Effect of Nb-Ti Double Microalloys on the Microstructure Control and Mechanical Properties of FeCrAl As-Cast Alloys

The effects of Nb-Ti double microalloys on the microstructure control of Fe-18Cr-6Al-0.2Ti-0.2Nb cast alloys under different cooling processes and temperatures provide valuable insights into the relationships among the alloy composition, cooling conditions, and microstructure. Nb-rich and Ti-rich precipitates at grain boundaries play a key role in inhibiting grain growth, particularly at temperatures below 1200 °C. The dissolution of precipitates led to obvious grain growth at 1250 °C. Additionally, the increase in the holding time from 1 to 6 hours affected the movement of the precipitates, resulting in slight grain growth over time. The grain size of the furnace-cooled samples is larger than that of the air-cooled samples, and these size differences do not change with the changes in the heat treatment temperature and holding time. Greater content of Nb-rich and Ti-rich precipitates in the matrix hinders the formation of Cr₂₃C₆ particles, as more carbon likely combines with Nb/Ti than with Cr. This finding indicates that the addition of Nb and Ti to the FeCrAl alloy can be used to simultaneously significantly refine the grain size and reduce the amount of Cr₂₃C₆. The synergistic effect of niobium (Nb) and titanium (Ti) can increase the dissolution temperature of titanium carbide (TiC) and niobium carbide (NbC) precipitates in the as-cast alloy, thus enabling effective control of the grain size at elevated temperatures.