Factors influencing solute segregation of spray-atomized palladium alloy powders
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These results show that for V # 1 mm/s, the growth occurs by two-dimensional nucleation process, whereas a transition in growth mechanism occurs for V . 1 mm/s. The presence of oscillations at low velocities could be related to the time required to form a new layer by the nucleation process. As the nucleus forms and spreads along the interface, the interface slows. However, the motion of the isotherm during this time will cause an increase in the interface undercooling, and thus cause the interface to move rapidly until it catches up with the isotherm. At this time, the velocity decreases and the process of nucleation begins again, giving rise to the oscillations in the position of the interface with time.
The author acknowledges many valuable discussions with R. Trivedi. Fig. 4—The relationship between V and DT for steady-state planar interface growth conditions.[3]
a constant interface velocity approximation precludes the possibility of an oscillatory growth, and a dynamical model of the transient process by Warren and Langer[4] has shown that diffusive growth of a planar interface can indeed show an oscillatory mode under certain growth conditions.[4] A similar conclusion was reached by Caroli et al.[7] through a more rigorous model of the transient regime. Both models[4,7] predict that, under local equilibrium conditions, there can be an oscillatory motion of the planar interface. However, the velocity required for the oscillations is found to be near or above the critical velocity for planar interface ability, so that the oscillations are generally not observed for a planar interface growth below critical velocity. This is confirmed by the absence of oscillations at low velocities in the succinonitrile-acetone system, as shown in Figure 2. Consequently, the oscillations that were observed in the naphthalene system are not due to the diffusive boundary layer, but they arise due to interface kinetics effects. In order to further characterize the oscillations, the position vs time of oscillation was measured for different velocities, and the oscillations were found to be periodic. Figure 3(a) shows the plot of the time period, t, between the oscillations as a function of the external velocity. A systematic variation in t with velocity was observed. The variation in dimensionless time period, Dt/V 2, can now be examined, by plotting t/V 2 vs external velocity, and the resulting plot is shown in Figure 3(b). An interesting behavior is seen in that the plot of t/V 2 with velocity exhibits a stepped behavior, with a sharp change in t/V 2 when the external velocity is increased above 1 mm/s. Experimental data presented in Figure 1 also show that the oscillatory behavior is very small or absent for V . 1 mm/s. This result indicates that a change in growth behavior occurs at a velocity above 1 mm/s. In order to examine the growth mechanism, detailed measurements of kinetic undercooling at the interface as a function of external velocity, for V # 1 mm/s, were carried out and the result is shown in Figure 4. The re
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