Growth behavior and strengthening mechanism of Cu-rich particles in sanicro 25 austenitic heat-resistant steel after aging at 973 K

Abstract

Precipitation and growth behaviors of Cu-rich particles in Sanicro 25 austenitic heat-resistant steel after aging at 973 K were systematically investigated by using transmission electron microscope (TEM) and three-dimensional atom probe (3DAP), and the strengthening mechanism of Cu-rich particles was discussed. The results show that many fine and well-dispersed Cu-rich particles precipitate at the early stage of aging at 973 K. This is because Cu atoms are supersaturated in the matrix after solid solution treatment and Cu-rich particles are coherent with austenitic matrix. The growth of Cu-rich particles during aging at 973 K follows diffusion-controlling LSW theory. The reduction of interfacial energy due to the segregation of Mn is estimated to be 8.1, 8.4, 14.7 and 16.7 mJ/m2 after aging for 10, 50, 100 and 1000 h, respectively. The segregation of Mn at interfaces of Cu-rich particle/matrix retards the growth of Cu-rich particles by reducing the interfacial energy, and the retarding effect increases with increasing aging time. The strengthening mechanism of Cu-rich particles in Sanicro 25 transforms from the combination of coherency strain and stacking-fault strengthening to Orowan strengthening when the average radius is about 13 nm. Cu-rich particles play an important role in affecting the strength and hardness of Sanicro 25 during aging. Increasing the content of Cu properly is beneficial to improve the strength of Sanicro 25 from the viewpoint of increasing the volume fraction of Cu-rich particles. Also, the addition of Mn is useful to improve the strength of Sanicro 25 since it can reduce the interfacial energy of Cu-rich particle/matrix.