Ultrafine-grained Titanium Research Articles

Microstructural evolution in ultrafine-grained titanium processed by high-pressure torsion under different pressures

A grade 2 commercially pure (CP) titanium was processed by high-pressure torsion (HPT) at pressures of 3.0 and 6.0 GPa in order to achieve improved strengths. The microhardness values for these Ti samples were plotted against the imposed strain, and the plots show that a higher saturation microhardness of 320 Hv is achieved for the sample processed at 6.0 GPa compared to a microhardness of 305 Hv when using a pressure of 3.0 GPa. The omega ω-phase has been reported in some earlier HPT investigations of pure titanium, but it was not detected in this investigation even after processing at 6.0 GPa. The absence of the ω-phase is attributed to the relatively high level of oxygen (0.25 wt%) in these CP titanium samples. The higher saturation hardness for the 6.0 GPa sample is consistent with the smaller average grain size of ~105 ± 12 nm compared with the measured grain size of ~130 ± 18 nm after processing with an imposed pressure of 3.0 GPa.

Gradient ultrafine-grained titanium: Computational study of mechanical and damage behavior

A computational model of ultrafine-grained (UFG) titanium with random and gradient distribution based on Voronoi tessellation and the composite model of nanomaterials is developed. The effect of grain size, non-equilibrium state of the grain boundary phase (characterized by the initial dislocation density and diffusion coefficient) and gradient of grain sizes on the mechanical behavior and damage initiation of the UFG titanium are studied in numerical experiments. Using computational experiments, the authors determined the likely damage criterion (dislocation-based model) and found several effects that can positively influence the mechanical response and strength of UFG titanium (homogeneity of grain sizes, dispersoids/precipitates in grain boundaries and initial dislocation density in grain boundaries). It is shown that the homogeneous structures of UFG titanium ensure higher yield stress and lower likelihood of damage than gradient structures. The availability of dispersoids or precipitates in UFG titanium changes its damage mechanisms and delays the evolution of damage.

Electrochemical and cellular behavior of ultrafine-grained titanium in vitro

The electrochemical and cellular behavior of commercially pure titanium (CP-Ti) with both ultrafine-grained (UFG) and coarse-grained (CG) microstructure was evaluated in this study. Equal channel angular pressing was used to produce the UFG structure titanium. Polarization and electrochemical impedance tests were carried out in a simulated body fluid (SBF) at 37°C. Cellular behaviors of samples were assessed using fibroblast cells. Results of the investigations illustrate the improvement of both corrosion and biological behavior of UFG CP-Ti in comparison with the CG counterpart.

Manufacturing of ultrafine-grained titanium by caliber rolling in the laboratory and in industry

The mechanical properties and the microstructure of Grade 2 titanium semi-products processed by warm caliber rolling in both laboratory and industrial environments are studied. It is shown that this technology yields ultrafine-grained (UFG) microstructure with high tensile strength and good ductility at room temperature. Finite element modelling (FEM) suggests that the effectiveness of caliber rolling in grain refinement is mainly caused by the large, homogeneous imposed strain, similar to conventional severe plastic deformation (SPD) methods. It is proved that the mechanical and microstructural properties of titanium processed by the industrial equipment are similar to the characteristics of the material manufactured in the laboratory. This observation suggests that caliber rolling carried out in industrial environments may be a candidate technology in mass-production of UFG titanium with improved mechanical properties.

Microstructure and Mechanical Properties of Nanostructured and Ultrafine-Grained Titanium and the Zirconium Formed by the Method of Severe Plastic Deformation

Results of investigation of the microstructure, mechanical properties, and thermostability of bioinert titanium VT1-0 and zirconium E110 in nanostructured and ultrafine-grained states formed by combined methods of severe plastic deformation, including abc pressing in a press-mould or without it and multipass rolling in grooved or smooth rolls, are presented. It is demonstrated that the combined severe plastic deformation method allows titanium and zirconium billets in nanostructured and ultrafine-grained states to be obtained that provides substantial improvement of the mechanical properties comparable to the properties of titanium alloys, for example, VT6 and VT16 ones. The yield strength and the microhardness of titanium and zirconium obey the Hall–Petch relationship.

