Liquid Film Migration Research Articles

Characterization of constitutional liquid-film migration in Alloy 718

Alloy 718 is a nickel-based superalloy which consists of an FCC solid solution of Ni, Fe, and Cr strengthened with γ″ precipitates. The composition of the alloy is shown in Table 1. It has excellent high temperature mechanical properties and oxidation resistance. However, Alloy 718 is susceptible to hot cracking in weld heat affected zones (HAZ) as a result of the formation of intergranular liquid. This metastable grain boundary liquid may be due to “constitutional liquation” of second phase particles, known as constitutional liquid film migration (CLFM). Radhakrishnan and Thompson report that CLFM can occur in situ in the weld HAZ of alloy 718 as a result of constitutional liquation.Constitutional liquation can be described in terms of a binary alloy with a phase diagram such as theone shown in Figure 1. If an alloy of composition C° is rapidly heated above theeutectic temperature, TE, to temperature T, the AB particles will not have sufficient time to dissolve, as dictated by equilibrium.

The influence of solidification mode on heat affected zone microfissuring in a nickel-iron base superalloy

The heat affected zone (HAZ) microfissuring of thermomechanically processed Incology 903 with a duplex grain structure, has been examined with a view to understand the mechanism(s) of, and to reduce the incidence of microfissuring. Extensive formation of liquid films on the HAZ grain boundaries primarily due to the complete constitutional liquation of preexisting MNP phosphides, MC carbides and partial constitutional liquation of primary MX carbide insolubles was observed. The liquid films resulted in considerable microfissuring on the long warm worked grain boundaries and extensive grain boundary liquid film migration (LFM) on the fine recrystallized grain boundaries. The liquid films formed on the warm worked boundaries were observed to have undergone normal solidification accompanied with the formation of dendritic gamma and terminal interdendritic constituents. The exclusive occurrence of LFM only on the fine grains and not on warm worked grains is attributed to the increased driving force and velocity for LFM arising due to substantial interface curvature of the fine grains. It is shown that HAZ microfissuring is minimized if the liquid films on the HAZ grain boundaries can partially equilibrate by LFM instead of by normal solidification.

Kinetics of grain growth in the weld heat-affected zone of alloy 718

Grain-boundary liquation occurs in the weld heat-affected zone (HAZ) of the Ni-base superalloy 718 at locations where the peak temperatures are greater than about 1200 ‡C. The evolution of the grain structure at these HAZ locations depends upon the interaction between the grains and the grain-boundary liquid. The evolution of grain structure in the presence of grain-boundary liquid was simulated by subjecting samples to controlled thermal cycles using resistance heating. A measurement of grain size as a function of isothermal hold at two peak temperatures of 1200 ‡C and 1227 ‡C indicated that in alloy 718, the kinetics of grain growth depended upon the prior thermal history of the alloy. In the solution-treated alloy, the presence of grain-boundary liquid did not arrest grain growth at either peak temperature. In the homogenized and aged alloy, a grain refinement was observed at the peak temperature of 1227 ‡C, while an arrest of grain growth was observed at a peak temperature of 1200‡C. Liquid film migration (LFM) and subgrain coalescence, either acting alone or simultaneously, are shown to explain most of the observed microstructural phenomena and the kinetics of grain growth in the alloy.

Composition change by liquid film migration in a sintered iron alloy for camshafts: H.S. Shin et al (Korea Advanced Inst of Science and Techynology, Korea)

Composition change by liquid film migration in a sintered iron alloy for camshafts: H.S. Shin et al (Korea Advanced Inst of Science and Techynology, Korea)

The effect of elastic anisotropy on the migration of intergranular liquid films and the precipitation of a liquid phase in a CoCu alloy

When a Co20Cu (wt%) alloy prepared by liquid phase sintering at 1300°C is heat-treated at 1150°C, the intergranular liquid films and grain boundaries migrate, leaving behind a new solid solution with reduced Cu content. The phenomenon is identical to that observed previously in MoNi. The calculated driving force for the migration is almost equal to the coherency strain energy in the solute diffusion zone ahead of the migrating boundaries. The migrating liquid films show faceting with curved planes and edges. Their shape closely resembles the growth shapes predicted on the basis of the coherency strain energy with anisotropic elastic constants. During the heat-treatment, liquid precipitates form within grains and at grain boundaries, and they also show faceting, which is consistent with orientation dependent coherency strain energy in the diffusion zone around the precipitates.

