Effects of thickness and texture on mechanical properties anisotropy of commercially pure titanium thin sheets

Simultaneous effects of thickness and texture on the anisotropy of mechanical properties and fracture behaviors of commercially pure titanium thin sheets were studied. The activation of different deformation systems, due to the split distribution of basal texture, led to mechanical properties anisotropy. The crack initiation and propagation energies, when the loading direction was parallel to the initial rolling direction, decreased with increasing thickness ranges from 0.25 to 1 mm. The changes of size, shape and distribution of dimples with increasing thickness confirmed the restriction of deformation systems and the development of triaxial stress state and plane-strain condition at the notch tip. However, in transverse-directed specimens, the energy release rate increased with increasing specimen thickness up to 0.75 mm and then decreased. The fractography of these specimens explained the simultaneous effects of thickness and texture on structural stability and high accommodated plastic deformation at the notch tip.     

Cryogenic equal channel angular pressing of commercially pure titanium: microstructure and properties

Cylindrical samples of CP Titanium (Grade 2) were deformed by one, two and three passes of equal channel angular pressing (ECAP) each at temperatures 77, 300 and 575 K, respectively. The microstructure of samples processed at 77 K shows retardation of recrystallisation, high density of dislocations and deformation twins, diffuse and obscure grain boundaries compare to microstructure of samples processed at room and high temperature, where recrystallised ultrafine equiaxed grains are observed. Mechanical properties for all structural states of Ti were studied by microhardness measurements at 300 K and uniaxial compression at temperatures 300, 170, 77 and 4.2 K. Higher levels of ECAP deformation (more passes of ECAP) lead to higher values of strength and hardness at all studied temperatures. Decrease of ECAP temperature leads to increase of strength characteristics in all cases. Influence of ECAP and compression temperatures on possible changes of deformation mechanism are discussed.     

Corrosion and in vitro biocompatibility properties of cryomilled-spark plasma sintered commercially pure titanium

Ti alloys, such as Ti6Al4V, are currently used in biomedical and dental implant applications. Ti alloys are used because they are stronger than commercially pure (CP) Ti due to the presence of alloying elements. However, toxicity of alloying elements during long-term use of implants is of concern. Another means of increasing the strength of materials is grain size refinement. In this study, ultrafine-grained (UFG, ~250 nm to 1 μm) CP Ti was produced by cryomilling followed by spark plasma sintering (SPS). Electrochemical impedance spectroscopy (EIS) and cell culture experiments were performed to compare the corrosion and biocompatibility properties of coarse grained (CG) Ti and UFG Ti. It was found that UFG Ti exhibited corrosion resistance comparable to CG Ti in Ringers solution. In addition, UFG Ti exhibited a reduced inflammatory response and enhanced cell adhesion compared to CG Ti. Investigation of surface roughness provided an explanation for enhanced cell adhesion.     

Formation mechanism of hydride precipitation in commercially pure titanium

Since titanium has high affinity for hydrogen and reacts reversibly with hydrogen,the precipitation of titanium hydrides in titanium and its alloys cannot be ignored.Two most common hydride precipitates in α-Ti matrix are γ-hydride and δ-hydride,however their mechanisms for precipitation are still unclear.In the present study,we find that both γ-hydride and δ-hydride phases with different specific orienta-tions were randomly precipitated in the as-received hot forged commercially pure Ti.In addition,a large amount of the titanium hydrides can be introduced into Ti matrix with selective precipitation by using electrochemical treatment.Cs-corrected scanning transmission electron microscopy is used to study the precipitation mechanisms of the two hydrides.It is revealed that the γ-hydride and δ-hydride precipita-tions are both formed through slip + shuffle mechanisms involving a unit of two layers of titanium atoms,but the difference is that the γ-hydride is formed by prismatic slip corresponding to hydrogen occupy-ing the octahedral sites of α-Ti,while the δ-hydride is formed by basal slip corresponding to hydrogen occupying the tetrahedral sites of cα-Ti.     

Notch-texture strengthening mechanism in commercially pure titanium thin sheets

Simultaneous effects of notch and texture on strengthening mechanisms of rolled thin sheets of commercially pure titanium were investigated. The presence of notch led to the restriction of deformation systems and different fracture behaviors compared to un-notched specimens. The loss of material’s ability to accommodate plastic deformation at the notch tip with increase in rolling reductions changed the notch strengthening phenomenon to the notch weakening one. At medium levels of deformation, due to the simultaneous development of a triaxial stress state and strong basal texture at the notch tip, a new strengthening mechanism which is called “notch-texture strengthening mechanism” led to a significant enhancement of tear strength. However, the lack of stress triaxiality in un-notched tensile specimens and a strong basal texture component in other notched specimens reduced the impact of strengthening. It was found that the restriction of deformation systems due to the c-axis compression condition at the notch tip was responsible for this strengthening mechanism.     

