Ultrauniform, strong, and ductile 3D-printed titanium alloy through bifunctional alloy design.

Coarse columnar grains and heterogeneously distributed phases commonly form in metallic alloys produced by three-dimensional (3D) printing and are often considered undesirable because they can impart nonuniform and inferior mechanical properties. We demonstrate a design strategy to unlock consistent and enhanced properties directly from 3D printing. Using Ti-5Al-5Mo-5V-3Cr as a model alloy, we show that adding molybdenum (Mo) nanoparticles promotes grain refinement during solidification and suppresses the formation of phase heterogeneities during solid-state thermal cycling. The microstructural change because of the bifunctional additive results in uniform mechanical properties and simultaneous enhancement of both strength and ductility. We demonstrate how this alloy can be modified by a single component to address unfavorable microstructures, providing a pathway to achieve desirable mechanical characteristics directly from 3D printing.

Fabrication and Properties of Biodegradable Akermanite-Reinforced Fe35Mn Alloys for Temporary Orthopedic Implant Applications.

As a representative of the biodegradable iron (Fe)-manganese (Mn) alloys, Fe35Mn has been investigated as a promising biodegradable metal biomaterial for orthopedic applications. However, its slow degradation rate, though better than pure Fe, and poor bioactivity are concerns that retard its clinical applications. Akermanite (Ca2MgSi2O7, Ake) is a silicate-based bioceramic, showing desirable degradability and bioactivity for bone repair. In the present work, Fe35Mn/Ake composites were prepared via a powder metallurgy route. The effect of different contents of Ake (0, 10, 30, 50 vol %) on the microstructure, mechanical properties, degradation, and biocompatibility of the composites was investigated. The ceramic phases were found to be evenly distributed in the metal matrix. The Ake reacted with Fe35Mn and generated CaFeSiO4 during sintering. The addition of Ake increased the relative density of pure Fe35Mn from ∼90 to ∼94-97%. The compressive yield strength (CYS) and elastic modulus (Ec) increased with increasing Ake, with Fe35Mn/50Ake exhibiting the highest CYS of ∼403 MPa and Ec of ∼18 GPa. However, the ductility decreased at higher Ake concentrations (30 and 50%). Microhardness also showed an increasing trend with the addition of Ake. Electrochemical measurements indicated that higher concentrations of Ake (30 and 50%) could potentially increase the corrosion rate of Fe35Mn from ∼0.25 to ∼0.39 mm/year. However, all of the compositions tested did not show measurable weight loss after immersion in simulated body fluid (SBF) for 4 weeks, attributed to the use of prealloyed raw material, high sintered density of the fabricated composites, and the formation of a dense Ca-, P-, and O-rich layer on the surface. Human osteoblasts on Fe35Mn/Ake composites showed increasing viability with increasing Ake content, indicating improved in vitro biocompatibility. These preliminary results suggest that Fe35Mn/Ake can be a potential material for biodegradable bone implant applications, particularly Fe35Mn/30Ake, if the slow corrosion of the composite can be addressed.

In Situ Observation of Liquid Solder Alloys and Solid Substrate Reactions Using High-Voltage Transmission Electron Microscopy.

The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilizing creative setups to "freeze" the reactions at different stages during the reflow soldering process. However, full understanding of the dynamics of the process is difficult due to the lack of direct observation at micro- and nano-meter resolutions. In this study, high voltage transmission electron microscopy (HV-TEM) is employed to observe the morphological changes that occur in Cu6Sn5 between a Sn-3.0 wt%Ag-0.5 wt%Cu (SAC305) solder alloy and a Cu substrate in situ at temperatures above the solidus of the alloy. This enables the continuous surveillance of rapid grain boundary movements of Cu6Sn5 during soldering and increases the fundamental understanding of reaction mechanisms in solder solid/liquid interfaces.

Sintering and biocompatibility of blended elemental Ti-xNb alloys.

