In-situ TEM/STEM observations of intermetallic growth and reverse transformation of ultra-fine bainitic steel

Abstract

Advanced bainitic steels are an intensively developing research field for demanding industrial applications. The novel grade cementite-free, ultra-fine BainNiAlCu bainitic steel was designed to introduce intermetallic strengthening at elevated temperatures. The principal aim of the research was to understand the sequence of precipitation phenomena and decomposition mechanisms of the metastable bainitic structure as a function of temperature. The studies used the MEMS heating holder and dynamic electron microscopy STEM/TEM observations. The applied thermal cycles enabled the observation of the precipitation processes of the B2-ordered intermetallic phase and the reverse transformation of the bainitic structure into austenite. The dynamic precipitation processes of the L12 and B2 phases were evaluated as a function of temperature, where the most intensive precipitation of the B2 phase occurred at 550 °C. The metastable bainitic matrix was thermally stable up to 500 °C, then decomposition processes were observed. Moreover, the dynamic processes of reverted austenite growth are observed at 650 °C. Austenite growth was noticed at 700 °C at the previous prior austenite grain boundary and may constitute a premise for the “austenite memory” effect. The designed steel is characterized by enhanced thermal stability compared to conventional bainitic steels and indicates the promising alloy design concept for advanced steels. It was confirmed, that in-situ electron microscopy provides an immense capability for better understanding the crucial solid-phase phenomena.