Coarse-grained Titanium Research Articles

Особенности обрабатываемости резанием новых высокопрочных титановых сплавов с ультрамелкозернистой структурой для деталей авиационной техники

The results of experimental studies of the cutting parameters effect (cutting speed, feed, depth-of-cut) on the roughness of Ra, the microstructure of the surface layer of Ti-6Al-4V alloy samples with a conventional coarse-grained (CG) and ultrafine-grained (UFG) structure obtained by equal-channel angular pressing are presented. In framework of solving the problem connected with studying the machinability of a new VT6 alloy having an UFG structure, the development of a methodology for experimental research under turning was carried out. Physical parameters such as vibrations and noise under cutting, power consumption, numerical parameters of roughness, the magnitude and the pattern of residual stresses distribution and others are determined according to cutting modes and the tool used for the operation. It is shown that when turning at a low cutting speed, the roughness of CG sample is better than UFG of the alloy. With an increase in the cutting speed by 1,5 times, on the contrary, a sample with an UFG structure has a lower roughness Ra compared to a CG sample. Differences in the morphology and microstructure of the chips formed within comparative machining are discussed, taking into account types of microstructure of the alloy being treated, namely, presence of plastic flow lines in the chip microstructure after turning an UFG sample, the formation of large shifts, chips and tiny fractures in the chips after turning a CG alloy. It is found that new titanium alloys under turning obtain better cutting machinability in terms of such parameters as surface roughness, noise and vibration under cutting, and a more favorable character of residual stresses and riveting in the surface layer. Studying these factors, it is possible to predict a significant reduction of tool wear and stabilization of its durability compared to conventional coarse-grained titanium treatment.

Variational mode decomposition based self-adaptive denoising imaging method for ultrasonic array testing of coarse-grained titanium alloys processed by additive manufacturing

The coarse columnar grain structure in titanium alloys processed by additive manufacturing leads to strong grain noise in ultrasonic testing, which affects the accurate detection and quantification of its defects. In this paper, a self-adaptive denoising imaging method for ultrasonic array testing is proposed based on variational mode decomposition (VMD). The proposed method uses VMD to decompose complex ultrasonic signals in full matrix capture (FMC) data collected by a linear ultrasonic array transducer. According to the particle swarm optimization (PSO) method, VMD parameters for FMC methods of the total focusing method (TFM), the plane wave imaging (PWI), and the Hadamard spatial encoding are self-adaptively determined. The evaluation parameters are set to select the intrinsic mode function components by VMD to complete the denoising reconstruction and imaging of FMC data. An additive-manufactured titanium alloy specimen was used to verify the improvement of defect detection capability by PSO-VMD denoising method. For a defect at large thickness in the specimen, compared with the conventional TFM and PWI methods, VMD denoising method based on the Hadamard spatial encoding can improve the signal-to-noise ratio by 7.7 dB and 9.0 dB, and reduce the quantitative error by 74 % and 68 %.

Microstructure evolution and grain fragmentation induced by the co-activation of slip and [formula omitted] nanotwin during cryo-forging of a Ti-6Al-4V alloy

The nanostructured titanium alloy usually exhibits superior mechanical properties than the coarse-grained titanium alloy. In the present work, the α-grain fragmentation induced by slip/nanotwin co-activation during cryo-forging was studied in a bimodal Ti-6Al-4V alloy, in which the nanosized (022‾1‾) deformation twin was revealed. The results show that the lamellar α-grains fragmented to small globular grains after 25% compression at 0.1 mm/s and 0.5 mm/s. The high strain rate and large deformation increased the dislocation density and flow stress, therefore strengthened the deformation texture and promoted the grain fragmentation. The (0001) <112‾0> basal slip system was regarded as the dominant slip mode during cryo-forging, which activated more intensely in lamellar α-grains than that in equiaxed α-grains, and was promoted as the deformation ratio and stain rate increased. In addition, the activation of basal slip system was inhomogeneous in lamellar α-grains. The partial lattice rotation caused high angle grain boundary (HAGB) formation and grain fragmentation. On the other hand, the (022‾1‾) nanosized deformation twins appeared within fragmented α-grains after 25% cryo-forging at 0.5 mm/s, in which the (011‾1) planes between matrix and twin existed a ∼27° misorientation. The (022‾1‾) deformation twins contributed to grain fragmentation by twin boundary grooving and twin-twin intersection. This work demonstrated an available route to achieve refined or gradient grained structure in Ti-6Al-4V alloy by slip/twinning co-activation.

