RESEARCH ON METASTABLE STRUCTURES USING HIGH-ENERGY BALL-MILLING AT NORTH-CAROLINA-STATE-UNIVERSITY

This paper reviews the research of the author's group at North Carolin a State University over the last ten years in the field of metastable materials prepared by mechanical attrition. A brief historical perspec tive of the author on the beginnings of this sub-field is presented. M uch of the research discussed is devoted to the crystalline-to-amorpho us phase transformation induced by mechanical attrition. This includes mechanical alloying (MA) where dissimilar powders react and material transfer occurs. Equilibrium structures prepared by MA are also discus sed. Amorphization by mechanical milling (MM), where a single composit ion (e.g. intermetallic compound) is milled requires the accumulation of defects to induce the transformation. It is concluded that anti-sit e chemical disorder and nanocrystalline grain boundaries are the impor tant defects in this regard. A brief description of studies on MM of n ormally immiscible systems is presented. Non-equilibrium solid solubil ities can be achieved by MM of systems such as Ge-Sn and Si-Sn. The pa per concludes with a summary of recent work on nanocrystalline materia ls prepared by ball milling. The questions of 1) the mechanism for syn thesis of nanocrystalline microstructures, and 2) the thermal stabilit y of such microstructures are discussed.