CRYSTALLITE GROWTH IN Fe-25%Y2O3 ALLOY PRODUCED BY MECHANICAL ALLOYING

Abstract

Oxide dispersion strengthened steels are under exploration for their use in the nuclear power reactors. Superior properties of Oxide Dispersion Strengthened (ODS) steels produced by mechanical alloying (MA) are resistance to oxidative and nitration environments, high strength at high temperatures, lower swelling and less prone to embrittlement upon high speed neutron irradiation etc., have qualified these ODS steels for their use in nuclear power system. Currently, in the nuclear reactor 9Cr ferritic and ferritic-martensitic steels having higher resistances to creep and swelling are in use. In order to sustain more harsh environmental conditions of future generation nuclear reactors, properties of 9Cr- steels needs to be further improved. Because of this, dispersion of nanosized yttria to develop oxide dispersion strengthened (ODS) steel clad tube with long- term creep strength is the current focus of materials research. The enhanced properties of ODS steels have been attributed to the role of relatively stable dispersed oxide particles in slowing down the grain growth, diffusion rates of solutes, favoring more dispersion of developing product phases upon heat treatment and oxide/ferrite interfaces acting as traps for vacancies. In commercial ODS steels a maximum of about 1 wt.% oxide dispersion is known to give optimum properties. Due to the small amount of dispersed second phase it is not easy to follow the changes in the microstructure of dispersed oxide particles during heat treatments (i.e., we cannot get statistically reliable data set to trace for example the crystallite size of oxide particles as a function of heat treatments using X-ray diffraction line profile analysis). In order to overcome this difficulty of characterizing the state of oxide dispersion, in this study we have increased the amount of Y2O3 dispersion to 25 wt.% so that we can get the X-ray diffraction peaks of both ferrite matrix and Y2O3 particles which can be used for tracing the microstructural changes. X-ray line profile analysis techniques such as Williamson-Hall, modified Williamson- Hall, Single Line analysis and Halder-Wagner have been used to extract the microstructural information from XRD data. In order to avoid the interference from possibly ongoing martensitic transformation in commercial ODS steel on the observed microstructural changes, pure iron is used as the matrix in place of Cr-alloyed ferrite. Because of this we can only expect usual ferrite to austenite transformation, therefore the role of Y2O3 dispersion on crystallite growth can be traced easily. Mechanical alloying was carried using ball milling of elemental Fe powder and pure iv Y2O3 powder in a planetary ball mill. Heat treatments usually carried on commercial 9Cr-ODS alloy has been employed on the model alloy of this study; which involves heating to 1200oC for a holding time of 30 min in flowing H2 atmosphere followed by air cooling and then further heating the same alloyed powder to 1060oC for 30 min followed by air cooling. It is found that the crystallite size has considerably decreased due to the ball milling with increase in lattice micro strain. Upon heat treatment at 12000C, it is found that the crystallite size has increased considerably with simultaneous decrease in microstrain. Rate of crystallite growth is faster for ferrite matrix as compared to that of Y2O3 and no clear evidence of significant solubility of Y2O3 in ferrite has been evidenced from the lattice parameter change of ferrite matrix. Ferrite matrix grain size has reduced after second heat treatment at 1060oC which has been attributed to the smaller grain size of austenite at 1060oC as compared to that at 1200oC. The powders were also analyzed using transmission electron microscopy to verify the crystallite size obtained from X-ray diffraction line profile analysis