Diffusion-Induced Grain Boundary Migration and Role of Defects

AbstractA model is presented to explain various aspects of diffusion-induced grain boundary migration (DIGM). The driving energies of DIGM are identified as the free energy of mixing and the interface free energy, the former being predominant in most cases of DIGM. The grain boundary migrates due to thermally activated motion of atoms across the interface under the influence of the driving energies. An expression for the velocity of migration is derived. It is shown that this depends parabolically on the solute concentration, in agreement with experimental observations in the case of liquid film migration (LFM), which is analogous to DIGM. Furthermore, the velocity is proportional to lattice diffusivity ahead of the boundary. Recent results of enhancement of DIGM by ion bombardment is explained by radiation-enhanced lattice diffusivity due to introduction of point defects by the ions. The model also explains that diffusion-induced recrystallization (DIR) is due to a free energy decrease associated with the transformation from the amorphous phase in the grain boundary layer to the crystalline phase.

Grain boundary liquid film migration during welding of incoloy 903

The phenomenon of migration is reported for a supersaturated liquid film that had been created via the constitutional liquation of heat-affected zone (HAZ) grain boundary-associated Nb-rich carbides during the welding of Incoloy 903. The supersaturation is relieved by solute transport in the liquid film; this process dissolves the gamma matrix lying ahead of the migration, and deposits an Nb-enriched gamma solid solution behind it. Supersaturation-relief can therefore be accomplished by both liquid film migration and Nb-enriched gamma formation, as well as through normal resolidification over a larger temperature range. 26 refs.

Chemically-induced liquid film migration with low lattice diffusivity relative to the migration rate in MoNi(W)

When a 90Mo-10Ni alloy (by wt) liquid phase sintered at 1400°C is heat-treated at 1400°C after replacing the matrix with a melt of 44Ni-34Mo-22W (by wt), the liquid films between the grains migrate, leaving behind an Mo alloy enriched with W. The ratio of the lattice diffusivity of W in Mo, D, to the initial migration velocity, v, ( D/ v) is estimated to be between 0.03 and 0.18 Å. Hence, it appears that there is no lattice diffusion of W ahead of the migrating liquid film, and in such a case the driving force has been suggested to be the chemical free energy. But the observed v is approximately same as that to be expected if the driving force is assumed to be diffusional coherency strain energy. Likewise, a previous study of den Broeder and Nakahara shows that the rate of chemically-induced grain boundary migration in CuNi shows a smooth variation with temperature as D/ v decreases from values much larger than the interatomic spacing to values much smaller with decreasing temperature. The coherency strain energy thus appears to be a general driving force for the migration even when the apparent diffusion length indicated by D/ v is smaller than the interatomic spacing.

Diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM) in an Al-2.07 wt% Cu alloy

Diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM) in an Al-2.07 wt% Cu alloy during isothermal annealing after down and up quenching from an equilibrated state at 620°C in the α + liquid phase field has been studied by light and scanning electron microscopy. Down quenching to 520, 540 and 560°C resulted in grain boundaries migrating by DIGM against their curvature from one grain into another leaving behind alloyed zones. Down quenching to 590 and 605°C resulted in liquid films migrating against their curvature from one grain into another leaving behind alloyed zones. Boundaries were also observed to migrate at these temperatures. Up quenching to 630 and 640°C resulted in liquid films migrating from one grain into another leaving behind dealloyed zones. The migration distance, s, for both boundaries and liquid films was observed to decrease monotonically with annealing time ( t) and to obey a power law relationship, s = Kt n , with annealing time. The exponent n falls between 0.20 and 0.25. For a given annealing time the migration rates of liquid films by LFM were about twice as fast as those of grain boundaries by DIGM. With respect to driving force for boundary and liquid film migration, the coherency strain energy developed in the grain being consumed does not seem to be great enough to drive the reactions against the curvature observed.

