Ultra–high strength oxide dispersion strengthened austenitic steel

Abstract

In this work, nitrogen–containing (0.36–0.38 wt %) and nickel alloyed austenitic oxide dispersion strengthened (AODS) steel was produced using two–stage ball milling under nitrogen atmosphere. The second stage milling (3, 4 and 5 h) was aimed at synthesizing a homogeneous austenite phase with reasonable powder yield. The milled powder was consolidated by hot extrusion followed by solutionization at 1075 °C for 2 h. Microstructure was characterized by X–ray diffraction, field emission scanning electron microscopy (FE–SEM) equipped with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. Mechanical properties at room temperature were determined by tensile tests. A fully homogenous austenitic phase was obtained upon solutionization from the powders milled for 3 h, due to the presence of higher Ni content in combination with nitrogen. The results showed that the microstructure consists of an ultrafine grain size (0.34–0.37 μm) with bimodal distribution. High resolution TEM (HRTEM) studies revealed that most of the oxide particles were of pyrochlore cubic Y2Ti2O7, with an average size of ~7.5 ± 3.2 nm along with coarse Cr2N precipitates. The processed AODS steel exhibited the yield strength of 1232 ± 24 MPa and a fracture strain of 21 ± 3% at room temperature, which is higher than the similar AODS steels reported so far. In addition, the yield strength was quantitatively estimated and compared with experimentally obtained values. The major contributions to the strengthening of the alloy were attributed to grain boundary and Orowan strengthening mechanisms.