Role of the Pulse Current Duty Cycle during Titanium Tension

The effect of a pulsed current on titanium tensile deformation obtained by postdeformation annealing after cold rolling of the coarse-grained and ultrafine-grained states has been considered. The effect of the duty cycle of the pulse current over a wide range on the shape of the stress–strain curves and mechanical properties has been studied. It is shown that an increase in the duty cycle results in an enhancement in the thermal effect of the current and a decrease in the flow stresses, strength, and plasticity, as well as in intense necking. A decrease in the duty cycle leads to the absence of heating and the occurrence of the electroplastic effect and an increase in the strength and plasticity, which depends on the structural state of coarse-grained titanium and the method of titanium production. The possible physical mechanisms of hardening associated with twinning, strain aging, and low-cycle fatigue have been considered.

Deformation Behavior under Tension with Pulse Current of Ultrafine-Grain and Coarse-Grain CP Titanium

The problem of the real existence of the electroplastic effect during deformation of metallic materials of different nature is still relevant. At the same time, the influence of structure refinement is not considered enough. In this work, the deformation behavior of ultrafine-grained (UFG) titanium Grade 4 is compared with that of coarse-grained (CG) titanium under tension with pulse current of the low duty cycle. The deformation curves of both structure states are presented for different regimes of pulsed current and thermal heating from an external source. Structure studies by optical and scanning electron microscopy, as well as microhardness measurements have been carried out. It is shown that Grade 4 titanium under tension accompanied by pulsed current exhibits electroplastic effect (EPE) in the form of a flow stress reduction. EPE in UFG state is much stronger than in CG state. An increase in the density and duration of the current pulse leads to a multiple decrease in the flow stresses in CG and UFG titanium. The contribution in the flow stress reduction from heating by an external source was less than that from tension with pulse current at the same temperatures. The impact of pulsed current during tension does not influence microhardness and grain size.

Superior properties and behaviour of coatings produced on nanostructured titanium by PEO coupled with the EPD process

Surface modification of commercially pure Grade 4 coarse-grained titanium (CG-Ti) and nano-grained titanium (n-Ti) by plasma electrolytic oxidation (PEO) and plasma electrolytic oxidation conjugated with electrophoretic deposition (PEO-EPD) processes is reported in the present study. Two different coatings were developed on each CG-Ti and n-Ti in phosphate-based electrolytes without and with the incorporation of hydroxyapatite (HA) nanoparticles. The phase composition, morphology (surface and cross-sectional), corrosion resistance, surface roughness, and scratch-resistance of the fabricated coatings were thoroughly studied and analysed. The L-929 fibroblast cells were used for assessing the in-vitro cell viability. The L-929 cells cultured on PEO-EPD treated CG-Ti, and n-Ti samples exhibited higher cell growth than PEO treated CG-Ti and n-Ti samples. Among all the PEO and PEO-EPD treated samples, the PEO-EPD treated n-Ti sample showed significantly better corrosion resistance (icorr = 8.85 × 10−7 mA cm−2), lower contact angle (40°), and good adhesion strength (Lc = 29 N), demonstrating the importance of the nanostructuring of the titanium substrate for the properties of the coating. The origin of the discovered enhancement in the properties of the modified PEO coating produced on nanostructured titanium was examined and discussed. After soaking in SBF for 14 days, the PEO-EPD treated sample is wholly covered with apatite layer indicating its good bioactivity

Influence of the structural state and oxide coating on the mechanical stability of VT1-0 titanium under its cyclic loading

It was found that the fatigue properties of submicrocrystalline titanium are significantly higher than those for its coarse-grained state. The application of the oxide coating leads to a slight increase in these properties for titanium with a submicrocrystalline and coarse-grained structure. Some features of fatigue fracture of submicrocrystalline and coarse-grained titanium are analyzed. Keywords: submicrocrystalline and coarse-grained titanium, VT1-0, fatigue, microarc oxidation.

Эволюция дефектной структуры в процессе длительного нагружения ультрамелкозернистого титана ВТ1-0, полученного при воздействии интенсивной пластической деформации

For high-strength titanium VT1-0, the ultrafine-grained structure of which was obtained by mechanical-thermal treatment using the methods of helical and longitudinal rolling followed by annealing to relieve stress of the first kind. The effect of long - term loading in the creep mode at elevated temperature on the size and shape of grains has been studied. A similar study was previously carried out for recrystallized coarse-grained titanium transferred from the ultrafine-grained state by isothermal annealing. Based on the data obtained in the work and the previous results of the authors, the factors affecting the mechanical stability (durability) of ultrafine-grained metals obtained under severe plastic deformation were analyzed.

