Grain size stabilization of Fe and Fe-Ni nanostructures developed by mechanical alloying

Abstract

The mechanical alloying (MA) technique has been used to prepare nanocrystalline Fe, Fe-5wt. %Ni, 7 wt. % Ni and 10 wt. % Ni alloys from elemental powder mixtures. Fe-5wt. %Ni and Fe-7 wt. % Ni were detected to form complete solid solution after MA for 1500 min. as per the X-ray diffraction (XRD) analysis. Out these two solid solutions, Fe-7 wt. % Ni alloy was chosen to investigate its grain size stabilization by addition of minute amount of yttrium. Fe is also stabilized by yttrium. All the alloys were annealed at different temperatures up to 1200ºC under high purity argon atmosphere. XRD analysis and microhardness measurement were carried out for all the specimens to analyze their stability after annealing. XRD analysis was carried out to measure the lattice parameter variation due to alloying, which is a vital data to detect whether the solid solution has been formed. Crystallite size was calculated by using Scherer formula and the value of the as-milled sample was below 15 nm. Atomic force microscopy would also be carried for selected specimens to investigate the microstructural features. The experiment showed yttrium stabilizes Fe and Fe-Ni grain size at higher temperature in nano range. Annealing decreases lattice strain for annealing temperature below 900°C but it suddenly increases while annealing at higher temperature for ball milled Fe. With the increase in Ni content upto 5 wt.% lattice strain increases for all annealing temperature except at 700°C and 1200°C. This in term increases its hardness and hence other mechanical properties. Stable crystallite size (less than 100 nm) and constant microhardness of the annealed samples can dictate its usefulness for high strength and other functional applications. Hardness of as milled sample of stabilized Fe obtained to be 9.933 GPa and of Fe‐7Ni to be 5.99 GPa. Crystallite size increases with increase in temperature. It becomes stable at higher temperature for Fe-Ni.