Microstructure evolution and thermal properties in nanocrystalline Fe during mechanical attrition Original Research Article

The microstructural evolution and thermal properties of nanocrystalline (nc) Fe during mechanical attrition were investigated by using quantitative X-ray diffraction and thermal analysis techniques. Upon milling of the Fe powders with coarse grains, grain refinement takes place gradually and a steady-state grain size in the nanometer regime is reached after a certain period of milling. With the further milling of the nc Fe within the stage of the steady-state grain size, we observed a grain boundary relaxation process that was manifested by evident decreases in the thermal expansion coefficient and the stored enthalpy, as well as slight decreases in the lattice strain and the Debye–Waller parameter. The grain boundary enthalpy of the nc Fe was estimated, showing a decreasing tendency with the milling time. The present work indicated with clear experimental evidence that the nc materials with the same grain size may exhibit very different properties that depend upon the microstructure of the numerous metastable grain boundaries.