Evolution of the defective structure during long-term loading of ultrafine-grained titanium VT1-0 obtained by intensive plastic deformation

For high-strength titanium VT1-0, the ultrafine-grained structure of which was obtained by mechanical-thermal treatment using the methods of helical and longitudinal rolling followed by annealing to relieve stress of the first kind. The effect of long-term loading in the creep mode at elevated temperature on the size and shape of grains has been studied. A similar study was previously carried out for recrystallized coarse-grained titanium transferred from the ultrafine-grained state by isothermal annealing. Based on the data obtained in the work and the previous results of the authors, the factors affecting the mechanical stability (durability) of ultrafine-grained metals obtained under severe plastic deformation were analyzed. Keywords: creep, durability, nanopores, ultrafine-grained metals, titanium, recrystallization.

Прочность и механизм разрушения при кручении технически чистого титана с ультрамелкозернистой структурой

The mechanical behavior and the mechanism of torsional fracture of medical grade titanium Grade4 with an ultrafine-grained (UFG) structure in comparison with a coarse-grained (CG) structure were investigated. CG titanium (dav = 25 μm) was investigated in the hot-rolled state. To obtain the UFG structure, two processing regimes were used. In the first regime, homogenization annealing was carried out at a temperature of 680°C for 1 hour, then equal-channel angular pressing (6 passes) according to the “Conform” scheme (ECAP-С) at a workpiece and tool temperature of 250°C (route Bc, φ =120°). The average grain size is equal to 0.4 μm. In the second regime, after homogenization annealing and six ECAP-C passes, the workpieces were drawn at a temperature of 200°C (ECAP-C+D). The average grain size is equal to 0.2 μm. The torsion test of samples with a diameter of 3 mm (close to the diameter of screws for fixing plates in traumatology) was carried out on a KTS 403‑20‑0.5 installation in accordance with GOST 3565-80. The torque, number of revolutions and angle of twisting of the samples made from CG and UFG titanium, as well as the mechanical properties of titanium during torsion were evaluated. Torsion tests have shown that the torsional strength and the torsional yield strength of UFG titanium increase, and the relative shear decreases in comparison with similar properties of CG titanium. In addition, the mode of nanostructuring (ECAP-C+D) provides higher values of the torsional strength properties of titanium compared to ECAP-C, which is favorable from the standpoint of resistance to fracture of titanium screws by torsion. On the surface of the fractures, there are three regions differing in microrelief: the central part of the fracture, the middle (transitional) and the peripheral parts. In the central part of the fractures, the microrelief consists of equiaxed dimples and structureless areas. In the middle part, shear dimples dominate, and in the peripheral part of fractures, areas of shear dimples alternate with areas of the rubbed surface.

Влияние структурного состояния и оксидного покрытия на механостабильность титана ВТ1-0 при его циклическом нагружении

It was found that the fatigue properties of submicrocrystalline titanium are significantly higher than those for its coarse-grained state. The application of the oxide coating leads to a slight increase in these properties for titanium with a submicrocrystalline and coarse-grained structure. Some features of fatigue fracture of submicrocrystalline and coarse-grained titanium are analyzed.

The effects of ultra-fine-grained structure and cryogenic temperature on adiabatic shear localization in titanium

The deformation at low temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium is investigated and its effect on adiabatic shear localization is established. In comparison with coarse-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain hardening approaches zero. Our results show that shear localization in dynamic deformation is also altered. The width of the shear band of coarse-grained titanium decreases from 30 to 18 μm (by 40%) with decreasing the initial deformation temperature to 77 K. In contrast, for 100 nm titanium, the width of shear band decreases more significantly, from 4 μm at room temperature to 1 μm (a 75% decrease) at 77 K. This difference is attributed to the combined effects of the decrease in the thermal conductivity and specific heat capacity, and the increase in thermal softening rate. These changes in the width are consistent with the predictions of the Grady and Bai-Dodd theories. Ultrafine- and nano- recrystallized grains are observed inside the bands which are dependent on initial grain size and initial deformation temperature. The dislocation evolution is evaluated for the different conditions using a Kocks-Mecking formulation; the rotational dynamic recrystallization mechanism responsible for forming the ultrafine/nanosized grains (40–250 nm) is successfully modeled incorporating the differences in initial temperature and grain size. Our results and analysis are important in enhancing the understanding of the structural evolution processes under high strain-rates and cryogenic temperatures.

Surface modification of ultrafine-grained titanium: Influence on mechanical properties, cytocompatibility, and osseointegration potential.