Quantitative bone tissue response to commercially pure titanium implants

Commercially pure titanium implants were inserted in rabbit tibia for 3, 6 and 12 weeks. Each rabbit had two implants inserted, one for removal torque measurements and the other for histomorphometrical analysis. Light microscopic observations revealed that there was a continuing bone remodelling, with new bone formation in the periosteal region after 3 and 6 weeks, which diminished with time, i.e. up to 12 weeks of follow-up. A higher removal torque was observed with increasing time of implant insertion. The removal torque values for the 12 week samples were converted to three different shear forces depending on three different theoretically calculated implant to bone attachment levels. The mean shear forces related to the entire length of the implant surface was 0.6 N mm^-2. If considering the length of the implant inside the cortical bone only, the mean shear force was 1.9 N mm^-2, and if the bone-metal contact length as related to an estimate of the bone-implant contact a mean shear force of 14.8 N mm^-2 was calculated. Histomorphometrical measurements revealed more bone-metal contact as well as a larger bone area in the threads with increasing time of insertion. Statistically significant differences were observed between all measurements of the 3 and 6 week samples and between the 3 and 12 week samples. For the 6 and 12 week sections a statistically eignificant difference could be demonstrated only when comparing bone areas in the three best consecutive threads located in the cortical region.     

Orientation dependent slip in polycrystalline titanium

The factors determining the active slip systems in cold-rolled polycrystalline titanium sheet were investigated. The texture of such a sheet has an important role in determining the active slip systems. Equi-Schmid factor lines for different slip modes were calculated, and transmission electron microscopy proved that pile ups of dislocations of the predicted systems are formed. The active primary slip system was found to be the prismatic a type slip {1¯100} 11¯20 while the secondary system is either prismatic or pyramidal type I {10¯11}. Basal slip of a dislocations could in certain orientations of load direction be the primary slip systems. Dislocations of the (c + a) type play no significant role in the plastic deformation of polycrystalline titanium sheet.     

Formation of face-centered cubic titanium in laser surface re-melted commercially pure titanium plate

Micron-scale face-centered cubic titanium phase(named as δ phase) were noticed in the re-melted zone of laser surface re-melted commercially pure titanium plate.The morphology,sub-structure,orientation and distribution of δ phase were investigated by scanning electron microscopy,electron back-scattered diffraction and transmission electron microscopy.Three kind formation processes of δ phase were put forward based on the investigation.The first one is α'→δ transformation which takes place in single α'grains and leads to the orientation relationship {001}δ//{0001}α' 110 δ// 1120 α'.The second one is β→α'+ δ transformation which takes place at α'/α'interfaces and leads to the orientation relationship{001}δ//110β110 δ//111β.The third one is another kind of β→α'+ δ transformation that takes place at α'/α'interfaces and leads to the orientation relationship111δ//110β 110 δ// 111 β.It is believed that the transformations of δ phase are stress assistant ones and in the present investigation,the phase transformation stress of β→α'transformation acts as the assistant driving force for the formation of δ phase.     

Osseointegration evaluation of laser-deposited titanium dioxide nanoparticles on commercially pure titanium dental implants

The nanotechnology field plays an important role in the improvement of dental implant surfaces. However, the different techniques used to coat these implants with nanostructured materials can differently affect cells, biomolecules and even ions at the nano scale level. The aim of this study is to evaluate and compare the structural, biomechanical and histological characterization of nano titania films produced by either modified laser or dip coating techniques on commercially pure titanium implant fixtures. Grade II commercially pure titanium rectangular samples measuring 35?×?12?×?0.25?mm length, width and thickness, respectively were coated with titania films using a modified laser deposition technique as the experimental group, while the control group was dip-coated with titania film. The crystallinity, surface roughness, histological feature, microstructures and removal torque values were investigated and compared between the groups. Compared with dip coating technique, the modified laser technique provided a higher quality thin coating film, with improved surface roughness values. For in vivo examinations, forty coated screw-designed dental implants were inserted into the tibia of 20 white New Zealand rabbits’ bone. Biomechanical and histological evaluations were performed after 2 and 4 weeks of implantation. The histological findings showed a variation in the bone response around coated implants done with different coating techniques and different healing intervals. Modified laser-coated samples revealed a significant improvement in structure, surface roughness values, bone integration and bond strength at the bone-implant interface than dip-coated samples. Thus, this technique can be an alternative for coating titanium dental implants.     

