EFFECT OF COMPOSITION ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ODS STEELS DEVELOPED THROUGH HOT POWDER FORGING ROUTE

Abstract

Advanced Generation IV nuclear reactors are being developed to produce high energy with minimal waste and a longer reactor life. These reactors will operate at high temperatures and intense neutron flux, and the performance of core structural materials will play a critical role in their effective operation. Currently, austenitic stainless steels are used in present-day nuclear reactors due to their superior creep resistance, but their use is limited by void swelling at radiation doses higher than 120 dpa. Ferritic steels are being developed as an alternative for these advanced reactors, as they offer high swelling resistance and relatively high thermal conductivity and lower thermal expansion coefficients. However, ferritic steels have a drawback in their inferior long-term creep rupture strength. The creep resistance can be improved by incorporating yttria into the ferrite matrix through mechanical alloying. Oxide Dispersion Strengthened (ODS) ferritic steels are being developed as candidate materials for clad tubes in Generation IV nuclear reactors. The preferred method for producing ferritic oxide dispersion strengthened (ODS) steels is through mechanical alloying powder processing. Subsequently, the milled powder may be consolidated through hot isostatic pressing (HIP), hot extrusion (HE), or spark plasma sintering (SPS) techniques. However, HIP consolidation results in residual porosity and the presence of prior particle boundaries (PPBs), while HE consolidation causes the presence of extrusion bands parallel to the extrusion direction, as well as elongated grains that may result in anisotropic properties. SPS processing produces small-sized products, thereby limiting its use. Further thermomechanical treatments are required to achieve the desired properties in ODS steels produced through these conventional routes. The clad tube of the fuel rod experiences a biaxial state of stress. Hence, the isotropic properties of ODS steels will play a very important role in advanced nuclear reactors (Gen-IV). The clad tube having band/stringers may fail in a transverse direction. To overcome this issue, this work aims to develop an alternative consolidation route, namely hot powder forging, for producing ODS steels. Consolidation through hot powder forging may improve the life of the power plant by producing alloys with isotropic mechanical properties and avoiding prior particle boundaries and extrusion bands. In the present study, elemental powders of iron (Fe), chromium (Cr), tungsten (W), titanium (Ti), and nanosized yttria (Y2O3 ̴30nm) were utilized as the starting materials to prepare three different alloy compositions: Fe-18Cr-2W (Alloy-0), Fe-18Cr-2W-0.285Ti-0.5Y2O3 (Alloy-1), and Fe-18Cr-2W-0.571Ti-1Y2O3 (Alloy-2). The flexibility in choosing the alloy composition was made possible by using elemental powders. The compositions were mechanically alloyed (MA) in a Simoloyer attritor. The optimal milling time was determined by the complete dissolution of the alloying elements. The MA powders were characterized through X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM). The MA powders were then consolidated through powder forging at a temperature of 1473K in a flowing hydrogen atmosphere, using a friction screw forging press. The powder forged alloys exhibited a ferritic matrix with fine recrystallized grains. A significant reduction in grain size was observed on addition of Ti and yttria in the alloys (Alloy-1 and Alloy-2). Hardness values measurement along longitudinal transverse (LT), transverse short (TS) and longitudinal short (LS) sections showed isotopic behaviour of the powder forged alloys. All the alloys exhibited significant strength and ductility at room temperature and at high temperatures. In this work, the formation of nano oxide particles in 18Cr oxide dispersion strengthened (ODS) steels containing higher yttria are discussed. The ODS steel performance is determined by the size, distribution and volume fraction of the various oxide particles formed. Therefore, a detailed characterization of various oxide particles formed in the Alloy-1 and Alloy-2 have been done. The nano-size oxide formed in this powder-forged ODS steels was characterized by TEM, STEM, and HR-TEM. The crystal structure and coherency of these various oxide particles are determined. The number fraction, average size, and size distribution of these oxide particles were determined for both alloys. Cr-rich shells were observed around most of the particles, which may prevent the coarsening of oxide particles at elevated temperatures. The present work also reports the creep behaviour of Alloy-0, Alloy-1 and Alloy-2. The creep tests were performed in a temperature range of 873K to 1023K and a stress range of 120MPa to 425MPa. The Microstructure of as forged samples revealed an equiaxed strain-free grain structure in the alloys. The microstructure of Ti and yttria containing alloys remains stable after creep with no significant coarsening of nano-size oxide particles. The creep mechanisms in these test regimes are identified using TEM. Samples sections close to creep fracture surfaces revealed void formation at triple junctions followed by crack propagation normal to the loading direction leading to final failure. All the samples exhibited mixed mode (cleavage + ductile dimple) failure. A significant improvement in creep life is observed on addition of Ti and yttria in the alloys. The results obtained in this work suggest that powder forging could be a potential alternative method for fabricating ODS steels.