Diffusionless Growth Research Articles

Kinetics of the amorphous to icosahedral structure transition in Al-Cu-V and Al-Mn-Si alloys

Transformation kinetics of rapidly quenched Al 75Cu 15V 10 and Al 55Mn 20Si 25 amorphous alloys to icosahedral phases were examined by differential scanning calorimetry and transmission electron microscopy. The transformation is analogous to the polymorphous transformation by homogenous nucleation and diffusionless growth. The Avrami exponent and activation energy for transformation were measured to be 1.7–2.0 and 2.5 eV, respectively, for both alloys. The activation energy for growth during the transformation was estimated to be about 2.6 eV for Al 75Cu 15V 10 and 2.7 eV for Al 55Mn 20Si 25. The difference in free energy between the amorphous and crystallines phases was estimated to be 1.6 times larger than that between the amorphous and quasicrystalline phases.

Coupled diffusional/displacive transformations

Transformations involving crystalline phases containing both substitutional and interstitial elements have been studied theoretically to establish the conditions under which plates of the product phase can form with just partial redistribution of the interstitial element during nonequilibrium nucleation and growth. The rate at which an interface moves depends both on its intrinsic mobility and on the ease with which the interstitial element diffuses ahead of the moving interface. The two processes are coupled so that a stable velocity can be calculated by matching the velocity solutions for diffusion and for interfacial motion. Interfacial mobility is modelled in terms of the dislocation theory of glissile martensitic interfaces. A thermodynamic model incorporating the effect of interfacial dissipation has enabled interfacial compositionsto be deduced for growth involving partial supersaturation. These data are then used to calculate the corresponding diffusion field velocity. As a possible model for bainitic transformation, calculations for an Fe-0.4C wt% alloy show that the maximum rates for both nucleation and growth can occur for incomplete partitioning of solute. At high undercoolings, diffusionless growth can occur, even when nucleation still requires some partitioning of carbon.

A simplified treatment of the transition from diffusion controlled to diffusion-less growth

The nature of the deviation from local equilibrium during a phase transformation is discussed. It is shown that a reaction occurring under non-local equilibrium conditions can be treated by the ordinary local equilibrium formalism by adding suitable corrections to the thermodynamic properties of the growing phase. This enables us to use well established methods and computer software to treat transformations where there is a deviation from local equilibrium. In particular, a simplified method of treating the deviation from local equilibrium caused by solute drag is suggested and applied to the edge-wise growth of a ferritic plate in a binary FeC alloy at 600°C. The new treatment shows essentially the same features as the more ambitious treatment by Hillert and Sundman. However, an important difference is caused by the inclusion of a finite interface mobility in the model. As the supersaturation is increased above a critical level a diffusion controlled reaction will gradually develop into diffusionless massive reaction having a flat interface. The new treatment is compared with methods recently presented by Olson et al. and by Aziz. It is found that those methods suffer from some serious drawbacks and thus the present method is to be preferred.

Thermodynamic and kinetic aspects of crystallization of supercooled AgCu liquids

When performing thermodynamic calculations at temperatures outside the temperature range where the liquid is the stable phase and where its properties can be measured, it is important to extrapolate the properties in a reasonable way. Many different methods have been proposed over the years. The methods are based on various approximations on how the difference in heat capacity between the liquid and the solid ΔC P L− S varies with temperature T. The simplest approach is ΔC P L− S = 0 or perhaps a constant value. Both of these approximations have recently been used by Murray and by Hayes et al. in their evaluations of the thermodynamic properties of the AgCu system. As these approximations are known to be less satisfactory for large supercoolings we have re-evaluated the thermodynamic properties of the AgCu system by applying a new model, recently presented by Ågren. It is a phenomenological model suitable for the thermodynamic description of a highly undercooled liquid. The new model, previously applied only to pure elements, was extended here to alloy melts. After having established a sound thermodynamic basis of the AgCu system the kinetics of solidification were considered. We calculated the growth rate of a dendritic tip, taking into account the effect of interface kinetics, the Gibbs-Thomson effect and the so-called solute-drag effect. This allowed us to model the transition from diffusion-controlled growth, occurring at low supersaturations, to diffusionless growth, occurring at high supersaturations. It was found that when the ratio between the solute mobility at the interface and the solute diffusivity in the liquid was small the transition occurred close to the so-called T 0 line. However, if the ratio was large the transition to diffusionless growth occurred close to the solidus.

Mechanism of dendritic side‐branches coarsening under isothermal conditions

AbstractThe process of dendritic branches transformation of succinonitrile and some other organic substances has been studied under the isothermal holding of two‐component melt. The dendritic branches have been shown to be separated from the steam in the specimen with the thickness comparable with the diameter of the dendrite. Dendritic structure coarsening observed in the bulk specimen is not associated with the separation of the branches. In the region of the concentration gradient the dendritic growth has been shown to result in partial dissolution of as‐crystallized material under the isothermal holding after the end of crystallization. This effect seems to result from the diffusionless growth in the two‐phase region of the phase diagram.

High temperature metal-induced crystallization of r.f. sputtered amorphous Si thin films

R.f. sputter-deposited amorphous Si (a-Si) films have been found to exhibit an interconnected fine structure of low and high density regions. The porosity decreases as the film thickness increases in agreement with results for evaporated a-Ge previously published by Donovan. At an annealing temperature of 800°C, a-Si films crystallize by the relatively slow diffusionless growth of crystallites nucleated within the a-Si matrix. Similar a-Si films containing a buried layer 30 Å thick of Al in an a-Si/Al/a-Si structure exhibited greatly enhanced nucleation and crystallization rates. Crystallite growth in these films occurred by two separate mechanisms both involving the precipitation of crystalline Si from an Al + Si liquid region. The first growth mechanism was observed during the early stages of crystalline formation in which crystalline islands grew around isolated liquid regions. Later, Al + Si liquid boundary layers were established between crystalline Si and a-Si regions, and further crystallization occurred by the motion of this boundary layer.

The theory of diffusionless crystal growth

The difference between the processes of diffusion growth and diffusionless growth of a new phase during phase transformation in a binary system is discussed. Transformation is analysed in a one-dimensional model. Diffusionless transformation (with the coefficients of diffusion in both phases equal to zero) is investigated and it is shown that the distribution of the components with respect to the phase boundary has the same qualitative nature as in the case of the diffusion transformation (specifically, the concentrations of the components are unequal on either side of the phase boundary). The analysis is made with allowance for diffusion in the initial phase. The transition from diffusion growth to diffusionless growth is traced on the basis of the derived relationship between the degree of short-range order in the new phase and the magnitude of the coefficient of diffusion. In the one-dimensional system examined here, this transition is smooth.