Room-temperature creep behavior on crack tip of commercially pure titanium

The room-temperature creep behavior on crack tip of compact tensile (CT) specimen for commercially pure titanium (CP-Ti) was studied by experiment and finite element (FE) simulation in this paper. The experimental results indicated that the time-dependent deformation was observed on the crack tip of CP-Ti CT specimen at room temperature, which agreed with the primary creep, and crack propagation was not observed. In order to consider the creep behavior on crack tip, time-dependent J-integral was used to characterize the stress fields near crack tip. The room-temperature creep behavior on crack tip was analyzed by FE simulation, which was verified by experimental results. Then, the strain fields under different stress states were analyzed by FE simulation. The influences of the locations to crack tip and load on the room-temperature creep were analyzed, which shows that the creep significance on crack tip is enhanced with increasing of load and decreasing of distance to crack tip. The estimation formula of creep strain value along ligament direction of CP-Ti CT specimen was established and verified by FE simulation results.     

Improvement of mechanical properties of cast pure titanium by repeated heat treatment

ABSTRACT

Pure titanium components fabricated by casting have a coarse grain microstructure. To improve the mechanical strength of pure titanium components by refining the grain size, the cast samples were repeatedly heat-treated. During the heat treatment, the titanium samples were repeatedly heated above the alpha-to-beta (α–β) transition temperature and cooled to room temperature to undergo phase transformation. The heating cycle was performed 1, 3, 5, and 7 times. As the number of heating cycles increased, the grain size decreased. The tensile strength was 267.9?MPa in the as-cast sample and improved to 343.4?MPa after 7 heat-treatment cycles owing to the grain size refinement, while the elongation was maintained during the heat treatment.

This paper is part of a thematic issue on Titanium.     

Diffusion bonding between commercially pure titanium and micro-duplex stainless steel

Diffusion-bonded joints between commercially pure titanium and micro-duplex stainless steel were prepared in the temperature range of 800–950 °C for 1.5 h under 3 MPa uniaxial load in vacuum. The diffusion bonds were characterized using light and scanning electron microscopy. The composition of the reaction products were determined by energy dispersive spectroscopy. Up to 850 °C, -Fe + λ and λ + FeTi phase mixtures were formed at diffusion interface; however -Fe + λ, λ + FeTi and FeTi + β-Ti phases mixtures were formed at 900 °C and above. The presence of these intermetallics was confirmed by X-ray diffraction technique. The maximum tensile strength of 96% of Ti and shear strength of 81% of Ti along with 6.9% ductility were obtained for the diffusion couple processed at 850 °C due to the finer width of intermetallic phases. With a rise in the joining temperature the bond strength drops owing to an increase in the width of reaction products.     

Influence of texture on the mechanical properties of commercially pure magnesium prepared by powder metallurgy

In the present work the influence of texture on the mechanical properties up to 500 °C of commercially pure magnesium prepared by PM was determined. Extrusion of magnesium powders was carried out between 250 and 450 °C. All extruded materials exhibited an intense fibre texture with the basal planes parallel to the extrusion direction whose intensity increased in line with the extrusion temperature. The microstructure consisted of highly elongated magnesium powder particles. All the materials presented a heterogeneous grain size resulting from the size distribution of the original magnesium powder particles. In addition, small MgO particles were found mainly decorating the original powder boundaries. The best mechanical properties corresponded to the materials extruded at 400 and 450 °C. This behaviour was associated particularly with the intense fibre texture of these materials.     

Microstructural and mechanical properties of commercially pure aluminum subjected to Dual Equal Channel Lateral Extrusion

This paper aims to provide an introductory insight about “Dual Equal Channel Lateral Extrusion”, a counterpart of “Equal Channel Angular Extrusion”. The process is implemented to severely refine the microstructure of aluminum slabs. Comparisons of macroscopic parameters as average straining and large scale distribution of strain, as well as process loads reveal the supremacies and short comings of DECLE with respect to ECAE. DECLE shares a relatively similar geometry of deformation with that of ECAE. The advantages of this process with respect to ECAE are: (i) more intensive strains attainable per pass, and (ii) less extruding power needed for a given sample size. Nonetheless, less homogeneous strain per pass is seen in case of DECLE. TEM inspections revealed remarkable refinement of the microstructure through out the process and also some recrystallization at the final passes. Hardness tends to increase through successive passes to a limiting value beyond which there appeared a decline associated with intense recovery and the recrystallization observed. Compression tests exhibit the same trend, viz. a general rise in strength followed by a decrease in work hardening with increasing number of passes, leading to uniform microstructure and hardness after 9 passes.     

