Amorphization and Disordering of The Ni₃Ai 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 → disordered fcc → 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 (~2 nm diameter) dimensions. The calculated interfacial free energy of the grain boundaries is consistent with the estimated free energy difference between the fee and amorphous phases in Ni₃Al.