The Structure and Entropy of Alloys Studied by Mossbauer Diffractometry and Nuclear Resonant Scattering

9816617 Fultz 35年来，几乎所有的穆斯堡尔效应在材料科学中的应用都利用了它作为光谱学的能力。 许多研究者已经从穆斯堡尔原子的不同晶相或化学环境中识别和量化了光谱成分。 然而，穆斯堡尔散射也可以是相干的，使其能够用于材料的第四种衍射实验（其他三种是X射线，电子和中子衍射）。 穆斯堡尔衍射的独特之处在于它的光谱选择性。 本课程利用穆斯堡尔衍射的化学环境选择性来研究金属合金中的无序。 穆斯堡尔光谱法的化学灵敏度用于选择特定化学环境中的原子，然后从具有该特定环境的原子测量衍射图案。 现在已有一种以电荷耦合器件摄影机为基础的区域探测器，法兰克福的一个小组正在为研制一种新型的充气区域探测器申请资金。 研究了具有不完全化学有序的57 Fe 3Al和57 FeRh合金。 在这些研究中，测量的空间周期性的不规则的化学环境（Fe网站以外的Fe网站的DO 3和B2有序结构）。 使用法兰克福探测器，可以测量准晶Al-Cu-Fe合金中不同Fe环境的衍射图样。 如果探测器的问题得到解决，一些同步辐射实验穆斯堡尔衍射将在本研究过程中进行。 在过去的几年里，第三代同步辐射源的进步使得测量伴随声子激发的核激发成为可能。 这些非弹性谱可用于获得材料中Fe原子的声子分态密度。 非弹性核共振散射的这项工作将显示如何振动的Fe原子有助于不同的合金相的振动熵。 最近的非弹性核共振散射表明，Fe 3Al的振动熵几乎完全取决于化学短程有序（而不是长程有序）。 该建议描述了要进行的测量，即Fe原子的振动如何取决于Pt 3Fe和FeRh中的化学顺序，Fe振动如何取决于FeAl中的点缺陷浓度，以及Fe振动如何不同于准晶和晶态Al-Cu-Fe。在大约25，000篇关于材料的穆斯堡尔谱研究的出版物之后，穆斯堡尔效应得到了很好的建立，并且非常适合材料科学中的许多研究。 它的扩展到非弹性核共振光谱法应该是有用的小样品，如薄膜的中子方法是不实用的原子振动的研究。 三种衍射方法（X射线、电子和中子）的互补性使它们在材料科学中得到了广泛的应用。 还有第四种穆斯堡尔衍射法的空间。

9816617FultzFor 35 years, almost all applications of the Mossbauer effect in materials science have utilized its capabilities as a spectroscopy. Many investigators have identified and quantified spectral components from different crystallographic phases or chemical environments of the Mossbauer atom. Mossbauer scattering can also be coherent, however, enabling its use for a fourth type of diffraction experiment on materials (the other three being X-ray, electron, and neutron diffraction). The unique feature of Mossbauer diffraction is its spectroscopic selectivity. This program studies disorder in metallic alloys by the chemical environment selectivity of Mossbauer diffraction. The chemical sensitivity of Mossbauer spectrometry is used to select an atom in a particular chemical environment, and a diffraction pattern is then measured from atoms having that particular environment. An area detector based on a CCD camera is now available, and funds are requested for a new type of gas-filled area detector developed by a group in Frankfurt. Studies are on alloys of 57Fe3Al and 57FeRh having imperfect chemical order. In these studies, measurements are made of the spatial periodicities of irregular chemical environments (Fe sites other than the Fe sites of the DO3 and B2 ordered structures). With the Frankfurt detector, it should be possible to measure diffraction patterns from different Fe environments in a quasicrystalline Al-Cu-Fe alloy. If detector issues are resolved, some synchrotron experiments on Mossbauer diffraction will be performed during the course of this research. In the last couple of years, advances at third generation synchrotron sources have made it possible to measure nuclear excitations accompanied by phonon excitations. These inelastic spectra can be used to obtain the phonon partial densities of states of Fe atoms in the material. This work on inelastic nuclear resonant scattering will show how vibrations of Fe atoms contribute to the vibrational entropy of different alloy phases. Inelastic nuclear resonant scattering recently showed that the vibrational entropy of Fe3Al depends almost entirely on chemical short-range order (as opposed to long-range order). This proposal describes measurements to be made on how the vibrations of Fe atoms depend on chemical order in Pt3Fe and FeRh, how Fe vibrations depend on point defect concentrations in FeAl, and how Fe vibrations differ for quasicrystalline and crystalline Al-Cu-Fe.%%%After approximately 25,000 publications on Mossbauer spectrometry studies of materials, the Mossbauer effect is well established and well suited for many studies in materials science. Its extension to inelastic nuclear resonant spectrometry should be useful for studies of atom vibrations in small samples such as thin films for which neutron methods are not practical. The complementary nature of the three diffraction methods (X-ray, electron, and neutron) has sustained their widespread use in materials science. There is room for a fourth method of Mossbauer diffractometry.