Deformation Mechanisms of NiP/Ni Composite Coatings on Ductile Substrates

NiP/Ni composite coatings with different thicknesses were prepared on coarse-grained Ni substrates by electrodeposition. The tensile tests show that compared with the substrate, the toughness and strength of the samples with multilayer composite coatings are greatly improved. The uniform elongation is increased from 24% to 43%, and the yield strength is increased from 108 to 172 MPa. In the deformation process, the geometrically necessary dislocations accumulate, resulting in long-range back stress, leading to strain hardening, showing synergistic strength and ductility. The mechanical properties of composite coatings are strongly affected by the layer thickness. Molecular dynamics studies show that there is a more uniform distribution of the shear strain in thinner coatings, and the propagation of shear transformation zones (STZs) is restrained, preventing the formation of a large shear band. With the decrease of thickness, the deformation of the NiP layer changes from shear fracture to the coexistence of uniform deformation and shear deformation. The interface resistance of the multilayer structure increases the resistance of crack propagation and alleviates the effects of NiP layer cracking on substrate cracking. Multilayer amorphous/crystalline coatings therefore may increase the toughness of the Ni substrate.

Controlling shear band instability by nanoscale heterogeneities in metallic nanoglasses

Strain localization during plastic deformation drastically reduces the shear band stability in metallic glasses, ultimately leading to catastrophic failure. Therefore, improving the plasticity of metallic glasses has been a long-standing goal for several decades. In this regard, nanoglass, a novel type of metallic glass, has been proposed to exhibit differences in short and medium range order at the interfacial regions, which could promote the formation of shear transformation zones. In the present work, by introducing heterogeneities at the nanoscale, both crystalline and amorphous, significant improvements in plasticity are realized in micro-compression tests. Both amorphous and crystalline dispersions resulted in smaller strain bursts during plastic deformation. The yield strength is found to increase significantly in Cu–Zr nanoglasses compared to the corresponding conventional metallic glasses. The reasons for the mechanical behavior and the importance of nanoscale dispersions to tailor the properties is discussed in detail. Graphic Abstract

Collisions between amorphous carbon nanoparticles: phase transformations

Context. Collisions of nanoparticles (NPs) occur in dust clouds and protoplanetary disks. Aims. Sticking collisions lead to the growth of NPs, in contrast to bouncing or even fragmentation events and we aim to explore these processes in amorphous carbon NPs. Methods. Using molecular-dynamics simulations, we studied central collisions between amorphous carbon NPs that had radii in the range of 6.5–20 nm and velocities of 100–3000 m s−1, and with varying sp3 content (20–55%). Results. We find that the collisions are always sticking. The contact radius formed surpasses the estimate provided by the traditional Johnson-Kendall-Roberts model, pointing at the dominant influence of attractive forces between the NPs. Plasticity occurs via shear-transformation zones. In addition, we find bond rearrangements in the collision zone. Low-sp3 material (sp3 ≤ 40%) is compressed to sp3 > 50%. On the other hand, for the highest sp3 fraction, 55%, graphitization starts in the collision zone leading to low-density and even porous material. Conclusions. Collisions of amorphous carbon NPs lead to an increased porosity, atomic surface roughness, and changed hybridization that affect the mechanical and optical properties of the collided NPs.

Effect of Annealing on Strain Rate Sensitivity of Metallic Glass under Nanoindentation

The influence of isothermal annealing on the strain rate sensitivity (SRS) of a Zr-based bulk metallic glass (BMG) was investigated by nanoindentation. A more positive SRS is observed with a decrease in the content of the free volume (FV) of the sample. Furthermore, the SRS becomes nearly constant with increasing annealing time when the FV is annealed out. By taking into consideration the FV-assisted activation and combination of the shear transformation zones (STZs), the underlying mechanism is well understood. The current work may offer useful insights into the correlation between the microstructure and mechanical properties of BMGs.

Soft Ferromagnetic Bulk Metallic Glass with Potential Self-Healing Ability

A new concept of soft ferromagnetic bulk metallic glass (BMG) with self-healing ability is proposed. The specific [Fe36Co36B19.2Si4.8Nb4]100−x(Ga)x (x = 0, 0.5, 1 and1.5) BMGs prepared by copper mold casting were investigated as a function of Ga content. The Ga-containing BMGs still hold soft magnetic properties and exhibit large plastic strain of 1.53% in compression. Local melting during shearing produces molten droplets of several µm size throughout the fracture surface. This concept of local melting during shearing can be utilized to produce BMGs with self-healing ability. The molten regions play a vital role in deflecting shear transformation zones, thereby enhancing the mechanical properties.

Effect of Hydrogen Charging on Pop-in Behavior of a Zr-Based Metallic Glass

In this work, structural and mechanical properties of hydrogen-charged metallic glass are studied to evaluate the effect of hydrogen on early plasticity. Hydrogen is introduced into samples of a Zr-based (Vit 105) metallic glass using electrochemical charging. Nanoindentation tests reveal a clear increase in modulus and hardness as well as in the load of the first pop-in with increasing hydrogen content. At the same time, the probability of a pop-in occurring decreases, indicating that hydrogen hinders the onset of plastic instabilities while allowing local homogeneous deformation. The hydrogen-induced stiffening and hardening is rationalized by hydrogen stabilization of shear transformation zones (STZs) in the amorphous structure, while the improved ductility is attributed to the change in the spatial correlation of the STZs.