Microstructural evolution of 9Cr ‐ ODS steel during high temperature deformation

Abstract

Scanning and transmission electron microscopy (TEM) was applied to study the structure of oxide dispersion strengthened (ODS) martensitic 9Cr steel before and after mechanical deformation at elevated temperatures. The tensile tests were performed in the temperature range of 25°C to 800°C at a nominal strain rate of 10 ‐3 s ‐1 . High angle annular dark field (HAADF) scanning TEM (STEM) with energy‐dispersive X‐ray (EDX) spectroscopy for the determination of the composition and conventional TEM with selected area diffraction for the crystallographic investigations were applied. Figure 1 shows an overview HAADF STEM image (a) with corresponded schematic (b) of the investigated steel in tempered conditions. Owning to the composition and heat treatments, the structure of the steel typically consists of tempered martensite with a high density of hierarchically organized internal interfaces. During quench, the prior austenite grains are sub‐structured by martensitic laths and packets. This transformation causes a large local deformation of the matrix, resulting in a high dislocation density. The following tempering process allows the recovery of the martensitic structure with transformation into a ferritic structure, precipitation of solute atoms and recovery of the dislocation cell structure into subgrain structure. Therefore, the final microstructure consists of some portions that are composed of typical laths (with several µm in length and 0.25 ± 0.06 µm in width) and others portions that are filled with subgrains (with size of 0.29 ± 0.13 µm). The non‐regular shaped M 23 C 6 carbides precipitate are arranged preferentially at prior austenite grain boundaries, packet and lath boundaries. HRTEM analysis of M 23 C 6 (here not shown) reveals that many carbides have fcc structure. The relation between the carbide and ferrite is: M 23 C 6 || Fe. The detail EDX analysis shows that carbide consist of Fe and Cr. The fine ODS particles are mostly revealed inside the boundaries of laths and subgrains, yet on them as well. Figure 2 shows HAADF STEM image of the ODS particle (a) with linescan EDX profiles across the particle (b). The detail EDX analysis of many ODS particles shows that they have composition of Y 2 Ti 2‐x O 7‐2x . For investigation on the deformed state, TEM samples were prepared from the gauge section region near the fracture surface of the tempered martensitic state specimens. At room temperature, the deformed microstructure shows similar characteristics as those typically observed for an as‐received state, but the dislocations are mostly pinned to the ODS particles. At elevated temperatures, structural evolution becomes prominent with the large decrease in dislocation density and the appearance of polygonal subgrains that replace the original lath structure. In addition, coarsening of M 23 C 6 carbides was observed with increase of testing temperatures. Figure 3 presents TEM bright‐field (a) and (110) dark‐field (b) images from the same place of sample obtained after the tensile test at 800°C. The equiaxed α‐Fe has high dislocation density at the grain boundaries (area 1) and dislocations that are pinned to the ODS particles at various locations (marked by arrows). The partially illuminated grain in Figure 3b confirms the presence of low‐angle grain boundary LGB that appears to be undergoing recovery process.