Analysis of grain growth in a two-phase gamma titanium aluminide alloy

Microstructure evolution during annealing of a wrought near-gamma titanium aluminide alloy, Ti-45.5Al-2Nb-2Cr (at. pct), in the temperature range 1200 °C to 1320 °C was investigated. The mean grain size of the alpha phase as well as the volume fraction and size of the gamma particles were evaluated as a function of annealing temperature and time. Isothermal annealing at temperatures above the alpha transus, T _α=1300 °C, led to rapid grain growth of the alpha phase, the kinetics of which could be described by a simple power-law type expression with a grain growth exponent p=2.3. Alpha grain growth was significantly retarded during annealing at subtransus temperatures (1200 °C≤T≤1300 °C) by the pinning influence of gamma-phase particles. Limiting grain size values predicted by computer simulation models applicable for high-volume fractions of precipitates/particles were in good agreement with experimental findings. The kinetics of alpha grain growth in the presence of gamma particles were analyzed, and the results showed that a grain growth exponent of p≈2.6 could satisfactorily account for the experimental results.     

Modeling of Solutionizing and Solute Redistribution in a Co-Cast Bi-Layer Al Alloy System

The effect of high-temperature treatment on solutionizing and solute diffusion in a co-cast X609-AA3003 alloy system is examined via a coupled dissolution and diffusion model using finite-element analysis. The model describes the kinetics of the dissolution of intermetallic particles of Mg₂Si and Si along with the diffusion of alloying elements of Mg, Si, and Cu across the interface between the two alloy layers. The results are verified using electron probe microanalysis (EPMA) measurements.     

Grain growth and carbide precipitation in superalloy, UDIMET 520

The results of an experimental study on the grain coarsening behavior, M₂₃C₆ carbide precipitation, and secondary MC carbide precipitation kinetics in UDIMET 520 are presented. Primary MC carbides and M (C, N) carbonitrides strongly influence the grain growth, with their dissolution near 1190 °C and 1250 °C, respectively, resulting in two distinct grain coarsening temperatures (GCTs). M₂₃C₆ carbides precipitate in the alloy over a wide range of temperatures varying between 600 °C and 1050 °C. A discrete M₂₃C₆ grain boundary carbide morphology is observed at aging temperatures below 850 °C. Secondary MC carbides formed at temperatures ranging between 1100 °C and 1177 °C, in specimens in which primary MC dissolution had been obtained at solution treatment temperatures of 1190 °C to 1250 °C. A schematic time-temperature-transformation (TTT) diagram for understanding the microstructure and precipitation inter-relationships in UDIMET 520 alloy is also presented.     

Kinetics and morphology of deformation bands at the initial stage of the loss of a stable plastic flow in the 5456 alloy

High-speed video filming is used to study the kinetics and morphology of the first deformation bands on the surface of the 5456 alloy having various initial microstructures. The mobility and morphology of the bands is found to change sharply after annealing in the vicinity of the solvus temperature. The dissolution of β(Al₃Mg₂) particles causes a transition from a plane into a branching morphology of the first bands and is accompanied by a sharp (almost an order of magnitude) increase in their mobility. The depinning of grain boundaries via the removal of β-phase particles is assumed to favor grain-boundary sliding, which causes branching of the bands.     

Modeling Creep Strength of Welded 9 to 12 Pct Cr Steels

The influence of weld-simulated heat treatments of 9 to 12 pct steels is evaluated by a fundamental model for creep. The heat-affected microstructure is predicted by considering particle coarsening, particle dissolution, and subgrain coarsening. Particle coarsening is predicted for a multicomponent system, showing significant M₂₃C₆ coarsening in the bcc matrix. Dissolution simulations of MX and M₂₃C₆ are performed by considering a size distribution of particles, indicating that the smallest particles can be dissolved already at relatively low welding temperatures. Recovery in dislocation networks will take place due to the coarser particles. Creep rate modeling is performed based on the heat-affected microstructure, showing strength reduction of weld-simulated material by 12 pct at 1123 K (850 °C) and 30 pct at 1173 K (900 °C). The main cause of this degradation is believed to be the loss of the smallest carbonitrides.     