Chemically Induced Migration of Liquid Films and Grain Boundaries in TiN—Ni—(TiC) Alloy

Liquid films and grain boundaries in the TiN–Ni system migrate when carbon atoms are added as TiC. Ti(NC) solid solution grows at the expense of TiN solid through liquid film migration (LFM), and its general characteristics are similar to those of the previously investigated Mo–Ni and W–Ni systems. But, in this system, precipitating solid–liquid interfaces of migrating liquid films show faceting, which is caused by orientation‐dependent interface‐controlled reactions. With an increase of added TiC, the migration rate increases parabolically with the observed lattice parameter difference between the TiN and Ti(NC) solid, which is in agreement with the prediction of the coherency strain model. Chemically induced grain‐boundary migration (CIGM) also occurs, together with LFM. The results in this study show that LFM and CIGM occur in a system having relatively strong covalent bonding.

On the thermodynamic driving force for diffusion-induced grain boundary migration, discontinuous precipitation and liquid film migration in binary alloys

An expression for the net thermodynamic driving force (free energy change per unit distance of reaction front migration per unit area of reaction front) for the migration of reaction fronts during diffusion-induced grain boundary migration (DIGM), discontinuous precipitation (DP) and liquid film migration (LFM) in binary alloys is determined from a two-dimensional model which considers individual segments of the migrating reaction front as open systems which can receive solute and solvent atoms from or give them up to the surrounding material. In the case of DIGM and DP the reaction front is treated as a separate grain boundary phase of thickness δ. In the case of LFM it is treated as a liquid film of thickness δ. The magnitude of the force on the reaction front is calculated for DIGM in AuCu and for LFM in NiMo using available thermodynamic data. Finally, a criterion for the stability of a reaction front, when a small change in composition across a reaction front occurs and the reaction front composition is slightly changed, is established which shows that a driving force for reaction front migration can develop spontaneously as the result of solute and solvent diffusion along it, as long as the curvature of the free energy curve for the reaction front is positive.

Diffusion-induced migration of grain boundaries and liquid films

Abstract A model is proposed to explain diffusion-induced migration of a grain boundary or a liquid film on a unified basis. The driving energies for both phenomena are identified as the interface free energy and the free energy of mixing, the latter being predominant in most cases. The grain boundary or the liquid film migrates owing to thermally activated motion of atoms across the interface under the influence of the driving energies. An expression for the velocity of migration is derived, and it is shown that this depends parabolically on solute concentration. The model explains numerous experimental facts.

The discontinuous precipitation of a liquid phase in MoNi induced by diffusional coherency strain

When a liquid phase sintered MoNi alloy is heat-treated at a temperature lower than that used for sintering where the solid and liquid phases coexist, the liquid films and grain boundaries between the grains migrate, leaving behind a new solid solution somewhat depleted of Ni. Since some liquid is produced during this migration, it resembles a discontinuous precipitation of the liquid phase. It is demonstrated experimentally that the driving force for this discontinuous precipitation arises from the coherency strain produced by Ni atom diffusion out of the grains. When two solute atom species simultaneously diffuse into or out of crystals in a ternary system, the resulting coherency strain depends on the size and concentration of the diffusing atoms and can be varied independently of the free energy of mixing. In particular, the coherency strain can be reduced to zero when the strain effects of the two atomic species exactly cancel each other. In this study, series of 90Mo10Ni alloy (by wt%) have been prepared by liquid phase sintering at temperatures between 1400 and 1520°C in order to produce solid MoNi grains of varying Ni concentration that are in equilibrium with the surrounding liquid matrix. The migration of liquid films and grain boundaries in the sintered specimens is induced by heat-treating them at 1400°C after adding various amounts of Fe to the liquid matrix. The migration does not occur when the estimated coherency strain is close to 0, although the free energy of mixing is finite. This result is a definitive demonstration that the driving force for the steady state migration is the coherency strain energy. The evidence that the grain boundaries exist in the sintered specimens and remain as grain boundaries during the migration is discussed.

