Impacts of Temperature and Time on Direct Nitridation of Aluminium Powders for Preparation of AlN Reinforcement.

Aluminium nitride (AlN) is an important technical ceramic with outstanding strength and thermal conductivity that has important applications for advanced heat sink materials and as a reinforcement for metal-based composites. In this study, we report a novel, straightforward and low-cost method to prepare AlN powder using a vacuum tube furnace for the direct nitridation of loose aluminium powder at low temperatures (down to 500 ∘C) under flowing high-purity nitrogen. Small amounts of magnesium powder (1 wt.%), combined with aluminium, promote nitridation. Here, we characterise the effects of time (up to 12 h) and temperature (490 to 560 ∘C) on nitridation with the aim to establish an effective regimen for the controlled synthesis of an aluminium nitride reinforcement powder for the production of metal matrix composites. The extent of nitridation and the morphology of the reaction products were assessed using scanning electron microscopy and X-ray diffraction analyses. AlN was detected for all nitriding temperatures ≥ 500 ∘C, with the highest yields of 80% to 85% obtained at 530 ∘C for times ≥ 1 h. At this temperature, nitridation proceeded rapidly, and there was extensive agglomeration of the reaction products making it difficult to reprocess into powder. At lower temperatures around 510 ∘C, a relatively high proportion of AlN was attained (>73% after 6 h) while retaining excellent friability so that it could be manually reprocessed to powder. The synthesised reinforcement consisted of micro- or nano-crystalline AlN comingled with metallic aluminium. The ratio of AlN and metallic aluminium can be readily controlled by varying the nitriding temperature. This provides a flexible and accessible method for the production of AlN-reinforcement powders suited to the production of metal matrix composites.

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.

Effect of Fe addition on properties of Ti-6Al-xFe manufactured by blended elemental process.

The mechanical properties of titanium alloys produced by powder metallurgy (PM) are dependent on the amount of porosity within the fabricated component. The space between powder particles and the behaviour of alloying elements during sintering contribute to the formation of pores. Iron (Fe) is well known to be a cost-effective alloying element for titanium alloys which acts to stabilise the ß-phase. This study aims to investigate the effects of Fe addition on the sintering response of titanium alloys containing aluminium. Ti-6Al-xFe(x = 1, 3, and 5 wt %) alloy systems were manufactured by press and sinter PM from blended-elemental powders. The density, mechanical properties, microstructures and pore distribution in the sintered parts were evaluated. The compressive strength of the alloys was positively correlated to the levels of Fe. Grain boundary and solid solution strengthening accounted for the strength improvements. Furthermore, Ti-6Al-3Fe exhibited the highest strength/modulus ratio. Evaluation of the pore distributions revealed that the number of fine pores was reduced significantly as the amount of Fe was increased, though concomitantly the number of larger pores increased. It is argued that the increasing number of larger pores with higher levels of Fe is due to coalescence of fine Kirkendall porosity during the latter stages of sintering. With excessive iron additions, large pores counteract any beneficial impacts on the sintering response. It is suggested to limit the amount of Fe additions to around 3 wt% to reduce adverse effects from large pores and to maximise the strength/modulus ratio.

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.

Effects of deformation twinning on the mechanical properties of biodegradable Zn-Mg alloys.

To satisfy the property requirements for biodegradable medical implants, Zn alloyed with low levels of Mg (≤0.8 wt%) has attracted increased research interest. In the present study, deformation twinning was observed in tensile tests and twinning appears to have an adverse impact on ductility. The profuse twinning in the as-cast Zn-Mg alloys accelerated crack growth in tension due to twinning impingement which caused local stress concentrations and initiates cracking. As-rolled Zn-Mg alloys have better ductility than their as-cast counterparts due to the inhibition of twinning by the refined Mg2Zn11 intermetallic phase and the finer grain size.

A biocompatible thermoset polymer binder for Direct Ink Writing of porous titanium scaffolds for bone tissue engineering.

There is increasing demand for synthetic bone scaffolds for bone tissue engineering as they can counter issues such as potential harvesting morbidity and restrictions in donor sites which hamper autologous bone grafts and address the potential for disease transmission in the case of allografts. Due to their excellent biocompatibility, titanium scaffolds have great potential as bone graft substitutes as they mimic the structure and properties of human cancellous bone. Here we report on a new thermoset bio-polymer which can act as a binder for Direct Ink Writing (DIW) of titanium artificial bone scaffolds. We demonstrate the use of the binder to manufacture porous titanium scaffolds with evenly distributed and highly interconnected porosity ideal for orthopaedic applications. Due to their porous structure, the scaffolds exhibit an effective Young's modulus similar to human cortical bone, alleviating undesirable stress-shielding effects, and possess superior strength. The biocompatibility of the scaffolds was investigated in vitro by cell viability and proliferation assays using human bone-marrow-derived Mesenchymal stem cells (hMSCs). The hMSCs displayed well-spread morphologies, well-organized F-actin and large vinculin complexes confirming their excellent biocompatibility. The vinculin regions had significantly larger Focal Adhesion (FA) area and equivalent FA numbers compared to that of tissue culture plate controls, showing that the scaffolds support cell viability and promote attachment. In conclusion, we have demonstrated the excellent potential of the thermoset bio-polymer as a Direct Ink Writing ready binder for manufacture of porous titanium scaffolds for hard tissue engineering.