Spark plasma sintering behaviour of commercially pure titanium micro-alloyed with Ta-Ru

ABSTRACT

Spark plasma sintering (SPS) technology was used to consolidate Ti-Ta-Ru powders and the effects of sintering parameters on the densification, corrosion and wear performance of the sintered compacts were investigated. Results showed that addition of Ta with small amount of Ru had significant influence on densification, hardness and corrosion behavior of the sintered alloy. When 9 vol.% and 1 vol.% Ru were added, the sintered density and hardness were 92.07% and 330 HV_0.1 respectively. Furthermore, the addition of tantalum and ruthenium improve the corrosion and wear behaviours of Ti with a significant effect on the corrosion potential, E_corr, and corrosion current density, I_corr, in 1?M HCl solution. The COF trend decreases upon the addition of Ta-Ru with relative improvement in wear resistance in Ti-10Ta and Ti-9Ta-1Ru as compared with commercially pure Ti. This decrease in COF is more gradual which may be attributed to the solid solution hardening offered by Ta and Ru in the matrix.     

Nanostructured hydroxyapatite/TiO(2) composite coating applied to commercially pure titanium by a co-sputtering technique

We demonstrate an approach for the coating of nanostructured hydroxyapatite(HAP)/TiO(2) composite on commercially pure Ti (CP-Ti) by a co-sputtering process. HAP/TiO(2) composite film was obtained by controlling the processing pressure. It was observed that decomposition of HAP into CaO was easily induced during sputtering at 0.53?Pa, a typical sputtering condition for film deposition. However, HAP/TiO(2) composite film was obtained with the sputtering pressure of 2.67?Pa. The Ca/P ratio was nearly maintained at 1.66 by sputter deposition at 2.67?Pa. We further confirmed by analysis of plasma spectral emission that the variation of the hydroxyl (OH) radical present was due to the Ar pressure during sputtering. It has been shown that HAP coatings are dependent on the processing pressure, which the hydroxyl radical requires in order to create HAP.     

Characterization of transition joints of commercially pure titanium to 304 stainless steel

Solid-state direct bonding between commercially pure titanium and type 304 austenitic stainless steel has been carried out in the temperature range of 850–950 °C, under a uniaxial pressure of 3 MPa for 1 h. The diffusion bonds have been evaluated using light microscopy, electron probe microanalysis (EPMA), X-ray diffraction (XRD) technique and tensile testing. Light microscopy shows that different intermediate layers are formed in the reaction zone, and the width of these layers increases with an increase in bonding temperature. EPMA revealed that, at any particular bonding temperature, Ti traverses a minimum distance in the 304 stainless steel side, whereas Fe, Cr and Ni travel comparatively larger distances in the Ti side. This microanalysis also indicated different step formations in the concentration profile of Ti, Fe and Cr over different composition ranges in the diffusion zone indicating formation of intermetallic phases that were detected by XRD. Brittle intermetallic phases lower the strength and ductility of the diffusion bonded couples significantly. Best room temperature tensile strength, 217 MPa, has been obtained at 850 °C processing temperature due to minimal deleterious effects.     

Long-term evaluation of titania-based ceramics compared with commercially pure titanium in vivo

Fifty-four cylinders (2.8 mm in diameter) machined from hot isostatically pressed titania (TI) and titania-hydroxyapatite (TI/HA-15 vol%) sintered at 925°C, as well as commercially pure titanium (c.p. Ti), were implanted in the fermoral cortical bone of New Zealand white rabbits for 1, 3 and 12 months. The shear strength between bone and implant was measured by a push-out test. The TI/HA composite showed a significantly higher bonding strength to bone compared to c.p. Ti at all times, while no differences were observed between TI and c. p. Ti at 1 and 3 months after implantation. Titania-based materials had a significantly higher bonding strength than that of c.p. Ti one year after implantation. The results indicate that bioactivity of HA in TI/HA composite contributes to the early bone apposition reflected by high bonding strength, while the stability of the oxide, determines the development of long-term bonding strength. Both effects may be explained by the level and type of ions released from the ceramic implant. HA has a positive conduction to bone ingrowth while TI has a limited interaction to the bone apposition due to the extraordinary low ion release in vivo. Under light microscopy, similar patterns of bone-implant interfaces were seen from titania-based materials and c.p. Ti in 3-month samples, indicating high biocompatibility of these materials. However, histological evaluation by light microscope cannot identify the differences between physical contact and chemical bonding of implant-bone interface, and thus does not give information on bonding mechanism and the level of shear stresses developed.