Metals2016, 6(9), 200; doi:10.3390/met6090200 (registering DOI) - published 25 August 2016 Show/Hide Abstract
Abstract: Viral templates are highly versatile biotemplates used for the synthesis of nanostructured materials. Rotavirus VP6 self-assembles into nanotubular hollow structures with well-defined diameters and variable lengths, serving as a nucleic acid-free biotemplate to synthesize metal nanoparticles of controlled size, shape, and orientation. Molecular docking simulations show that exposed residues (H173-S240-D242 and N200-N310) of VP6 have the ability to specifically bind Pd(II) ions, which serve as nucleation sites for the growth and stabilization of palladium nanoclusters. Using VP6 nanotubes as biotemplates allows for obtaining small Pd particles of 1–5 nm in diameter. Advanced electron microscopy imaging and characterization through ultra-high-resolution field-emission scanning electron microscopy (UHR-FE-SEM) and spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) at a low voltage dose (80 kV) reveals, with high spatial resolution, the structure of Pd nanoparticles attached to the macromolecular biotemplates.
Metals2016, 6(9), 196; doi:10.3390/met6090196 - published 24 August 2016 Show/Hide Abstract
Abstract: Pre-stretching was carried out to modify the microstructure of Mg-Nd-Zn-Zr alloy to enhance its age strengthening. The results indicated that more heterogeneous nucleation sites can be provided by the high density of dislocations caused by the plastic pre-stretching deformation, as well as speeding up the growth rate of precipitates. Comparison of microstructure in non-pre-stretched specimens after artificial aging showed that pre-stretched specimens exhibited a higher number density of precipitates. The fine and coarse plate-shaped precipitates were found in the matrix. Due to an increase in the number density of precipitates, the dislocation slipping during the deformation process is effectively hindered, and the matrix is strengthened. The yield strength stabilizes at 4% pre-stretching condition, and then the evolution is stable within the error bars. The 8% pre-stretched specimens can achieve an ultimate tensile strength of 297 MPa. However, further pre-stretching strains after 8% cannot supply any increase in strength. Tensile fracture surfaces of specimens subjected to pre-stretching strain mainly exhibit a trans-granular cleavage fracture. This work indicated that a small amount of pre-stretching strain can further increase strength of alloy and also effectively enhance the formation of precipitates, which can expand the application fields of this alloy.
Metals2016, 6(9), 198; doi:10.3390/met6090198 - published 24 August 2016 Show/Hide Abstract
Abstract: The influence of grain growth inhibitors and powder size on the microstructure and mechanical properties of ultrafine and nanostructured cemented carbides was researched. Three different WC powders, with an addition of different type and content of grain growth inhibitors GGIs, VC and Cr3C2 and with dBET grain sizes in the range from 95 to 150 nm were selected as starting powders. Four different mixtures with 6 and 9 wt. % Co were prepared. The consolidated samples are characterized by different microstructural and mechanical properties with respect to the characteristics of starting powders. Increased sintering temperatures led to microstructural irregularities in the form of a discontinuous WC growth, carbide agglomerates and abnormal grain growth as a consequence of coalescence via grain boundary elimination. The addition of 0.45% Cr3C2 contributed to microstructure homogeneity, reduced discontinuous and continuous grain growth, and increased Vickers hardness by approximately 70 HV and fracture toughness by approximately 0.15 MN/m3/2. The reduction of the starting powder to a real nanosize of 95 nm resulted in lower densities, and significant hardness increase, with a simultaneously small increase in fracture toughness. The consolidation of real nanopowders (dBET < 100 nm) solely by conventional sintering in hydrogen without isostatic pressing is not preferred.
Metals2016, 6(9), 197; doi:10.3390/met6090197 - published 24 August 2016 Show/Hide Abstract
Abstract: In this paper, machining aeronautical aluminum alloy AA2024-T351 in dry conditions was investigated. Cutting forces, chip segmentation, and built-up edge formation were analyzed. Machining tests revealed that the chip formation process depends on cutting conditions and tool geometry. So continuous and segmented chips are generated. Under some cutting conditions, built-up edge formation occurs. A predictive machining theory, based on a finite elements method (FEM), was applied to reproduce and explain these phenomena. Thermomechanical behaviors of the work material and the tool-work material interface were considered. Results of the proposed modelling were compared to experimental data for a wide range of cutting speed. It was shown that the feed force is well reproduced by the ALE-FE (arbitrary lagrangian-eulerian finite element) formulation and highly underestimated by the lagrangian finite element (LAG-FE) one. While, the periodic localized shear band, leading to a chip segmentation, is well reproduced with the Lagrangian FE formulation. It was found that the chip segmentation can be correlated to the cutting force evolution using the defined chip segmentation intensity parameter. For the built-up edge (BUE) phenomenon, it was shown that it depends on the contact/friction at the tool-chip interface, and this is possible to simulate by making the friction coefficient time-dependent.
Metals2016, 6(9), 195; doi:10.3390/met6090195 - published 23 August 2016 Show/Hide Abstract
Abstract: A new copper–zirconium oxide was synthesized at ambient pressure in air during a thermal treatment. Its crystal structure was analyzed using X-ray Powder Diffraction, and the atomic ratio between copper and zirconium was found performing a Rietveld analysis. An accurate analysis, also comparing this new compound with others present in the literature and which present a similar structure, enables us to characterize the new mixed oxide well. Scanning electron microscopy and thermogravimetric analyses were also performed in order to completely characterize this new material, which is interesting both from an academic point of view for its crystal structure and from an industrial one due to the formation of copper–zirconium-based shape memory alloys during thermal treatment.
Metals2016, 6(8), 193; doi:10.3390/met6080193 - published 22 August 2016 Show/Hide Abstract
Abstract: The effect of titanium on the carbides and mechanical properties of martensitic stainless steel 8Cr13MoV was studied. The results showed that TiCs not only acted as nucleation sites for δ-Fe and eutectic carbides, leading to the refinement of the microstructure, but also inhibited the formation of eutectic carbides M7C3. The addition of titanium in steel also promoted the transformation of M7C3-type to M23C6-type carbides, and consequently more carbides could be dissolved into the matrix during hot processing as demonstrated by the determination of extracted carbides from the steel matrix. Meanwhile, titanium suppressed the precipitation of secondary carbides during annealing. The appropriate amount of titanium addition decreased the size and fraction of primary carbides in the as-cast ingot, and improved the mechanical properties of the annealed steel.