Optimization of milling parameters, processing and characterization of nano-crystalline oxide dispersion strengthened ferritic steel

Abstract

Ferritic steel powder was mechanically milled in a dual drive planetary ball mill, under different milling conditions to optimize the milling parameters. The resulted powder was characterized, using particle size analyzer, X-rays and electron microscope. X-ray peak broadenings were investigated to estimate crystallite size, lattice strain and deformation stress. Better Pearson's coefficient was observed for uniform stress deformation model (USDM) (0.988) in comparison to uniform deformation model (UDM) (0.64) which shows better estimation of lattice parameter. An increase in fineness was observed with an increase in ball to powder ratio as well as for an increase in rotational speed. At the optimized condition, ferritic steel powder, together with Y2O3, was milled in the dual drive mill to produce oxide dispersion strengthened ferritic steel powder, suitable to be used in nuclear applications. Convoluted morphology, desired for better alloying, was confirmed using an electron microscope. A significant increase in per unit surface area was noticed due to milling using BET surface area analysis. Negligible contamination was observed due to milling atmosphere and mill container. The steel powder produced, was sintered using spark plasma sintering and its density and hardness were measured. High hardness and lower crystallite size were recorded using spark plasma sintering. Addition of Y2O3 shows decreases in the thermal expansion coefficient. Effect of added titanium was studied and an adverse effect on oxide dispersion strengthened steel was noticed.