Titanium-niobium (Ti-Nb) alloys have great potential for biomedical applications due to their superior biocompatibility and mechanical properties that match closely to human bone. Powder metallurgy is an ideal technology for efficient manufacture of titanium alloys to generate net-shape, intricately featured and porous components. This work reports on the effects of Nb concentrations on sintered Ti-xNb alloys with the aim to establish an optimal composition in respect to mechanical and biological performances. Ti-xNb alloys with 33, 40, 56 and 66 wt% Nb were fabricated from elemental powders and the sintering response, mechanical properties, microstructures and biocompatibility assessed and compared to conventional commercial purity titanium (CPTi). The sintered densities for all Ti-xNb compositions were around 95%, reducing slightly with increasing Nb due to increasing open porosity. Higher Nb levels retarded sintering leading to more inhomogeneous phase and pore distributions. The compressive strength decreased with increasing Nb, while all Ti-xNb alloys displayed higher strengths than CPTi except the Ti-66Nb alloy. The Young's moduli of the Ti-xNb alloys with ≥40 wt% Nb were substantially lower (30-50%) than CPTi. In-vitro cell culture testing revealed excellent biocompatibility for all Ti-xNb alloys comparable or better than tissue culture plate and CPTi controls, with the Ti-40Nb alloy exhibiting superior cell-material interactions. In view of its mechanical and biological performance, the Ti-40Nb composition is most promising for hard tissue engineering applications.

Additively manufactured iron-manganese for biodegradable porous load-bearing bone scaffold applications.

Selective laser melting (SLM) can produce complex hierarchical architectures paving the way for highly customisable biodegradable load-bearing bone scaffolds. For the first time, an in-depth analysis on the performance of SLM-manufactured iron-manganese bone scaffolds suitable for load-bearing applications is presented. Microstructural, mechanical, corrosion and biological characterisations were performed on SLM-manufactured iron-manganese scaffolds. The microstructure of the scaffold consisted primarily of γ-austenite, leading to high ductility. The mechanical properties of the scaffold were sufficient for load-bearing applications even after 28 days immersion in simulated body fluids. Corrosion tests showed that the corrosion rate was much higher than bulk pure iron, attributed to a combination of the manufacturing method, the addition of Mn to the alloy and the design of the scaffold. In vitro cell testing showed that the scaffold had good biocompatibility and viability towards mammalian cells. Furthermore, the presence of filopodia showed good osteoblast adhesion. In vivo analysis showed successful bone integration with the scaffold, with new bone formation observed after 4 weeks of implantation. Overall the SLM manufactured porous Fe-35Mn implants showed promise for biodegradable load-bearing bone scaffold applications. STATEMENT OF SIGNIFICANCE: Biodegradable iron scaffolds are emerging as a promising treatment for critical bone defects. Within this field, selective laser melting (SLM) has become a popular method of manufacturing bespoke scaffolds. There is limited knowledge on SLM-manufactured iron bone scaffolds, and no knowledge on their application for load-bearing situations. The current manuscript is the first study to characterise SLM manufactured iron-manganese bone scaffolds for load-bearing applications and also the first study to perform In vivo testing on SLM produced biodegradable iron scaffolds. In this study, for the first time, the mechanical, corrosion and biological properties of an iron-manganese scaffold manufactured using SLM were investigated. In summary the SLM-manufactured porous iron-manganese implants displayed great potential for biodegradable load-bearing bone scaffolds.

Comparison of the Microstructure and Biocorrosion Properties of Additively Manufactured and Conventionally Fabricated near ß Ti-25Nb-3Zr-3Mo-2Sn Alloy.

The microstructure and biodegradability of a near ß Ti-25Nb-3Zr-3Mo-2Sn alloy produced by laser engineered net shaping have been compared to those of alloys produced via casting and cold rolling in order to identify the key effects of processing pathways on the development of microstructure and biocorrosion properties. Results confirm the significant influence of processing technique on microstructure and concomitant biocompatibility of the alloy. Tests using mesenchymal stem cells confirm the ability of the additively manufactured alloy to support cell adhesion and spreading.

Porous Titanium Scaffolds Fabricated by Metal Injection Moulding for Biomedical Applications.

