Direct observation of atomic structure disordering in Mn+1AXn phases

Feb 27, 2019
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Ion irradiation is one means of producing such disorder via the production of large numbers of defects. Therefore, ion beam irradiation is commonly used to simulate radiation effects in materials used in advanced nuclear energy systems. Ion beam irradiation can also be used to synthesize materials with unique properties. Highly ordered, compositionally complex ceramics, such as Mn+1AXn phases, are especially prone to disordering under irradiation. Understanding the mechanism of the order-to-disorder transformation in such materials is critical to their technological applications. However, this disorder is difficult to observe because it is complex and occurs at the atomic scale.

In previous studies, it was reported that ion/neutron irradiation drives formation of new fcc-structured phases in Mn+1AXn phases. Conventional wisdom suggests that this atomic structure evolution is driven by decomposition from the initial ternary hexagonal Mn+1AXn phases to binary MX phases, with A atoms out-diffusing to grain boundaries or surfaces of the materials (Acta Mater., 2015, 85, 132–143; Acta Mater., 2014, 66: 317-325). Although the migration energies of A atoms in these materials are relatively low, it is still hard to believe that such a large amount of A atoms can be pushed out of the material, particularly at relatively low radiation doses. However, because conventional high-resolution transmission electron microscopy (HRTEM) imaging only illustrates the atomic stacking sequences of the cations in the materials (M and A atoms, but not X), and cannot distinguish M from A atoms, the precise nature of this transformation could not previously be determined (Scr. Mater., 2017, 133, 19-23; J. Am. Ceram. Soc., 2016, 99(5), 1769-1777; Acta Mater., 2015, 98, 197-205). This left two main questions: (1) Where do A atoms go when fcc-structured phases are formed? (2) Where are the X anions located after irradiation?

In order to answer these questions, we used an advanced imaging technique, scanning TEM (STEM) high-angle annual dark-field (HAADF) and annual bright-field (ABF) imaging. These techniques enable direct atom-by-atom imaging and chemical identification in Mn+1AXn phases. The intensity, or brightness, of atomic columns imaged by STEM HAADF and ABF are proportional to ~Z2 and ~Z0.33, respectively, making STEM HAADF ABF imaging sensitive to relatively heavy atoms (like the cations M and A) and light atoms (like the anion X), respectively.

As shown in the figure below, atoms in the Ti layers of Ti3AlC2 are brighter than those in the Al layers of a HAADF image (Fig. c). The C anions can be clearly observed between the Ti layers in the ABF image (Fig. f). In contrast, in the irradiation-induced fcc phase, the contrast of each atomic column in the HAADF image (Fig. k) becomes identical. This means that Ti and Al are mixed onto the same cation sites, demonstrating disorder in the form of Ti/Al cationic antisite defects. Furthermore, the ABF image (Fig. n) shows that C anions have not been lost, and are instead located at the octahedral sites between the rearranged cation sites. Irradiation triggers the formation of cationic antisite defects, anionic rearrangement, and also the accumulation stacking faults, driving the phase transformation from the initial hexagonal phase to a new fcc phase, i.e. an fcc-(Mn+1A)Xn phase.

Supporting this interpretation, atom probe tomography (APT) results show that all the individual elements (M, A, and X atoms) are uniformly distributed throughout the material. This confirms the solid solution nature of the fcc-(Mn+1A)Xn phase and excludes the previously-hypothesized possibility of irradiation-induced decomposition.

Our study reveals the underlying, atomic-scale mechanism of irradiation-induced order-to-disorder phase transformations process in Mn+1AXn materials. These findings provide a starting point for the optimization of material performance under extreme radiation environments, such as those encountered in nuclear reactors, while also demonstrating a new strategy for the synthesis of new derivatives of the Mn+1AXn phases with tailored structural disorder. Furthermore, the STEM HAADF and ABF imaging techniques used in our study are powerful and suitable for directly observing atomic order/disorder in many other materials, such as the complex ceramics used in Li batteries, thermoelectrics, and nuclear waste forms. In future experiments, we will investigate the phase stability of the new fcc-(Mn+1A)Xn phases at high temperature and response of Mn+1AXn phases to higher energy radiation.

Our study has been published in Nature Communications, and can be read and downloaded from the link below:

https://www.nature.com/articles/s41467-019-08588-1

Chenxu Wang

Postdoc, Stanford University

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