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.

Microstructure, elastic deformation behavior and mechanical properties of biomedical ß-type titanium alloy thin-tube used for stents.

Cold-deformability and mechanical compatibility of the biomedical ß-type titanium alloy are the foremost considerations for their application in stents, because the lower ductility restricts the cold-forming of thin-tube and unsatisfactory mechanical performance causes a failed tissue repair. In this paper, ß-type titanium alloy (Ti-25Nb-3Zr-3Mo-2Sn, wt%) thin-tube fabricated by routine cold rolling is reported for the first time, and its elastic behavior and mechanical properties are discussed for the various microstructures. The as cold-rolled tube exhibits nonlinear elastic behavior with large recoverable strain of 2.3%. After annealing and aging, a nonlinear elasticity, considered as the intermediate stage between "double yielding" and normal linear elasticity, is attributable to a moderate precipitation of α phase. Quantitive relationships are established between volume fraction of α phase (Vα) and elastic modulus, strength as well as maximal recoverable strain (εmax-R), where the εmax-R of above 2.0% corresponds to the Vα range of 3-10%. It is considered that the "mechanical" stabilization of the (α+ß) microstructure is a possible elastic mechanism for explaining the nonlinear elastic behavior.

Evolution of the microstructure and mechanical properties during fabrication of mini-tubes from a biomedical ß-titanium alloy.

The processing of Ti-25Nb-3Mo-3Zr-2Sn tubes with outside diameters of 5.6-8.0 mm and wall-thicknesses of 0.7-1.0 mm were investigated in order to study the evolution of microstructure and mechanical properties and their impact on the processing of the tubes. The annealed small tubes with single ß phase microstructures exhibit double yielding during tensile tests. The onset of martensitic phase transformation was observed to occur after the lowest point of the strain hardening. Cold rolling also activates the formation of the stress induced martensitic α″ phase. Its volume fraction increased with increasing ε. The rate of strain hardening and the modulus of the tubes are related to the stress induced transformation of the ß phase to the α″ phase. The stress induced α″ slightly improves the yield strength of the tubes at low levels of strain. However, larger strains result in grain growth during annealing, which diminishes the mechanical properties.

The cold-rolling behaviour of AZ31 tubes for fabrication of biodegradable stents.

Mg alloys are receiving considerable attention for biomedical stents due to their combination of good mechanical properties and high biodegradability. Cold rolling is necessary to process Mg alloy tubes before final drawing and fabrication of the magnesium stents. In this paper, cold-rolled tubes were subjected to a cross-sectional reduction rate (ε) of up to 19.7%, and were further processed at various ratios of wall-thickness to diameter reduction (Q) from 0 to 2.24 with a constant ε of 19.7%. The results show that the cold-rolled tubes exhibited a rise in ultimate tensile strength (UTS), yield strength (YS), and a reduction in elongation as ε increased from 5.5% to 19.7%. UTS, YS and elongation decreased when Q was increased from 0 to 2.24. Mechanical twinning was observed and analysed. Extension twins increased with increasing ε and were almost saturated at a ε of 16.5%. Extension twins play an important role in determining the evolution of mechanical behaviour in the case of increasing ε, whilst contraction/double twins and secondary extension twins have a large effect on mechanical behaviour in the case of varying Q. The results indicate that the proportions and types of twins play a major role in determining the mechanical behaviour of the AZ31 tubes.

Effects of phase stability and processing on the mechanical properties of Ti-Nb based ß Ti alloys.

The influence of ß phase stability on mechanical properties, deformation behaviours and phase composition were investigated for a series of Ti-24Nb-3Zr-2Sn-xMo alloys in response to hot and cold rolling. For the hot rolled alloys, the phase composition and deformation behaviours were largely consistent with those predicted on the basis of a Bo-Md plot and the Ms estimates. The deformation mechanisms involve growth and/or reorientation of plate-like martensite and/or twins. However, these are largely restricted in the cold rolled alloys due to the effects of grain refinement and residual stress. The cold rolled alloys exhibit the highest strengthening in combination with more limited ductility, which increased with increasing ß phase stability. The moderately stable alloy, B, with e/a around 4.18 and Moeq∼10wt% gave the greatest strengthening in response to cold rolling, which was related to intense localised grain refinement.

Strength enhancement of a biomedical titanium alloy through a modified accumulative roll bonding technique.

The strength of a biomedical ß-type alloy, Ti-25Nb-3Zr-3Mo-2Sn, was enhanced through severe plastic deformation using a modified accumulative roll bonding technique. Incremental strength increases were observed after each cycle, while ductility initially fell but showed some recovery with further cycles. After 4 cycles there was a 70% improvement in the ultimate tensile strength to 1220 MPa, a two-fold increase in the 0.5% proof stress to 946 MPa and the ductility was 4.5%. The microstructure comprised of ultrafine grain ß grains heavily elongated in the rolling direction with a fine dispersion of nanocrystalline α phase precipitates on the ß grain boundaries. Shear bands formed in order to accommodate large plastic strains during processing and the grains within the bands were significantly finer than the surrounding matrix.