Structural impact on the hall-Petch relationship in an Al-5Mg alloy processed by high-pressure torsion

The evolution of microstructure and microhardness was studied in a commercial 5483 Al-5 Mg alloy processed by high pressure torsion (HPT) under a pressure of 6.0 GPa up to 10 turns. Significant grain size refinement was observed even after 1/4 turn and additional processing led to a further grain size reduction and a shift in the distribution of grain boundary misorientation angles towards higher values. An essentially fully homogeneous microstructure was reached after 10 turns with a final grain size of ~70 nm, a saturation Vickers microhardness of Hv?240 which was attained at and above equivalent strains of ~150, a relatively narrow grain size distribution and a fraction of ~80% of high-angle grain boundaries. Analysis shows the Hall-Petch plot deviates from the conventional linear relationship for samples processed through small numbers of turns but after 3 or more turns there is a direct correlation between the results obtained in HPT processing and coarse-grained sample

Evolution of microstructure and hardness in an AZ80 magnesium alloy processed by high-pressure torsion

An AZ80 magnesium alloy with an initial grain size of ?25 ?m and a hardness of Hv ? 63 was processed by high-pressure torsion (HPT) at room temperature for up to 10 turns under an imposed pressure of 6.0 GPa. After processing, the specimens were examined by optical microscopy and transmission electron microscopy and measurements were taken of the Vickers microhardness along diameters of the HPT discs. The results show that the grains are refined to ?200 nm after 5 and 10 turns of HPT and the hardness increases to Hv ? 120 at an equivalent strain of ?30. There is a saturation condition and no further hardening at additional equivalent strains up to >200

Fast-degrading PLA/ORMOGLASS fibrous composite scaffold leads to a calcium-rich angiogenic environment

The success of scaffold implantation in acellular tissue engineering approaches relies on the ability of the material to interact properly with the biological environment. This behavior mainly depends on the design of the graft surface and, more precisely, on its capacity to biodegrade in a well-defined manner (nature of ions released, surface-to-volume ratio, dissolution profile of this release, rate of material resorption, and preservation of mechanical properties). The assessment of the biological behavior of temporary templates is therefore very important in tissue engineering, especially for composites, which usually exhibit complicated degradation behavior. Here, blended polylactic acid (PLA) calcium phosphate ORMOGLASS (organically modified glass) nanofibrous mats have been incubated up to 4 weeks in physiological simulated conditions, and their morphological, topographical, and chemical changes have been investigated. The results showed that a significant loss of inorganic phase occurred at the beginning of the immersion and the ORMOGLASS maintained a stable composition afterward throughout the degradation period. As a whole, the nanostructured scaffolds underwent fast and heterogeneous degradation. This study reveals that an angiogenic calcium-rich environment can be achieved through fast-degrading ORMOGLASS/PLA blended fibers, which seems to be an excellent alternative for guided bone regeneration

Incremental ECAP as a method to produce ultrafine grained aluminium plates

In this work, we propose a new approach to producing ultrafine grained plates using a modified ECAP method, namely incremental ECAP. Unlike conventional ECAP, incremental ECAP works step by step whereby deformation and feeding are performed with two different tools acting asynchronously. Incremental processing reduces forces and allows to process relatively large billets. The major advantage of this technique is that the specimens are in the form of plates with a rectangular shape, which makes them suitable for further processing, e.g. via deep drawing. This paper reports a study on microstructure development, mechanical properties and their anisotropy in aluminium plates processed by means of incremental ECAP. Eight passes applied (with the accumulated strain of 9.2) with the rotation about the Z axis brought about the reduction in the grain size down to 600 nm with the 80% fraction of high angle grain boundaries and a very homogenous equiaxial microstructure. This, in turn, resulted in a significant increase in mechanical strength with the ultimate tensile strength reaching 200 MPa and, more importantly, very low anisotropy with respect to the rolling direction

Influence of reduction conditions of NiO on its mechanical and electrical properties

