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

9816617Fultz 35年来，穆斯堡尔效应在材料科学中的几乎所有应用都利用了它作为光谱的能力。许多研究人员已经确定并量化了穆斯堡尔原子不同晶相或化学环境中的光谱成分。然而，穆斯堡尔散射也可以是相干的，这使得它能够用于材料的第四种类型的衍射实验(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光谱的选择性。本程序通过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。穆斯堡尔谱学的化学灵敏度被用来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。一种基于电荷耦合器件的区域探测器现已问世，法兰克福的一个小组正在开发一种新型的充气区域探测器，目前正在申请资金。对化学有序不完善的57Fe3Al和57FeRh合金进行了研究。在这些研究中，测量了不规则化学环境的空间周期性(DO_3和B_2有序结构中的Fe位以外的Fe位)。使用法兰克福探测器，应该可以测量准晶Al-Cu-Fe合金中不同Fe环境下的衍射图。如果探测器问题得到解决，在本研究过程中将进行一些穆斯堡尔衍射的同步辐射实验。在过去的几年里，第三代同步加速器的进步使测量伴随声子激发的核激发成为可能。这些非弹谱可以用来获得材料中铁原子态的声子偏密度。这项关于非弹性核共振散射的工作将说明Fe原子的振动如何对不同合金相的振动熵做出贡献。最近的非弹性核共振散射表明，Fe3Al的振动熵几乎完全依赖于化学短程有序(而不是长程有序)。这项建议描述了如何测量Fe原子的振动如何依赖于Pt3Fe和FeRh中的化学顺序，Fe振动如何依赖于FeAl中的点缺陷浓度，以及准晶和晶态Al-Cu-Fe的Fe振动如何不同。%在大约25,000篇关于材料的穆斯堡尔谱学研究的文章之后，穆斯堡尔效应得到了很好的证实，并非常适合于材料科学的许多研究。将其推广到非弹性核共振光谱学，对于研究中子方法不切实际的薄膜等小样品中的原子振动应该是有用的。三种衍射法(X射线、电子和中子)的互补性支持了它们在材料科学中的广泛使用。穆斯堡尔衍射法还有第四种方法的空间。

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.