THERMAL STABILIZATION OF NANOCRYSTALLINE Fe-Cr ALLOYS DEVELOPED BY MECHANICAL ALLOYING

Abstract

This work examines the influence of insoluble alloying element Y (up to 1 at.%) on the thermal stability of nanocrystalline Fe-Cr alloys prepared by mechanical alloying (MA). The ball milled samples were subjected to isochronal heat treatments up to 1200°C. Crystallite size based on Xray diffraction (XRD) analysis along with microhardness measurements were carried out to investigate their thermal stability, Microstructural evolutions were analyzed using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The Gibbs free energy change and XRD analysis showed that I at. % Y could be dissolved in Fe-Cr matrix after 25 h of MA. Feasibility of formation of Fe-Cr-Y solid solutions has been explained from the Gibbs free energy change using Miedema's model and Toop's model. Thermodynamic analysis portrayed that the energy barrier required for formation of disordered solid solution exceeded by the supplementary energy produced by strain dislocations energy and grain boundary defects. Grain growth was restricted for alloys with higher Y (1 at.%) content and nanocrystalline grains (within 50 nm) were retained up to 1000°C. Alloys with lower Y% displayed less stability but relatively better than that of the Fe-Cr alloys without Y additions. Grain size analysis by AFM and TEM is well corroborated with XRD crystallite size. The high thermal stability as well as large strengthening effect is discussed in the light of solid solution strengthening due to formation of disorder solid solution, precipitation hardening and grain boundary pinning by solute drag effect and/or segregation of Y.