Control of Structure and Formation of Amorphous and Nonequilibrium Crystalline Metals by Mechanical Milling (Overview)

Abstract

This review first describes a rod milling method developed by us. Using this method we utilize shear force more effectively than impact force for both comminution to nano-crystallites and amorphization, and minimize contamination problems. X-ray diffraction, transmission electron microscopy, neutron scattering and thermal analyses for milled powders elucidate the elemental processes of solid-state amorphization: 1) preferential mutation of octahedral units to tetrahedral units is indispensable for amorphization, where reconnection of tetrahedral units changes from vertex-sharing to face- and/or edge-sharing, 2) layered morphology produced in the initial stage of MA is highly energetic, leading to amorphization even by heating (thermally assisted solid state amorphization), 3) comminution to nano-crystallites is attributed to the glide of dislocations induced by shear force, and 4) in the ternary system, preferential alloying of binary constituents initially forms an intermetallic and such heterogeneous mixture gradually become a homogeneous amorphous phase. Mechanical milling is also powerful to produce nonequilibrium disordered crystalline alloys, whose equilibrium crystalline counter parts show immiscibility and/or very narrow primary solid solution ranges. Since milled powers is very fine and contains a large amount strain and defects, Al is removed from nonequilibrium crystalline and amorphous Al-transition metal alloys, leaving metastable transition metal powders.