3. Apparent and hidden mechanochemistry

Two phenomena will be considered. Phase transformations (PTs) and chemical reactions (CRs) under compression and shear of materials in rotational diamond anvil cell (RDAC) represent examples of apparent effects of plastic shear. We recently predicted the new phenomenon that crystal-crystal and crystalamorphous PTs can occur through virtual melting (VM) significantly below (> 1000K!) the melting temperature. This hidden mechanochemical phenomenon is caused by the energy of the internal elastic stresses, induced by solid-solid PT. It increases the driving force for melting and reduces the melting temperature. Melting releases the stresses, and the unstable melt solidifies into the crystalline or amorphous phase (above or below the glass transition temperature, respectively). Some indications were found about the existence of the virtual amorphization (V A), and that VM and/or V A may serve as mechanisms of various structural changes: plastic flow under high-strain-rate and shock loading of HMX crystal, metals, and metallic nanowires; fracturing; nanofriction; sublimation; and grain growth and sliding. One more example of hidden mechanochemistry is related to the melt-dispersion mechanism of the reaction of Al nanoparticles. Correspondence/Reprint request: Dr. Valery I. Levitas, Department of Mechanical Engineering, Department of Aerospace Engineering, Department of Material Science and Engineering 2008 Black Engineering Building Iowa State University Ames, Iowa 50011-2161, USA. E-mail: [email protected]