Simulated Annealing Model and Martensitic Transformation

本项目的部分目的是开发从头算分子动力学(MD)方法，这种方法在半导体物理中非常成功，因此也适用于过渡金属系统。虽然还没有让分子动力学真正运行，但我们相信，从某种意义上说，我们可以朝着这个目标稳步迈出一步，这个项目真的很成功。由于上述MD方法尚未完成，在研究马氏体相变时，我们不得不使用相当传统的方法：一种是基于唯象方法，另一种是通过电子结构计算。到目前为止，该项目取得的新进展概括如下：i)我们建立了全势KKR方法，作为一种实用而方便的工具，用于金属和复杂体系的精确电子结构计算。结果表明，这一新技术在…上是可行的。更直接地应用于模拟退火法模型或分子动力学，从而使我们也能够对过渡金属执行类似CAR-Parrinello的过程。我们的初步计算表明，用这种方法处理一个含有-10个原子的晶胞，在典型的超级计算机上需要大约1分钟的CPU时间。ii)在Landau唯象理论的基础上，讨论了面心立方晶格中空间调制的第一类反铁磁结构的周期与正交晶格扭曲之间的关系。结果表明，磁调制的周期与晶格失真的周期相同或两倍。(3)为了揭示Fe-Pd系在Pd30%区附近连续马氏体相变的微观来源，我们进行了KKR-CPA-LSD电子结构计算。准确计算了面心立方和体心立方结构沿程的总能量变化(贝恩失真)。结果表明，当Pd为30%时，面心立方结构实际上变得不稳定。然而，计算未能证明FCT相的存在，该相很可能在实验上存在，即使在T=OK时也是如此；相反，它预测了从面心立方结构到体心立方结构的一级类跃迁。较少

The purpose of the present project was, partly, to develop the ab-initio molecular dynamics (MD) method, which had been so successful for semi-conductor physics, such that it was also applicable to transition metal systems. Though not yet having got the molecular dynamics actually run, the project, we believe, is really successful in the sense that we could make a steady step to this end. Since the above MD method had not been completed, we were obliged, in studying martensitic transformations, which itself formed another part of the present project, to use rather conventional methods : one based on a phenomenological approach, another by electronic structure calculations. New developments which have been made so far concerning the project are summarized in the following :i) We have established the full-potential KKR method as a realistic and convenient tool for accurate electronic structure calculation of metallic and complex systems. It was shown that this new technique was applicabl … More e to the simulated annealing model, or the molecular dynamics, rather straightforwardly, and thus enabled us to perform a Car-Parrinello-like Procedure also for transition metals. Our preliminary calculations showed that dealing with an unit cell containing-10 atoms in it by this method would cost around 1 minute of CPU time on a typical super-computer.ii) Relation between periods of spatially modulated first-kind antiferromagnetic structure and orthorhombic lattice distortion in an fcc lattice were discussed on a basis of Landau's phenomenological theory. The period of the magnetic modulation turned out either the same or twice as long as that of the lattice distortion. The magnetic structure of gamma-Fe precipitated in Cu was discussed in the light of this result.iii) To reveal the microscopic origin of the successive martensitic transformation seen in Fe-Pd system near Pd 30% region, we performed the KKR-CPA-LSD electronic structure calculation. The total energy change was calculated accurately along the pass connecting the fcc and bcc structures (Bain's distortion). The result showed that the fcc structure actually became unstable around Pd 30%. The calculation, however, failed to prove the existence of the fct phase, which is likely to exist experimentally even at T=OK ; instead it predicted a 1st order-like jump from the fcc to the bcc structure. Less