surfaces of prior Al particles in the powder bed, which made contact with the infiltrating melt. In addition, unreacted Mg₃N₂ was observed outside the composite, where the Al melt did not come into direct contact. Fine AlN particles formed in situ resulted in significant strengthening, even in the control alloys with no addition of BN. In the composite reinforced with BN, additional AlN was formed by the interfacial reaction of the BN and Al melt as well as by the in situ reaction. Consequently, both the BN particles and the additional AlN particles formed by the interfacial reaction led to a further strengthening in the composite, as compared to the control alloy, which was strengthened only by the AlN particles formed in situ. In addition, the flake shape of BN may have lent considerable strength, due to the high aspect ratio it demonstrates, as compared with that of a spherically shaped particle.     

Thermodynamics of oxide-sulfate melts: The system PbO-PbSO4

Activities in the PbO-PbSO4 melts at 1253 K have been measured by emf and gas-equilibration techniques. The activity of PbO was directly obtained from the emf of the solid oxide cell, Pt, Ni-NiO/CaO-ZrO2/Auo.92PbO.08, PbOx-PbSO4(1-x), Ir, Pt for 1.0 >XPbO > 0.6. The melt and the alloy were contained in closed zirconia crucibles. Since the partial pressure of SO2 gas in equilibrium with the melt and alloy was appreciable (>0.08 atm) atXPbO < 0.6, activities at lower PbO concentrations were derived from measurements of the weight gain of pure PbO under controlled gas streans of Ar + SO2 + O2. The partial and integral free energies of mixing at 1253 K were calculated and found to fit a subregular model: ΔGEPbO =X2PbSO4 {-42,450 + 20,000X2PbSO4} J mol-1 ΔGEPbO =X2pbSO {-12,450 - 20,000XPbS} J mol-1 ΔGEpbSOXPbSO4 {-32,450XPbS - 22,450XPbSO4 } J mol-1. The standard free energy of formation of liquid PbSO4 from pure liquid PbO and gaseous SO3 at 1 atm at 1253 K was evaluated as -88.02 (±0.72) kJ mol-1.     

Self-propagating high-temperature synthesis of composite targets based on titanium carbonitride,silicide, and aluminide for ion-plasma deposition of multifunctional coatings

The macrokinetic features of combustion of the mixtures in the Ti-Al-Si₃N₄-C system calculated for the formation of compact ceramic materials (CCMs), the composition of which is described by the general formula X(TiAl₃) + (100 − X)(0.448TiC_0.5 + 0.552(Ti₅Si₃ + 4AlN) with mixture parameter X = 10–50%, are investigated. Compact CCM samples with the main structural components in the form of TiC _x N _y grains and binding phases TiAl3 and Ti5Si3 are fabricated by the technology of forced self-propagating high-temperature synthesis (SHS) compaction. An increase in X promotes the formation of the M _n +1AX _n phase with the composition Ti₃SiC₂ in the synthesis products. Complex investigations into the physicomechanical properties of the obtained ceramics are performed. Based on their results, the inverse dependence of the density and hardness of compact materials on parameter X is established. Tests of the samples for oxidation resistance showed that the obtained CCMs based on titanium carbonitride, silicide, and aluminide possess excellent resistance to high-temperature oxidation, and their oxidation rate in air at t = 900°C for 30 h does not exceed 7.8 × 10⁻⁵ g/(m² s).     

