Liquid Film Migration Research Articles

Isothermal grain coarsening of spray formed alloys in the semi-solid state

Grain coarsening of alloys in the semi-solid state is important in controlling grain sizes in spray forming and in other processes in which an alloy is formed or cast in the semi-solid state. Until now, this coarsening has been analysed in terms of classic LSW theory or in terms of the migration of grain boundary liquid films. These analyses suggest that the coarsening rate should increase with increasing solid fraction, f S, but this suggestion conflicts with previous experimental results which show that coarsening rate decreases with increasing f S for f S>0.7. This paper shows, for the first time, that coarsening rate does increase with f S for f S less than approximately 0.75 in agreement with the liquid film migration model, and then decreases again with further increasing f S for f S greater than approximately 0.75. A modified model of liquid film migration is proposed which takes into account the reducing area of the liquid film as f S increases for high f S. The formation of intragranular liquid droplets, and the pinning of grain boundary liquid films by dispersoids during coarsening are also discussed.

Microstructure evolution and electrical properties of yttria and magnesia stabilized zirconia

The paper contains the results of microstructure and chemical composition analyses of micrograins, which were formed on initial grain boundaries in ZrO2–Y2O3 and ZrO2–Y2O3–MgO systems. It has been found that the micrograins appear in the process of diffusion induced by grain boundary migration (DIGM). The observed processes can be described as both liquid film migration (LFM) and chemically induced grain boundary migration (CIGM). New micrograins had an increased content of Y2O3 and a cubic symmetry. Zirconia–yttria solid solutions with magnesia particulate addition showed an increased amount of migration nuclei and bigger size of new grains. However, no change in the chemical composition of the grains has been detected. The ionic conductivity measurements have shown that the activation energy (Ea) of conductivity at lower temperatures is connected to a DIGM-like process and to the distance of grain boundary migration. In the case of materials with dominating LFM process an increased grain boundary migration distance leads to a lowering of the activation energy of conductivity. Contrary to that, in the materials with dominating CIGM process an increase of migration zone causes increase of Ea values. The data obtained with respect to the type of DIGM process (LFM or CIGM) indicate that the grain boundary conductivity contribution increases with the CIGM distance.

Improvement of dielectric properties of SrTiO3-based materials through grain-boundary engineering

When an oxide melt is infiltrated into sintered SrTiO3, the liquid films formed between SrTiO3 grains are often observed to migrate. This paper shows two techniques for controlling the liquid film migration during the melt infiltration. An oxidation treatment of Nb2O5-doped SrTiO3 between a sintering in 95N2-5H2 and a melt infiltration in air suppressed the migration as did a proper ratio (∼1) of BaO and CaO in an infiltrant. The observed migration suppression was attributed to the formation of a thin oxidized layer whose lattice parameter was similar to that of bulk SrTiO3 grain. The migration suppression resulted in a considerable increase in dielectric properties.

Characterization of Constitutional Liquid Film Migration In Nickel-Base Alloy 718

When multiphase alloys are rapidly heated, it is possible to cause melting of the interface between phases. This is called constitutional liquation if, during melting, the bulk composition is in a nonliquid region of the phase diagram but the tie-line between the liquating phases passes through a liquid region. The liquid produced during constitutional liquation can spread along grain boundaries and promote liquid film migration (LFM). This is known as constitutional liquid film migration (CLFM), which is thermodynamically similar to liquid film migration; however, mechanistically there are significant differences. Nickel-base alloy 718 has been studied to show the features of migration that are unique to CLFM. Experimentation consisted of heat-treating rods of alloy 718 to promote the trapping of niobium carbide particles on the grain boundaries. These samples were then subjected to isothermal treatments above their constitutional-liquation temperature, which produced CLFM of the grain boundaries. The movement of the liquid films away from their centers of curvature, the formation of a new solid solution behind the migrated liquid films, and the reversals of curvature of the migrated liquid films confirmed that CLFM was the phenomenon observed. The concentration of niobium behind the migrated liquid films for isothermal treatments below the solidus temperature was shown to be greater than the niobium concentration in the matrix. Above the solidus temperature, there was no increase in niobium concentration. The validity of the coherency strain hypothesis as the driving force for CLFM in alloy 718 is discussed.

