A succinonitrile (SCN)-3.6 wt% acetone (ACE) alloy was unidirectionally solidified with a constant temperature gradient G = 5.7 K mm(-1) in the growth rate ranges V = 6.5-113 µm s(-1) and a constant growth rate V = 6.5 µm s(-1) in the temperature gradient ranges G = 3.5-5.7 K mm(-1). The primary dendrite arm spacings, secondary dendrite arm spacings, dendrite tip radius and mushy zone depth were measured as a function of growth rate and temperature gradient. Theoretical models for the dendrite arm spacing and tip radius have been compared with the experimental observations, and a comparison of our results with the current theoretical models and previous experimental results has also been made. The stability constant (σ) for this alloy system was measured and this result was compared with various similar organic transparent alloys.

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AA7075 and AA7020 alloys were prepared using a vacuum melting furnace and a casting furnace. Microstructural properties of the alloy samples were examined in both as-cast and heat-treated conditions. In order to investigate the effect of heat treatment, various designed AA7075 and AA7020 samples were homogenized in two steps (300 oC/12h+475 oC/12h) and then aged under different regimes. The effects of heat treatment on the microstructures were studied using optical microscopy (OM), scanning electron microscopy (SEM)/energy-dispersive X-ray (EDX) elemental analysis, transmission electron microscopy (TEM), and X-ray diffraction (XRD) analysis. The microstructural properties of both alloy systems (AA7075 and AA7020) were compared both within themselves and with each other, depending on how they were exposed to heat treatment. The analysis of EDX and XRD patterns reveal that the aged AA7075 and AA7020 alloys contain α-Al matrix phase, MgZn2 and Al2CuMg intermetallics, with the MgZn2 IMC phase being the only one detected in the aged AA7020 alloy sample, which is evident from the TEM micrographs showing polyhedral shaped Al2CuMg particles about 30-50 nm in size and spherical shaped MgZn2 particles about 10-30 nm in size dispersed in the eutectic phase and at the grain boundaries.

EffEct of Growth ratE on coarsEninG of sEcondary dEndritE arm spacinGs in dirEctionally solidifiEd of al-8.8la-1.2ni tErnary alloy One of the most important factors directly affecting microstructure and mechanical properties in directional solidification process is secondary dendrite arm spacing (sdas). it is very important to measure the sdas and examine the factors that may affect them. to investigate the effect of growth rate on the sdas, the alloy specimens were directional solidified upward with different growth rates (V = 8.3-83.0 μm/s) at a constant temperature gradient (G = 4 K/mm) in a Bridgman-type growth apparatus. after the specimens are directionally solidified, they were exposed to metallographic processes in order to observe the dendritic solidification structure on the longitudinal section of the specimens. Coarsen secondary dendrite arm spacings (λ 2C ) were measured excluding the first arms near the tip of the dendrite. local solidification times (t f ) were calculated by ratio of spacings to growth rates. it was determined that the t f values decreased with increasing V values. the relationships between t f and λ 2C were defined by means of the binary regression analysis. Exponent values of t f were obtained as 0.37, 0.43, 0.46 and 0.47 according to increasing V values, respectively. These exponent values are close to the exponent value (0.33) predicted by the Rappaz-Boettinger theoretical model and good agreement with the exponent values (0.33-0.50) obtained by other experimental studies.