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The effect of fluctuation microscopy constraints on reverse Monte Carlo models of metallic glass structure.

机译：波动显微镜约束对金属玻璃结构反向蒙特卡洛模型的影响。

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摘要

Although discernible atomic ordering in amorphous materials is not thought to extend much beyond two nanometers, such length scales are beyond the limits of what a two-body correlation from x-ray and neutron scattering experiments can clearly illuminate. In order to incorporate nanometer scale order into current models of amorphous solids, we must turn to experimental characterization techniques that are sensitive to higher order correlation functions.;One such technique is fluctuation transmission electron microscopy (FEM) which is sensitive simultaneously to two-, three-, and four-body correlation functions and has been shown to successfully differentiate between models of amorphous materials that differ on the nanometer scale. We set out to generate a simulated atomic model of a metallic glass that is not only consistent with a number of complementary experimental two-body scattering spectra, but also with measured fluctuation electron microscopy patterns using reverse Monte Carlo computer simulations.;This computational technique refines an atomic structure until it is consistent with experimental scattering spectra within errors without introducing any structure into the model a priori. Resonant x-ray scattering, neutron scattering, and FEM spectra were measured from Pd40Ni 40P20 bulk metallic glass samples and used as constraints on RMC simulations. To determine the influence of the higher order correlation functions from FEM on an atomic simulation, models were run with only two-body constraints and with two-body plus FEM constraints. It was found that a RMC model constrained only by two body-correlations could reproduce qualitative features of the FEM experiment. The two models turned out to be nearly statistically equivalent out to the nanometer scale, suggesting that the short range structural information contained in two-body functions is sufficient to produce nanometer-scale order.;Reverse Monte Carlo was also used to successfully identify subtle structural differences between Pd40Ni40P20 bulk samples quenched at cooling rates. Both samples displayed mostly icosahedral local ordering, with the slowly cooled sample model showing sharpened structural features in the first and second coordination shells. This observation is consistent with the notion that high cooling rates applied to metallic glasses quench in a more disordered atomic structure.
机译：尽管人们认为非晶态材料中可辨别的原子序号不会扩展到超过两纳米，但是这样的长度尺度却超出了x射线和中子散射实验的两体相关性可以清晰阐明的范围。为了将纳米级的量级纳入当前的无定形固体模型中，我们必须转向对高阶相关函数敏感的实验表征技术。其中一种技术是对两个原子同时敏感的波动透射电子显微镜（FEM）。三体和四体相关函数，已被证明可以成功地区分纳米尺度上不同的非晶材料模型。我们着手生成一个金属玻璃的模拟原子模型，该模型不仅与许多互补的实验性两体散射光谱一致，而且与反向蒙特卡洛计算机仿真所测得的波动电子显微镜图样一致。原子结构，直到与误差内的实验散射光谱一致为止，而无需先验地将任何结构引入模型。从Pd40Ni 40P20大块金属玻璃样品中测量了共振X射线散射，中子散射和FEM光谱，并将其用作RMC模拟的约束。为了确定FEM的高阶相关函数对原子模拟的影响，仅在两体约束以及两体加FEM约束下运行模型。结果发现，仅受两个身体相关性约束的RMC模型可以重现FEM实验的定性特征。两种模型在统计学上几乎等效于纳米尺度，这表明包含在两体函数中的短程结构信息足以产生纳米尺度的顺序。反向蒙特卡洛法也用于成功地识别微妙的结构Pd40Ni40P20大块样品之间的差异在冷却速率下淬火。两种样品均显示出大多数二十面体局部有序，而缓慢冷却的样品模型则显示出第一和第二配位壳的锐化结构特征。该观察结果与这样的观点是一致的，即应用于金属玻璃的高冷却速率会淬灭更无序的原子结构。

著录项

  • 作者

    Hruszkewycz, Stephan O.;

  • 作者单位

    The Johns Hopkins University.;

  • 授予单位 The Johns Hopkins University.;
  • 学科 Physics Condensed Matter.;Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2009
  • 页码 244 p.
  • 总页数 244
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 工程材料学;
  • 关键词

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