Lanthanum-containing hydroxyapatite coating on ultrafine-grained titanium by micro-arc oxidation: A promising strategy to enhance overall performance of titanium

Titanium is widely used in biomedical materials, particularly in dental implants, because of its excellent biocompatibility and mechanical characteristics. However, titanium implant failures still remain in some cases, varying with implantation sites and patients. Improving its overall performance is a major focus of dental implant research. Equal-channel angular pressing (ECAP) can result in ultrafine-grained titanium with superior mechanical properties and better biocompatibility, which significantly benefits dental implants, and without any harmful alloying elements. Lanthanum (La) can inhibit the acidogenicity of dental plaque and La-containing hydroxyapatite (La-HA) possesses a series of attractive properties, in contrast to La-free HA. Micro-arc oxidation (MAO) is a promising technology that can produce porous and firmly adherent hydroxyapatite (HA) coatings on titanium substrates. Therefore, we hypothesize that porous La-containing hydroxyapatite coatings with different La content (0.89%, 1.3% and 1.79%) can be prepared on ultrafine-grained (~200–400 nm) titanium by ECAP and MAO in electrolytic solution containing 0.2 mol/L calcium acetate, 0.02 mol/L β-glycerol phosphate disodium salt pentahydrate (β-GP), and lanthanum nitrate with different concentrations to further improve the overall performance of titanium, which are expected to have great potential in medical applications as a dental implant.

Microstructure–texture–mechanical properties relationship in multi-pass warm rolled Ti–6Al–4V Alloy

In the present study, high strength bulk ultrafine-grained titanium alloy Ti–6Al–4V bars were successfully processed using multi-pass warm rolling. Ti–6Al–4V bars of 12mm diameter and several metres long were processed by multi-pass warm rolling at 650°C, 700°C and 750°C. The highest achieved mechanical properties for Ti–6Al–4V in as rolled condition were yield strength 1191MPa, ultimate tensile strength of 1299MPa having an elongation of 10% when the rolling temperature was 650°C. The concurrent evolution of microstructure and texture has been studied using optical microscopy, electron back scattered diffraction and x-ray diffraction. The significant improvement in mechanical properties has been attributed to the ultrafine-grained microstructure as well as the morphology of α and β phases in the warm rolled specimens. The warm rolling of Ti–6Al–4V leads to formation of 〈101¯0〉α//RD fibre texture. This study shows that multi-pass warm rolling has potential to eliminate the costly and time consuming heat treatment steps for small diameter bar products, as the solution treated and aged (STA) properties are achievable in the as rolled condition itself.

Influence of annealing on ductility of ultrafine-grained titanium processed by equal-channel angular pressing–Conform and drawing

Abstract

Effect of Texture and Grain Size on Bio-Corrosion Response of Ultrafine-Grained Titanium

The bio-corrosion response of ultrafine-grained commercially pure titanium processed by different routes of equal-channel angular pressing has been studied in simulated body fluid. The results indicate that the samples processed through route Bc that involved rotation of the workpiece by 90 deg in the same sense between each pass exhibited higher corrosion resistance compared to the ones processed by other routes of equal-channel angular pressing, as well as the coarse-grained sample. For a similar grain size, the higher corrosion resistance of the samples exhibiting off-basal texture compared to shear texture indicates the major role of texture in corrosion behavior. It is postulated that an optimum combination of microstructure and crystallographic texture can lead to high strength and excellent corrosion resistance.

