Solid-state amorphization and contamination of NiTa powders during ball-milling

Abstract

High-energy ball-milling is generally used to mechanically alloy metallic powders and develop microstructural refinement as a means to enhance structure-dependent properties. The process involves the repeated fracture, welding and fine-scale layering of the particulate. In some instances it may also induce transformations to highly metastable phases. In particular, when applied to alloy systems with negative heats-of-mixing of the constituent elements and one element that diffuses through the lattice at an anomalously fast rate, the ball-milling almost invariably results in a solid-state amorphization reaction (SSAR). That is, the crystalline material can be driven into a superheated condition which is unstable with respect to the amorphous phase because the free energy of the glass lies below that of the corresponding crystalline solid solution of the same composition. This method of forming the metallic glass state has certain advantages over those involving rapid solidification or vapor deposition. SSAR is convenient to control, usually leads to a much wider glass formation range, and the glasses often have high thermal stability than that of melt-spun ribbons. In this paper the authors chose to ball-mill mixtures of Ni and Ta crystalline powders because they satisfy the requirements mentioned above and SSAR had not been attempted beforemore » with these elements. The authors were successful in producing Ni-Ta amorphous powder alloys of two compositions using a milling container and balls made of WC+Co.« less