Microstructure characterization of nickel alloy 718 with automated optical image processing

Abstract

The microstructure of a vacuum arc remelted (VAR) nickel alloy 718 ingot was characterized using macro photography and studied with quantitative image analysis techniques. Microstructure quantification was used to produce information that may aid the determination of how melt parameters influence solidification microstructures. Such information is expected to eventually support new understandings of how solidification defects, such as freckle channels and white spots, form. Two slabs were sectioned from the center of a laboratory-scale alloy 718 VAR ingot. Each slab was ground and etched to reveal dendritic solidification microstructures. Custom automated instrumentation was used to acquire 454 digital images of these slabs at a 1:1 magnification. Images were subsequently stitched and blended to produce two mosaic images, one 562 and the other 579 megapixels in size. Digital image analysis techniques were used to identify primary and secondary dendrite arms. These image analysis techniques and the results they produced are presented.