Isothermal omega assisted alpha phase precipitation and microstructural evolution of an aged Ti-30Nb-3Fe alloy

The mechanical behavior of metastable β Ti alloys can be controlled through heat treatments. Thus, the relationship between the precipitation of α phase and the mechanical properties of these alloys is of special interest. In this work, the microstructure evolution of Ti-30Nb-3Fe alloy during aging heat treatments was evaluated using optical microscopy, scanning electron microscopy and X-ray diffraction. Moreover, Vickers hardness and elastic modulus were measured as a function of aging time. Finally, the ultimate strength and ductility of the alloy aged at 500 °C was assessed by tensile tests. In comparison to a Ti-30Nb alloy, the addition of Fe lowered the β-transus temperature, decreased the martensite start temperature to a value below room temperature, increased the precipitation temperature and reduced the dissolution temperature of ω phase, and lastly, decreased the α phase precipitation temperature. Low heating rates enabled isothermal ω phase precipitation and growth, providing favorable conditions for α phase precipitation and increasing the amount of α phase precipitates. Compared to the solution heat-treated and water-quenched condition, aging heat-treated Ti-30Nb-3Fe alloy presented higher Vickers hardness and mechanical strength, without significant loss of ductility233CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP405054/2016-588887.357955/2019-002016/24693-3The authors gratefully acknowledge the LNNano (National Nanotechnology Laboratory) at the CNPEM (National Center for Research on Energy and Materials) for allowing access to its SEM facilities. We also acknowledge the financial support of the Brazilian research funding agencies FAPESP (State of São Paulo Research Foundation) for Grant #2016/24693-3, CNPq (National Council for Scientific and Technological Development) for Grant #405054/2016-5, and CAPES/PNPD (Coordination for the Improvement of Higher Education Personnel) for Grant #88887.357955/2019-00. We thank the Brazilian Niobium Mining and Processing Company CBMM for supplying the Nb used in this stud

Transformações de fase, microestrutura e propriedades mecânicas de novas ligas de titânio beta metaestáveis do sistema Ti-Nb-Zr-Fe (TNZF) para aplicação biomédica

Orientador: Rubens Caram JuniorTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia MecânicaResumo: A obtenção de ligas de titânio de baixo custo com elevada razão entre tensão de escoamento e módulo de elasticidade (EAS) é um dos grandes desafios na área de biomateriais estruturais. Neste estudo, nós respondemos à esse desafio em duas frentes. Inicialmente, exploramos o sistema Ti-Nb-Fe com o objetivo de encontrar composições otimizadas, que apresentassem um baixo módulo de elasticidade com um teor de Nb reduzido. Lingotes das ligas Ti-(31-4x)Nb-(1+0.5x)Fe foram preparados, enquanto o Nb foi sendo substituído por Fe, a partir da liga Ti-31Nb-1.0Fe até a liga Ti-11Nb-3.5Fe. As amostras foram solubilizadas e testadas sob três diferentes condições: resfriada em água, resfriada em forno e envelhecida (diversas temperaturas). As microestruturas resultantes foram analisadas por meio de difração de raios-X, calorimetria exploratória diferencial, microscopia eletrônica de varredura e de transmissão. Entre as ligas ternárias, a liga Ti-19Nb-2.5Fe foi a que apresentou a melhor combinação de resistência mecânica e módulo de elasticidade. As amostras envelhecidas revelaram dados interessantes sobre a formação e crescimento da fase alfa neste sistema durante o envelhecimento. Em geral, a formação de alfa se dá preferencialmente nos contornos de grão, mesmo em tratamentos isotérmicos de curta duração (1 min). Uma microestrutura composta de grãos de beta e fase alfa fina e dispersa foi obtida para a maioria das ligas através de envelhecimentos à 450 ºC. A segunda parte do projeto consistiu na exploração de ligas quaternárias Ti-Nb-Fe-Zr, com adições de 4 a 13% (peso) de Zr. Uma liga adicional, baseada na liga Ti-19Nb-2.5Fe, foi preparada com adição de 6% de Sn. Além de todo o trabalho de caracterização executado, ensaios de tração a temperatura ambiente comprovaram que ligas do sistema Ti-Nb-Fe-Zr podem atingir EAS próxima de 1,5, com boa ductilidade. As adições de Zr e Sn foram uteis na supressão da fase omega após resfriamento em água. Além disso, Zr e Sn são igualmente particionados entre matriz e precipitados durante o envelhecimento. Enquanto a difusão de Nb e Fe foi favorecida pelos contornos de grão, ela parece ser inibida na presença de Zr e Sn. Como consequência, adições de Sn resultam em uma maior resistência mecânica e precipitação de alfa mais refinada. Finalmente, um mapa de seleção de materiais é apresentado com o objetivo de ajudar futuros autores a comparar ligas de aplicação biomédica tendo como parâmetros a razão tensão-módulo (EAS) e o custoAbstract: An open challenge on structural biomaterials is to obtain low-cost Ti-alloys with a high elastic admissible strain (the ratio of yield strength to elastic modulus). In this study, we addressed the presented challenge via two working directions. Firstly, we explored the Ti-Nb-Fe system to find an optimal, cost-effective composition with a compromise between a low elastic modulus and low added Nb contents. Ti-(31-4x)Nb-(1+0.5x)Fe ingots were prepared, and Nb was substituted with Fe, starting at Ti-31Nb-1.0Fe and going down to Ti-11Nb-3.5Fe (wt%). The samples were solution-treated and tested under three conditions: water-quenched, furnace-cooled and step-quenched to different temperatures. Resultant microstructures were analyzed with the aid of X-ray diffraction, differential scanning calorimetry, scanning, and transmission electron microscopy. Among the ternary alloys, Ti-19Nb-2.5Fe (wt. %) presented the best combination of mechanical strength and elastic modulus. The heat-treated samples provided useful insights into how the alpha-phase formation starts and develops in this system during aging. In general, alpha-phase precipitation starts at the grain boundaries, even after very short-time isothermal heat-treatments (1 min). Overall, an optimized microstructure composed of beta-grains and fine and dispersed alpha-phase was obtained for most of the experimental alloys after aging at 450 ºC. The second part of this project comprises the exploration of Ti-Nb-Fe-Ze quaternary alloys, with additions of 4-13 wt.% of Zr. One additional alloy, based on Ti-19Nb-2.5Fe, was prepared with additions of 6 wt.% of Sn. Beyond all the characterization work, tensile tests performed at room-temperature confirmed that Ti-Nb-Fe-Zr alloys could reach an elastic admissible strain close to 1.5, with relatively good ductility. Regarding the Zr and Sn additions, they helped suppress omega formation after water-quenching. Also, Zr and Sn were equally distributed between matrix and precipitates during aging. While the diffusion of Nb and Fe were enhanced via grain-boundaries, they seemed to be inhibited by the presence of Zr and Sn. As a result, these elements allow higher yield-strengths and more refined secondary ?-phase. In the end, a materials selection chart is presented to help future researchers to compare materials for orthopedic implants considering the elastic admissible strain and cost as significant guidelinesDoutoradoMateriais e Processos de FabricaçãoDoutor em Engenharia Mecânica2014/24449-0; 2016/22714-3FAPES

