A metastable β titanium alloy Ti–5Al–5Mo–5V–3Cr (Ti-5553) was friction stir processed (FSP) in both the β and α+β phase fields. Homogenous microstructures were obtained in both phase fields by ...adjusting FSP parameters. The mean β grain size after FSP varied from 15 to ~0.7 μm from the single-phase β to α+β conditions, respectively. At the α+β conditions, a near-globular α phase (50–200 nm in size) was observed. Microstructure and microtexture evolution in β and α+β conditions were investigated with the aid of scanning and transmission electron microscopy. Ultra-fine intra-granular α and duplex α microstructures were generated at both conditions by duplex aging of the FSP samples. The FSP alloys display excellent combinations of strength and ductility.
Work hardening of quaternary powder metallurgy Ti alloys Al-hajiri, M.; Yang, F.; Bolzoni, L.
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
April 2024, 2024-04-00, Volume:
897
Journal Article
Peer reviewed
Open access
The concurrent addition of Al as α stabiliser and Nb and Cu or Mn as β stabilisers was used to design new powder metallurgy quaternary Ti alloys and investigate their manufacturability and consequent ...performance. It is found that the powder blend's compressibility decreases with the total amount of alloying elements due to the characteristics of the powders used. Consequently, the sintered density of the quaternary Ti alloys decreases and slightly higher values are achieved if Mn instead of Cu is used, despite the higher diffusivity of Cu at the sintering temperature. The quaternary Ti alloys are characterised by a lamellar microstructure comprising α grain boundaries and α+β lamellae where the amount and type of alloying elements determine the coarseness of the microstructural features. This results in stronger and harder but less ductile quaternary Ti alloys for higher alloying elements contents and for stronger β stabilisers, although of their overall elastoplastic behaviour. Because of their lamellar microstructure, the quaternary Ti alloys show similar deformation behaviour but the actual work hardening rate is governed by the fineness of the microstructure.
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Selective laser melting (SLM) has enabled the production of porous titanium structures with biological and mechanical properties that mimic bone for orthopedic applications. These ...porous structures have a reduced effective stiffness which leads to improved mechanotransduction between the implant and bone. Triply periodic minimal surfaces (TMPS), specifically the sheet-based gyroid structures, have improved compressive fatigue resistance due lack of stress concentrations. Sheet-based gyroid microarchitectures also have high surface area, permeability, and zero mean curvature. This study examines the effects of the gyroid microarchitectural design in parallel with SLM parameters on structure and function of as-built titanium alloy (Ti6Al4V ELI) scaffolds. Scaffold design was varied by varying unit cell size and wall thickness to produce scaffolds with porosity within the range of trabecular bone (50–90%). Manufacturer’s default and refined laser parameters were used to examine the effect of input energy density on mechanical properties. Scaffolds exhibited a stretching-dominated deformation behavior under both compressive and tensile loading, and porosity dependent stiffness and strength. Internal void defects were observed within the walls of the gyroids structure, serving as sites for crack initiation leading to failure. Refinement of laser parameters resulted in increased compressive and tensile fatigue behavior, particularly for thicker walled gyroid microarchitectures, while thinner walls showed no significant change. The observed properties of as-built gyroid sheet microarchitectures indicates that these structures have potential for use in bone engineering applications. Furthermore, these results highlight the importance of parallel design and processing optimization for complex sheet-based porous structures produced via SLM.
Selective laser melting (SLM) is an additive manufacturing technology which produces complex porous scaffolds for orthopedic applications. Titanium alloy scaffolds with novel sheet-based gyroid microarchitectures were produced via SLM and evaluated for mechanical performance including fatigue behavior. Gyroid structures are function based topologies have been hypothesized to be promising for tissue engineering scaffolds due to the high surface area to volume ratio, zero mean curvature, and high permeability. This paper presents the effects of scaffold design and processing parameters in parallel, a novel study in the field on bone tissue scaffolds produced via additive manufacturing. Additionally, the comparison of compressive and tensile behavior of scaffolds presented is important in characterizing behavior and failure mechanisms of porous metals which undergo complex loading in orthopedic applications.
