Effects of Sorbed Water on the Crystallization of Indomethacin from the Amorphous State

Time dependent nucleation theory has been applied to calculate temperature vs incubation time curves of competing phases, the nucleation temperatures and the critical cooling rates required for the formation of primary phases in undercooled melts. The incubation time is controlled by melt undercooling and diffusitivity in the melt especially at large undercooling. Larger melt undercoolings are required for the suppression of intermetallic phases with increasing solute content. The predicted hierarchy of phase selection in Al-TM (TM = Cr, Ti) alloys is in agreement with experimental results. The calculated phase boundaries on nucleation maps of Al-TM alloys are in agreement with experimental results, especially when heterogeneous volume nucleation is considered. The fit to experiment of calculations where the diffusion coefficient in the melt is related to viscosity is improved by considering T g = 0.25 T 0 .

Prediction of phase selection in rapid solidification using time dependent nucleation theory

The present study was undertaken to provide insight into the dynamic interactions that occur when ceramic particles are placed in intimate contact with a metallic matrix undergoing a phase change. To that effect, Al-4 wt pct Si/SiCp composite droplets were synthesized using a spray atomization and coinjection approach, and their solidification microstructures were studied both qualitatively and quantitatively. The present results show that SiC particles (SiCp) were incor- porated into the matrix and that the extent of incorporation depends on the solidification con- dition of the droplets at the moment of SiC particle injection. Two factors were found to affect the distribution and volume fraction of SiC particles in droplets: the penetration of particles into droplets and the entrapment and/or rejection of particles by the solidification front. First, during coinjection, particles collide with the atomized droplets with three possible results: they may penetrate the droplets, adhere to the droplet surface, or bounce back after impact. The extent of penetration of SiC particles into droplets was noted to depend on the kinetic energy of the particles and the magnitude of the surface energy change in the droplets that occurs upon impact. In liquid droplets, the extent of penetration of SiC particles was shown to depend on the changes in surface energy, ΔEs, experienced by the droplets. Accordingly, large SiC particles encoun- tered more resistance to penetration relative to small ones. In solid droplets, the penetration of SiC particles was correlated with the dynamic pressure exerted by the SiC particles on the droplets during impact and the depth of the ensuing crater. The results showed that no pene- tration was possible in such droplets. Second, once SiC particles have penetrated droplets, their final location in the microstructure is governed by their interactions with the solidification front. As a result of these interactions, both entrapment and rejection of SiC particles occurred during droplet solidification. A comparison of the present results to those anticipated from well-established kinetic and thermodynamic models led to some interesting findings. First, the models proposed by Boiling and Cisse[24] and Chernovet al.[58] predict relative low critical interface velocities necessary for entrapment, inconsistent with the present experimental findings. Second, although the observed correlation between the critical front velocity and droplet diameter was generally consistent with that predicted by Stefanescuet a/.’s model,[27] the dependence on the size of SiC particles was not. In view of this discrepancy, three possible mechanisms were proposed to account for the experimental findings: nucleation of α-Al on SiC particles, entrapment of SiC particles between primary dendrite arms, and entrapment of SiC particles between secondary dendrite arms.

Interaction mechanisms between ceramic particles and atomized metallic droplets

A method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the <45 μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al13Cr2 intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellular α aluminium, (iii) α aluminium plus fine spherical precipitates rich in chromium and manganese, and (iv) α aluminium plus Al13Cr2 primary intermetallic particles. The solidification of these powders is discussed in terms of so...

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 1 Microstructure of high pressure gas atomized powders

Laser cladding of aluminium-base quasicrystalline alloys

A previous treatment of nucleation in gas atomised liquid aluminium alloys has been extended to model more realistically the cooling conditions in a gas atomiser. Previous predictions of the phase which will predominantly nucleate in powder particles, as a function of powder size and composition, are reinforced. Predicted undercoolings are now slightly higher than in the earlier work. Results from this work are compared with recent experiments.MST/1013

N. Saunders

Papers

Preferential nucleation in undercooled liquid aluminium alloys