A microstructure detector and in-situ method for real-time determination of the microstructure of a material undergoing alloying or other phase transformation. The method carried out by the detector includes the steps of: (a) detecting light emitted from a plasma plume created during phase transformation of a material; (b) determining at least some of the spectral content of the detected light; and (c) determining an expected microstructure of the transformed material from the determined spectral content. Closed loop control of the phase transformation process can be carried out using feedback from the detector to achieve a desired microstructure.