The columnar to equiaxed transition (CET) of grain structures has been a great challenge during titanium alloy additive manufacturing (AM), especially for wire arc additive manufacturing (WAAM) with ...the highly localized heat input and large temperature gradient. In this work, three combined methods (ultrahigh frequency pulse arc + growth restricting solutes (GRS) additions (HP–S method), ultrasonic vibration + GRS additions (UV–S method) and low frequency pulse arc + GRS additions (LP-S method)) were investigated and compared for the capacity of refining β columnar grains and weakening α texture during the titanium alloy WAAM process. Compared with the other two methods, the LP-S method has a dramatic effect on achieving CET and weakening α texture. During the solidification process of LP-S method, three stages can be divided: (1) nucleation stage (2) competitive growth stage (3) re-melting stage, which promote CET and weaken α texture. It can be anticipated that further optimization in the selection of alloy elements and the process parameters of the low frequency pulse arc (base current, peak current and pulse frequency, etc.) can achieve grain morphology control and mechanical properties improvement.
Revealing the microstructural parameters-mechanical properties relationship is very critical for the microstructure tailoring and properties optimization of titanium alloy. In this work, the ...dependence of mechanical properties on microstructural parameters of TA15 titanium alloy with tri-modal microstructure was investigated. The results indicate that at a certain lamellar α (αl) content (28%), both of yield strength (σ0.2) and ultimate tensile strength (σb) decrease continuously with the increase of equiaxed α (αp) content, while the decrease speed slows down at higher αp content. At a certain content of αp of 20%, σ0.2 and σb both increase first and then decrease as the content of αl increases. The above effect laws result from the competition between the softening effect caused by increasing the softer αp and αl contents and the strengthening effect relating to αl/βt interfaces and βt hardening. As for the elongation (δ) and reduction of area (Φ), they show an increase trend with αp content increasing at a certain αl content of 28%. However, they decrease first and then increase with αl content increasing at a certain αp content of 20%. The early plasticity decrease at lower αl content is due to the popular rapid void nucleation and cleavage fracture along αl/βt interfaces. When αp or αl content increases to higher level, the deformation compatibility and homogeneity improves, which suppress the rapid void nucleation and cleavage fracture along αl/βt interfaces. Meanwhile, the fracture mechanism changes from the coexistence of void coalescence and cleavage fracture to the void coalescence dominated fracture, thus the plasticity increases. Moreover, a back-propagation neural network model was developed to correlate the mechanical properties with microstructural parameters of tri-modal microstructure. The prediction results suggest that better combination of strength and plasticity can be achieved by controlling the contents of αp and αl in the range of 10–15% and 22–27%, respectively.
Hot compression features of Ti-55511 alloy are investigated by high-temperature compression tests in α+β region. It is found that the flow stress and softening mechanisms are obviously influenced by ...deformation conditions. The true stress decreases with the reduced strain rate or the raised temperature. The spheroidization of α phase and dynamic recrystallization (DRX) of β phase easily occur at low temperatures such as 973, 1003 and 1033 K, while the dynamic recovery (DRV) of β phase mainly occurs at high temperatures such as 1063 K because of the transformation from α phase to β phase at relatively high temperatures A dislocation density-based constitutive model, which is associated with DRV, work hardening mechanisms and the spheroidization of α phases, is established and validated to describe flow behavior. The correlation coefficient (R) and average absolute relative error (AARE) of the established model are 0.9924 and 6.8%, respectively. 3D power dissipation efficiency maps and processing maps are established to determine the appropriate processing window, i.e., too low temperatures (lower than 973 K) or too high strain rates (higher than 1 s−1) easily induce flow instability. Therefore, the medium temperature (1003–1063 K) and the low strain rate (0.001–0.1 s−1) are applicable for thermal compression of the studied titanium alloy.
•Hot compression features of Ti-55511 alloy is investigated by high-temperature compression tests in α+β region.•DRX easily occurs at low temperatures, while DRV mainly occurs at high temperatures.•A accurate dislocation density-based constitutive model considering the spheroidization of α phases is established.•The optimal forming temperature and strain rate are 1003–1063 K and 0.001–0.1 s−1.
Many commercial Ti-alloys contain 6 wt% Aluminium and these alloys are prone to precipitation of α2 (Ti3Al). Here, we investigate and quantify the effect of α2 precipitation on strain localisation ...behaviour for Ti-6Al-4V with an equiaxed microstructure using High Resolution Digital Image Correlation (HR-DIC) in combination with Electron Back Scatter Diffraction (EBSD). HR-DIC has enabled us to quantify strain localisation, which shows that at 1% applied strain the strain heterogeneity in terms of maximum shear strain is about twice in the sample containing α2 precipitates compared to the α2-free sample. Theoretical slip trace angles for all possible slip systems were calculated using Electron Back Scatter Diffraction (EBSD) orientation data and cross-correlated with experimental slip trace angles measured from nanoscale shear strain maps recorded by HR-DIC to predict the active slip domain. It has been found that while slip type activity in terms of frequency is strongly dependent on texture in respect to loading direction, the actual shear strain contribution from prismatic slip does increase significantly in the presence of α2 precipitation. This experimental observation supports previous calculations of Anti-Phase Boundary (APB) energies for α2 precipitates 1 where widely dissociated partial dislocations on the prismatic plane show a lower APB energy than the APB energy associated with shearing on the basal plane in α2.
