Kinetics of formation of the TiAl3 phase in the Ti-Al system

Conclusions In a general case the solid-phase reaction of aluminum with titanium leading to the formation of TiAl₃ is controlled by dual kinetics. In the initial period the rate of the TiAl₃ is controlled by dual kinetics. In the initial period the rate of the TiAl₃ formation process at the interface between titanium and aluminum is constant with time. The temperature dependence of the formation rate constant under kinetic conditions obeys Arrhenius' equation. The energies of activation of TiAl₃ formation in the linear stage, E_s = 170 ± 30 in the solid-phase reaction and e₁ = 127 ± 30 kJ/mole in the reaction of titanium with liquid aluminum, match, allowing for errors in the determination of E_s and e₁, the standard heat of formation of TiAl₃, H₂₉₈ = 142 ± 4 kJ/mole [11]. It is therefore reasonable to conclude that the mechanism of contact reaction in the linear stage of layer growth is the same in both cases and is determined not by diffusional transport but by chemical kinetics. Differences between values of rate constants of the reactions of titanium with solid and liquid aluminum are apparently mainly due to the method employed in processing experimental data. The true area of the reaction surface between titanium and liquid aluminum is considerably larger than the surface area of the starting titanium specimen. Consequently, calculation in this case yields larger values of reaction rate constants. During the reaction of titanium with solid aluminum the growing thickness of the TiAl₃ phase layer increasingly hinders the supply of aluminum to the reaction front, and this then becomes the limiting stage of the process. As a result, the conditions of layer growth change from kinetic to diffusional. When titanium reacts with liquid aluminum, the thickness of the thin layer of columnar TiAl₃ crystals adjacent to titanium and the number of capillaries crossing this layer do not vary as functions of reaction time (up to 5.5 h at 850°C). The rate of growth of this layer is therefore equal to the rate of its disintegration on the outer boundary. In this case, since the length of the capillaries does not vary owing to constancy of the layer thickness, the flow of aluminum to the titanium surface remains unchanged. Thus, during the reaction of titanium with liquid aluminum the intermetallic compound layer grows according to a law which is always linear, never changing to parabolic.Translated from Poroshkovaya Metallurgiya, No. 2(290), pp. 26–31, February, 1987.The authors wish to thank Dr. F. J. J. Van Loo of Holland for providing them with unpublished as well as published data on the reaction of titanium with solid aluminum.     

The stable and metastable Ti-Nb phase diagrams

The phase transformations which occur in the Ti-Nb binary alloy system have been discussed in two recent papers. The phase relationships were investigated by varying alloy composition and thermal history. In this paper, these results are summarized in complete and thermodynamically consistent calculations of the stable and metastable phase diagrams. The calculations of the metastable equilibria are relevant to the Ti-V and Ti-Mo systems, as well as to several other titanium and zirconium-based transition metal alloy systems. Formerly with the National Bureau of Standards, Gaithersburg, MD 20899.     

Structure and formation of the metastable phasem-TiBe

The metastable ordered phasem-TiBe was discovered in the course of studying liquidquenched Ti-Be and Ti-Be-Zr alloys. m-TiBe forms, together with the α-Ti solid solution, on heating binary (and ternary) metallic glasses to 700°C and is also observed directly in as-quenched alloys lying outside the composition limits for glass formation. This phase has a B2 (CsCl-type) crystal structure. Its existence can be justified in terms of crystal chemical factors leading to the conclusion that TiBe most likely does not appear as an equilibrium phase in normally cooled alloys due to the greater stability of the Laves phase TiBe₂.     

Solidification of a binary eutectic in a three-component system

The change in mass and composition of the phases during the equilibrium solidification of eutectic alloys in three-component systems is considered. For the model system A-B-C, we show that equilibrium solidification of binary eutectics in three-component systems is determined by processes of decomposition and interaction, as in the crystallization of a solid solution. Eutectic transformation is generally associated with equilibrium solidification, since decomposition dominates over interaction. These findings are confirmed by studying solidification in the Al-Si-Cu system.     

Stable and metastable phase equilibria in the Al-Mn system

The aim of the present investigation was resolution of certain obscure features of the Al-Mn phase diagram. The experimental approach was guided by assessment of the previous literature and modeling of the thermodynamics of the system. It has been shown that two phases of approximate stoichiometry “Al₄Mn” (λ and μ) are present in stable equilibrium, λ forming by a peritectoid reaction at 693 ± 2 °C. The liquidus and stable equilibrium invariant reactions as proposed by Goedecke and Koester have been verified. A map has been made of the successive nonequilibrium phase transformations of as-splat-quenched alloys. Finally, the thermodynamic calculation of the phase diagram allows interpretation of complex reaction sequences during cooling in terms of a catalogue of all the metastable invariant reactions involving (Al), Al₆Mn, λ, μ, ?, and Al₁₁Mn₄ phases.     

