Synthesis of intermetallics by mechanical alloying

Mechanical alloying (MA), a solid-state powder processing method, is a “far from equilibrium” synthesis technique which allows the development of novel crystal structures and microstructures, leading to enhanced physical and mechanical properties. The application of MA to the synthesis of intermetallics in the TiAl(Nb), AlFe, NbAl, TiMg, AlZr(Fe) and AlMg systems is presented. The ability to synthesize a variety of alloy phases, including supersaturated solid solutions, nanocrystalline structures, amorphous phases and intermetallic compounds themselves, is discussed. No extension of solubility using MA was observed in the intermetallics studied, unlike the situation using rapid solidification (RS). Nanostructured grains were observed in all compositions, their rate of decrease in size and minimum size being related to the following partially interrelated parameters: stability of the intermetallic, grain boundary energy, melting point and the balance between defect creation/recovery. Long-time milling generally resulted in amorphous phase formation largely because of the increase in grain boundary energy per mole with reduced grain size; good agreement with the Miedema model for amorphization was obtained in the AlFe system. Generally, annealing was required to form the intermetallic after MA; however, intermetallics with a large negative enthalpy of formation were detected in the mechanically alloyed condition. Low-temperature compaction allowed the retention of the fine microstructure in the nanometer range, giving an interesting capability to enhance ductility in the normally brittle intermetallics.