On the deformation mechanism of ultrafine-grained titanium with multilayered hierarchical structure

The ultrafine-grained Ti with a multilayered hierarchical structure (MHS) has been fabricated by using cryorolling combined with surface mechanical attrition treatment technique. The deformation behavior of this material is a result of superimposition of the different structural layers (and their interfaces) each of which has its specific deformation and failure mechanism. The deformation mechanisms include copious shear banding in the outer amorphous/nanocrystallite layer, grain boundary mediated deformation in the nanograined layer and shear localization in the ultrafine-grained core. The mechanical gradations in the junctions between layers are believed to be responsible for the mitigated interface failure pattern, crack-stopping effect and the enhanced loading resistance. Such a complex plastic deformation processes operating within the individual layers benefits the high strength and work hardening of the MHS Ti.

Indentation and scratch testing of DLC-Zr coatings on ultrafine-grained titanium processed by high-pressure torsion

High-pressure torsion was employed to refine the microstructure of grade 2 Ti under an imposed pressure of 3.0GPa at room temperature. The microhardness of grade 2 Ti increased from 1.82GPa for the coarse grain state to 3.05GPa after high-pressure torsion processing, where this value is very close to the hardness of the Ti–6Al–4V alloy. Subsequently, several diamond-like carbon (DLC) coatings with thicknesses of ∼1.4μm were deposited on as-received Ti, high-pressure torsion processed Ti and Ti–6Al–4V samples via physical vapour deposition. Both indentation and scratch tests showed a much improved adhesion of DLC-7Zr, DLC:H-7Zr and DLC-9Zr coatings with high-pressure torsion processed Ti as the substrate by comparison with the same coatings on coarse-grained Ti. The results suggest that commercial pure Ti processed by high-pressure torsion and coated with a diamond-like carbon coating provides a potential candidate material for bio-implant applications.

The Ultrafine-Grained Titanium and Biomedical Titanium Alloys Processed by Severe Plastic Deformation (SPD)

The ultrafine-grained materials processed by severe plastic deformation (SPD) can be tailored to achieve superior properties and performances. Recently, the SPD methods, emerging as an effective way to grain refinement, have become attractive for fabrication of the ultrafine-grained biomedical materials, which can be adjusted to possess both favorable mechanical properties and excellent biocompatibility. Biomedical titanium alloys have become one of the most promising biomedical metallic materials, due to their high strength, low density, good biocompatibility and excellent corrosion resistance. Compared with traditional titanium alloys, the ultrafine-grained biomedical titanium alloys possess higher strength, better corrosion resistance and fatigue performance. Moreover, the ultrafine-grained biomedical titanium alloys, which are usually used for orthopedic and dental implants, can induce in-growth of bone tissues, increase the interfacial strength and accelerate the repair process. This review examines recent developments related to fabricating ultrafine-grained titanium and biomedical titanium alloys by various kinds of SPD methods, such as equal channel angular pressing (ECAP), high pressure torsion (HPT), accumulative roll bonding (ARB) and friction stir processing (FSP). More specifically, the mechanical properties and performances of biomedical titanium alloys processed by ECAP, HPT, ARB and FSP have been investigated. It can be expected that in the near future, these techniques will be utilized as methods for continuous productions of the UFG biomaterials in large scale industrial applications.

Processing of an ultrafine-grained titanium by high-pressure torsion: An evaluation of the wear properties with and without a TiN coating

A commercial purity (CP) Grade 2 Ti was processed by high-pressure torsion (HPT) using an imposed pressure of 3.0GPa at room temperature. The HPT processing reduced the grain size from ∼8.6μm in the as-received state to ultra-fine grains (UFG) of ∼130nm after HPT. Tensile testing showed the HPT-processed Ti exhibited a good combination of high ultimate tensile strength (∼940MPa) and a reasonable elongation to failure (∼23%). Physical vapour deposition was used to deposit TiN coatings, with a thickness of 2.5μm, on Ti samples both with and without HPT processing. Scratch tests showed the TiN coating on UFG Ti had a critical failure load of ∼22.5N whereas the load was only ∼12.7N for the coarse-grained Ti. The difference is explained using a simple composite hardness model. Wear tests demonstrated an improved wear resistance of TiN coating when using UFG Ti as the substrate. The results suggest that CP Ti processed by HPT and subsequently coated with TiN provides a potentially important material for use in bio-implants.