A test for diffusional coherency strain hypothesis in the discontinuous precipitation in Mg-Al alloy

Discontinuous precipitation (DP) occurs in many alloy systems under certain conditions. Although solute supersaturation is the chemical driving force for DP, this has to be coupled with another driving force for grain boundary migration. This was identified to be diffusional coherency strain ahead of the moving boundary in the case of diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM). In the present work, the validity of diffusional coherency strain hypothesis is verified in Mg-Al alloy, which exhibits discontinuous precipitation. Samples were tested with an applied stress simultaneously with discontinuous precipitation and it was found that the velocity of the boundaries both parallel and transverse to the stress axis obeys the model for diffusional coherency strain. This work can be used as a conclusive evidence for diffusional coherency strain hypothesis for the occurrence of discontinuous precipitation in Mg-Al alloys.

The Mechanism of Braze Metal Penetration by Migration of Liquid Films in Aluminium

Samples of braze-clad AA3003 alloy, have been examined in various conditions after heat treatments simulating stages of the brazing process. Sheet material was produced in the annealed temper using both homogenised and unhomogenised ingots and was subsequently strained to different degrees before brazing. A critical degree of deformation leads to unfavourable penetration of braze metal into the core alloy. This process involves the dragging of liquid films under the action of the stored energy of deformation, analogous to the grain boundary tension effect reported previously, and is different from the phenomenon of liquid film migration driven by alloying and associated coherency stresses.

Grain Boundary Melting or Liquid Film Migration in Brazing Sheet

A study has been carried out to increase understanding about the nature of core penetration, grain boundary melting or liquid film migration. In the literature all three phrases are used for probably the same metallurgical observation. This phenomenon is frequently observed in O-temper brazing sheet. In this study Differential Scanning Calorimetry (DSC) measurements were correlated with the fillet sizes formed in a reverse T-sample, together with the occurrence of grain boundary penetration in the same sample. The conclusion is that DSC measurements provide reliable data to study grain boundary penetration. The DSC method is discriminating enough to measure small differences in grain sizes. DSC proves to be a simple but powerful tool to study changes in the core induced by the interaction of the clad alloy with the core alloy.

Forced velocity cells and liquid film migration

Forced velocity cells and liquid film migration

Liquid film migration as a broken symmetry due to an unbalanced anisotropy of surface tension

A new quantitative phenomenological theory of asymmetric solidification or liquid film migration (LFM) is presented. This is designed to replace a currently popular phenomenology which is demonstrated to be faulty in two crucial aspects. Firstly, the planar quasi-steady state diffusion model contradicts the leading interface mass balance and must accordingly be replaced by a Boltzmann-type parabolic structure suggested in part by the definitive experiments of Kuo and Fournelle. Secondly, while a proposed coherency strain effect may be involved in the initial symmetry-breaking, it cannot as a necessary constant corollary of the usual diffusion model be fully sustained beyond the initiation stage. The configuration, as a consequence of the modern theory of layered films, is seen to be strongly unstable to formation of mobile misfit dislocations, especially as it lies near the melting point. It is demonstrated that a typical critical film thickness (∼1 μm) is very much less than the actual film thickness (∼the grain size). It is suggested that anisotropy of surface tension expressed as a decreased mobility of the frontal interface is the essential cause of asymmetry. The experimental record is salutary to the new mechanically relaxed structure. In particular, it is predicted that a time 1/2 dependency of the melting front motion which, allowing for a damping effect of developing curvature, is consistent with the observations of Kuo and Fournelle. Since the diffusion and interface reaction laws remain close to the linearity required by Onsager's irreversible thermodynamics, the global selection to the asymmetric parabolic state is seen to be a quantitative illustration of the integral principle of minimum dissipation as a thermokinetic imperative.

On liquid film migration in aluminium–copper alloys

Experimental and numerical modelling results are presented for the migration of liquid lenses, formed on melting of grain-boundary θ precipitates in Al–3.4 wt% Cu alloys, which had been previously equilibrated at 400°C. Liquid films of comparable thickness, formed upon the melting of intragranular Widmanstätten precipitates, were found not to migrate. The modelling studies are based on the hypothesis that the leading and trailing solid–liquid interfaces are each characterized by a constrained local equilibrium, as determined by chemical, coherency and capillary terms, and that the process is driven by the liquid concentration gradient resulting from different equilibrium concentrations at the two interfaces. The model is utilized to explore the sensitivity of the process to variations in physico-chemical parameters, and ultimately as an aid in the understanding of the migration of the liquid lenses and its cessation. It is concluded that the capillary term has a strong influence on the migration process, even for liquid pool dimensions of several microns.

