Crystallite size and microstrain in mechanically alloyed and heat treated Fe-25 wt.% Y2O3

Abstract

The main problem in characterizing the commercial ODS steel to understand fundamentally the underlying microstructural changes is associated with the very small volume fraction of Y2O3 which obstructs the application of, statistically reliable, bulk material characterization techniques like X-ray diffraction. Furthermore, the commercial ODS steels have other precipitation reactions which also complicates the understanding of the sole influence of Y2O3 on the microstructure evolution. In order to overcome these difficulties, in this study the high Y2O3 containing Fe-25 wt.% Y2O3 model alloy was chosen which allows the application of X-ray diffraction line profile analysis to trace the evolution of both ferrite and Y2O3 crystallite sizes and microstrains as a function of different processing steps like ball milling and the following heat treatments. No significant change in the lattice parameter of ferrite matrix has been observed after ball milling suggesting the negligible solubility of Y2O3 in ferrite matrix even after ball milling. During heat treatment at 1200°C the micro strain of the ferrite matrix has decreased with simultaneous increase in crystallite size and the same phenomena was also observed for Y2O3 as well. During the following second heat treatment at 1060°C (i.e., the powder heat treated at 1200°C was further heat treated at 1060°C) no significant change in crystallite size of Y2O3 occurred whereas the ferrite crystallite size has significantly decreased. This decrease in crystallite size of ferrite during second heat treatment has been attributed to the smaller austenite grain size after second heat treatment at 1060°C than the first heat treatment at 1200°C.