In-situ microstructural evolutions of 5Mn steel at elevated temperature in a transmission electron microscope

Abstract

The microstructural evolutions of 5Mn steel during various heat treatments have been investigated by in-situ transmission electron microscopy (TEM). The specimen of 5Mn steel was prepared using focused ion beam (FIB) milling, which allowed the selection of specific morphology of interest prior to the in-situ observation. The complete austenization at 800 °C was verified at the atomic scale by minimizing thermal expansion and sample drift in a heating holder based on micro-electro-mechanical-systems. During annealing at 650 °C, the formation of reverted austenite was dynamically observed, while the morphologies of austenite laths of 5Mn steel after in-situ heating were quite similar to that after ex-situ intercritical annealing. During annealing at 500 °C, the morphological evolution of cementite and associated Mn diffusion were investigated. It was demonstrated that a combination of FIB sampling and high temperature in-situ TEM enables us to probe the morphological evolution and elemental diffusion of specific areas of interest in steel at high spatial resolution.