ChemInform Abstract: Direction of Liquid Film Migration Induced by Chromic Oxide in Alumina‐ Anorthite.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Direction of Liquid Film Migration Induced by Chromic Oxide in Alumina‐Anorthite

Three kinds of single‐crystalline alumina plates with the crystallographic planes of C, m, and R were diffusion‐bonded with liquid‐phase sintered (98)alumina‐(2)anorthite (in wt%) plates (P) and then heat‐treated at 1600°C under a Cr203‐containing atmosphere. During the heat treatment, for all of the specimens studied, the anorthite liquid films between alumina plates migrated to grains with surface orientation corresponding to higher coherency strain energy. This result is in better agreement with a prediction based on the coherency strain theory than the previous one obtained for grain‐boundary migration. The discrepancy between the predicted and previously observed migration directions of some grain boundaries in alumina may therefore be attributed to an effect of grain‐boundary structure and stress transmission across the boundary.

Characterization of constitutional liquid film migration in nickel-base alloy 718

When multiphase alloys are rapidly heated, it is possible to cause melting of the interface between phases. This is called constitutional liquation if, during melting, the bulk composition is in a nonliquid region of the phase diagram but the tie-line between the liquating phases passes through a liquid region. The liquid produced during constitutional liquation can spread along grain boundaries and promote liquid film migration (LFM). This is known as constitutional liquid film migration (CLFM), which is thermodynamically similar to liquid film migration; however, mechanistically there are significant differences. Nickel-base alloy 718 has been studied to show the features of migration that are unique to CLFM. Experimentation consisted of heat-treating rods of alloy 718 to promote the trapping of niobium carbide particles on the grain boundaries. These samples were then subjected to isothermal treatments above their constitutional-liquation temperature, which produced CLFM of the grain boundaries. The movement of the liquid films away from their centers of curvature, the formation of a new solid solution behind the migrated liquid films, and the reversals of curvature of the migrated liquid films confirmed that CLFM was the phenomenon observed. The concentration of niobium behind the migrated liquid films for isothermal treatments below themore » solidus temperature was shown to be greater than the niobium concentration in the matrix. Above the solidus temperature, there was no increase in niobium concentration. The validity of the coherency strain hypothesis as the driving force for CLFM in alloy 718 is discussed.« less

Liquid film migration in a Mo(Ni) bicrystal

The liquid film migration process has been studied during the penetration of a Ni-rich liquid phase along a symmetrical 70degree(011) {011} tilt grain boundary in a Mo bicrystal at 1380 C. The experimentally determined average migration rate of the film is two orders of magnitude lower than expected from an estimate made according to coherency-strain theory. This discrepancy can be explained under the assumption that the actual film migration time is much lower than the annealing time.

The effect of initial microstructure on heat-affected zone microfissuring in Incoloy 903

Studies were undertaken on electron beam welded thermomechanically processed Incology 903 with varying proportions of duplex grain size distributions, to determine quantitatively the effect of variation in the volume fraction of fine grains to warm worked grains on heat-affected zone (HAZ) microfissuring. Extensive microfissuring was observed in samples consisting primarily of large warm worked grains, which, however, could be minimized by increasing the proportion of the fine recrystallized grains outlining the large warm worked grains. Microfissuring was also observed to be reduced by decreasing the welding speed for any given value of electron beam current. A solidification structure composed of γ/carbide eutectic reaction was observed on the boundaries that developed microfissures. The fine grain boundaries that were devoid of microfissures exhibited liquid film migration and formation of niobium-enriched γ phase and were devoid of γ/carbide eutectic constituents. It is suggested that one of the major causes of minimal microfissuring on fine grains in Incoloy 903 is due to the occurrence of liquid film migration towards curvature on some of the grain boundaries. The existence of substantial mean grain boundary curvature on the fine grains is thus the major factor responsible for the reduced microfissuring in Incoloy 903.

The liquid film migration in a sintered Fe-Cr-C base alloy

In an Fe-5Cr-lMo-2Cu-0.5P-3C (wt pct) alloy prepared by liquid-phase sintering at 1120 °C, the intergranular liquid films migrate during cooling and isothermal heat treatment at temperatures where the solid and liquid phases coexist. The liquid film migration (LFM) occurs extensively even during rapid cooling, apparently because of a large driving force. The migrating liquid films solidify to form network carbides. In the regions swept by the migrating liquid films, the concentrations of the substitutional solute atoms are slightly different from those in the original grains. When cooled rapidly, martensite is abundant in these regions. It is possible that the driving force for LFM in this alloy stems in part from the C concentration change associated with the concentration changes of the slowly diffusing substitutional solute atoms.