Selective laser melting of high strength-to-modulus ratio tntz and tntz-o alloys tailored for load-bearing biomedical applications

Titanium alloys, combined with human-friendly β stabilizers such as niobium, molybdenum, and tantalum have been investigated for low-modulus biomedical applications. This project characterised the relationships between microstructure, mechanical properties, and biocompatibility of selective laser melting (SLM) manufactured β titanium alloys. In the study of TNT and TNTZ alloy design and manufacturing, the authors investigated Ti-Nb-Ta based β alloys with different zirconium additions (0, 5, 9 wt. %) manufactured by SLM. BF-TEM images combining SAD patterns of TNT(Z) alloys show single β phase obtained inside the beta matrix; a low level of as-fabricated defects is obtained and zirconium functions as a neutral element in these high β-stabilized Ti-Nb-Ta based alloys. Corrosion ions of TNT(Z) alloys released from immersion testing at each time intervals show extremely small concentrations (<10 μg/L). The TNTZ post-processing treatment work shows the existence of single beta grain matrix and alpha precipitates along grain boundary in SLM+HIP manufactured TNT5Zr alloy, and ellipsoidal nano-sized intragranular α'' precipitates were introduced after aging treatment. Including the inferior notch-like surface of the test-pieces, slip-band cracking occurs in this ductile SLM+HIP+aging manufactured TNT5Zr alloy are regarded as the main factors to determinate its fatigue strength (170 MPa). In vitro short-term biocompatibility evaluation reveals that almost no significant difference of biocompatibility results between TNT5Zr alloy and the reference biomaterial (Ti-6Al-4V). In the study of Ti-34Nb-13Ta-5Zr-0.2O (TNT5Zr-0.2O, wt. %) post-processing treatment, advanced HIP subjected to high and intermediate cooling rate (HCR & ICR) were exploited to close keyholes and tune the microstructure of SLMed TNT5Zr-0.2O alloys. High-angle annular dark-field (HAADF) micrographs show discrete large Ti-rich α grain boundary precipitates in TNT5Zr-0.2O-ICR alloy. Tensile properties show that TNT5Zr-0.2O-AF alloy possessed high UTS of 975 ± 12 MPa, and elongation of 4.9% ± 0.3%; the TNT5Zr-0.2O-ICR alloy obtained slightly higher UTS (1036 ± 26 MPa) and lower elongation (3.0% ± 0.3%). Advanced HIP subjected to intermediate cooling rate functions well to close SLM-processed keyholes but the resistance to fatigue is not markedly enhanced. In the study of TNT5Zr-0.2O surface treatment, the authors investigated the feasibility of thermal oxidation (TO) for improving the wear and fatigue properties of TNT5Zr-0.2O alloys manufactured by SLM. A mixture of rutile, Nb2O5, Ta2O5, and ZrO phases were formed as an oxide layer after TO. Plain fatigue strength of CE treated alloy (150 MPa) was 1.5 times higher than the value of CE+TO treated alloy (60 MPa), as a result of multiple premature fatigue cracks possibly developing in the compounds region after TO. In vitro biocompatibility results showed no significant differences in metabolic activity of pre-osteoblasts seeded on the treated surfaces. Overall, though the oxide layer is corrosion-resistant in the aggressive environment (3M HCl solution), showing potential application of TO in additively manufactured titanium medical devices. 3D porous structures have been receiving more attention for orthopedic implant development mainly due to their lower elastic moduli to prevent aseptic loosening, however, their low yield strengths may increase failure risk in load-bearing implants. In the study of TNT5Zr-0.2O lattice design and manufacturing, scaffolds infilled with sheet-based triply periodic minimal surface (TPMS) unit cells were manufactured with different design porosity via SLM. Quasi-static compression tests showed that low elastic moduli (10~22 GPa) and high yield strengths (358~1045 MPa) were obtained in the varying TPMS cylindrical specimens. A good balance of high strength and low modulus is obtained in low-porosity TNT5Zr-0.2O TPMS scaffold implants, which potentially work well in human body and provides long service time