Nanostructural titanium: Its applications, structure, and properties

In most cases, medical implants are made from titanium alloys or stainless steel with good mechanical properties. However, these materials contain toxic ele� ments such as nickel, aluminum, and vanadium. For medical applications, it would be preferable to use titanium (1, 2), zirconium, niobium and their alloys, with distinctive physicomechanical and biological properties. The widespread use of titanium in implants is mainly hindered by its poor mechanical properties, including its durability under periodic and cyclic loads. This problem may be resolved by using nano� structural (ultrafinegrain) titanium. At present, we are able to produce large blanks of nanostructural tita� nium with good mechanical properties (3). The meth� ods employed are based on intense plastic deformation (3-7): equalchannel angular pressing (3, 4), abc pressing (5-7), and so on. By intense plastic deforma� tion, a nanostructural state may be established over the whole volume of the blank. That ensures mechanical properties matching those of moderately complex tita� nium alloys such as VT6 alloy. The creation of nanostructure permits fundamen� tal change in the mechanical properties of metals: the yield point and strength, the fatigue life, the wear resis� tance, the cyclic durability, etc. Note that at least two successive methods of intense plastic deformation must be used to obtain a nanostructural state of tita� nium (6, 7). Those methods may be combined in a sin� gle cycle (3). Twostage intense plastic deformation was pro� posed in (6, 7): abc pressing in a mold; and multipass rolling. In that approach, the initial upsetting in the mold involves successively changing the compression axis three or four times (analogously to multistage abc pressing (5)). In the first stage, the blank is deformed in a hydraulic press at 10 -3 -10 -1 s -1 . At specified tem� perature, each cycle includes onetime 40-50% upsetting, with subsequent change of the deformation axis by 90° rotation of the blank around the longitudi� nal axis. The temperature of the blank is reduced in stages in the range 700-400°C on passing to the next cycle. In the second stage, the blank is deformed by rolling at room temperature; the rollers may be grooved or smooth. The accumulated strain in rolling is 90%. Rolling produces blanks in the form of rods or plates. The final blanks are annealed in argon at 250 or 300 °C to remove the internal stress and increase the plasticity. As a result of twostage treatment and lowtemper� ature annealing, uniform grain-subgrain nanostruc� ture is formed in the blank (Fig. 1). The mean size of the elements (grains, subgrains, fragments) is less than 100 nm. This nanostructural state ensures plasticity of 6-10%, yield point of 1100 MPa, and strength of 1160 MPa.

High performance ultrafine-grained Ti-Fe-based alloys with multiple length-scale phases

In order to simultaneously enhance the strength and plasticity in nanostructured / ultrafine-grained alloys, a strategy of introducing multiple length scales into microstructure (or called bimodal composite microstructure) has been developed recently. This paper presents a brief overview of the alloy developement and the mechanical behavior of ultrafine-grained Ti-Fe-based alloys with different length-scale phases, i.e., micrometer-sized primary phases (dendrites or eutectic) embedded in an ultrafine-grained eutectic matrix. These ultrafine-grained titanium bimodal composites could be directly obtained through a simple single-step solidification process. The as-prepared composites exhibit superior mechanical properties, including high strength of 2000-2700 MPa, large plasticity up to 15-20% and high specific strength. Plastic deformation of the ultrafine-grained titanium bimodal composites occurs through a combination of dislocation-based slip in the nano-/ultrafine scale matrix and constraint multiple shear banding around the micrometer-sized primary phase. The microstructural charactersitcs associated to the mechanical behaivor have been detailed discussed.

Machining of coarse grained and ultra fine grained titanium

Machining of titanium is quite difficult and expensive. Heat generated in the process of cutting does not dissipate quickly, which affects tool life. In the last decade ultra fine grained (UFG) titanium has emerged as an option for substitution for more expensive titanium alloys. Extreme grain refinement can be readily performed by severe plastic deformation techniques. Grain refinement of a material achieved in this way was shown to change its mechanical and physical properties. In the present study, the microstructure evolution and the shear band formation in chips of coarse grained and UFG titanium machined to three different depths and three different feeding rates was investigated. A change in thermal characteristics of commercial purity Ti with grain refinement was studied by comparing heating/cooling measurements with an analytical solution of the heat transfer boundary problem. It was demonstrated that an improvement in the machinability can be expected for UFG titanium.