The main objective of this study was to demonstrate that dental implants made from ultrafine-grain titanium (UFG-Ti) can be created that replicate state of the art surfaces of standard coarse-grain titanium (Ti), showing excellent cytocompatibility and osseointegration potential while also providing improved mechanical properties. UFG-Ti was prepared by continuous equal channel angular processing (ECAP), and surfaces were treated by sandblasting and acid etching. Mechanical properties (tensile and fatigue strength), wettability, and roughness parameters were evaluated. Human trabecular bone-derived osteoblast precursor cells (HBCs) were cultured on all samples to examine cytocompatibility and mineralization after 4 and 28days, respectively. Biomechanical pull-out measurements were performed in a rabbit in vivo model 4weeks after implantation. Both yield and tensile strength as well as fatigue endurance were higher for UFG-Ti compared to Ti by 40%, 45%, and 34%, respectively. Fatigue endurance was slightly reduced following surface treatment. Existing surface treatment protocols could be applied to UFG-Ti and resulted in similar roughness and wettability as for standard Ti. Cell attachment and spreading were comparable on all samples, but mineralization was higher for the surfaces with hydrophilic treatment with no significant difference between UFG-Ti and Ti. Pull-out tests revealed that osseointegration of surface-treated UFG-Ti was found to be similar to that of surface-treated Ti. It could be demonstrated that existing surface treatments for Ti can be translated to UFG-Ti and, furthermore, that dental implants made from surface-treated UFG-Ti exhibit superior mechanical properties while maintaining cytocompatibility and osseointegration potential.

Ultrasonic Burnishing of Titanium Alloys

This paper presents research results concerning the influence of ultrasonic burnishing on the structure and mechanical properties of ultra-fine grain titanium alloys: commercial pure titanium BT1-0 and an over stoichiometric alloy with the shape memory Ti49.3Ni50.7. It was shown by the methods of optical microscopy and transmission electronic microscopy that in the surface layer of 20 μm thickness, the ultrasonic burnishing in coarse-grained titanium forms a nanostructure with a grain size of 100 nm, and it additionally decreases the size of crystals from 100 to 30 nm in the nanostructural titanium nickelide. The ultrasonic treatment of alloys greatly increases the strength and micro- and nanohardness of the surface layer, decreases the roughness, forms the gradient nanostructure, improves the lifetime, and expands the functionalities of the items.

Microstructural evolution and mechanical properties of a friction-stir processed Ti-hydroxyapatite (HA) nanocomposite

In this research, a new metal matrix nanocomposite with enhanced capability for biomedical applications was fabricated by incorporation of nano-sized hydroxyapatite (HA) particles within the titanium substrate using multi-pass friction-stir processing (FSP). These n-HA particles were dispersed effectively within the titanium matrix. Titanium metal-matrix was processed without introducing the HA nanoparticles, as well, for the aim of comparison. The results showed the formation of different regions with various microstructural features and mechanical property across the processed materials. A thin layer with ultra-fine grain structure and indentation hardness value of up to ~400 HV was formed on the surface after FSP modification of coarse-grained titanium substrate. This was due to severe shear deformation induced by the rotating shoulder as well as the surface absorption of N and O elements from the atmosphere inside the layer. Incorporation of nanoparticles and subsequent grain structural refinement owing to operative dynamic recrystallization mechanisms leads to a maximum hardness improvement of up to ~250 HV in the lower regions (as compared to the average hardness value of base metal ~150 HV). The FSP modified pure titanium exhibited a good combination of strength and ductility by refining the grain structure with a well-developed dimple-like structure on the fracture surface. For the nanocomposite specimen, the trend of the tensile property was found deteriorative showing the impaired features on the fracture surface. This is attributed to the complex structure of HA compound and low quality of interfacial bonding between the nanoparticles and titanium matrix.

Strain-Rate Dependence of Compressive Yield Strength of Titanium Grade 4 with Coarse-Grained and Ultrafine-Grained Structures: Experimental Results and Theoretical Calculation

The nanocrystalline (NC) and ultrafine-grained (UFG) structures of metallic materials can lead to their extraordinary high strength. However, most of the papers on this topic consider deformation parameters of NC and UFG materials only for the case of quasi-static tensile tests. Characteristics of dynamic strength and fracture of such materials remain unexplored. This paper presents a study of the mechanical behavior of pure titanium Grade 4 with a coarse-grained (CG) and UFG structure under uniaxial compression with different strain rates. The UFG structure was provided using the method of equal-channel angular pressing. The dynamic compression was carried out on a setup with the Split-Hopkinson pressure bar. It is found that in the observed range of strain rates 10–3-3×103 s–1, the yield stress of the CG titanium increases by 20%, and does not exceed the yield stress of the UFG titanium. However, the yield stress of the UFG titanium remains close to a quasi-static value. It is shown that these strain-rate dependencies of the yield strength can be predicted by the incubation time approach. The calculated curves show that at strain rates above 104 s–1 the yield stress of the CG titanium becomes higher than the yield strength of the UFG titanium.

