Surendra N. Tewari

The engulfment of foreign particles by a freezing interface

The interactions of second-phase particles, liquid droplets or gas bubbles with a solidification front form the basis of various materials synthesis and purification processes and the design of microstructures in cast metal-matrix composites, as well as frost heaving and biological cell interactions. The physical mechanisms of this interaction phenomenon are based upon surface thermodynamic factors, solidification parameters, and fluid dynamic effects such as fluid drag and buoyancy. An overview is presented of the role of various factors which determine the nature as well as the kinetics of foreign particle-solidification front interactions, and the current status and limitations of the various theoretical models of the phenomenon.

Effect of Magnetic Field on the Microstructure and Macrosegregation in Directionally Solidified Pb-Sn Alloys

An investigation into the influence of a transverse magnetic field (0.45 T) on the mushy zone morphology and macrosegregation in directionally solidified hypoeutectic Pb-Sn alloy shows that the field has no influence on the morphology of dendritic arrays. The field does, however, cause severe distortion in the cellular array morphology. Cellular arrayed growth with the magnetic field results in an extensive channel formation in the mushy zone, as opposed to the wellaligned and uniformly distributed cells formed in the absence of the field. The channels are produced due to the anisotropy in the thermosolutal convection caused by the magnetic field. Macrosegregation, however, along the length of the directionally solidified samples is not influenced by this magnetic field for either the cellular or dendritic arrays.

Reactive Infiltration of Silicon Melt through Microporous Amorphous Carbon Preforms

The kinetics of unidirectional capillary infiltration of silicon melt into microporous carbon preforms has been investigated as a function of the pore morphology and melt temperature. The infiltrated specimens showed alternating bands of dark and bright regions, which corresponded to the unreacted free carbon and free silicon regions, respectively. The decrease in the infiltration front velocity for increasing infiltration distances is in qualitative agreement with the closed-form solution of capillarity-driven fluid flow through constant-cross-section cylindrical pores. However, drastic changes in the thermal response and infiltration front morphologies were observed for minute differences in the preform’s microstructure. This suggests the need for a dynamic percolation model that would account for the exothermic nature of the silicon-carbon chemical reaction and the associated pore-closing phenomenon.

Macrosegregation During Dendritic Arrayed Growth of Hypoeutectic Pb-Sn Alloys: Influence of Primary Arm Spacing and Mushy Zone Length

Thermosolutal convection in the dendritic mushy zone occurs during directional solidification of hypoeutectic lead tin alloys in a positive thermal gradient, with the melt on the top and the solid below. This results in macrosegregation along the length of the solidified samples. The extent of macrosegregation increases with increasing primary dendrite spacings for constant mushy zone length. For constant primary spacings, the macrosegregation increases with decreasing mushy zone length. Presence of convection reduces the primary dendrite spacings. However, convection in the interdendritic melt has significantly more influence on the spacings as compared with that in the overlying melt, which is caused by the solutal buildup at the dendrite tips.

Thermosolutal convection and macrosegregation

Thermosolutal convection caused by the solute build up ahead of growing arrays of cells and dendrites results in macrosegregation along the length of Pb-Sn alloy (10 to 58 wt pct Sn) specimens when they are directionally solidified in a positive thermal gradient(melt on top, solid below, and gravity pointing down). At a constant thermal gradient, the extent of macro-segregation increases with decreasing growth speed as the microstructure changes from dendritic to cellular and to planar. An empirical parameter, effective partition coefficient (keff), obtained from the dependence of the longitudinal macrosegregation on fraction distance solidified can be used to represent the extent of macrosegregation. The extent of macrosegregation appears to be related to a parameter, [g(Ct - Co) Dl/RCo]1/2, where Co is the tin content of the alloy, Ct is the tin content of melt at the array tips, R is the growth speed, Dl is the diffusivity of tin in the melt, and g is the acceleration due to gravity.

Interfacial shear strength of cast and directionally solidified NiAl-sapphire fiber composites

The feasibility of fabricating intermetallic NiAl-sapphire fiber composites by casting and zone directional solidification has been examined. The fiber-matrix interfacial shear strengths measured using a fiber push-out technique in both cast and directionally solidified composites are greater than the strengths reported for composites fabricated by powder cloth process using organic binders. Microscopic examination of fibers extracted from cast, directionally solidified (DS), and thermally cycled composites, and the high values of interfacial shear strengths suggest that the fiber-matrix interface does not degrade due to casting and directional solidification. Sapphire fibers do not pin grain boundaries during directional solidification, suggesting that this technique can be used to fabricate sapphire fiber reinforced NiAl composites with single crystal matrices.

Durability of TBCs with a surface environmental barrier layer under thermal cycling in air and in molten salt

Abstract In order to protect the thermal barrier coatings (TBCs) from oxidation and hot corrosion attack, several barrier coatings on top of the yittria-stabilized zirconia (YSZ) TBC have been evaluated. Thermal cycling tests in air and hot corrosion resistance tests in molten Na 2 SO 4 were used to determine the durability of these coatings. Barrier layers investigated include mullite (3Al 2 O 3 ·2SiO 2 ), BAS (BaO·Al 2 O 3 ·2SiO 2 ) and calcium silicate (1.8CaO·SiO 2 ). Mullite and BAS barrier coatings showed the best promise as potential hot corrosion barriers. TBCs with mullite or BAS barrier showed life similar to that of standard YSZ TBC under thermal cycling in air, while demonstrating the ability to limit seepage of salt into YSZ in hot corrosion test. In order to avoid the coating damage due to seepage of salt through the edge, there is a need to examine the hot corrosion resistance on coupons that are completely enclosed by the coating.

Interfacial and capillary phenomena in solidification processing of metal-matrix composites

Abstract Chemical and hydrodynamic aspects of wetting and interfacial phenomena during the solidification processing of metal-matrix composites are reviewed. Significant experimental results on fibre-matrix interactions and wetting under equilibrium and non-equilibrium conditions in composites of engineering interest have been compiled, based on a survey of the recent literature. Finally, certain aspects of wetting relevant to stir-casting and infiltration processing of composites are discussed.

Break-down of a planar liquid-solid interface during directional solidification; influence of convection

Abstract The influence of convection on the development of morphological instability at the liquid-solid interface during directional solidification in a positive thermal gradient has been examined in Pb-10wt%Sn and succinonitrile-1.9wt%acetone. The onset of interfacial breakdown occurs at higher growth speeds in the presence of convection. The linear stability analysis due to Favier and Rouzaud which uses the “deformable” mass flow boundary layer concept shows a good agreement with the experimentally observed behavior.

A critical examination of the dendrite growth models Comparison of theory with experimental data

Abstract Dendrite tip temperature, dendrite tip radius and primary arm spacing data, and their variation with the growth speed and temperature gradient for directionally solidified succinonitrile-acetone, succinonitrile-salol, aluminum-copper, and lead-paladium alloys have been examined against their qualititative and quantitative fit with predictions from several dendrite growth models. The Burden and Hunt analysis while predicting the proper quantitative behavior, does not in general, yield a good quantitative agreement with experimental data. Models due to Trivedi, and more recently, Laxmanan (minimum dendrite tip undercooling approach as well as the tip stability approach) show a very good quantitative fit with the experimental data. Predictions of dendrite tip temperature and tip composition in the liquid have been shown to be inadequate to distinguish between the models within the experimentally feasible directional solidification conditions. Therefore, in order to determine which model is most appropriate, additional directional solidification experiments involving simultaneous measurements of dendrite tip radius, tip temperature, tip composition, and primary arm spacing in the low growth velocity regime are suggested.