Activities in the spinel solid solution Fe X Mg1−X Al2O4

Activities in the spinel solid solution Fe _X Mg_1−X Al₂O₄ saturated with α-Al₂O₃ have been measured for the compositional range 0<X<1 between 1100 and 1350 K using a bielectrolyte solid-state galvanic cell, which may be represented as Pt, Fe + Fe _X Mg_1−X Al₂O₄+α-Al₂O₃//(Y₂O₃)ThO₂/(CaO)ZrO₂//Fe + FeAl₂O₄+α-Al₂O₃, Pt Activities of ferrous and magnesium aluminates exhibit small negative deviations from Raoult’s law. The excess free energy of mixing of the solid solution is a symmetric function of composition and is independent of temperature: ΔG ^E=−1990 X(1−X) J/mol. Theoretical analysis of cation distribution in spinel solid solution also suggests mild negative deviations from ideality. The lattice parameter varies linearly with composition in samples quenched from 1300 K. Phase relations in the FeO-MgO-Al₂O₃ system at 1300 K are deduced from the results of this study and auxiliary thermodynamic data from the literature. The calculation demonstrates the influence of intracrystalline ion exchange equilibrium between nonequivalent crystallographic sites in the spinel structure on intercrystalline ion exchange equilibrium between the monoxide and spinel solid solutions (tie-lines). The composition dependence of oxygen partial pressure at 1300 K is evaluated for three-phase equilibria involving the solid solutions Fe + Fe _X Mg_1−X Al₂O₄+α-Al₂O₃ and Fe + Fe _y Mg_1−Y O+Fe _X Mg_1−X Al₂O₄. Dependence of X, denoting the composition of the spinel solid solution, on parameter Y, characterizing the composition of the monoxide solid solution with rock salt structure, in phase fields involving the two solid solutions is elucidated. The tie-lines are slightly skewed toward the MgAl₂O₄ corner.     

Thermodynamics of phosphorus in molten Si-Fe and Si-Mn alloys

The thermodynamics of phosphorus in molten Si-Fe and Si-Mn alloys has been investigated at 1723 K by equilibrating the alloys in a controlled phosphorus partial pressure. The activity coefficient of phosphorus in each alloy shows a maximum value at a certain composition due to a strong interaction between silicon and iron and between silicon and manganese. Interaction coefficients between phosphorus and iron in molten silicon were found to be ε _P ^Fe =7.43 and ρ _P ^Fe =−16.4 (0≦X _Fe≦0.65), and those between phosphorus and manganese were ε _P ^Mn =12.0 and ρ _P ^Mn =−22.2 (0≦X _Mn≦0.5). Further discussion has revealed that the Si-Fe-P and Si-Mn-P systems approximately conform to a regular solution within the composition ranges investigated in the present work.     

Fabrication and characteristics of AA6061/Si3N4p composite by the pressureless infiltration technique

The tensile properties and microstructures of AA6061/Si₃N₄ particle composites fabricated by pressureless infiltration under a nitrogen atmosphere were analyzed. In addition, the control AA6061 without Si₃N₄ particles fabricated by the same method was investigated to separate the effect of Si₃N₄ particle addition. It was found that AlN particle layers formed on the surface of Al particles in the powder bed, which replaced the Mg₃N₂ coated layers through the following reaction: Mg₃N₂ + 2Al → 2AlN + 3Mg. Thus, the spontaneous infiltration results from a great enhancement of wetting via the formation of Mg₃N₂ by the reaction of Mg vapor and nitrogen gas. The increased tensile strength and 0.2 pct offset yield strength in the control AA6061 were largely due to fine AlN particles formed by the aforementioned in situ reactions, as compared to commercial AA6061. In the composite reinforced with Si₃N₄ particles, of course, the AlN was also formed through the following additional reaction at the Si₃N₄ particle/Al melt interfaces: Si₃N₄ + 4Al → 4AlN + 3Si. However, this AlN may not contribute to the increase in strength because its formation is compensated by the consumption of Si₃N₄ particles. Consequently, the strength increase of the composite fabricated by the present method is attributed to the fine AlN particles formed in situ, as well as the fine reinforcing Si₃N₄ particles, as compared to commercial AA6061.     