Coarsening of SiO2 particles in copper and MnS inclusions in steel

A model to simulate the diffusion-controlled coarsening and dissolution kinetics of particles within a metallic matrix is formulated. With an arbitrary size distribution of particles, the model can be used to calculate the change in the size distribution of particles during coarsening or dissolution. Other system parameters, such as average radius of particles, volume fraction, average distance between particles, surface area, and matrix composition are also calculated. An important result is that kinetics do not generally obey the often-applied Lifshiftz-Slyozov-Wagner theory for diffusion controlled coarsening based upon concentration profiles around isolated spheres. In such a formulation, the direct effect of the surrounding particles is neglected. In our model, which is a modification of the coarsening kinetics described by Weins and Cahn, the effect of surrounding particles is incorporated because the system is taken to be a system of point potentials, each with a potential according to its radius of curvature. Calculations are on silica particles in a copper matrix and on manganese sulfide inclusions in iron, with emphasis on the latter, in order to predict their behavior during homogenization or soaking treatments. The effect of the composition of manganese, from 0.1 to 1.2 wt pct, on the coarsening of sulfides in a “high” sulfur (0.017 wt pct) steel and a “low” sulfur (0.003 wt pct) steel was investigated. As expected, the model predicts that manganese strongly reduces the rate of coarsening, particularly for times of ten hours or less in the temperature range of 1100 to 1400 °C. Calculated results also indicate that the rate of dissolution is very low at temperatures greater than the solvus for manganese sulfide inclusions in austenite.     

Prediction of Bubble Size Distribution in Aluminium Foam as a Function of %Titanium Hydride Addition

Foaming of liquid aluminium by addition of foaming agent (TiH₂ particles) is numerically simulated using population balance equations. Phenomena such as hydrogen release by the TiH₂ particles, heterogeneous nucleation of bubbles in oxide surface cavities, and diffusion based bubble growth are modelled. A simple mass transfer coefficient, which varies inversely with the bubble size is used to estimate the bubble growth rate. Simulation is performed to study the effect of TiH₂ content on the final bubble size distribution, total number of bubbles and average bubble size. In general, the average properties of the predicted distributions are close to the experimental values, whereas the spread in the bubble size is observed to be considerably narrower for the predicted values. The deviation in the spread of the distributions is attributed to the inverse bubble size dependent growth rate and non-inclusion of bubble coalescence in the model.     

Wetting behaviour in the Al-Si/SiC system: interface reactions and solubility effects

The sessile drop technique was used to study the wetting behaviour of Al-Si alloys on SiC sintered ceramic substrates under vacuum in the 700–1100°C temperature interval. Al-Si alloys with Si concentrations up to 50% were tested. An expected non-wetting/wetting transition was observed at 900–1000°C due to the presence of an alumina film surrounding the molten alloy. At higher temperatures wetting was observed and the Si concentration of the alloy has a marked effect on the measured contact angles, θ. At 1100°C θ decreases from 55° to 25° when instead of pure al and A112.3%Si or an A116.6%Si alloy is used. The suppression of the formation of a continuous Al₄C₃ layer at the interface and a process of dissolution and reconstruction of the SiC surface, due to the increased Si concentration of the Al-Si alloys, are the key factors to explain the observed behaviour.     

Kinetics of discontinuous precipitation and dissolution in Cu–Ag alloys

The kinetics of discontinuous precipitation and dissolution of the cellular precipitate have been studied in Cu-3 at.% Ag and Cu-4 at.% Ag alloys. The growth rates of the cells were measured using optical microscopy. The interlamellar spacings of the primary cells and compositions of the depleted matrix were measured using scanning electron microscope and X-ray diffraction, respectively. The Cu–Ag alloys were observed to decompose into a lamellar structure consisting of alternate lamellae of the α (Cu-rich) and β (Ag-rich) phases when a solid solution of the alloy was aged below the solvus temperature. The rod shaped morphology of the β phase dispersed in the matrix of α was observed at all temperatures. The primary cell growth data were analysed using the theories of Cahn, Hillert, Sundquist, Turnbull and Petermann and Hornbogen. From the diffusivity values, it has been shown that the growth of primary cells occurs by the diffusion of Ag along the grain boundaries. The results are consistent with the diffusivity values reported in the literature in the same temperature range.The discontinuous dissolution of the primary cells occurred above the solvus temperature as well as 30–40 K below it. The dissolution occurred primarily at the prior positions of grain boundaries as well as at the primary cells intersection, at least in the initial stages of dissolution. The discontinuous dissolution occurred by the diffusion of Ag along the interface boundary between the primary and dissolution cells. The diffusivity and mobility values obtained during dissolution are one order of magnitude smaller than those obtained for the discontinuous precipitation. This has been explained by volume diffusion ahead of the interface and not all of the free energy being used to drive the interface boundary.     