Biocompatible titanium scaffolds with up to 40% interconnected porosity were manufactured through the metal injection moulding process and the space holder technique. The mechanical properties of the manufactured scaffold showed a high level of compatibility with those of the cortical human bone. Sintering at 1250 °C produced scaffolds with 36% porosity and more than 90% interconnected pores, a compressive yield stress of 220 MPa and a Young's modulus of 7.80 GPa, all suitable for bone tissue engineering. Increasing the sintering temperature to 1300 °C increased the Young's modulus to 22.0 GPa due to reduced porosity, while reducing the sintering temperature to 1150 °C lowered the yield stress to 120 MPa, indicative of insufficient sintering. Electrochemical studies revealed that samples sintered at 1150 °C have a higher corrosion rate compared with those at a sintering temperature of 1250 °C. Overall, it was concluded that sintering at 1250 °C yielded the most desirable results.

Biocompatible porous titanium scaffolds produced using a novel space holder technique.

We describe a new fabrication strategy for production of porous titanium scaffolds for skeletal implants which provides a promising new approach to repair and remodel damaged bone tissue. The new strategy involves powder sintering of titanium powder, employing pharmaceutical sugar pellets as temporary space holders, to facilitate production of porous scaffolds with structures optimized for mechanical performance and osseointegration of implants. The spherical sugar pellets, with controlled size fractions and excellent biocompatibility, are easily removed by dissolution prior to sintering providing an ideal space holder material for controlled synthesis of titanium scaffolds with desired porosities and pore sizes. The scaffolds contain pores with high degrees of sphericity and interconnectivity which impart excellent mechanical properties and superior biocompatibility to the structures. Scaffolds with 40% porosity and a pore size range of 300-425 µm exhibited an effective Young's modulus of 16.4 ± 3.5 GPa and strength of 176 ± 6 MPa, which closely mimics the properties of human bone, and were also able to support cell adhesion, viability and spreading in cell culture tests. Porous titanium scaffolds manufactured by this approach have excellent potential for hard tissue engineering applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2796-2806, 2018.

Mechanical properties and biocompatibility of porous titanium scaffolds for bone tissue engineering.

Synthetic scaffolds are a highly promising new approach to replace both autografts and allografts to repair and remodel damaged bone tissue. Biocompatible porous titanium scaffold was manufactured through a powder metallurgy approach. Magnesium powder was used as space holder material which was compacted with titanium powder and removed during sintering. Evaluation of the porosity and mechanical properties showed a high level of compatibility with human cortical bone. Interconnectivity between pores is higher than 95% for porosity as low as 30%. The elastic moduli are 44.2GPa, 24.7GPa and 15.4GPa for 30%, 40% and 50% porosity samples which match well to that of natural bone (4-30GPa). The yield strengths for 30% and 40% porosity samples of 221.7MPa and 117MPa are superior to that of human cortical bone (130-180MPa). In-vitro cell culture tests on the scaffold samples using Human Mesenchymal Stem Cells (hMSCs) demonstrated their biocompatibility and indicated osseointegration potential. The scaffolds allowed cells to adhere and spread both on the surface and inside the pore structures. With increasing levels of porosity/interconnectivity, improved cell proliferation is obtained within the pores. It is concluded that samples with 30% porosity exhibit the best biocompatibility. The results suggest that porous titanium scaffolds generated using this manufacturing route have excellent potential for hard tissue engineering applications.

Manufacturing of graded titanium scaffolds using a novel space holder technique.

To optimize both the mechanical and biological properties of titanium for biomedical implants, a highly flexible powder metallurgy approach is proposed to generate porous scaffolds with graded porosities and pore sizes. Sugar pellets acting as space holders were compacted with titanium powder and then removed by dissolution in water before sintering. The morphology, pore structure, porosity and pore interconnectivity were observed by optical microscopy and SEM. The results show that the porous titanium has porosity levels and pore size gradients consistent with their design with gradual and smooth transitions at the interfaces between regions of differing porosities and/or pore sizes. Meanwhile, the porous titanium has high interconnectivity between pores and highly spherical pore shapes. In this article we show that this powder metallurgy processing technique, employing the novel sugar pellets as space-holders, can generate porous titanium foams with well-controlled graded porosities and pore sizes. This method has excellent potential for producing porous titanium structures for hard tissue engineering applications.

Structural studies of group 1 metal 4-azapentalenyl complexes.

The first structurally authenticated 4-azapentalenyl complexes are reported, along with DFT studies, that reveal tendencies for their cyclopentadienyl isostructural nature and an ability to exhibit binding modes of varying hapticity.