A modified Hall–Petch relationship in ultrafine-grained titanium recycled from chips by equal channel angular pressing

Equal channel angular pressing was used to consolidate commercially pure titanium chips into fully dense bulk material with high strength (up to 650 MPa), due to an ultrafine grain size as low as 0.8 μm, and good ductility (∼16%). Electron backscattered diffraction was employed to reveal low-angle grain boundaries (LAGBs), enabling a modified Hall-Petch relationship to be analyzed in consideration of strengthening contributions from LAGBs in particular, but also interstitial solutes, high-angle grain boundaries and potential oxide dispersion.

Thermal stability and annealing behaviour of ultrafine grained commercially pure titanium

Abstract Ultrafine grained (UFG) commercially pure titanium was fabricated by 8 successive passes of equal channel angular pressing (ECAP) via route B C . The deformed structure was studied by transmission electron microscopy (TEM). The material was then annealed at a range of temperature (450–600 °C) for up to 6 h. The microstructure evolution and the grain growth behaviour were investigated by electron back scattered diffraction (EBSD) technique. The UFG titanium was found to be thermally stable up to 450 °C. Based on the experimental results, the grain growth kinetics was characterized by calculating the grain growth activation energy ( Q ) and the time exponent ( n ). The value of Q was found to be close to the self diffusion activation energy of titanium, while n was found to be lower than the time exponent reported for grain growth in coarse grained pure titanium.

Evolution of the temperature field during deformation and fracture of specimens of coarse-grained and ultrafine-grained titanium

Studies of the deformation and evolution of temperature fields in flat specimens of BT1-0 titanium in the Coarse-Grained (CG) and nanostructured/ultrafine-grained (NS/UFG) states and BT6 titanium alloy were performed. The yield and ultimate stresses for NS/UFG BT1-0 titanium are twice as high as those for the CG state and are comparable with the characteristics of the BT6 alloy. It was found that the ultimate strain before damage of NS/UFG titanium specimens that are tensioned at a constant deformation rate of 6.5 × 10−3 s−1 decreases by a factor of 2. In the region of a macroscopic localization of a plastic deformation the temperature abruptly rises and reaches the maximum values in the crack formation zone. An abrupt temperature increase in the zones of localization of plastic deformations and crack formation is observed in specimens with both the CG and NS/UFG states. Comparing the experimental data on the temperature distributions on the surfaces of strained CG and NS/UFG titanium specimens shows that a macroscopic plastic deformation develops more uniformly in BT1-0 titanium in the NS/UFG state. Under identical loading conditions, the heat release rate is higher for BT1-0 titanium in the NS/UFG state.

Enhanced in vitro biocompatibility of ultrafine-grained titanium with hierarchical porous surface

Bulk ultrafine-grained Ti (UFG Ti) was successfully fabricated by equal-channel angular pressing (ECAP) technique in the present study, and to further improve its surface biocompatibility, surface modification techniques including sandblasting, acid etching and alkali treatment were employed to produce a hierarchical porous surface. The effect of the above surface treatments on the surface roughness, wettability, electrochemical corrosion behavior, apatite forming ability and cellular behavior of UFG Ti were systematically investigated with the coarse-grained Ti as control. Results show that UFG-Ti with surface modification had no pitting corrosion and presented low corrosion rate in simulated body fluids (SBF). The hierarchical porous surface yielded by surface modification enhanced the ability of UFG Ti to form a complete apatite layer when soaked in SBF and promoted osteoblast-like cells attachment and proliferation in vitro, which promises to have a significant impact on increasing bone-bonding ability and reducing healing time when implanted due to faster tissue integration.