Sudden loss of thermal stability in Fe-based nanocrystalline alloys

  The inherent instability of nanocrystalline materials with respect to grain growth can be mitigated by adding an atomic species that segregates to the grain boundaries. Our investigations of binary Fe-based alloys show that this method is effective in stabilizing the nanocrystalline state, but only up to about 900 °C. Above this threshold, microstructural stability is suddenly lost. In situ X-ray diffraction measurements incriminate the α-to-γ transformation, and focused ion beam microscopy reveals a growth morphology reminiscent of abnormal grain growth.