Stable and metastable phase equilibria in the Al-Mn system

The aim of the present investigation was resolution of certain obscure features of the Al-Mn phase diagram. The experimental approach was guided by assessment of the previous literature and modeling of the thermodynamics of the system. It has been shown that two phases of approximate stoichiometry “Al₄Mn” (λ and μ) are present in stable equilibrium, λ forming by a peritectoid reaction at 693 ± 2 °C. The liquidus and stable equilibrium invariant reactions as proposed by Goedecke and Koester have been verified. A map has been made of the successive nonequilibrium phase transformations of as-splat-quenched alloys. Finally, the thermodynamic calculation of the phase diagram allows interpretation of complex reaction sequences during cooling in terms of a catalogue of all the metastable invariant reactions involving (Al), Al₆Mn, λ, μ, ϕ, and Al₁₁Mn₄ phases.     

Solidification and phase equilibria in the Fe-C-Cr-NbC system

A solidification model is developed and experimentally checked for Fe-C-Cr-Nb alloys in the white cast irons range. It is based on a partial quaternary Fe-C-Cr-NbC phase diagram and predicts the possible solidification paths for the alloys containing γ, with (Fe,Cr)₇C₃ and NbC as the microconstituents at room temperature. The dendritic γ to massive (Fe,Cr)₇C₃ transition in experimental alloy microstructures with NbC contents up to 22 pet is explained by this model. Thermal analysis is also used to compare the solidification paths and model approach.     

Phase equilibria investigations on the aluminum-rich part of the binary system Ti-Al

The binary system Ti-Al has been reinvestigated in the composition range of 50 to 76 at. pct Al by X-ray diffraction, metallography, electron probe microanalysis (EPMA), and differential thermal analysis (DTA). Heat-treated alloys (600°C to 1300°C) as well as the as-cast alloys were investigated. Seven stable intermetallic phases were observed: TiAl, Ti_1−x Al_1+x , Ti₃Al₅, TiAl₂, Ti₅Al₁₁, TiAl₃ (h), and TiAl₃ (1); two metastable phases, TiAl₂ (m) and TiAl₃ (m), were also found. For each of these phases, the homogeneity range and the crystal chemical parameters were determined. The temperatures of the solid-state phase reactions were re-established. On the basis of the experimental results, an improved version of the equilibrium phase diagram has been drawn and critically compared with earlier versions presented in the literature.     

Solidification of binary hypoeutectic alloy matrix composite castings

We consider a binary hypoeutectic alloy casting which solidifies in dendritic form in an unreinforced engineering casting and seek to predict its microstructure in a metal matrix composite. We focus on the case where the reinforcement is fixed in space and fairly homogeneously distributed. We assume that the reinforcement does not catalyze heterogeneous nucleation of the solid. We show that the reinforcement can cause several microstructural transitions in the matrix alloy, depending on the matrix cooling rate, the width, A, of interstices left between reinforcing elements, and the initial velocityV of the solidification front. These transitions comprise the following: (1) coalescence of dendrite arms before solidification is complete, causing solidification to proceed in the later stages of solidification with a nondendritic primary phase mapping the geometry of interstices delineated by reinforcement elements; (2) sharp reduction or elimination of microsegregation in the matrix by diffusion in the primary solid matrix phase; and (3) a transition from dendrite to cell formation, these cells featuring significant undercoolings or a nearly plane front configuration when reinforcing elements are sufficiently fine. Quantitative criteria are derived for these transitions, based on previous work on composite solidification, observations from directional solidification experiments, and current solidification theory. Theory is compared with experimental data for aluminum-copper alloys reinforced with alumina fibers and for the dendrite to cell transition using data from directional succinonitrile-acetone solidification experiments. Theory and experiment show good agreement in both systems. This article is based on a presentation made at the “Analysis and Modeling of Solidification” symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2–6, 1994, under the auspices of the TMS Solidification Committee.     

