Multiscale analysis of crystalline defect formation in rapid solidification of pure aluminium and aluminium–copper alloys

Rapid solidification leads to unique microstructural features, where a less studied topic is the formation of various crystalline defects, including high dislocation densities, as well as gradients and splitting of the crystalline orientation. As these defects critically affect the material’s mechanical properties and performance features, it is important to understand the defect formation mechanisms, and how they depend on the solidification conditions and alloying. To illuminate the formation mechanisms of the rapid solidification induced crystalline defects, we conduct a multiscale modelling analysis consisting of bond-order potential-based molecular dynamics (MD), phase field crystal-based amplitude expansion simulations, and sequentially coupled phase field–crystal plasticity simulations. The resulting dislocation densities are quantified and compared to past experiments. The atomistic approaches (MD, PFC) can be used to calibrate continuum level crystal plasticity models, and the framework adds mechanistic insights arising from the multiscale analysis. This article is part of the theme issue ‘Transport phenomena in complex systems (part 2)’.

The Growth Mechanism of Lithium Dendrites and its Coupling to Mechanical Stress

Operando high-resolution light microscopy with extended depth of field is used to observe large regions of an electrode during electrodeposition of lithium. The analysis of the morphology of the evolving deposit reveals that besides electrochemistry, mechanics and crystalline defects play a major role in the growth mechanism. Based on the findings, a growth mechanism is proposed that involves the diffusion of lithium atoms from the lithium surface into grain boundaries and the insertion into crystalline defects in the metal. Crystalline defects are a result of plastic deformation and hence mechanical stimulation augments the insertion of lithium.

Application and Optimization of Automated ECCI Mapping to the Analysis of Lowly Defective Epitaxial Films on Blanket or Patterned Wafers

The physical limits of CMOS scaling, as predicted by Moore's Law, should have already been reached several years ago. However, the scaling of transistors is still ongoing due to continuous improvements in material quality enabling the fabrication of complex device structures with nm-size dimensions. More than ever, the structural properties and the eventual presence of crystalline defects in the various semiconductor materials (SiGe, III/V) play a critical role. Electron channeling contrast imaging (ECCI) is a powerful defect analysis technique developed in recent years. The technique allows for fast and non-destructive characterizations with the potential for extremely low detection limits. The analysis of lowly defective materials requires measurements over large areas to obtain statistically relevant data. Automated ECCI mapping routines enable the quantification of crystalline defect densities as low as ~1e5 cm-2, e.g., Si0.75Ge0.25 strain relaxed buffers (SRB) epitaxially grown on a Si substrate. Methods to reduce the total measurement time without compromising its sensitivity will be discussed. The measurement routine has also been optimized to detect extended crystalline defects in III/V layers, selectively grown on shallow trench isolation patterned Si wafers. Throughout these examples, this study demonstrates the great potential of ECCI as a versatile and industry-relevant technique for defect analysis.

Welding of high-entropy alloys and compositionally complex alloys—an overview

High-entropy alloys (HEAs) and compositionally complex alloys (CCAs) represent new classes of materials containing five or more alloying elements (concentration of each element ranging from 5 to 35 at. %). In the present study, HEAs are defined as single-phase solid solutions; CCAs contain at least two phases. The alloy concept of HEAs/CCAs is fundamentally different from most conventional alloys and promises interesting properties for industrial applications (e.g., to overcome the strength-ductility trade-off). To date, little attention has been paid to the weldability of HEAs/CCAs encompassing effects on the welding metallurgy. It remains open whether welding of HEAs/CCAs may lead to the formation of brittle intermetallics and promote elemental segregation at crystalline defects. The effect on the weld joint properties (strength, corrosion resistance) must be investigated. The weld metal and heat-affected zone in conventional alloys are characterized by non-equilibrium microstructural evolutions that most probably occur in HEAs/CCAs. The corresponding weldability has not yet been studied in detail in the literature, and the existing information is not documented in a comprehensive way. Therefore, this study summarizes the most important results on the welding of HEAs/CCAs and their weld joint properties, classified by HEA/CCA type (focused on CoCrFeMnNi and AlxCoCrCuyFeNi system) and welding process.