XCT gives information that cannot be obtained any other way from opaque materials and objects. It gives us an “eye inside” so that features, of a certain size range, can be seen. The Division’s XCT instruments can reconstruct 3D images with a voxel size down to about 0.5 micrometers, at the smallest, and about 50 micrometers, at the largest. The typical size of the reconstructed images are 1000 x 1000 pixels for each cross-sectional slice, and usually at least 1000 and usually more slices. Many problems have detail on these length scales, and so analysis of materials properties is aided by these kinds of interior images. For example, when investigating the metallic powders used in laser-melting additive manufacturing instruments, one can see the small pores contained inside some of the metallic powder particles, which can possibly be transferred to the finished product, built by the additive manufacturing process, as mechanical defects.
XCT sample preparation is usually quite simple. One takes a piece of whatever needs to be examined, stabilizes it mechanically on some kind of platform that can be gripped by a standard instrument holder, and then inserted in the machine. The mechanical stability is important because scans can take several hours or more, and the least movement, even only of micrometers in size, will badly blur the images that have voxel sizes of only micrometers themselves. For powders, usually they are dispersed in an epoxy and the mixture, while still fluid, is drawn into a small diameter plastic tube by a low-powered vacuum pump. After curing, the solid tube can then be easily inserted in the XCT and scanned and the particles picked out computationally. The voltage of the X-ray tube itself can be adjusted, depending on which instrument is used, from either 40 kV to 100 kV or 30 kV to 160 kV, which changes the penetrating power of the X-rays. The exposure times can also be adjusted, so that materials from 90 % porosity polymeric foams to 1 % porosity stainless steels can be interrogated. Using the XCT to scan a sample and reconstruct the sample interior is actually the easy part – the hard part is analyzing the masses of image data obtained.