Liquid film migration (LFM) in the weld heat affected zone (HAZ) of a ni-base superalloy

Liquid film migration (LFM) in the weld heat affected zone (HAZ) of a ni-base superalloy

Dynamic recrystallization and chemical evolution of clinoamphibole from Senja, Norway

Clinoamphibole from a mylonitic amphibolite exhibits microstructures characteristic of dynamic recrystallization, including porphyroclasts in a finer grained matrix of needle-shaped amphibole. The matrix amphibole defines an LS fabric and porphyroclasts have core and mantle structures with a core containing undulose to patchy extinction and (100) deformation twinning surrounded by a mantle of recrystallized grains. In addition intragranular grains also occur within the cores. TEM analyses of the porphyroclasts reveal that they contain a wide variety of lattice defects including high densities (5 × 108cm−2) of free dislocations and dislocation arrays, dissociated dislocations, stacking faults, and (100) micro-twins. TEM also shows that matrix grains and intragranular grains have relatively low defect densities, and that the intragranular new grains occur at localities in the porphyroclasts characterized by high densities of dislocations. These observations along with the chemical and orientation relationships between the recrystallized grains and porphyroclasts indicate that the new grains may have formed by heterogeneous nucleation and that further growth probably occurred by both strain assisted and chemically induced grain boundary migration or liquid film migration. This recrystallization event is interpreted to be synkinematic based on the fact that no recrystallization textures are present in the matrix grains and that the matrix grains define an LS fabric. However, the low defect densities in the matrix grains and the lack of intracrystalline strain in other phases indicate that post-kinematic recovery processes were active.

Melting of CuIn solid solutions at small superheating by droplet formation and liquid film migration

The melting of homogenized, polycrystalline α-CuIn solid solutions was studied by Differential Thermal Analysis and metallographic methods. At small superheating melt forms by two mechanisms. The formation of homogeneously distributed droplets in the interior of the grains and the subsequent diffusion of the excess indium to these nuclei dominates, if the solid solution is heated up rapidly (≈ 20 K s −1) up to 50 K above its solidus temperature. At small heating rates of ≈ 8.3·10 −1 K s −1, homogenization takes place by the formation of thin liquid films at the grain boundaries and their subsequent migration. The moving liquid films accumulate the excess indium from the dissolved supersaturated solid solution and leave a solid solution of equilibrium concentration in their wake. From the correspondence between the experimental migration velocities and calculated values it is concluded, that coherency strains in the solid adjacent to the liquid film provide the driving force for the second mechanism.

Comment on the process of liquid film migration

Comparaison des caracteristiques entre la migration d'un film liquide et la migration des joints de grains induite chimiquement. Determination des conditions d'etat stable ou quasi stable de la migration des films liquides

Migration of liquid film and grain boundary in MoNi induced by W diffusion

The liquid films and grain boundaries in liquid phase sintered Mo-Ni alloy are observed to migrate during heat-treatment after adding W to the liquid matrix. Behind the migrating boundaries forms Mo-Ni-W solid solution with the W concentration decreasing with the migration distance because of W depletion in the liquid matrix. The migration rate during the heat-treatment at 1540°C after adding W decreases with the decreasing pretreatment sintering temperature. When the sintering temperature is 1420°C, the migration rate is almost reduced to 0. Under this condition, the coherency strain due to the simultaneous diffusion of W and Ni into the grain surfaces is estimated to be almost 0. The results thus show that the coherency strain due to lattice diffusion is the driving force for the liquid film and grain boundary migration.

The Coherency Strain Induced Instability of Solid-Liquid Interfaces in the Mo-Ni System

An instability at solid-liquid interfaces induced by coherency strain has been studied in the Mo-Ni system. When the lattice parameter of a solid solution varies with compositions, a coherency strain develops if there is diffusion in the solid adjacent to the liquid. As a result of this strain, the originally planar solid-liquid interface becomes unstable by dissolution of the initial solid and precipitation of the new solid, forming a sinusoidal interface shape. In a previous study, the coherency strains have been shown to be the driving force for the instability. In this paper we analyze the evolution of the instability with time, from initiation to healing of the instability. Using the equations developed for liquid film migration (LFM) the change of microstructure is explained qualitatively. In addition, the relationships between this instability, diffusion induced grain boundary migration (DIGM) and LFM are discussed.

On the selectivity of certain experiments on diffusion induced grain boundary migration

In recent months some papers have appeared in the literature describing systematic studies of diffusion induced grain boundary migration (DIGM) or related phenomena such as liquid film migration (LFM). The results appear to lend support to different theories regarding the origins of the DIGM phenomenon. This note explores the nature of this dichotomy and suggests how the apparently conflicting results can be reconciled.