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 develop ment of novel crystal structures and microstructures, leading to enhan ced physical and mechanical properties. The application of MA to the s ynthesis 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 va riety of alloy phases, including supersaturated solid solutions, nanoc rystalline structures, amorphous phases and intermetallic compounds th emselves, is discussed. No extension of solubility using MA was observ ed in the intermetallics studied, unlike the situation using rapid sol idification (RS). Nanostructured grains were observed in all compositi ons, their rate of decrease in size and minimum size being related to the following partially interrelated parameters: stability of the inte rmetallic, grain boundary energy, melting point and the balance betwee n defect creation/recovery. Long-time milling generally resulted in am orphous phase formation largely because of the increase in grain bound ary energy per mole with reduced grain size; good agreement with the M iedema model for amorphization was obtained in the Al-Fe system. Gener ally, annealing was required to form the intermetallic after MA; howev er, intermetallics with a large negative enthalpy of formation were de tected in the mechanically alloyed condition. Low-temperature compacti on allowed the retention of the fine microstructure in the nanometer r ange, giving an interesting capability to enhance ductility in the nor mally brittle intermetallics.