Effects of extrusion temperature on the nano-mesoscopic structure and mechanical properties of an Al-alloyed high-Cr ODS ferritic steel

The nano-mesoscopic structure of a newly developed Al-alloyed high-Cr oxide-dispersion-strengthened (ODS) ferritic steel, which was extruded at 1150 and 1050 °C, has been characterized by transmission electron microscopy to correlate with microhardness at room temperature. The grain size decreases, the number density of the oxide particles increases and their size decreases, and the microhardness increases, with decreasing extrusion temperature. After extrusion at 1150 °C, most of the oxide particles in the ODS steel are semi-coherent (78%). The semi-coherent particles were divided into two types with almost equal fraction according to the spacings of surrounding misfit moiré fringes, which are about 2 nm and 1.65 nm, indicating the misfit strains to be 0.075 and 0.092, respectively. After 1050 °C extrusion, most of the oxide particles are coherent (86%). The misfit strains of the coherent particles and a few semi-coherent particles are about 0.035 and 0.075, respectively. The strengthening mechanism is briefly discussed.