Microstructural evolution of nanocrystalline Fe–Zr alloys upon annealing treatment

• We show clear evidence of precipitation of Fe3Zr phase above 600 °C.
• Stabilization of nanostructure is not solely controlled by thermodynamic mechanism.
• Above 600 °C, Zener pinning plays an important role in stabilizing nanostructure.

Nanocrystalline Fe–Zr alloys exhibit an extraordinary thermal stability at elevated temperatures, which enables their potential applications in various fields. However, there remain concerns regarding the controlling stabilization mechanisms responsible for their thermal stability. In this work, two nanocrystalline Fe–Zr alloys containing 1 at.% Zr and 5 at.% Zr were annealed at various temperatures (Tann) up to 900 °C. Microstructural evolution of the alloys upon annealing was investigated by means of an X-ray diffractometer equipped with a 2-dimensional detector and transmission electron microscopy. Below 600 °C, microstructures of the two alloys consist of single nanocrystalline ferrite whose grain size is rather stable upon annealing treatments. Above 600 °C, accompanying the precipitation of Fe3Zr phase, an apparent grain coarsening of ferrite is observed, whereas the thermal stability of the alloys is still considerably higher than that of nanocrystalline pure Fe. Based on the experimental results, it was claimed that stabilization of the nanocrystalline Fe–Zr alloys should not be totally ascribed to the thermodynamic stabilization mechanism due to the reduction in grain boundary energy as suggested in earlier investigations [K.A. Darling et al., Scr. Mater. 59 (2008) 530 and K.A. Darling et al., Mater. Sci. Eng. A527 (2010) 3572]; when Tann is higher than 600 °C, along with the precipitation of Fe3Zr, the effect of thermodynamic stabilization is weakened, the kinetic effect arising from Zener pinning of Fe3Zr precipitates turns to be an important mechanism contributing to the stabilization of the nanoscale grain size.