The strengthening effect of voids

Strengthening gold with dispersed nanovoids

Materials often fail prematurely or catastrophically under load while containing voids, posing a challenge to materials manufacturing. We found that a metal (gold) containing spherical voids with a fraction of up to 10% does not fracture prematurely in tension when the voids are shrunk to the submicron or nanometer scale. Instead, the dispersed nanovoids increase the strength and ductility of the material while simultaneously reducing its weight. Apart from the suppressed stress or strain concentration, such structure provides enormous surface area and promotes surface-dislocation interactions, which enable strengthening and additional strain hardening and thus toughening. Transforming voids from crack-like detrimental defects into a beneficial "ingredient" provides an inexpensive and environmentally friendly approach for the development of a new class of lightweight, high-performance materials.

Targeted Additive Micromodulation of Grain Size in Nanocrystalline Copper Nanostructures by Electrohydrodynamic Redox 3D Printing

The control of materials' microstructure is both a necessity and an opportunity for micro/nanometer-scale additive manufacturing technologies. On the one hand, optimization of purity and defect density of printed metals is a prerequisite for their application in microfabrication. On the other hand, the additive approach to materials deposition with highest spatial resolution offers unique opportunities for the fabrication of materials with complex, 3D graded composition or microstructure. As a first step toward both-optimization of properties and site-specific tuning of microstructure-an overview of the wide range of microstructure accessed in pure copper (up to >99.9 at.%) by electrohydrodynamic redox 3D printing is presented, and on-the-fly modulation of grain size in copper with smallest segments ≈400 nm in length is shown. Control of microstructure and materials properties by in situ adjustment of the printing voltage is demonstrated by variation of grain size by one order of magnitude and corresponding compression strength by a factor of two. Based on transmission electron microscopy and atom probe tomography, it is suggested that the small grain size is a direct consequence of intermittent solvent drying at the growth interface at low printing voltages, while larger grains are enabled by the permanent presence of solvent at higher potentials.

The Study of Nanosized Cu–Mn Precipitates Contribution to Hardening in Body Centered Cubic Fe Matrix

In order to study the contribution of manganese (Mn) atoms in copper (Cu) precipitates to hardening in body centered cubic (BCC) structure iron (Fe) matrix, the interactions of a 1/2 〈111〉 {110} edge dislocations with nanosized Cu and Cu–Mn precipitates in BCC Fe have been investigated by using molecular dynamics method (MD). The results indicate that the critical resolved shear stresses (τc) of the Cu–Mn precipitates are larger than that of Cu precipitates. Meanwhile, τc of the Cu–Mn precipitates show a much more significant dependence on temperature and size compared to Cu precipitates. Mn atoms exhibit strong attraction to dislocation segment in Cu precipitate and improve the fraction of transformed atoms from BCC phase to nine rhombohedron (R) phase for big size precipitates. Those all lead to the higher resistance to the dislocation glide. Eventually, these features confirmed that the appearance of Mn atoms in Cu precipitates greatly facilitates the hardening in BCC Fe matrix.

Idealized vs. Realistic Microstructures: An Atomistic Simulation Case Study on γ/γ' Microstructures

Single-crystal Ni-base superalloys, consisting of a two-phase γ/γ′ microstructure, retain high strengths at elevated temperatures and are key materials for high temperature applications, like, e.g., turbine blades of aircraft engines. The lattice misfit between the γ and γ′ phases results in internal stresses, which significantly influence the deformation and creep behavior of the material. Large-scale atomistic simulations that are often used to enhance our understanding of the deformation mechanisms in such materials must accurately account for such misfit stresses. In this work, we compare the internal stresses in both idealized and experimentally-informed, i.e., more realistic, γ/γ′ microstructures. The idealized samples are generated by assuming, as is frequently done, a periodic arrangement of cube-shaped γ′ particles with planar γ/γ′ interfaces. The experimentally-informed samples are generated from two different sources to produce three different samples—the scanning electron microscopy micrograph-informed quasi-2D atomistic sample and atom probe tomography-informed stoichiometric and non-stoichiometric atomistic samples. Additionally, we compare the stress state of an idealized embedded cube microstructure with finite element simulations incorporating 3D periodic boundary conditions. Subsequently, we study the influence of the resulting stress state on the evolution of dislocation loops in the different samples. The results show that the stresses in the atomistic and finite element simulations are almost identical. Furthermore, quasi-2D boundary conditions lead to a significantly different stress state and, consequently, different evolution of the dislocation loop, when compared to samples with fully 3D boundary conditions.