Linear Friction Welding of an AZ91 Magnesium Alloy and the Effect of Ca Additions on the Weld Characteristics.

Solid-state welding offers distinct advantages for joining reactive materials, such as magnesium (Mg) and its alloys. This study investigates the effect of linear friction welding (LFW) on the microstructure and mechanical properties of cast AZ91 (Mg-9Al-1Zn) and AZ91-2Ca alloys, which (to the best knowledge of the authors) has not been reported in the literature. Using the same set of LFW process parameters, similar alloy joints-namely, AZ91/AZ91 and AZ91-2Ca/AZ91-2Ca-were manufactured and found to exhibit integral bonding at the interface without defects, such as porosity, inclusions, and/or cracking. Microstructural examination of the AZ91/AZ91 joint revealed dissolution of the Al-rich second phase in the weld zone, while the Mn containing phases remained and were refined. In the AZ91-2Ca/AZ91-2Ca joint, the weld zone retained Ca- and Mn-rich phases, which were also refined due to the LFW process. In both joint types, extensive recrystallization occurred during LFW, as evidenced by the refinement of the grains from ~1000 µm in the base materials to roughly 2-6 µm in the weld zone. These microstructural changes in the AZ91/AZ91 and AZ91-2Ca/AZ91-2Ca joints increased the hardness in the weld zone by 32%. The use of digital image correlation for strain mapping along the sample gage length during tensile testing revealed that the local strains were about 50% lower in the weld zone relative to the AZ91 and AZ91-2Ca base materials. This points to the higher strength of the weld zone in the AZ91/AZ91 and AZ91-2Ca/AZ91-2Ca joints due to the fine grain size, second phase refinement, and strong basal texture. Final fracture during tensile loading of both joints occurred in the base materials.

Atomic simulations of (101¯2), (101¯1) twinning and (101¯2) detwinning in magnesium.

(101¯2) and (101¯1) twinning and (101¯2) detwinning for Mg were investigated from the viewpoint of mobility of twinning dislocations and atomic shuffling. First-principles calculations suggested that the twinning dislocations glide more readily for the (101¯1) twinning than for the (101¯2) twinning. However, this conflicts with the experimental fact of easier (101¯2) twin formation. On the other hand, molecular dynamics simulations showed that the atomic shuffling was more activated for the (101¯2) twinning than for the(101¯1) twinning, which corresponds to the experimental fact. Therefore, it is suggested that the rate-controlling process for the twin formation is the atomic shuffling. Moreover, the calculations and simulations showed that the twinning dislocations glide more readily for the (101¯2) detwinning than for the (101¯2) twinning, whereas the atomic shuffling is less activated for the detwinning, suggesting that the detwinning occurs easily but is unstable, resulting in easy repetition of twinning-detwinning.

Fabrication of Mg alloy tubes for biodegradable stent application.

Though Mg alloys are promising candidates for biodegradable stents, it is very difficult to fabricate stent tubes with high dimensional accuracy using Mg alloys because of their low deformability. This study aimed to develop thin-walled, high-quality Mg alloy tubes with good performance in stent applications. Cold drawing with a fixed mandrel was carried out for extruded Mg-0.8%Ca and AZ61 alloy tubes using optimized drawing parameters and lubrication, and stent tubes with 1.5-1.8mm outer diameter and 150 µm thickness were fabricated. A dimensional evaluation showed that the tube dimensional errors were within 0.02-2.5%. Also, an immersion test of pure Mg with different crystal orientations showed that the crystal orientation affected the corrosion properties, results that are the same with other Mg alloys. The crystal orientation of the stent tube could be controlled by changing the deformation amount and direction in the drawing, showing that it is possible to further improve the biodegradability of stents by approaching their fabrication from a processing aspect.

Softening due to disordered grain boundaries in nanocrystalline Co.

Nanocrystalline Co consisting of fcc and hcp phases was processed by electrodeposition, and its mechanical properties were investigated by hardness tests. In addition, high-resolution transmission electron microscopy observations and molecular dynamics (MD) simulations were performed to investigate the grain boundary structure and dislocation nucleation from the grain boundaries. A large amount of disorders existed at the grain boundaries and stacking faults were formed from the grain boundaries in the as-deposited Co specimen. The as-deposited specimen showed a lower hardness than did the annealed specimen, although the grain size of the former was smaller than that of the latter. The activation volume of the as-deposited specimen (=1.5b(3)) was lower than that of the annealed specimen (=50b(3)), thus indicating that nucleation of dislocations from grain boundaries is more active in the as-deposited specimen than in the annealed specimens. The MD simulations showed that dislocation nucleation was closely related to a change in the defect structures at the boundary. Therefore, it is suggested that a significant amount of defects enhance changes in the defect structures at the boundary, resulting in softening of the as-deposited specimen.

Hydrogen embrittlement in a magnesium grain boundary: a first-principles study.

First-principles fully relaxed tensile and shear test simulations were performed on Σ10(1124)/[1100] tilt Mg grain boundary (GB) models, with and without H segregation, to investigate mechanisms of H embrittlement of Mg. Strengthening as a result of covalent-like characteristics of Mg-H bonds prevailed over weakening of Mg-Mg bonds resulting from charge transfer; as a result, an H atom strengthened the GB. In addition, because the strong Mg-H bonds suppressed macroscopic GB fracture, elongation to failure was not reduced by H segregation. However, the resistance to GB shearing was increased by H segregation. It is therefore suggested that H segregation enhances crack growth at the GB, because dislocation emission from the crack tip is suppressed, resulting in H embrittlement of Mg.

Electrochemical stability of self-assembled monolayers on nanoporous Au.

Desorption of thiolate self-assembled monolayers (SAMs) seriously limits the fabrication of thiol-based devices. Here we demonstrate that nanoporous Au produced by dealloying Au-Ag alloys exhibits high electrochemical stability against thiolate desorption. Nanoporous Au has many defective sites, lattice strain and residual Ag on the ligament surface. First-principles calculations indicate that these surface aspects increase the binding energy between a SAM and the surface of nanoporous Au.