Thermal stability, phase transformation and hardness of mechanically alloyed nanocrystalline Fe-18Cr-8Ni stainless steel with Zr and Y2O3 additions

Abstract

Nanocrystalline Fe-18Cr-8Ni (at%) stainless steels were produced by mechanical alloying from elemental powders with Zr and Y2O3 additions and subjected to annealing treatments at various temperatures. X-ray diffraction experiments, transmission electron microscopy and focused ion beam microscopy were used to investigate the microstructural evolution as a function of alloy composition and annealing temperature. The dependence of hardness on the microstructure was utilized to study the mechanical changes with temperature. It was found that the resulting microstructures by mechanical alloying were bcc solid solution, the so-called α′-martensite structure whereas, depending on the composition and temperature, partial reverse transformation from martensite to austenite was induced by annealing. As-milled nanocrystalline Fe-18Cr-8Ni stainless steel yielded grain growth upon annealing reaching to micron sizes at 1100 °C while addition of Zr and Y2O3 stabilized the microstructure below approximately 250 nm grain size and close to 5 GPa hardness after annealing at the same temperature. Such microstructural features may facilitate the consolidation process of nanocrystalline stainless steel powders as well as enabling the use of these materials at elevated temperatures.