Phase-Field Model for Melt-Mediated Solid-Solid Phase Transformation
Kasra Momeni (Iowa State University), Valery Levitas (Iowa State University)
Mechanics of Phase Transforming and Multifunctional Materials
Tue 2:40 - 4:00
CIT 219
Formation of a few nanometer sized intermediate phase at the stationary or moving phase interface between two phases is reported in a number of research studies. Examples include formation of a virtual melt (VM) between and phases of HMX energetic material at 120 K below the melting temperature and between tetragonal preperovskite and cubic perovskite phases in PbTiO3 nanofibers hundreds degrees below melting temperature, both during solid-solid phase transformation (PT). Here, A Ginzburg-Landau potential with advanced mechanics is developed. Then a phase field approach is pursued to study barrierless melting at the stationary and propagating solid-solid interface with applications to HMX explosives. The model is implemented in the finite element code COMSOL and study of mesh-dependence is performed. Effects of temperature, elastic energy, and solid-solid and solid-melt interface widths and energies on solid-solid PT and possible formation of an intermediate VM are studied. It is also shown that considering the interaction between two solid phases via an intermediate melt plays a key rule in determining the width of VM, well-possedness of the problem and mesh-independence of solutions. Numerical results illustrate a non-trivial effect of formation of VM on the interface velocity. Different interface energy (width) ratios between solid-solid and solid-melt are considered while interface width (energy) of solid-solid and solid-melt is fixed. Presence of two different solutions based on the chosen initial conditions in a specific range of interface energy (width) ratios is illustrated, which indicates the presence of a critical nucleus and the importance of thermal fluctuations for the PT. It is shown that the presence of elastic energy promotes formation of VM, and surface tension can move the location of molten nucleus from the intersection between solid-solid interface and free surface to the mid-plane of the sample.