Liotti E, Lui A, Connolley T, Dolbnya IP, Sawhney KJS, Malandain A, Wilson MD, Veale MC, Seller P, Grant PS. Mapping of multi-elements during melting and solidification using synchrotron X-rays and pixel-based spectroscopy. Scientific Reports 5, doi:10.1038/srep15988 (2015).
Solidification; Synchrotron X-ray; Radiography; XRF; Solute distribution.
A new imaging technique was designed and tested that maps the distribution of multiple elements during a dynamic process. The dynamic process explored here is the solidification of a metal alloy, but the technique has applications throughout metallurgy and materials science.
An aluminium alloy containing silver (Ag), zirconium (Zr), and molybdenum (Mo) was melted at 700 °C, before re-solidifying. As the metal freezes, metallic crystals (grains) grow. During this freezing, the alloying elements and impurities segregate and become concentrated in the remaining, rapidly vanishing liquid. In these last-to-freeze regions, highly concentrated clusters of elements freeze into different types of tiny crystals. These give the metal an irregular microstructure which can have strong negative effects on the alloy mechanical properties and recyclability. This frozen microstructure is hard to modify, making it commercially important to understand and manipulate the way elements distribute and segregate during solidification.Techniques for studying this problem work by contrasting the amount of X-ray absorption by the different elements, and are limited to twoelement systems. However, the new technique combined X-ray fluorescence (an element’s characteristic ‘secondary’ X-rays emitted from the sample) and X-ray transmission through the sample. Pairs of 2D maps were created from the data: one showing the elements, and one showing radiographs of the crystals. The experiment was piloted on Diamond's Test Beamline (B16), which is specially designed for trialling new techniques due to its flexible setup. The new technique successfully distinguished all three elements, providing pixel-based images of their distributions at each stage of the alloy freezing.
The microstructure of many cast components is formed during the liquid to solid transformation and often cannot be significantly altered afterwards. For this reason every year millions of pounds are spent by the metal industry to try to manipulate the casting conditions in order to obtain fine-scale and isotropic crystalline grains, which are beneficial for most of the mechanical properties, and to restrict the segregation and concentration of alloy impurities. Although it is well-known that the way alloying elements and impurities move and segregate ahead of the growing grains during solidification is critical in controlling the final microstructure, there are no experimental approaches that allow direct measurement of this behaviour, especially in commercial, multielement alloys. In recent years synchrotron X-ray radiography and tomography have been extensively used to study the real-time dynamics of solidification providing both solute distribution and grain morphology information, however this information is generally available only for model binary alloys1,2. In order to overcome this limitation, a new synchrotron based elemental imaging technique that combines X-ray radiography and fluorescence spectroscopy has been developed. The technique was used to study the spatial distribution of Ag, Zr and Mo alloying elements in an Al alloy during heating, melting and then cooling and re-solidification.
Experiments were carried out at the B16 test beamline and the experimental set up is schematically shown in Fig. 1. A foil sample was positioned in a furnace cell with the flat, exposed surface positioned at 45° to the incoming unfiltered white X-ray beam. Conventional transmission radiographs were collected using an AVG Manta G125B CCD camera combined with a scintillator mounted on an optic module, while fluorescence signal was magnified using a pinhole camera arrangement and recorded using a HEXITEC3 detector module. Materials Village Beamline B16 This detector was capable of collecting X-ray fluorescence spectra at each pixel and allowed the construction of 2D maps of several elements simultaneously. The spatial resolution was 3.75 μm/pixel in radiography and 13.75 μm/pixel in fluorescence mode. Samples consisted of 0.2 mm thick foil of Al-Cu base alloy with addition of Ag, Zr and Mo and were designed to facilitate the observation of the spatial distribution of multiple elements during either solid-state heating and melting, or cooling and solidification.
Figure 1: Schematic of the experimental setup (top view).
Figure 2: Radiograph (transmission image) and fluorescence maps taken at 570 °C and 660 °C showing Ag, Zr and Mo distributions. Three regions were detected (A, B and C) that were rich in Ag (A) and Zr (B and C).
During the re-solidification, the samples were held at 700 °C until fully liquid and then the temperature decreased in steps smaller than 1 °C with chemical maps and radiographs collected at each temperature. Fig. 3 shows a radiograph of the microstructure after solidification had completed at 550 °C with the smaller HEXITEC field of view superimposed in red and the Ag, Zr and Mo fluorescence maps shown to the side. Dendritic crystals grew from approximately left to right with some variation in spacing due to convective effects and some noise in the furnace control during cooling. Inter-dendritic contrast due to elemental segregation was clear in the radiographs and the chemical maps revealed the inter-dendritic regions to be rich in Ag and comparatively denuded in Zr and Mo, information unavailable in radiographic mode.
Figure 3: A radiograph at 550 °C showing the re-solidified dendritic microstructure along with corresponding Ag, Zr and Mo fluorescence maps. The fluorescence field of view is shown in red.
A novel synchrotron radiography-fluorescence technique has been developed to study dynamic phenomena in alloys containing more than two elements, and has been applied for the study of the melting and resolidification of a Al-Cu-Ag-Zr-Mo model alloy. The technique is a step forward from existing methods commonly utilised in materials science for chemical mapping, combining some of the advantages of room temperature electron microscopy with the dynamic capabilities offered by high flux X-rays. In this initial orientation, spatio-temporal resolution was sufficient to capture some details of element diffusion and segregation during melting and solidification. The combined radiography and pixel-based spectroscopy elemental imaging technique offers potential in the field of metallurgy and materials science to study a range of high temperature diffusion and elemental segregation phenomena.
We gratefully acknowledge the beamtime provided by Diamond Light Source under experiment MT10133. The authors also acknowledge the financial support of the UK Engineering and Physical Science Research Council (EPSRC grant: Centre for Innovative Manufacturing Research on Liquid Metal Engineering, EP/H026177/1).
Dr Enzo Liotti, University of Oxford, firstname.lastname@example.org.
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