AbstractThis paper describes non-destructive three-dimensional (3D) mapping of crystallographic phase distributions within the bulk (centimeter range) of samples with micrometer-scale resolution. The technique leverages diffraction contrast due to Bragg scattering and the large penetration power of neutrons through high-atomic-number-element-based structural materials. Our tomographic approach overcomes critical limitations of existing techniques by allowing spatially resolved phase mapping in bulk samples and offers a wide range of potential applications. The technique is demonstrated for (metastable 304L stainless) steel samples that exhibit strain-induced martensitic phase transformation after being subjected to tensile and torsional deformation. The distribution of phase fractions within the 3D reconstructed volumes was verified at select locations using neutron diffraction as well as EBSD, and results agree with transformation kinetics theory.