AN EXAFS STUDY OF STRUCTURAL-CHANGES INDUCED BY MECHANICAL MILLING OF THE NI3AL ORDERED INTERMETALLIC COMPOUND

Extended X-ray absorption fine-structure (EXAFS) data from the Ni3Al ordered intermetallic compound during structural evolution induced by mechanical milling (MM) are presented and discussed. X-ray diffraction and transmission electron microscopic observations confirm that the L12-Ni3Al ordered intermetallic compound changes to a disordered fcc form of Ni3Al at an early stage of milling and then changes to a two phase nanocrystalline and amorphous microstructure. Least-squares-fitting of the EXAFS data shows that both the Ni-Ni and the Ni-Al atomic distances increase slightly with milling time. During the first 5 h of milling, the Ni-Ni coordination decreased while the Ni-Al coordination increased. This observation is also indicative of an order-disorder transition. Subsequent milling produced a remarkable increase in the Ni-Ni coordination number as a nanocrystalline structure developed. Simultaneously, the Ni-Al coordination number decreased markedly, the total Ni coordination always remaining roughly constant. These EXAFS results suggest that segregation of Al from the Ni3Al occurred during the MM process, and eventually a partly amorphous structure evolved. After the longest milling time, a Ni-rich nanocrystalline phase was surrounded by an Al-rich amorphous phase. The Al segregation may be at the nanocrystalline grain boundaries and/or within the more extensive amorphous regions observed by transmission electron microscopy.