Thermal stability of nanocrystalline iron prepared by mechanical attrition

Mechanical attrition as obtained by high energy ball milling of powder particles has been found to be a versatile method in producing nanophase materials with a broad range of chemical compositions and atomic structures including nanocrystalline as well as amorphous states. In pure metals the severe plastic deformation results in a nanocrystalline metastable structure with a typical grain size of 10nm and 0.5-1% microstrain. The excess enthalpy due to the high density of defects is released at relatively low temperatures compared to conventional polycrystals. Relaxation and grain growth behaviour of nanocrystalline Fe has been investigated by isothermal and nonisothermal DSC measurements. The calorimetric data have been correlated with X-ray diffractograms, the grain size and microstrain being deduced from Xray line broadening. A comparison is drawn to the recrystallization of conventionally cold worked materials.