Levitas Valery I, Henson Bryan F, Smilowitz Laura B, Asay Blaine W
Center for Mechanochemistry and Synthesis of New Materials, Department of Mechanical Engineering, Texas Tech University, Lubbock, 79409, USA.
J Phys Chem B. 2006 May 25;110(20):10105-19. doi: 10.1021/jp057438b.
We theoretically predict a new phenomenon, namely, that a solid-solid phase transformation (PT) with a large transformation strain can occur via internal stress-induced virtual melting along the interface at temperatures significantly (more than 100 K) below the melting temperature. We show that the energy of elastic stresses, induced by transformation strain, increases the driving force for melting and reduces the melting temperature. Immediately after melting, stresses relax and the unstable melt solidifies. Fast solidification in a thin layer leads to nanoscale cracking which does not affect the thermodynamics or kinetics of the solid-solid transformation. Thus, virtual melting represents a new mechanism of solid-solid PT, stress relaxation, and loss of coherence at a moving solid-solid interface. It also removes the athermal interface friction and deletes the thermomechanical memory of preceding cycles of the direct-reverse transformation. It is also found that nonhydrostatic compressive internal stresses promote melting in contrast to hydrostatic pressure. Sixteen theoretical predictions are in qualitative and quantitative agreement with experiments conducted on the PTs in the energetic crystal HMX. In particular, (a) the energy of internal stresses is sufficient to reduce the melting temperature from 551 to 430 K for the delta phase during the beta --> delta PT and from 520 to 400 K for the beta phase during the delta --> beta PT; (b) predicted activation energies for direct and reverse PTs coincide with corresponding melting energies of the beta and delta phases and with the experimental values; (c) the temperature dependence of the rate constant is determined by the heat of fusion, for both direct and reverse PTs; results b and c are obtained both for overall kinetics and for interface propagation; (d) considerable nanocracking, homogeneously distributed in the transformed material, accompanies the PT, as predicted by theory; (e) the nanocracking does not change the PT thermodynamics or kinetics appreciably for the first and the second PT beta <--> delta cycles, as predicted by theory; (f) beta <--> delta PTs start at a very small driving force (in contrast to all known solid-solid transformations with large transformation strain), that is, elastic energy and athermal interface friction must be negligible; (g) beta --> alpha and alpha --> beta PTs, which are thermodynamically possible in the temperature range 382.4 < theta < 430 K and below 382.4 K, respectively, do not occur.
我们从理论上预测了一种新现象，即具有大转变应变的固 - 固相变（PT）可以通过在远低于熔点（超过100K）的温度下沿着界面的内应力诱导的虚拟熔化发生。我们表明，由转变应变引起的弹性应力能量增加了熔化驱动力并降低了熔化温度。熔化后，应力立即松弛，不稳定的熔体凝固。薄层中的快速凝固导致纳米级裂纹，这并不影响固 - 固转变的热力学或动力学。因此，虚拟熔化代表了固 - 固PT、应力松弛以及在移动的固 - 固界面处失去相干性的一种新机制。它还消除了非热界面摩擦，并消除了正 - 逆转变先前循环的热机械记忆。还发现与静水压力相反，非静水压缩内应力促进熔化。十六个理论预测与在含能晶体HMX中的PT所进行的实验在定性和定量上一致。特别是，（a）在β→δ相变期间，内应力能量足以将δ相的熔化温度从551K降低到430K，在δ→β相变期间，将β相的熔化温度从520K降低到400K；（b）预测的正、逆PT的活化能与β相和δ相相应的熔化能以及实验值一致；（c）正、逆PT的速率常数的温度依赖性均由熔化热决定；结果b和c对于整体动力学以及界面传播均得到；（d）如理论所预测的，相变伴随着在转变材料中均匀分布的大量纳米裂纹；（e）如理论所预测的，对于第一和第二β⇄δ相变循环，纳米裂纹不会明显改变PT的热力学或动力学；（f）β⇄δ相变在非常小的驱动力下开始（与所有已知的具有大转变应变的固 - 固转变相反），也就是说，弹性能量和非热界面摩擦必须可以忽略不计；（g）分别在382.4＜θ＜430K和低于382.4K的温度范围内热力学上可能的β→α和α→β相变不会发生。
