This proposal addresses the new phenomenon of crystal-crystal phase transformation and amorphization that can occur through virtual melting at temperatures significantly below (1000K) the melting temperature. Some indications were found that virtual melting may serve as mechanisms of various structural changes: plastic flow under high strain rate loading of metals and metallic nanowires, fracturing, and sublimation. The objective of this project is to develop multiple theoretical and computational approaches (coupled to experiment) to study the main features of virtual melting, and to explore the generality of the phenomena in various applications. General continuum thermodynamic and kinetic theories, as well as phase field theory and quantitative models, will be developed and applied to simulations of phase transformations, plastic flow under high strain-rate loading, fracture, and sublimation via virtual melting. Molecular dynamics simulations for the same phenomena and materials will be performed.
New mechanisms related to virtual melting are expected in various material systems, e.g., for phase transformation in HMX explosive, pharmaceuticals, and commercial nonlinear optical media and for high-pressure amorphization in geological, electronic, and superhard materials. Understanding the mechanism-based kinetics will lead to ability to control the transformations and look for hidden phases that need to be accessed (or avoided). One graduate student will be trained. Intense interdisciplinary collaboration will be developed. The PI will develop a new graduate course on multiscale modeling of phase transformations (available via distance learning) and will organize symposia devoted to the phase transformations.
The PI has recently suggested a new mechanism for crystal-crystal phase transformations, which resolved numerous puzzles from experimental measurements of phase transformation in HMX energetic crystals at 120K below its melting temperature. In the proposed mechanism, energy of elastic stresses that is induced by transformation strain, increases the driving force for melting and reduces the melting temperature. The stress-induced melt relaxes the internal stresses and eliminates the driving force for melting, which result in subsequent solidification of the unstable melt. Intellectual merit: Significant progress towards development of the models and experimental proof of various virtual melting scenarios is achieved. New types of the virtual melting as well as intermediate melting (a metastable melt within solid-solid interface supported by reduction of the interface energy) are revealed. Presence of virtual (intermediate) melting is proved experimentally and using atomistic simulations. Detailed phase-field models are developed. The outcomes include: (a) A new mechanism of plastic flow and stress relaxation under high strain rate loading via virtual melting 4000 K below the melting temperature is predicted thermodynamically and confirmed in molecular dynamics simulations. (b) Theoretical prediction and experimental confirmation of solid-solid phase transformation via surface-induced intermediate melting. (c) Development of the first phase field theory for interface stresses and advanced model of nonequilibrium coherent interface for multivariant martensitic phase transformations. (d) Development of an advanced phase field model for multivariant martensitic phase transformations at large-strains with two different choices of the order parameters. (e) Development of the first phase field theory of surface-induced pre-transformations and transformations, coupled to mechanics, for multivariant martensitic phase transformations. (f) Development of the first phase field theory for coherent solid-liquid interface with consistent interface stresses. (g) Introducing the concept of anisotropic transformation strain for melting and deriving the kinetic equation for it. (h) Development of the first phase field theory for solid-solid and solid-melt phase transformations, which takes into account the finite width of the external surface, and revealing strain-induced morphological transitions within external surface. (i) Development of FEM algorithm for the coupled phase field and mechanics problems on multivariant martensitic phase transformations, melting/solidification, and intermediate melting. (j) Revealing new scale effects, thermally activated surface nucleation and bi-stable states for melting/solidification of nanoparticles in terms of the width of the external surface. (k) Phase field study of solid-solid transformations via nanoscale intermediate interfacial melt: revealing multiple nanostructures, scale, kinetic, and mechanics effects. (l) Resolution of outstanding problem in the interface science on strict definition of the position of Gibbs dividing surface and proving the consistency of this definition for a specific phase field model. Broader Impact: The PI developed and taught a new graduate distance education course "Phase Transformations in Elastic Materials," which included phase field approaches to phase transformations in solids, melting, and surface phenomena, developed in the project. The PI has presented a short course (6 lectures) on Mechanically-Induced Phase Transformations and Chemical Reactions during Advanced School "Plasticity and Beyond: Microstructures, Crystal- Plasticity and Phase Transitions" at International Centre for Mechanical Sciences (CISM, Udine, Italy). The PI organized 5 symposia on mechanochemistry and on phase transformations and is in the process of organizing two more symposia. A female post-doc researcher N. Altukhova (currently lecturer at Iowa State University) worked on the project. Four PhD students were partially supported and three of which were graduated and received faculty positions at different universities. Six undergraduate US students and 5 female and minority students were involved in the project with support from REU supplement. Three PhD students who worked on project have received 7 different awards. Participants of the project published 30 papers mostly in highly ranked journals in mechanics and physics. They presented 48 conference talks (including 16 keynote and invited lectures) and gave 12 invited seminar presentations.