Microstructural stability during creep of Mo- or W-bearing 12Cr steels

The evolution and stability of particulate phases during creep of molybdenum- or tungsten-bearing 12Cr steels have been investigated in considerable depth. The important finding is that the performance of Laves-phase precipitation in the molybdenum-bearing alloy is significantly different from that in the tungsten-bearing alloy. It is generally believed that such differences in kinetics will influence creep behavior. Data on Laves-phase precipitation kinetics as a function of time and temperature were quantified using the Wert-Zener equation in conjunction with the proprietary Thermo-Calc software, to determine equilibrium solute concentrations in these complex steels. The progressive depletion of Mo and W from the matrix as the particles of Laves phase evolve has been quantitatively modeled using experimental data obtained on both steels over a range of times and temperatures. The Isothermal coarsening rates of M₂₃C₆ and MX carbide particles were measured and found to occur at a constant volume fraction, in accordance with Ostwald ripening kinetics, with no significant differences in rates found between the two steels. The coarsening rates of M₂₃C₆ particles, found on subgrain boundaries, were consistent with a third-power dependence on particle radius, with an activation energy similar to that of volume diffusion. The smaller MX particles, which lay on subgrain-interior dislocation lines, were better explained by dislocation pipe diffusion, with a fifth-power dependence on particle radius and an activation energy approximately half that of volume diffusion.     

Effect of post-homogenisation cooling rate and Mn addition on Mg2Si precipitation and hot workability of AA6060 alloys

ABSTRACT

The microstructure evolution for different post-homogenisation cooling rates and the flow stress behaviour in direct chill cast AA6060 alloys were studied. Results revealed that decreasing cooling rates reduced the flow stress owing to the precipitation of Mg₂Si and reduction of the solid solution level. Micro-alloying of Mn generated a distribution of α-Al(FeMn)Si dispersoids during the homogenisation, with the size and number density decreasing at higher homogenisation temperatures. TEM studies confirmed that the dispersoids acted as favourable nucleation sites for Mg₂Si and significantly promoted the precipitation of Mg₂Si during subsequent cooling. The high-temperature flow stress was controlled by the solid solution levels of Mg, Si, and Mn resulting from the interaction between dispersoids and Mg₂Si. The combination of the Mn addition, a low cooling rate, and a low homogenisation temperature provided the lowest flow stress, which improved the hot workability of the alloy and promoted ready dissolution of Mg₂Si during extrusion.     

High-Performance dispersion-strengthened Cu-8 Cr-4 Nb alloy

A new high-temperature-strength, high-conductivity Cu-Cr-Nb alloy with a Cr:Nb ratio of 2:1 was developed to achieve improved performance and durability. The Cu-8 Cr-4 Nb alloy studied has demonstrated remarkable thermal and microstructural stability after long exposures at temperatures up to 0.98 T_m. This stability was mainly attributed to the slow coarsening kinetics of the Cr₂Nb precipitates present in the alloy. At all temperatures, the microstructure consists of a bimodal and sometimes trimodal distribution of strengthening Cr₂Nb precipitates, depending on precipitation condition, i.e., from liquid or solid solution, and cooling rates. These precipitates remain in the same size range, i.e., large precipitates of approximately 1 μm and small precipitates less than 300 nm, and effectively pin the grain boundaries, thus retaining a fine grain size of 2.7 μm after 100 hours at 1323 K. This grain-boundary pinning and sluggish coarsening of Cr₂Nb particles explain the retention of good mechanical properties after prolonged holding at very high temperatures, e.g., twothirds of the original yield strength after aging for 100 hours at 1273 K. The main sources of strengthening are the Hall-Petch and Orowan mechanisms due mostly to small particles. The coarsening kinetics of the large precipitates are most likely governed by grain-boundary diffusion and, to a lesser extent, volume diffusion mechanisms.     