Reactive atomization of silicon to formin situ oxide sintering aids

The present investigation demonstrated the feasibility of using reactive atomization to produce Si powder within situ oxide sintering aids. With further process optimization, this powder may be an alternative starting material to the conventional, mechanically blended, Si-plus-oxide powder used to produce commercial sintered reaction bonded silicon nitride (SRBSN). In the reactive atomization approach, yttrium and aluminum additives were introduced into silicon metal during induction melting. Reactive atomization was accomplished using a N₂-5 pct O₂ mixture as the atomization gas. During atomization, oxygen in the atomization gas reacted with Y and Al in the Si melt to produce Y₂O₃ and Al₂O₃, which act asin situ sintering aids. The reactive atomized powder demonstrated a Gaussian distribution with a mean diameter of 36 μm. The powder fines (<38 μm) were used to produce cold isostatically pressed compacts that were subsequently reaction bonded and sintered. The results demonstrate that β-Si₃N₄ formed during reaction bonding and sintering. The density of the SRBSN was 77 pct of theoretical. Transmission electron microscopy (TEM) studies indicated the presence of a glassy phase on the grain boundaries, which is typical in SRBSN and indicative of the presence of thein situ sintering aids. A kinetic model was used to study the influence of processing parameters, such as droplet temperature and oxygen partial pressure, on the kinetics of oxide formation during reactive atomization. The results suggest that the volume fraction of oxides increases with increasing droplet temperature and oxygen partial pressure in the atomization gas mixture. This article is based on a presentation made in the “In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics” symposium, held February 12–16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).     

Crystallization kinetics of amorphous magnesium-rich magnesium-copper and magnesium-nickel alloys

Amorphous magnesium-rich alloys Mg _y X_1-y (X=Ni or Cu and 0.82<y<0.89) have been produced by melt spinning. The crystallization kinetics of these alloys have been determined by in situ X-ray diffraction (XRD) and isothermal and isochronal differential scanning calorimetry (DSC) combined with ex situ XRD. Microstructure analysis has been performed by means of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Crystallization of the Mg-Cu alloys at high temperature takes place in two steps: primary crystallization of Mg, followed by simultaneous crystallization of the remaining amorphous phase to Mg and Mg₂Cu. Crystallization of the Mg-Cu alloys at low temperatures takes place in one step: eutectic crystallization of Mg and Mg₂Cu. Crystallization of the Mg-Ni alloys for a Mg content, y>0.85, takes place in two steps: primary crystallization of Mg and of a metastable phase (Mg_∼5.5Ni, with Mg content y=0.85), followed by the decomposition of Mg_∼5.5Ni. Crystallization of the Mg-Ni alloys for a Mg content y<0.85 predominantly takes place in one step: eutectic crystallization of Mg and Mg₂Ni. Within the experimental window applied (i.e., 356 K<T<520 K and 0.82<y<0.89), composition dependence of the crystallization sequence in the Mg-Cu alloys and temperature dependence of the crystallization sequence in the Mg-Ni alloys has not been observed.     

The forming of Zr3Al-base alloys

Tubes have been formed from the intermediate phase Zr₃Al (L1₂ type) which is of potential use as a structural element in thermal nuclear power reactors. The procedure is to extrude Zr/Zr₂Al two-phase ingots at temperatures above 1270 K in theβ Zr + Zr₂Al two-phase field and then transform the extruded product at lower temperatures to Zr₃Al via the peritectoid transformation Zr + Zr₂Al → Zr₃Al. For small tubes (⪝ 3.2 cm OD) the extrusion constants at 1375 and 1425 K, respectively, are ≃ 400 and ≃ 300 MN/m² for the conditions chosen. The experimental extrusions indicate no fundamental barriers to forming pressure tubes from Zr-Al alloys containing 7.6 to 9.0 wt pct Al.     

The kinetics of S35 exchange between SO2/CO/CO2 gas mixtures and copper sulfide melts at 1523 K

The kinetics and mechanisms of oxidation of copper sulfide melts have been investigated using a radioisotope exchange technique. Copper sulfide melts were doped with S³⁵. The transfer of the radioisotope between the melt and SO₂/CO/CO₂ gas mixtures in chemical equilibrium with the melt was monitored by analyzing the changes in radioactivity of the gas. Analysis of the results indicates that the rate-limiting chemical reaction involves the formation of an activated complex SO, and the rate of exchange of the sulfur isotope at 1523 K is described by the relationshipR = 6.4(±2) (P _CO /P _CO2 )P _SO2 g atom S m⁻² s⁻¹. formerly Research Assistant, University of Queensland formerly Postdoctoral Fellow, University of Queensland     

