The columnar-to-equiaxed transition (CET) was studied in lead-tin alloys, which were solidified directionally from a chill face. The main parameters analyzed include the temperature gradients, solidification velocities of the liquidus and solidus fronts, and grain size. The transition was observed to occur when the temperature gradient in the melt decreased to values between Ϫ0.8 ЊC/cm and 1 ЊC/cm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The comparison with available analytical models and the observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites.

Understanding the interaction between the parameters involved in the columnar-to-equiaxed transition (CET) has gained considerable attention over the last two decades in the study of the structure of ingot castings. The present investigation was undertaken to investigate experimentally the directional solidification of Al-Zn and Zn-Al (ZA) alloys under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. The CET is observed; grain sizes are measured and the observations are related to the solidification thermal parameters: cooling rates, growth rates, thermal gradients, and recalescence determined from the temperature vs time curves. The temperature gradient in the melt, measured during the transition, is between -0.338°C/mm and 0.167°C/mm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites. The results are compared with those previously obtained in lead-tin alloys.

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The influence of sol-gel dip-coating and anodic oxidation process parameters in producing thin TiO2 films is studied. As the size of the films is in the order of nanometres (20-140 nm), to obtain a precise measurement of their thickness and analyse their crystalline structures, glancing incidence angle X-ray techniques (X-ray reflectometry and Xray diffraction) using synchrotron radiation are used. A relationship between the colour and thickness of the films was found. This enables the film thickness to be estimated by the film colour. Within the range of the parameters studied, both techniques produce thin films with smooth surfaces which at most reproduce the roughness of the polished substrate. Independently of the technique, thermally-treated films thicker than 30 nm presented different crystalline structures with anatase and rutile phases.

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The wear behaviors of five different zinc-aluminum (ZA)-based alloys containing silicon, copper, and 8 and 16 pct on volume of reinforcing silicon carbide (SiC) particles were analyzed. Hardness, dimensional stability, and wear tests were performed on these five alloy samples. Microstructural investigation and semiquantitative chemical analysis of the different alloying characteristics of the cast samples, the wear surface, and the wear debris were obtained by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDXA), and X-ray diffraction (XRD). The addition of Si, Cu, and SiC has a significant effect on the solidification process and final morphology of the alloys. The five cast alloys tested showed dimensional stability for a period of 1000 hours at 165°C Ϯ 2.5°C. The wear tests were performed using a pin-on-disc apparatus under dry and lubricated conditions. Loads of 29.43 N (3 kg), 49.05 N (5 kg), and 78.48 N (8 kg) and a velocity of 250 rpm (2 m/s) were used. The results indicate that the wear rate of ZA alloys is strongly dependent on test load in a nonlinear relationship and that the addition of SiC particles improved the wear properties of the matrix alloys. Under dry conditions, there was considerable loss of material, particularly in the nonreinforced alloys. In addition, the nonreinforced alloys presented substantial local plastic deformation and transfer of elements between the disc, the sample, and the debris. The amount of element transfer can be correlated with the elements presented. The proposed wear mechanisms are discussed.