Solute segregation and deviation from bulk thermodynamics at nanoscale crystalline defects

Liquid metal synthesis solvents for metallic crystals

In nature, snowflake ice crystals arrange themselves into diverse symmetrical six-sided structures. We show an analogy of this when zinc (Zn) dissolves and crystallizes in liquid gallium (Ga). The low-melting-temperature Ga is used as a "metallic solvent" to synthesize a range of flake-like Zn crystals. We extract these metallic crystals from the liquid metal solvent by reducing its surface tension using a combination of electrocapillary modulation and vacuum filtration. The liquid metal-grown crystals feature high morphological diversity and persistent symmetry. The concept is expanded to other single and binary metal solutes and Ga-based solvents, with the growth mechanisms elucidated through ab initio simulation of interfacial stability. This strategy offers general routes for creating highly crystalline, shape-controlled metallic or multimetallic fine structures from liquid metal solvents.

The Key Role of Tin (Sn) in Microstructure and Mechanical Properties of Ti2SnC (M2AX) Thin Nanocrystalline Films and Powdered Polycrystalline Samples

Layered ternary Ti₂SnC carbides have attracted significant attention because of their advantage as a M2AX phase to bridge the gap between properties of metals and ceramics. In this study, Ti₂SnC materials were synthesized by two different methods—an unconventional low-energy ion facility (LEIF) based on Ar⁺ ion beam sputtering of the Ti, Sn, and C targets and sintering of a compressed mixture consisting of Ti, Sn, and C elemental powders up to 1250 °C. The Ti₂SnC nanocrystalline thin films obtained by LEIF were irradiated by Ar⁺ ions with an energy of 30 keV to the fluence of 1.10¹⁵ cm⁻² in order to examine their irradiation-induced resistivity. Quantitative structural analysis obtained by Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed transition from ternary Ti₂SnC to binary Ti_0.98C carbide due to irradiation-induced β-Sn surface segregation. The nanoindentation of Ti₂SnC thin nanocrystalline films and Ti₂SnC polycrystalline powders shows that irradiation did not affect significantly their mechanical properties when concerning their hardness (H) and Young’s modulus (E). We highlighted the importance of the HAADF-STEM techniques to track atomic pathways clarifying the behavior of Sn atoms at the proximity of irradiation-induced nanoscale defects in Ti₂SnC thin films.

Terraced and Three-dimensional Pyramid-shaped Polymer Single Crystal via low temperature-Assisted Microfluidic Technology

Three-dimensional pyramidal polymer single crystals provide spatial gradient variations within the crystal molecules, and these variations facilitate the study of the relationship between structure and properties within the molecules of various complexes with anisotropic structures. As described herein, we propose a low-temperature-assisted microfluidic pore channeling approach to prepare structurally ordered polymer single crystals. A mixture of dichloromethane and dimethyl sulfoxide was used as a prepolymer, and a liquid microfluidic technique was employed to grow the end-functionalized polymers into three-dimensional polymer single crystals. Through the ordered growth of single crystals, a personalized pyramidal pattern with a homogeneous structure was formed. To evaluate the mesh node density, low-temperature growth time and substrate type were also investigated. Rectangular, pyramidal, and dendritic patterns were synthesized via low-temperature single crystal growth. This work shows that low temperature-assisted microfluidics provides a novel means to tune the three-dimensional structure of polymer single crystals. This article is protected by copyright. All rights reserved.

Atomic Pathways of Solute Segregation in the Vicinity of Nanoscale Defects

Using GeSn semiconductor as a model system, this work unravels the atomic-level details of the behavior of solutes in the vicinity of a dislocation prior to surface segregation in strained, metastable thin layers. The dislocations appear in the 3D atom probe tomography maps as columnar regions, 3.5-4.0 nm wide, with solute concentrations 3-4 times higher than the sounding matrix. During the initial stage of phase separation, the migration of solute atoms toward the dislocation is associated with a gradual increase in Sn concentration and in density of atomic clusters, which reach 175-190 per 10³ nm³ with 12-15 atoms/cluster close to dislocations. The latter provide, at advanced stages, fast diffusive channels for Sn mass-transport to the surface, thus bringing the matrix around the dislocation to the equilibrium concentration. In parallel, an increase in solute concentration (∼0.05 at. %/nm) and in the number of atomic clusters (12-16 clusters/33 nm) is observed along the dislocation core.

A defect-resistant Co-Ni superalloy for 3D printing

Additive manufacturing promises a major transformation of the production of high economic value metallic materials, enabling innovative, geometrically complex designs with minimal material waste. The overarching challenge is to design alloys that are compatible with the unique additive processing conditions while maintaining material properties sufficient for the challenging environments encountered in energy, space, and nuclear applications. Here we describe a class of high strength, defect-resistant 3D printable superalloys containing approximately equal parts of Co and Ni along with Al, Cr, Ta and W that possess strengths in excess of 1.1 GPa in as-printed and post-processed forms and tensile ductilities of greater than 13% at room temperature. These alloys are amenable to crack-free 3D printing via electron beam melting (EBM) with preheat as well as selective laser melting (SLM) with limited preheat. Alloy design principles are described along with the structure and properties of EBM and SLM CoNi-base materials.

Additive manufacturing promises a major transformation of the production of high economic value metallic materials. Here, the authors describe a new class of 3D printable superalloys that are amenable to crack-free 3D printing via electron beam melting as well as selective laser melting.

Unveiling the Re effect in Ni-based single crystal superalloys

Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO₂ emissions in air-traffic.

Adding minute amounts of rhenium to Ni-based single crystal superalloys extends their high temperature performance in engines, but the reasons behind that are still unclear. Here, the authors combine high resolution imaging and modelling to show that rhenium enriches and slows down partial dislocations to improve creep performance.