MICROSTRUCTURAL STABILITY AND MECHANICAL PROPERTIES OF 15CR FERRITIC ODS STEEL PRODUCED BY SPARK PLASMA SINTERING OF MECHANICALLY ALLOYED POWDERS

Abstract

Ferritic oxide dispersion strengthened (ODS) steels with uniformly dispersed oxides (Y2O3) are considered as a promising fuel cladding material for future nuclear reactors. The attractive properties of Y2O3 particles such as higher stability than any other oxides (ex. MgO, Al2O3) under neutron irradiation, efficient pinning of mobile dislocations as well as the mobile grain boundaries have made it the most viable reinforcement in the oxide dispersion strengthened (ODS) steels. Literature indicates further refinement of Y2O3 particles in its size by re-precipitation in a complex structure (nano-cluster) in presence of other minor alloying elements like Ti, Zr, Hf, Al, which are found even more favorable in enhancing the mechanical properties at elevated temperatures according to the particle-dislocation interaction theory. Addition of Ti and Zr is found the most viable amongst the studied solutes for the improved creep life without any adverse effect on mechanical properties. To achieve the uniform dispersion of nano-clusters in the matrix, mechanical alloying has gained considerable attention contrary to classical and conventional casting methods. The present study focusses on synthesizing the ODS ferritic steel powders via mechanical alloying followed by their consolidation via spark plasma sintering (SPS) technique. Further, different concentrations of Y2O3 particles are investigated in terms of their dispersion and subsequently their effect on grain coarsening kinetics and mechanical properties. Finally, nano-clusters precipitated out with the addition of Ti and Zr (separately as well as together) are investigated in terms of their ease of formation, size and effect on high temperature properties in the ferritic ODS steel. The powders/ samples are characterized via XRD, DTA, particle size analyzer, SEM, EBSD, TEM, and EDS. The SPSed samples are tested for Vicker hardness (5 kgf) at room temperature and for compressive strength at room as well as at high temperatures (i.e. 600 oC, and 700 oC). Concerning to above, an in depth analysis of dissolution of all the solutes in a ferritic stainless steel with the nominal composition of Fe-15Cr-2W was carried out and the corresponding mechanical alloying parameters were established in the first study. The study dictates that the dissolution of Cr took place at an earlier stage as compared to that of W and a complete solid solution was found to occur at 10 h of milling. Y2O3 was found to be stable (undissolved) and consistent till 10 h of milling though its size was refined. In the second study, densification behavior of alloy powders via SPS technique was investigated to obtain the best combination of density and mechanical properties. This study reveals the role of a higher pressure (60 MPa) to be dominant at lower sintering temperature (i.e. 950 oC) and the role of temperature activated diffusion to be dominant at higher sintering temperatures (≥1025 oC) in terms of densification. Further, a pressure of 50 MPa and sintering temperature of 1025 oC were found suitable for maximum density and good mechanical properties. In the third study, to analyze the grain growth kinetics, four SPSed compositions containing Y2O3 concentration of 0, 0.3, 0.7, and 1.0 wt% were annealed at 1100 oC for the dwell period of 0, 60, 120 mins and the same samples were further annealed at different temperatures i.e. 950 oC, 1100 oC, and 1250 oC for a fixed duration of 60 mins. Results indicate that the attributes of mechanical alloying as well as added Y2O3contents significantly influenced the grain growth exponent (n) and activation energy (Q), reaching ‘n’ to 11.52 and ‘Q’ to 612.91 kJ/mol in 1.0 wt% Y2O3 composition. Strength contributions of temperature dependent strengthening mechanisms suggest that the role of ultra-fine grains and dispersoids seemed to be predominant at room temperature and high temperatures respectively. In the final study, four SPSed compositions i.e. Fe-15Cr-2W-0.3Y2O3-x (x = 0, 0.3Ti, 0.3Zr, and 0.3Ti-0.3Zr) were studied to investigate the evolution of nano-clusters of different chemistry in correlation with pseudo-convex-hull of formation enthalpy diagram and their effect on mechanical properties. Results suggest that contrary to Ti, addition of Zr was found with the precipitation of dense and much finer nano-clusters (~3 nm) and observed to be more effective in enhancing the hardness and compressive strength at room as well as high temperatures. Composition containing both Ti and Zr solutes resulted in ultrafine grains of nearly uniform sizes and exhibited considerably enhanced hardness as well as compressive strength at all temperatures. Further, it favored the formation of Y-Zr-O nano-clusters instead of Y-Ti-O due to exhibiting lower formation enthalpy. As per the theoretically evaluated strength contributions, Orowan strengthening was found prevailed at high temperatures (600 oC, 700 oC) and it had a major contribution in compressive yield strength at room temperature as well.