The structure and properties of additively manufactured metastable-β Ti-15Mo

A bidirectional powder deposition strategy was employed to additively manufacture Ti-15Mo wt% using laser metal deposition. Phase identification, elemental analysis and microstructural characterisation were conducted in the As-Built condition and also after uniaxial tensile testing using X-ray diffraction and scanning electron microscopy along the different processing directions. In addition, electron backscattering diffraction and transmission electron microscopy were used to analyse deformation mechanisms. It was found that three distinct zones, namely the fusion, remelted and heat affected zones, evolved in all 25 deposited layers which predominantly comprised coarse columnar grains. These columnar grains were coarser up the build height due to increased distance from the substrate and slower cooling rates determined. Mo segregation was pronounced in the as-built microstructure. The fusion zone was the most solute enriched zone, followed by the remelted zone. The heat affected zone of each deposited layer featured inter-dendritic lamellas of molybdenum rich and lean inter-layers and this zone was the least solute-enriched. Deformation accommodation in β matrix was by a combination of slip, {332}〈113〉 and {112}〈111〉 twinning, α martensite and ωD formation contrarily to the expected twinning. The as-built alloy was subsequently subject to post-fabrication heat treatment. Microstructural characterisation was conducted in the heat-treated state and also after uniaxial tensile deformation. X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy were employed in the heat-treated state. Electron backscatter diffraction was used in investigating the deformed microstructure. Columnar β grain refinement was achieved by fragmentation from a combined contribution from precipitated phases and deformation induced products. The three distinct microstructural zones, namely the fusion, remelted and heat affected zones, observed in each deposited layer of the as-built microstructure were retained after sub-β-solvus heat treatment but completely erased in the super-β-solvus microstructure. Accommodation of plastic deformation in β matrix was by a combination of slip and primary α martensite which formed preferentially at grain boundaries. Elastic modulus decreased from 86.85±0.45 GPa in the as-built alloy to 72.8±0.65 GPa after heat treatment. Ultimate tensile strength of 1168± 1.12 MPa from the heat-treated sample represents only a marginal increase from that of the as-built sample of 1099± 2.3 MPa. This was accompanied by a small decrease in total elongation. The as-built alloy was also subject to post-fabrication uniaxial thermomechanical processing at strain rates of 0.00055s-1, 0.0011s-1, 1s-1, and 4s-1 to strains of 20% and 40%. Experiments were conducted at room and elevated temperatures. Phase identification, elemental and microstructural characterisation were conducted using x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The three distinct zones, namely the fusion, remelted and heat affected zones, identified in each deposited layer of the as-built microstructure were retained after thermomechanical processing. After processing, electron backscatter diffraction was used to analyse deformation mechanisms. Deformation accommodation in β matrix was predominantly by a combination of slip and α martensite which formed as a primary product in parent β and at grain boundaries. However, the operation of {332}〈113〉 and {112}〈111〉 β-twinning was also determined, howbeit with a very small surface fraction. This implies a small surface fraction of secondary α martensite forming within β-twins in the deformed microstructure. Compressive mechanical properties showed a strong dependence on strain rate as higher flow stress and compressive strength were obtained at higher strain rates