Amorphization by mechanical deformation

Abstract

Amorphization of crystalline structures is a ubiquitous phenomenon in metals, ceramics, and intermetallic compounds. Although the amorphous phase generally has a higher Gibbs free energy than its crystalline counterpart, there are many methods by which amorphization can be generated. The requirement to create an amorphous phase from a solid crystalline one is to increase its free energy above a critical level which enables this transition. In this review, our focus is on amorphization induced by mechanical deformation which can be imparted by a variety of means, prominent among which are tribological processes, severe plastic deformation, nanoindentation, shock compression, diamond anvil cell and ball milling/mechanical alloying. The deformation introduces defects into the structure, raising its free energy to the level that it exceeds the one of the amorphous phase, thus propitiating conditions for amorphization. Experimental observations of amorphization in metallic alloys, intermetallic compounds, ionically and covalently bonded materials are presented and discussed. There is also an observation of amorphization in a biological material: it is generated by impact deformation of hydroxyapatite in the mantis shrimp club. We also focus on the fundamental mechanisms of plastic deformation of amorphous materials; this is a closely linked process by which deformation continues, beyond amorphization, in the new phase. Observations and analyses of amorphization are complemented by computational simulations that predict the process of mechanically-induced amorphization and address the mechanisms of this transformation.