Characterization of metals in four dimensions

The ability to watch the three-dimensional (3D) evolution of structural materials is a breakthrough in non-destructive characterization. In particular, X-ray tomographic imaging techniques have found success in revealing the underlying mechanisms of microstructural transformations in partially and fully solidified metals. Here we review the most important developments in four-dimensional X-ray microscopy, focusing on absorption- and diffraction-based techniques in the laboratory and the synchrotron. In light of recent progress in this area, we identify critical issues that point to directions for future research in imaging the evolution of heterogeneous microstructures at extreme space and time scales. IMPACT STATEMENT Four-dimensional X-ray tomography has opened a new paradigm in physical metallurgy, allowing us to characterize the various epochs of microstructural evolution in 3D and as a function of time.