Secondary phases in the microstructure of cast alloys can greatly increase the complexity of functional film development, particularly when film growth is induced by diffusion. This paper examines the effect of secondary phases on the diffusion behaviors of aluminum, oxygen, and nitrogen, each of which plays an important role in the formation of functional films on cast aluminum and gray cast iron alloys. In general, a fine and evenly distributed phase morphology improves the uniformity of functional films regardless of whether the secondary phase accelerates or delays mass transfer during diffusion.

The dissolution of second phase particles in a 319-type (Al-Si- Cu-Mg) aluminum casting alloy has been quantified by image analysis of metallographic specimens as well as an electron microprobe technique. The initial phase content of the as-cast material, and the change in volume fraction of each phase following solution treatment for various times at 480°C and 505°C, was determined by analysis of particles observed by backscattered electron microscopy. Furthermore, the change in dendritic alloy content during solution treatment was measured using electron microprobe analysis in order to estimate the relative volume fraction of second phase particles dissolved. Finally, a non-isothermal dissolution model was used to predict the dissolution behaviour during solution treatment and comparisons are made between the model predictions and experimental measurements.

Current heat treatment standards do not adequately define tempers for thin-walled castings that solidify at high rates. Emerging casting processes, such as vacuum high-pressure die casting, benefit from rapid solidification rates, which result in fine microstructures and reduce the need for prolonged solution treatment times. Studies on rapidly solidified samples with secondary dendrite arm spacing between 35-10 μm were conducted, with solution times ranging from 30 minutes to 9 hours, and various aging parameters were evaluated. Metallurgical analysis revealed that increased microstructure refinement could reduce solution time by up to 50% without compromising the alloy’s mechanical properties. The highest strengths, with an ultimate tensile strength of 330 MPa (47.9 ksi) and a yield strength of 300 MPa (43.5 ksi), were achieved under T6 peak aging conditions. Additionally, thermal analysis and dilatometer results are presented to evaluate phase dissolution during solution treatment, aging kinetics, and dimensional stability.