Chip Morphology and Chip Formation Mechanisms During Machining of ECAE-Processed Titanium

Severe plastic deformation (SPD) processing such as equal channel angular extrusion (ECAE) has been pioneered to produce ultrafine grained (UFG) metals for improved mechanical and physical properties. However, understanding the machining of SPD-processed metals is still limited. This study aims to investigate the differences in chip morphology when machining ECAE-processed UFG and coarse-grained (CG) titanium (Ti) and understand the chip formation mechanism using metallographic analysis, digital imaging correlation (DIC), and nano-indentation. The chip morphology is classified as aperiodic saw-tooth, continuous, or periodic saw-tooth, and changes with the cutting speed. The chip formation mechanism of the ECAE-processed Ti transitions from cyclic shear localization within the low cutting speed regime (such as 0.1 m/s or higher) to uniform shear localization within the moderately high cutting speed regime (such as from 0.5 to 1.0 m/s) and to cyclic shear localization (1.0 m/s). The shear band spacing increases with the cutting speed and is always lower than that of the CG counterpart. If the shear strain rate distribution contains a shift in the chip flow direction, the chip morphology appears saw-tooth, and cyclic shear localization is the chip formation mechanism. If no such shift occurs, the chip formation is considered continuous, and uniform shear localization is the chip formation mechanism. Hardness measurements show that cyclic shear localization is the chip formation mechanism when localized hardness peaks occur, whereas uniform shear localization is operative when the hardness is relatively constant.

Effect of surface etching on the tensile behavior of coarse- and ultrafine-grained pure titanium

For metallic biomaterials, surface treatment is essential for improving osseointegration. Acid etching is the basic method used for removing undesirable contaminants and increasing surface roughness. Although the obvious effect of the etching process is a change in surface characteristics, interestingly, surface etching can also affect the tensile properties of the material. In this paper, the influence of etching on the tensile behavior of pure titanium was investigated. The surface morphology and tensile properties of coarse- and ultrafine-grained pure titanium specimens were examined after hydrofluoric acid etching, in terms of the etching time. The ultrafine-grained titanium was prepared using conform equal-channel angular pressing to enhance the mechanical properties of the pure titanium. Micro-scale pores were observed, and tensile strength was decreased in the 1min etched coarse-grained titanium. The 20min etched coarse-grained specimen and the ultrafine-grained specimen with relatively homogeneous pore structure maintained or slightly increased their tensile strengths. These results provide a new foundation for the production of reliable metallic biomaterials.

High-rate erosion of Ti–6Al–4V ultrafine-grained titanium alloy obtained via intensive plastic torsional deformation

Testing results for Ti–6Al–4V ultrafine-grained titanium alloy obtained via intensive plastic torsional deformation (IPTD) are presented. To estimate the effect of IPTD treatment on the behavior of this material under erosion conditions, special experimental techniques were developed. The ultrafine-grained alloy was tested alongside with the traditional coarse-grained titanium alloy in an erosion wind tunnel in an air flow with corundum particles as an abrasive material. The erosion resistance of the material was estimated from the mass loss of specimens. Despite a considerable increase in the static strength characteristics, the nanostructured material did not demonstrate any increase in its erosion resistance in comparison with the initial alloy.

Compressive responses of ultrafine-grained titanium within a broad range of strain rates and temperatures

Mechanical behavior of ultrafine-grained (UFG) materials is affected by many factors including “intrinsic” ones such as grain size, dislocation density and concentration of impurities, and “extrinsic” ones such as temperature, strain rate and strain. To evaluate those contributions to the yield and flow stress of commercial-purity titanium, compressive mechanical tests were conducted at strain rates from 10−4 to 3 × 103 s−1 and temperature from 77 to 473 K. Microstructure characterization was also carried out to pin-down the underlying mechanisms responsible for the observed mechanical behavior. Results show that the primary strengthening sources for coarse-grained (CG) titanium comes from interstitials, which contribute ∼64% of the yield stress. Grain boundary strengthening, dislocation interaction and interstitials contribute ∼38%, ∼32% and ∼30% of the yield stress of UFG-Ti, respectively. The flow stresses of both CG and UFG titanium depends positively on strain rate and negatively on temperature, but neither strain rate sensitivity (SRS) nor temperature sensitivity (TS) is constant over a wide range. The SRS of titanium is found to increase with increase in grain size and temperature.