Microstructural stability of a 9Cr oxide dispersion strengthened alloy under thermal aging at high temperatures

Abstract

High temperature heat treatment is one of the critical steps in hot compression bonding (HCB), which was recently developed to scale up the fabrication of oxide dispersion strengthened (ODS) alloys. Understanding the microstructural evolution during heat treatments is required to refine the fabrication process to produce large-scale homogeneous ODS alloys. In this study, the effect of high temperature thermal aging treatments at representative temperatures (1000 °C and 1200 °C, respectively) on the microstructural stability of a 9Cr ODS alloy was investigated. The results show that the microstructure is relatively stable during thermal aging at 1000 °C up to 500 h. Within 50 h of aging at 1200 °C, δ-ferrite appears and its content increases with aging time. However, δ-ferrite completely disappears and hexagonal TiO2 particles with the size of 1–3 µm are uniformly distributed in the matrix after aging for 200 h. The nano-sized oxide particles coarsen slightly at 1000 °C, and the average size is ∼4.3 nm after aging for 500 h, whereas, significant coarsening takes place at 1200 °C after aging for more than 20 h, and the particle size increased from 3.76 nm at 20 h to 6.22 nm at 200 h. Additionally, the average grain size grows obviously after aging to 200 h. These results indicate that the HCB and hot working processes of 9Cr ODS alloy should be less than 20 h at 1200 °C or kept at lower temperatures to maintain the stability of nano-sized oxide particles and the overall microstructure.