Aluminum nanoscale order in amorphous Al92Sm8 measured by fluctuation electron microscopy

Abstract

When annealed below T g , the melt-spun material exhibits a steadily decreasing nucleation rate, 4 which suggests heterogeneous nucleation. 5 The nucleation site has so far eluded structural or chemical detection, ruling out common sites like second phase interfaces or large impurity clusters. This suggests that the nucleation sites in quenched Al 92 Sm 8 may be a form of nanometer-length structure or medium-range order ͑MRO͒. Such structure is difficult to detect in amorphous materials using conventional techniques such as x-ray diffraction. 6 Here, we report fluctuation electron microscopy ͑FEM͒ ͑Refs. 7 and 8͒ measurements and simulations which find MRO associated with primary crystallization in amorphous Al 92 Sm 8 .FEM measures diffraction from nanoscale volumes using dark-field transmission electron microscopy ͑TEM͒ at a deliberately low ͑5-50 Å͒ image resolution. The magnitude of the spatial fluctuations in diffraction, measured by the normalized variance V as a function of scattering vector k, gives information about MRO at the length scale of the image resolution. 7,9 V͑k͒ depends on the three-and four-body atomic position correlation functions. 9 Peaks in V͑k͒ give information about the type of MRO from their position in k and the degree of MRO from their height. A polycrystalline sample is an extreme example of order: in a dark-field image each grain will appear brightest when it satisfies a Bragg condition in k, leading to a high peaks in V͑k͒ at the crystal reciprocal lattice k's.Samples of amorphous Al 92 Sm 8 were prepared by rapid quenching in a single wheel melt spinner at a tangential wheel speed of 55 m / s and by cold-rolling elemental foil multilayers at a 0.003 s −1 strain rate. Melt-spun ribbon samples were annealed at 130°C ͑ϽT g of 171°C͒ 3,10 under vacuum. TEM samples were prepared by electropolishing only, as ion milling can introduce spurious peaks in V͑k͒ of amorphous metals. 11 FEM was done in hollow-cone darkfield mode on a LEO 912 EFTEM at 120 kV and 16 Å resolution. Each V͑k͒ data set is the mean of measurements from at least seven areas of the sample, quoted with one standard deviation of the mean error bars.

Fluctuation Transmission Electron Microscopy: Detecting Nanoscale Order in Disordered Structures

“…Aluminum-like MRO was detected from a melt-spun Al 92 Sm 8 sample, whereas no Al-like order was detected from a sample amorphized by plastic deformation. [31] A small change in composition, such as adding a few percent of Ta into a Zr-Cu-Ni-Al alloy [32] [23] may change the MRO. It has also been found that the thinning method used to prepare the TEM sample may affect the MRO-ion thinning induces more structural order than electropolishing.…”

Interfacial mixing of nickel vanadium multilayers induced by cold rolling

“…The unique capability of MA is exemplified by several novel structures such as the extension of solid solubility in systems that are immiscible in equilibrium [2][3][4], the synthesis of metallic glass [5] and different kinetic pathways in solid solution formation [6]. These novel structures are not initiated by the usual thermally activated diffusion.…”

Electron beam induced crystallization of amorphous Al-based alloys in the TEM

“…Heterogeneous nucleation requires a high density of nanoscale structural precursor sites to account for the large number of nucleation events. We have recently shown using fluctuation electron microscopy (FEM) that one amorphous Al alloy, Al 92 Sm 8 , contains nanoscale aluminum-like order, which may be that heterogeneous nucleation site [9].…”

“…Initially, FEM was used to study amorphous semiconductors [10,15,16], but recently it has been used to study amorphous oxides [17,18], amorphous carbon films [19], and amorphous metals [9,20].…”