Diffusion of Au in the Intermetallic Compound Ti<Subscript>3</Subscript>Al

The Au diffusion in the Ti₃Al compound was investigated at six compositions from 25 to 35 at. pct Al by using the diffusion couples (Ti-X at. pct Al/Ti-X at. pct Al-2 at. pct Au; X = 25, 27, 29, 31, 32, and 35) at 1273 to 1423 K. The diffusion coefficients of Au in Ti₃Al ( D_\textAu^{\textTi₃ \textAl} ) \left( {D_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} } \right) are relatively close to those of Ti. The D_\textAu^{\textTi₃ \textAl} \texts {D}_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}} slightly increase with Al concentration within the same order of magnitude. The activation energies of Au diffusion, Q_\textAu^{\textTi₃ \textAl} \texts, Q_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}, evaluated from the Arrhenius plots were relatively close to those of Ti diffusion, Q_\textTi^{\textTi₃ \textAl} \texts, Q_{\text{Ti}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}, rather than those of Al diffusion, Q_\textAl^{\textTi₃ \textAl} \texts; {Q}_{\text{Al}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}; therefore, it was suggested that Au atoms diffuse by the sublattice diffusion mechanism in which Au atoms substitute for Ti sites preferentially in Ti₃Al and diffuse by vacancy mechanism on Ti sublattice. The influence of the D0₁₉ ordered structure (hcp base) of Ti₃Al on diffusion of Au and other elements is discussed by comparing the diffusivities in Ti₃Al and α-Ti.     

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.     

Numerical Simulation of Phosphorus Removal from Silicon by Induction Vacuum Refining

Phosphorus can be expected to evaporate preferentially from silicon melt by induction vacuum refining (IVR). In the present study, on the assumption of phosphorus evaporating from silicon melt as gas species P and P ₂, a numerical model of phosphorus removal from silicon by IVR was developed. The factors affecting phosphorus removal in decreasing order are temperature, chamber pressure, geometry of silicon melt, holding time, and original phosphorus concentration. Calculated phosphorus removal shows good agreement with the present experimental data.     

Coarsening of Al<Subscript>3</Subscript>Sc precipitates in an Al-0.28 wt pct Sc alloy

The Ostwald ripening of Al₃Sc precipitates in an Al-0.28 wt pct Sc alloy during aging at 673, 698, and 723 K has been examined by measuring the average size of precipitates by transmission electron microscopy (TEM) and the reduction in Sc concentration in the Al matrix with aging time, t, by electrical resistivity. The coarsening kinetics of Al₃Sc precipitates obey the t ^1/3 time law, as predicted by the Lifshitz-Slyozov-Wagner (LSW) theory. The kinetics of the reduction of Sc concentration with t are consistent with the predicted t ^−1/3 time law. Application of the LSW theory has enabled independent calculation of the Al/Al₃Sc interface energy, γ, and volume diffusion coefficient, D, of Sc in Al during coarsening of precipitates. The Gibbs-Thompson equation has been used to give a value of γ using coarsening data obtained from TEM and electrical resistivity measurements. The value of γ estimated from the LSW theory is 218 mJ m⁻², which is nearly identical to 230 mJ m⁻² from the Gibbs-Thompson equation. The pre-exponential factor and activation energy for diffusion of Sc in Al are determined to be (7.2±6.0)×10⁻⁴ m² s⁻¹ and 176±9 kJ mol⁻¹, respectively. The values are in agreement with those for diffusion of Sc in Al obtained from tracer diffusion measurements.     

Prevention of macrosegregation in squeeze casting of an Al-4.5 wt pct Cu alloy

The squeeze casting of an Al-4.5 wt pct Cu alloy was carried out to investigate the conditions for the formation and the prevention of macrosegregation. The effects of the process parameters, applied pressure, die temperature, pouring temperature, delay time, and humidity on the formation of macrosegregation were investigated in correlation with the evolution of macrostructure and shrinkage defects. Two critical applied pressures were defined: one is the critical applied pressure, P _SC , under which shrinkage defects form, and the other is the critical applied pressure, P _MS , above which macrosegregates form in the squeeze castings. A quantitative diagram describing the optimum process conditions was proposed for obtaining sound squeeze castings. It was found that the pouring temperature, the die temperature, the delay time, and the humidity are closely related to the two critical applied pressures P _SC and P _MS , in different manners. It was concluded that sound castings without macrosegregation and shrinkage defects can only be obtained when the applied pressure is in the range of P _SC <P<P _MS .