Effect of Growth Rate on Coarsening of Secondary Dendrite Arm Spacings in Directionally Solidified of Al-8.8La-1.2Ni Ternary Alloy

Abstract

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 ( tf) were calculated by ratio of spacings to growth rates. It was determined that the tf values decreased with increasing V values. The relationships between tf and λ 2C were defined by means of the binary regression analysis. Exponent values of tf 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.

Keywords

• directional solidification
• al-la-ni ternary alloys
• coarsening
• secondary dendrite arm spacing (sdas)