Studies on dissolution kinetics of dolime in electrothermal magnesium slag

The electrothermal process of magnesium metal production is a promising route, where large sized internally heated reactor is used for magnesium production resulting in less energy and labour intensive and high space-time yield process. However, the dissolution behavior of dolime in the electrothermal slag has been found critical for the process optimization. In this paper, the dissolution kinetics of the dolime in the slag was discussed. Quaternary slag (CaO-Al₂O₃-SiO₂-MgO) was prepared having basicity CaO/SiO₂ ≥ 1.8 and Al₂O₃/SiO₂ ≥ 0.26 for dolime dissolution studies by static hot dip method. Prior to the experiments, FactSage calculations were carried out varying temperatures and slag compositions. In the kinetic studies, dolime particles 10–15 mm size was added in slag melted at 1450, 1500 and 1550°C and samples were taken at various time intervals. The chemical analysis of slag sample was carried out to investigate the dissolution kinetics to establish the rate expression. The activation energy for the process was calculated for different models used in study and was found to be in the range of 130–270 kJ/mol. SEM analysis was done for surface analysis of reacted particles. This study would be helpful in optimizing the dolime charging rate during pilot scale trials for electrothermal magnesium production at CSIR-NML, Jamshedpur.     

Influence of a Silicon Additive on Resistivity and Hardness of the Al–1% Fe–0.3% Zr Alloy

Isothermal sections of the diagram of the Al–Fe–Si–Zr alloy at temperatures of 450 and 600°C, as well as polythermal sections at concentrations of silicon up to 2 wt % and zirconium up to 1 wt %, are analyzed using computational methods with the help of Thermo-Calc software. It is shown that the favorable phase composition consisting of the aluminum solid solution (Al), the Al₈Fe₂Si phase, and Zr (which completely enters the composition of the solid solution (Al) during the formation of the cast billet) can be attained in equilibrium conditions at silicon concentrations of 0.27–0.47 wt %. To implement the above-listed structural components in nonequilibrium conditions and ensure that Zr enters the (Al) composition, experimental ingots were fabricated at an elevated cooling rate (higher than 10 K/s). A metallographic analysis of the cast structure of experimental samples revealed the desired structure with contents of 0.25 wt % Si and 0.3 wt % Zr in the alloy. The microstructure of the Al–1% Fe–0.3% Zr–0.5% Si alloy also contains the eutectic (Al) + Al₈Fe₂Si; however, the Al₈Fe₂Si phase partially transforms into Al₃Fe. The structure of the alloy with 0.25 wt % Si in the annealing state at 600°C contains fragmented particles of the degenerate eutectic (Al) + Al₈Fe₂Si along the boundaries of dendritic cells. It is established that the Si: Fe = 1: 2 ratio in the alloy positively affects its mechanical properties, especially hardness, without substantially lowering the specific conductivity during annealing, which is explained by the formation of the particles of the Al₈Fe₂Si phase of the compact morphology in the structure. Moreover, silicon accelerates the decay of the solid solution by zirconium, which is evidenced by the experimental plots of the dependence of hardness and resistivity on the annealing step. The best complex of properties was shown by the Al–1% Fe–0.3% Zr–0.25% Si alloy in the annealing stage at 450°C with the help of the optimization function at specified values of hardness and resistivity.     

Decomposition and dissolution kinetics ofδ′ precipitation in Al-Li binary alloys

Aside from its technological importance, the Al-Li alloy system also exhibits interesting phase transformations involving both equilibrium and metastable states. Recent theoretical studies have shown that a supersaturated solid solution could take different transformation paths when it is quenched into theα +γ′ field. Suggestions were made that a rapidly quenched solution phase should first undergo a congruent ordering transformation before it decomposes into a two-phase mixture by either a secondary spinodal decomposition or the classical nucleation and growth process. Moreover, a metastable miscibility gap was predicted at lower temperatures. The objective of this research is to study the transformation paths and dynamics in Al-Li binary alloys of three compositions (5.2, 7.0, and 12.0 at. pct Li). This investigation emphasizes thein situ small-angle X-ray scattering (SAXS) observations on specimens subjected to various aging conditions. Special attention is paid to the early stages of the transformation in an attempt to characterize the various possible modes of phase separation on one hand and to study the dynamics of the precipitation process on the other. The following results are obtained: the congruent ordering precedes decomposition at low temperatures; the metastableγ′ solvus curve is reconfirmed; but the predicted metastable miscibility gap is not found. Guinier radii measurements of the particles showed Ostwald ripening is quickly reached upon heating to the aging temperatures. Slowing down behavior is seen at aging temperatures close to the solvus boundary. Activation energies for Li diffusion were obtained using the modified Lifshitz, Slyozov, and Wagner (MLSW) model. A test of dynamical scaling behavior is carried out for the Al-12.0 at. pct Li alloy. Formerly Visiting Scientists at the University of Illinois     

