First-Order Polymorphic and Melting Phase Transitions Under Shock Loading

Abstract

Some materials at high pressure have structural arrangements of atoms and molecules that are at a lower energy state than that at (P ¼ 0, v o , T-293 K) conditions. When this occurs a thermodynamic force on the material exists to change the configuration to the new one. This process is called a first-order phase transition that is characterized by the coexistence of several thermodynamic phases. It is well established that condensed matter can also undergo thermodynamic second order phase transitions in electric and magnetic properties that are not treated in this book. Readers interested in second order phase transitions seen in dynamic experiments consult the literature especially the articles by Keeler and Royce [1].Static studies of phase transitions provide the baseline for understanding many phase transitions. They can show if the mixed phase region is in thermodynamic equilibrium or in a metastable state. Inferring phase transformations under dynamic loading from experimental static data is very useful. The first order guess of the phase seen in dynamic experiments will be the same as observed in static experiments but this needs to be verified in real-time dynamic experiments. One must keep in mind that under dynamic loading, phases other than that seen in static experiments are a possibility. This was seen in a recent study of shocked polycrystalline iron [2] where fcc phases occurred in certain oriented crystal grains. Note that this shock study using intense x-rays directly confirmed experimentally the majority of the iron transition was martensitic in nature, which agrees with static studies.The kinetics of the transformation determines whether a new configuration occurs under shock loading. If the transformation takes many microseconds to occur it will not be seen in standard shock wave experiments and the material will be put into a thermodynamic metastable state. Since dynamic loading is different than static compression some transitions that require cooperative lattice motion can be accelerated due to the shear in a shock wave. This is what happens in the body center cubic (bcc or a) to hexagonal closed packed (hcp or ɛ) lattice transition in iron [3] at 130 kbar. Keep in mind that due to creation of defects,