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 Carolina State University over the last ten years in the field of metastable materials prepared by mechanical attrition. A brief historical perspective of the author on the beginnings of this sub-field is presented. Much of the research discussed is devoted to the crystalline-to-amorphous 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 discussed. Amorphization by mechanical milling (MM), where a single composition (e.g. intermetallic compound) is milled requires the accumulation of defects to induce the transformation. It is concluded that anti-site chemical disorder and nanocrystalline grain boundaries are the important defects in this regard. A brief description of studies on MM of normally immiscible systems is presented. Non-equilibrium solid solubilities can be achieved by MM of systems such as Ge-Sn and Si-Sn. The paper concludes with a summary of recent work on nanocrystalline materials prepared by ball milling. The questions of 1) the mechanism for synthesis of nanocrystalline microstructures, and 2) the thermal stability of such microstructures are discussed.