Phase transformations in the ordered Ni3Al (L12) compound upon mechanical alloying with Al

Solid-state synthesis of monoaluminide NiAl by mechanical alloying of Ni3Al with Al is studied. The mechanical alloying - co-milling of Ni3Al and Al - was carried out at room temperature in an argon atmosphere using a planetary ball mill. It was shown that structural changes in both the intermetallic compound and aluminum start immediately after the beginning of co-milling. The structural changes consist in the disordering of nickel aluminide and dissolution of Al in it, which are accompanied by an increase in the lattice parameter of Ni3Al and decrease in the Al lattice parameter owing to the dissolution of Ni in Al. At the same time, changes in the substructure - a decrease in the block size (DNi(Al) = 15 nm and DAl = 40 nm) and an increase in the micro-distortions 〈Ε2〉0.5 to 0.4 - 0.5 % - are observed. The subsequent milling leads to the precipitation of phases such as Al3Ni, Al2Ni3, and NiAl belonging to the Al-Ni3Al composition range. These phases are likely to be nucleated at the interfaces between Ni(Al) and Al(Ni) supersaturated solid solutions. Upon subsequent milling, the phases that have the highest negative enthalpy grow preferentially; because of this, two competitive phases - Al2Ni3 and NiAl - remain in the solid solution. Subsequently, the increase in the NiAl content is accompanied by an increase in the degree of long-range order from 0.5 (upon the nucleation) to 0.7 after the 120-min milling, when the monoaluminide remains a single phase. The lattice parameter of NiAl also increases. During further milling, the composition of NiAl becomes close to the stoichiometric composition. However, the degree of long-range order remains low because of the high content of point defects typical of the high-energy strain.