Decomposition and dissolution kinetics ofδ′ precipitation in Al-Li binary alloys

Aside from its technological importance, the Al-Li alloy system also exhibits interesting phase transformations involving both equilibrium and metastable states. Recent theoretical studies have shown that a supersaturated solid solution could take different transformation paths when it is quenched into theα +γ′ field. Suggestions were made that a rapidly quenched solution phase should first undergo a congruent ordering transformation before it decomposes into a two-phase mixture by either a secondary spinodal decomposition or the classical nucleation and growth process. Moreover, a metastable miscibility gap was predicted at lower temperatures. The objective of this research is to study the transformation paths and dynamics in Al-Li binary alloys of three compositions (5.2, 7.0, and 12.0 at. pct Li). This investigation emphasizes thein situ small-angle X-ray scattering (SAXS) observations on specimens subjected to various aging conditions. Special attention is paid to the early stages of the transformation in an attempt to characterize the various possible modes of phase separation on one hand and to study the dynamics of the precipitation process on the other. The following results are obtained: the congruent ordering precedes decomposition at low temperatures; the metastableγ′ solvus curve is reconfirmed; but the predicted metastable miscibility gap is not found. Guinier radii measurements of the particles showed Ostwald ripening is quickly reached upon heating to the aging temperatures. Slowing down behavior is seen at aging temperatures close to the solvus boundary. Activation energies for Li diffusion were obtained using the modified Lifshitz, Slyozov, and Wagner (MLSW) model. A test of dynamical scaling behavior is carried out for the Al-12.0 at. pct Li alloy. Formerly Visiting Scientists at the University of Illinois     

Experimental evaluation of three single-particle dissolution models

The dissolution of the 60-85-mesh fraction os recording, flow-through dissolution apparatus equipped with a dissolution cell; it was particularly suitable for kinetic analysis of multiparticulate systems. By using a time-scaling approach, experimental data are compared with theoretical calculations to evaluate, quantitatively, which of three single-particle dissolution models best describes the data and how well the multiparticulate kinetics can be explained mathematically. The nonspherical tolbutamide particles are replaced in the calculations by a hypothetical system of spherical particles that appears to be log-normally distributed. This procedure permits the calculation of the intrinsic dissolution profile, considering both size distribution and particle shape effects.     

Effect of Silicon on Carbide Precipitation after Tempering of H11 Hot Work Steels

The formation of secondary carbides during tempering of H11 hot work steels at 898 K (625 °C) was studied by transmission electron microscopy (TEM) and related to the previously established effects of Si content on mechanical properties. Lower Si contents (0.05 and 0.3 pct Si) and higher Si contents (1.0 and 2.0 pct Si) were observed to yield different carbide phases and different particle distributions. Cementite particles stabilized by Cr, Mo, and V in the lower Si steels were found to be responsible for similar precipitation hardening effects in comparison to the M₂C alloy carbides in the higher Si steels. The much higher toughness of the lower Si steels was suggested to be due to a finer and more homogeneous distribution of Cr-rich M₇C₃ carbides in the interlath and interpackage regions of the quenched and tempered martensite microstructure. The present effects of Si content on the formation of alloy carbides in H11 hot work steels were found to be the result of the retarding effect of Si on the initial formation of cementite, well known from the early tempering stages in low alloy steels.     

Melting of secondary-phase particles in Al-Mg-Si alloys

The melting of secondary-phase particles—or, more precisely, the melting of such particles together with the surrounding matrix—in two ternary Al-Mg-Si alloys has been studied. In the quasi-binary Al-Mg₂Si alloy, one melting reaction is found. In the alloy with an Si content in excess of that necessary to form Mg₂Si, three different melting reactions are observed. At upquenching temperatures above the eutectic temperature, the reaction rates are very high, and it is assumed that they are controlled by diffusion of the alloying elements in the liquid. Melting is also observed after prolonged annealing at temperatures below the eutectic temperature in these alloys, which is explained by the different diffusion rates of Mg and Si. The rate of the melting reaction is in this case assumed to be controlled by diffusion of the alloying elements in the solid α-Al phase. It is shown that calculation of the particle/matrix interface composition, which determines when melting is possible, cannot be made solely on the basis of the phase diagram, but must also include the rate of diffusion of Mg and Si. The melting temperatures observed differ somewhat from the accepted eutectic temperatures for these alloys. On prolonged annealing, the liquid droplets formed dissolve into the surrounding matrix and their chemical composition is found to change during dissolution. The resulting eutectic structure after quenching of a droplet is explained by the phase diagram and the different diffusion rates of Mg and Si as well as by the nucleation conditions of the constituents involved.