Local Structure Change around Ni Atoms in MgNi Alloys during Mechanical Alloying Process

摘要

Mg_(50)Ni_(50) amorphous alloy was prepared by mechanical alloying (MA) of elemental powder mixtures of Mg and Ni. A structural change in Mg_(50)Ni_(50) alloy was observed during solid state amorphization by x-ray diffraction (XRD) and Ni K-edge extended x-ray absorption fine structure (EXAFS). XRD profile showed that Bragg peaks of hcp-Mg disappeared in the early stage of MA (<7h), while that of fcc-Ni remained until the late stage of MA (～100h). Finally, an amorphous Mg_(50)Ni_(50) alloy was obtained after 200h of milling. The radial structure function (RSF) obtained from Ni K-edge EXAFS indicated that the local structure around the Ni atom changed from fcc-Ni to MgNi_2 intermetallic compound between 10h and 50h of milling time. The results of the quantitative analysis of Ni K-edge EXAFS indicated that the coordination number of the Mg atom is higher than that of the Ni atom. This result suggests that Mg_2Ni type local structure is also formed around the Ni atom in the amorphous Mg_(50)Ni_(50) alloy.