Yttria stabilized zirconia with a nickel catalyst (Ni-YSZ) is the most developed, widely used cermet anode for manufacturing Solid Oxide Fuel Cells (SOFCs). Its electro-catalytic properties, mechanical durability and performance stability in hydrogen-rich environments makes it the state of the art fuel electrode for SOFCs. During the reduction stage in initial SOFC operation, the virgin anode material, a NiO-YSZ mixture, is reduced to Ni-YSZ. The volume decrease associated with the change from NiO-YSZ to Ni-YSZ creates voids and causes structural changes, which can influence the physical properties of the anode. In this work, the structural, mechanical and electrical properties of NiO samples before and after reduction in pure H2 and a mixture of 5 vol. % H2-Ar were studied. The NiO to Ni phase transformations that occur in the anode under reducing and Reduction-Oxidation (RedOx) cycling conditions and the impact on cell microstruc-ture, strength and electrical conductivity have been examined. Results show that the RedOx treatment of the NiO samples influence on their properties controversially, due to structural transformation (formation of large amount of fine pores) of the reduced Ni. It strengthened the treated samples yielding the highest mechanical strength values of 25.7 MPa, but from another side it is resulting in lowest electrical conductivity value of 1.9×105 S m-1 among all reduced samples. The results of this investigation shows that reduction conditions of NiO is a powerful tool for influence on properties of the anode substrate

A novel hybrid nanofibrous strategy to target progenitor cells for cost-effective in situ angiogenesis

Although the impact of composites based on Ti-doped calcium phosphate glasses is low compared with that of bioglass, they have been already shown to possess great potential for bone tissue engineering. Composites made of polylactic acid (PLA) and a microparticle glass of 5TiO(2)-44.5CaO-44.5P(2)O(5)-6Na(2)O (G5) molar ratio have already demonstrated in situ osteo-and angiogenesis-triggering abilities. As many of the hybrid materials currently developed usually promote osteogenesis but still lack the ability to induce vascularization, a G5/PLA combination is a cost-effective option for obtaining new instructive scaffolds. In this study, nanostructured PLA-ORMOGLASS (organically modified glass) fibers were produced by electro-spinning, in order to fabricate extra-cellular matrix (ECM)-like substrates that simultaneously promote bone formation and vascularization. Physical-chemical and surface characterization and tensile tests demonstrated that the obtained scaffolds exhibited homogeneous morphology, higher hydrophilicity and enhanced mechanical properties than pure PLA. In vitro assays with rat mesenchymal stem cells (rMSCs) and rat endothelial progenitor cells (rEPCs) also showed that rMSCs attached and proliferated on the materials influenced by the calcium content in the environment. In vivo assays showed that hybrid composite PLA-ORMOGLASS fibers were able to promote the formation of blood vessels. Thus, these novel fibers are a valid option for the design of functional materials for tissue engineering applications

Towards 4th generation biomaterials: a covalent hybrid polymer-ormoglass architecture

Hybrid materials are being extensively investigated with the aim of mimicking the ECM microenvironment to develop effective solutions for bone tissue engineering. However, the common drawbacks of a hybrid material are the lack of interactions between the scaffold's constituents and the masking of its bioactive phase. Conventional hybrids often degrade in a non-homogeneous manner and the biological response is far from optimal. We have developed a novel material with strong interactions between constituents. The bioactive phase is directly exposed on its surface mimicking the structure of the ECM of bone. Here, polylactic acid electrospun fibers have been successfully and reproducibly coated with a bioactive organically modified glass (ormoglass, Si-Ca-P2 system) covalently. In comparison with the pure polymeric mats, the fibers obtained showed improved hydrophilicity and mechanical properties, bioactive ion release, exhibited a nanoroughness and enabled good cell adhesion and spreading after just one day of culture (rMSCs and rEPCs). The fibers were coated with different ormoglass compositions to tailor their surface properties (roughness, stiffness, and morphology) by modifying the experimental parameters. Knowing that cells modulate their behavior according to the exposed physical and chemical signals, the development of this instructive material is a valuable advance in the design of functional regenerative biomaterials

Using high-pressure torsion to fabricate an Al-Ti hybrid system with exceptional mechanical properties

A novel hybrid material was fabricated from the Al-Ti system using high-pressure torsion up to 50 turns. Microstructural observations revealed intermetallic phases and mixing zones enriched in Ti, consisting of grains of ~20 nm within an Al matrix. Microhardness measurements gave values higher than in the HPT-processed bulk aluminium alloy