Mechanical properties of a thermally-aged cast duplex stainless steel by nanoindentation and micropillar compression

Cast duplex stainless steels (CDSS) were investigated in order to understand the mechanical properties of their individual constituents, ferrite and austenite, after thermal aging at 475 °C for up to 2000 h, using nanoindentation and micropillar compression. Nanoindentation experiments indicate that the hardness of ferrite continuously increases with increasing aging time, while after annealing heat treatment the hardness drops to slightly higher than that of the as-received condition. The hardness in austenite did not change significantly with different aging conditions. The results of micropillar compression on both ferrite and austenite, with [1 1 1] orientation, reveal similar trends to the results from nanoindentation. After the long-term thermal aging process, spinodal decomposition takes place, as well as G-phase precipitates forming in the ferrite phase. Then, after annealing, the spinodal decomposition products were reduced and only the G-phases were left in the ferrite phase. Hardening in ferrite is mostly caused by the spinodal decomposition but also related to G-phase precipitates. The role of the G-phase can be explained via precipitation hardening theory, i.e., the small G-phases block the movement of dislocations throughout the ferrite lattice.