Nanostructures in a ferritic and an oxide dispersion strengthened steel induced by dynamic plastic deformation

Zhenbo Zhang

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Nanostructured metals and alloys are expected to have much higher strength and better irradiation tolerance than their counterparts with a coarse-grained structure. Therefore, nanostructuring is suggested to be a promising approach to improve the properties of structural materials for advanced fission and fusion reactors. In this study, two candidate steels for nuclear reactors, namely a ferritic/martensitic steel (modified 9Cr-1Mo steel) and an oxide dispersion strengthened (ODS) ferritic steel (PM2000), were nanostructured by dynamic plastic deformation (DPD). The resulting microstructure was characterized in detail, and the annealing behavior regarding the thermal stability and microstructural response was investigated across several length scales by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), high resolution TEM (HRTEM), and scanning TEM (STEM). A nanoscale lamellar structure with a <100>+<111> duplex fibre texture develops in both the modified 9Cr-1Mo steel and PM2000 during DPD to high strains. The strength is improved significantly, but the thermal stability is largely reduced. A very pronounced orientation dependent recovery and recrystallization take place, when both steels after DPD are annealed. Both oriented nucleation and oriented growth of <111> oriented lamellae are demonstrated to account for such an orientation dependence. The underlying mechanisms are discussed, including the differences in stored energy, structural variation, and recovery processes. Higher strain rates are demonstrated to be able to facilitate the structural refinement; nevertheless, the general annealing behavior resembles that of the material after deformation at low strain rate. In addition to the microstructure of the matrix materials, the oxide nanoparticles in PM2000 were systematically investigated with respect to their structure in the as-received, substantially deformed and annealed material. The majority of oxide nanoparticles in PM2000 are orthorhombic YAlO3 (YAP) and have a cuboid-on-cube orientation relationship with the ferrite matrix. Annealing at 1300 °C and above leads to considerable coarsening; the orientation relationship between YAP/Fe changes into a pseudo-cube-on-cube relationship. It is remarkable that YAP nanoparticles with diameters smaller than 20 nm are substantially deformed during DPD. The average equivalent strain of the YAP nanoparticles is about 1.2 estimated in the sample after DPD to a strain of 2.1. Mechanical twinning is found to be the dominant deformation mechanism for the YAP nanoparticles. Nanovoids are found to form at the YAP/Fe interface at low strain. With increasing strain, these nanovoids become closed around the smaller particles which are considerably deformed, whereas those voids associated with the undeformed, larger particles remain open. When a sample with substantially deformed YAP nanoparticles is annealed, the morphology of the deformed particles changes dramatically leading to the formation of much smaller YAP particles distributed in the ferrite matrix. These findings provide insight in the strengthening mechanism and ductility of ODS steels, and, most importantly, open a novel approach to refine oxide nanoparticles in ODS steels by a combination of thermomechanical treatments.
Original languageEnglish
PublisherDTU Wind Energy
Number of pages212
ISBN (Electronic)978-87-93278-27-1
Publication statusPublished - 2015
SeriesDTU Wind Energy PhD
Number0045(EN)

Keywords

  • DTU Wind Energy PhD-0045(EN)
  • DTU Wind Energy PhD-0045
  • DTU Wind Energy PhD-45

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