Theories and observations of chemically induced interface migration

AbstractThe diffusional coherency strain theory and its implications for chemically induced migration of liquid films and grain boundaries are reviewed and critically examined. The critical experiments in metals and oxides show the validity of this theory. The observed characteristics of this phenomenon: misfit dislocations at the initial boundary location; migration direction; faceting; dependence on grain boundary structure, on boundary curvature, and on temperature are also consistent with the coherency strain theory. Other theories are also described, but are mostly inconsistent with the observations, and there is very little experimental evidence for the validity of any of them. There was experimental evidence that the coherency strain theory is also valid for discontinuous precipitation. The significance of lattice diffusion at small distances in the order of the lattice spacing is examined. The related phenomena of diffusion induced recrystallisation and surface corrugation are also described. Fina...

Precipitate Coarsening by Liquid Film Migration in (Mg,Ca)‐PSZ's

Precipitate coarsening involving preferential growth of solute‐poor tetragonal ZrO2 precipitates along grain boundaries, combined with migration of grain boundaries containing a liquid silicate phase to form solute‐rich precipitate‐free cubic ZrO2, has been observed in grain boundary regions of MgO–CaO–ZrO2 and CaO–ZrO2 alloys during isothermal aging at 1400°C. Studies of alloys with 9.5 mol% total solute, but with varying MgO/CaO ratios, demonstrated a marked composition dependence of the phase separation process, in that such phase separation was rare in MgO‐rich materials. The migration process is analyzed using models developed for migration in less complex materials, and a mechanism incorporating orientation‐dependent precipitate strain energies is proposed.

The influence of electron-beam welding parameters on heat-affected-zone microfissuring in INCOLOY 903

The microfissuring in the heat-affected zone (HAZ) of electron-beam-(EB-) welded thermomechanically processed INCOLOY 903 has been studied with a view to reducing the incidence of microfissuring and to obtaining a better understanding of the influence of EB welding parameters on it. For a given heat of material, microfissuring susceptibility has been quantitatively related to EB welding parameters and the shape of the weld pool. Fractional factorial experimental study of welding parameters showed that a reduction in welding speed and an increase in EB current for a given heat input would minimize HAZ microfissuring in the alloy. It was observed that with lower travel speeds, bccause of the shallower temperature gradients in the HAZ, the amount of liquated grain boundary area is less, thus leading to decreased microfissuring. Considerable HAZ microfissuring was observed on the coarse grain boundaries of warm-worked grains. The microfissures appeared to initiate in regions slightly removed from the fusion line. Minimal microfissuring was observed on the grain boundaries of fine recrystallized grains. These boundaries, however, had a thickened appearance bccause of the formation of Nb-enrichedy phase by the process of grain boundary liquid film migration (LFM). The origin of the liquid on the grain boundaries is suggested to be due to the constitutional liquation of preexisting primary carbides (partial), fine MC carbides, and MNP-type phosphides. It is suggested that substantial occurrence of LFM in the HAZ minimizes microfissuring by decreasing the total temperature range of solidification and also by enabling the grain boundary liquid to solidify without the occurrence of low-melting terminal eutectic reaction.

The suppression of chemically induced liquid film migration in CoCu at high temperature

When a Co-20 wt% Cu alloy prepared by liquid phase sintering at 1150°C is heat-treated at 1300°C, some intergranular liquid films initially migrate and reverse their directions to return to their initial locations, while others do not migrate at all. Previously, extensive liquid film migration (LFM) has been observed when the sintering and the heat-treatment temperatures have been reversed. When the same alloy sintered at 1150°C is heat-treated at 1150°C in contact with a Cu-10Co-10Sn (wt%) powder mixture, extensive LFM occurs, producing Cu-depleted solid behind the migrating liquid films. When the same experiment is carried out at 1300°C, very little LFM is observed and the grain composition is equilibrated by lattice diffusion. Upon increasing the Sn content in the powder mixture to 15 and 20 wt%, extensive LFM occurs at both 1150 and 1300°C. These observations are consistent with the prediction of the coherency strain theory that there are high temperature limit and minimum critical driving force for LFM. At high temperatures and low driving forces, the ratio of the solute lattice diffusivity to the migration velocity is so large that the coherency in the frontal diffusion zone is expected to be broken, eliminating the driving force for LFM. The same principle should apply to the grain boundary migration.