Nanoscale Solute Partitioning in Bulk Metallic Glasses

Adv. Mater. 2008, 20, 1–4 2008 WILEY-VCH Verlag Gmb A T IO N Fundamental understanding of composition variations and morphology in the nanoscale is essential to the design of advanced materials. Partial crystallization or devitrification of bulk metallic glasses (BMGs) results in novel microstructures, with high density (10–10m ) nanocrystalline precipitates evenly distributed in a glassy matrix. These crystalline precipitates are known to impede the propagation of shear bands, and are promising candidates for improving the mechanical properties of BMG alloys. However, it has been an experimental challenge to determine the fine structure of these precipitates, and no one technique can provide all the answers. In this paper, we report the experimental study of a multicomponent BMG alloy, Zr52.5Cu17.9Ni14.6Al10Ti5, utilizing several state-of-the-art characterization techniques. Nanoscale solute partitioning due to strong chemical order is revealed at unprecedented detail by a new wide-field atom probe. This level of details is crucial for understanding the interference peaks observed in small-angle X-ray and neutron scattering experiments, an unsolved mystery for over a decade. A core/shell structure is formed as a result of nanoscale solute partitioning, which poisons the growth and helps stabilize the nanocrystalline particles. Zr52.5Cu17.9Ni14.6Al10Ti5 is a widely studied BMG with excellent glass forming ability. Upon devitrification, crystalline precipitates of 10–20 nm diameter emerge, as evidenced by high-resolution transmission electron microscopy. Moreover, Z-contrast imaging, a technique more sensitive to composition distribution, showed high densities of distinct crystalline particles of similar sizes but with fuzzy boundaries. Nanoscale composition fluctuations have been detected by atom probe tomography (APT), and were attributed to nanocrystalline particles. The structure of devitrified Zr52.5Cu17.9Ni14.6Al10Ti5 has also been investigated by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). While microscopy reveals structural details in a restricted field of view or analysis volume, small-angle scattering yields the average structure over the scattering volume. SANS and SAXS profiles of Zr52.5Cu17.9Ni14.6Al10Ti5 are both characterized by an interference peak. However, there has been no satisfactory analysis of the experimental data that could identify the underlying structure. Although composition fluctuations due to spinodal decomposition can produce interference peaks, no composition wave with a characteristic wavelength was detected experimentally. Instead, well-defined crystalline particles were reported by microscopy experiments. Mathematically, the interference peak could also be generated by second-phase particles with a depleted diffusion zone. However, experimental determination of the fine-scale composition variations in zirconium-based alloys is difficult with traditional voltage-pulsed APT, due to their poor electrical conductivities and brittleness at cryogenic temperatures. As a result, the structure of nanocrystalline particles in BMG alloys has remained a mystery. We have recently conducted an experimental study using a new atom-probe equipped with a high-repetition pulsed laser, together with in situ SAXS. With these two complementary techniques, and making use of previously reported SANS data, the structure of nanocrystalline particles of devitrified Zr52.5Cu17.9Ni14.6Al10Ti5 is established. Two specimens were characterized by APT; one in the as-cast condition, and the other after isothermal annealing for 900min at 663K. For the as-cast sample, the APT analysis revealed a homogeneous microstructure characteristic of a completely amorphous state. On the other hand, the sample annealed for 900min at 663K showed a two-phase microstructure, consisting of lens-shaped precipitates and the surrounding matrix. Portions of several impinging precipitates, contained in a 70 nm 70 nm 200 nm box, were extracted from the atom-probe data, and are shown in Figure 1a. Many of the precipitates are partially obscured by others, and the full extent of the precipitates is cropped by the bounding box. The red isoconcentration surface is constructed at 52% Zr, a concentration that is representative of the precipitate/matrix interface. This isoconcentration surface reveals the lenticular shape of each precipitate. The size, position, and orientation of the atom-probe data were selected to reveal the full extent of the upper precipitate. The upper portion of the red Zr isoconcentration surface of this precipitate has been removed, in order to reveal the Al-enriched core in the interior of the precipitate. The core region is denoted by the yellow isoconcentration surface, constructed of 5.4% Al. A rough estimate indicates a precipitate number density on the order of 10 m .