Nucleation and growth kinetics of stable and metastable eutectics in FeSiB metallic glasses

Two iron-based metallic glasses, Fe₇₈Si₁₀B₁₂ and Fe₇₂Si₁₀B₁₈, have been examined in detail after partial crystallization at a series of temperatures. The number and size of the eutectic cells—of both stable and metastable eutectics—have been determined as functions of time and temperature and the results compared with theoretical nucleation models. It has been found that the extent to which quenched-in nuclei from the original planar flow casting operation influence the subsequent crystallization behaviour depends on alloy composition and on the annealing temperature. The results indicate that it is possible to control the type, number and size of the crystallization products in the microstructure by suitable choice of annealing conditions.     

Modelling the influences of solid-state interdiffusion and dissolution on transient liquid phase sintering kinetics in a binary isomorphous system

A simple model was developed to predict the impact that solid-state interdiffusion and dissolution have on liquid formation and its duration during transient liquid phase sintering (TLPS). The model predicts that solid-state interdiffusion can dramatically reduce the amount of liquid initially formed during heating. This reduction is dependent on the heating rate and initial base metal particle size. In cases of sintering above the additive phase melting point, the model predicts that base metal dissolution increases liquid phase formation and that this additional melting reduces the base metal particle size. The model predicts that longer times are required to solidify isothermally the greater amounts of liquid formed at higher temperatures (because of dissolution). This agreed qualitatively with experimental results for a Ni-65 wt pct Cu TLPS mixture sintered at 1090 °C and 1140 °C. Quantitative comparisons between the model and experiment were good at 1140 °C; however, the rate of isothermal solidification was underestimated by the model for intermediate sintering times at 1085 °C.     

Solidification and spangle formation of hot-dip-galvanized zinc coatings

Solidification of hot dip coatings was studied regarding thermal conditions. Optical phenomena occurring at the liquid zinc surface were documented and the solid zinc surface was characterized in respect to optical and microscopic appearance, distribution of Pb and Al, crystal orientation, and topography. Resulting from these observations, a solidification model can be derived: zinc nucleation occurs at the steel/zinc interface. Due to thermal conditions in the slightly undercooled liquid zinc film, solidification occurs by rapid sideways dendritic expansion of the nucleated grains along the steel/zinc interface. Dendritic growth is controlled by interaction of crystal orientation of the nucleated zinc grain and thermal conditions in the undercooled layer. This leads to formation of different shaped grains with thicker and thinner sectors. The mechanism of sideways expansion continues until the entire interface is covered with dendritic zinc grains. Even though the zinc outer surface is still a liquid phase, final spangle size, as well as surface appearance and shape of the grains, is already determined at that early stage of solidification. Further growth only leads to a thickening of the solid layer; however, its relief remains almost unchanged. Thickening occurs relatively slowly due to the fact that marginal heat flow toward the surface now represents the limiting factor. Growth of the solid zinc layer results in continuous enrichment of Pb and Al in the residual liquid. Then, outer surface solidification starts as segments of single grains emerge. Distribution of the enriched residual melt in between the already solid areas depends on the relief of the solid layer. Finally, eutectic Zn-Pb reaction with precipitation of Pb particles takes place, which defines the dull appearance of these regions. Solidification for “lead-free” coatings is essentially the same, except that the final eutectic Zn-Pb reaction is missing. Additional investigations of dendritic secondary arm spacing indicate that Pb does not act by suppressing zinc nucleation. Pronounced dendritic growth is proposed to be favored by a change in interfacial energy. The new solidification model is applicable for a wide range of processing conditions and explains the origin of the typical spangle structure.     

Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels

An investigation of phase transformations in hydrogen-charged metastable austenitic stainless steels was carried out. Solution-annealed, high-purity, ultralow-carbon Fel8Crl2Ni (305) and laboratory-heat Fel8Cr9Ni (304) stainless steels were examined. The steels were cathodically charged with hydrogen at 1, 10, and 100 mA/cm², at room temperature for 5 minutes to 32 hours, in an lN H₂SO₄ solution with 0.25 g/L of NaAsO₂ added as a hydrogen recombination poison. Changes in microstructure and hydrogen damage that resulted from charging and subsequent room-temperature aging were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Hydrides from hydrogen charging (hep ε* in 305 SS and fcc γ* and hcp ε* in 304 SS) were observed. The evidence suggests the following mechanisms for hydride formation during charging: (1)γ → ε → ε* hydride and (2) γ → γ* hydride. These hydrides were found to be unstable and decomposed during room-temperature aging in air by the following suggested mechanisms: (1)ε* hydride (hcp) → expanded ε (hcp) phase →α′ (bcc) phase and (2) γ* hydride →γ phase. The transformation from ε* toα′, however, was incomplete, and a substantial fraction of ε was retained. A kinetics model for hydride decomposition and the accompanying phase transformation during aging is proposed.