Amorphization and disordering of the Ni3Al ordered intermetallic by mechanical milling

The ordered fcc intermetallic compound Ni3Al was mechanically milled in a high energy ball mill. The severe plastic deformation produced by milling induced transformations with increasing milling time as follows: ordered fcc &rarr disordered fcc &rarr nanocrystalline fcc + amorphous. The milling time for complete disordering occurred at 5 h for stoichiometric Ni3Al milled at ambient temperature compared to 50 h for the first observation of an amorphous structure. The structural and microstructural evolution with milling time was followed by x-ray diffraction, TEM, hardness, and calorimetry. The major defect believed responsible for inducing the crystalline-to-amorphous transformation is the fine grain boundary structure with nanometer (ca.2 nm diameter) dimensions. The calculated interfacial free energy of the grain boundaries is consistent with the estimated free energy difference between the fcc and amorphous phases in Ni3Al.