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Implants with bioactive coatings are becoming increasingly popular in bioengineering. The incorporation of silver nanoparticles into the oxide layer supports the antibacterial effect. This article ...describes the surface modification of Ti15Mo alloy subjected to the plasma electrochemical oxidation (PEO) process in baths containing Ag compounds: Ag2O, Ag3PO4, and Ag3PO4+ Ca(PO4)2 to incorporate antibacterial silver particles into the oxide layer on the surface of the tested alloy. Scanning electron microscopy revealed changes in the surface porosity of the obtained oxide layers. Energy dispersive analysis, Raman spectroscopy, and X-ray electron spectroscopy determined the chemical composition and distribution of elements on the sample's surface. The tests showed the presence of embedded silver particles and uniform distribution on the surface. The bioactivity of the obtained layer was tested by adhesion tests of bacteria and fungi: Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922), and Candida albicans (ATCC 2091).
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•Two different Ag sources were used to form PEO layers.•PEO baths consisted of Ag-containing suspensions.•Bioactive Ca, P and Ag were successfully introduced into Ti15Mo surface during PEO.•Ag incorporated PEO coatings exhibited bacteriostatic effect.
In this work, the correlations between the growth mechanism and properties of micro-arc oxidation (MAO) coatings on titanium (Ti) alloy were studied using different electrolytes. The adhesion and ...tribological properties of MAO coatings were evaluated by thermal shock tests and ball-on-disk friction tests, respectively. Results show that the growth mechanism as well as adhesion and tribological properties of MAO coatings are greatly influenced by electrolytes. In silicate electrolyte, the growth of MAO coatings is dominated by the deposition of silicate oxides and mostly characterized by outward growth. As a result, the coatings exhibit poor adhesion, but the presence of silicate oxides in the coatings is beneficial for improving the wear resistance. In phosphate electrolyte, the coating growth mainly results from the oxidation of the substrate and is more characterized by inward growth, resulting in high adhesion but low wear resistance. Employing the mixed silicate and phosphate electrolyte, however, is a feasible way of optimization to get relatively both high adhesion and improved wear resistance.
•The properties of MAO coatings are determined by growth mechanism.•The coating growth mechanism is influenced by electrolytes.•Silicate and phosphate electrolytes are comparatively studied.•The outward growth and inward growth are emphasized.
The main challenge in electrochemical machining (ECM) of titanium alloys is not processing efficiency, but the difficulty in achieving high-quality machined surfaces. Titanium alloys are particularly ...susceptible to pitting, and there are obvious dissolution differences among different phases in ECM, which greatly affect the machining accuracy and surface quality. This paper aims to eliminate the dissolution disparity between different phases and enhance the surface quality of titanium alloys through the synergistic effect of a similar normal distribution current waveform and passivation film. A new experimental phenomenon was found that the presence of a passivation film resulted in a remarkably smooth pit during the initial pitting stage, with no discernible dissolution differences among different phases. A superior surface quality can be achieved when the surface of the titanium alloy sample is composed of these smooth pits. From this perspective, the effect of frequency and passivation film on pitting process and surface quality of titanium alloys in pulse ECM was systematically investigated. In particular, at the frequency of 100 Hz, high surface quality was achieved on Ti-6.5Al-2Zr-1Mo-1 V (TA15) alloy with a surface roughness of Ra 0.69 μm while the dissolution disparity between different phases was significantly eliminated. The rate of material thickness removal was 0.09 mm/min. Successful applications on other titanium alloys, such as Ti-6Al-4 V (TC4), Ti-25 V-15Cr-0.3Si (Ti40), and Ti-5.8Al-4Sn-3.58Zr-0.7Mo-0.5 Nb-1.1Ta-0.4Si (Ti60), further validated that high-quality surfaces can be obtained with surface roughness values of Ra 0.31 μm, Ra 0.44 μm and Ra 0.92 μm respectively.
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•The synergistic effect of current waveform and passivation film improve the surface quality of titanium alloys in ECM.•The current waveform and frequency affect the corrosion morphology and repassivation process.•High quality surfaces of titanium alloys are achieved under optimized frequencies.
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