Comparison of Measurement Methods for Secondary Dendrite Arm Spacing

Secondary dendrite arm spacing (SDAS) is most commonly measured by what is referred to as the linear intercept method. However, substantial variation in the technique exists between researchers, and its influence on the measurements has not yet been elucidated. Given the strong correlations between SDAS and material properties, a consistent and accurate technique is essential for interstudy comparability and effective alloy design. In this study, the SDAS of an aluminum alloy cast at two different solidification rates was quantified using five methods typical in the literature. Each method enabled observation of the refinement in dendritic structure associated with faster solidification. Also, each method produced a very similar average SDAS value for castings at high solidification rates, since the error is reduced in fine and uniform microstructures. Yet, significant variation, inaccuracy, and inconsistency in SDAS values were found to be possible between the methods for castings at low solidification rates. The most accurate and precise of the five methods for both coarse and fine microstructures was identified, and its use is recommended to improve SDAS measurement practices in academia and the industry.