Room temperature fracture processes of a near-α titanium alloy following elevated temperature exposure

Near-α titanium alloys are used at higher temperatures than any other class of titanium alloys. As a consequence of thermal exposure, these components may develop locally elevated oxygen concentrations at the exposed surface which can negatively impact ductility and resistance to fatigue crack initiation. In this work, monotonic and fatigue fracture mechanisms of Ti–6Al–2Sn–4Zr–2Mo–0.1Si samples exposed to laboratory air at 650 °C for 420 h were identified by means of a combination of quantitative tilt fractography, metallographic sectioning, and electron backscatter diffraction. These mechanisms were compared and contrasted with those operative during similar tests performed on material is the as-received condition with uniform oxygen content. While faceted fracture was not observed during quasi-static loading of virgin material, locally elevated concentrations of oxygen near the surfaces of exposed samples were shown to change the fracture mode from ductile, microvoid coalescence to brittle facet formation and grain boundary separation at stresses below the macroscopic yield point. Similar features and an increased propensity for facet formation were observed during cyclic loading of exposed samples. The effects of this time-dependent degradation on monotonic and cyclic properties were discussed in the context of the effect of oxygen on crack initiation and propagation mechanisms.     

Preparation,microstructure and mechanical properties of porous titanium sintered by Ti fibres

Open-cell porous Ti with a porosity ranging from 35 to 84% was successfully manufactured by sintering titanium fibres. The microstructure of the porous titanium was observed by SEM and the compressive mechanical properties were tested. By adjusting the spiral structure of the porous titanium, the pore size can be controlled in a range of 150–600 μm. With the increasing of the porosity, compressive yield strength and modulus decrease as predicated. However, high mechanical properties were still obtained at a medium porosity, e.g. the compressive yield strength and the modulus are as high as 100–200 MPa and 3.5–4.2 GPa, respectively, when the porosity is in the range of 50–70%. It was suggested that the porous titanium be strong enough to resist handing during implantation and in vivo loading. It is expected to be used as biocompatible implant, because their interconnected porous structures permit bone tissues ingrowth and the body fluids transportation.     

Fatigue of VT20 titanium alloy with vacuum-plasma coatings at high temperatures

We study the influence of vacuum–plasma TiN, (TiAl)N, and (TiC)N coatings on the high-cycle fatigue of VT20 titanium alloy in the temperature range 350–640°C for a loading frequency of 10 kHz. It is shown that, in this temperature range, the fatigue limits of VT20 alloy with the indicated coatings 6 μm in thickness become 15–25% higher than for the material without coating. The possibility of replacement of steel blades with titanium blades with vacuum-plasma coatings is demonstrated. Translated from Problemy Prochnosti, No. 4, pp. 101–107, July–August, 2009.     

Influence of oxynitriding on the wear resistance of VT14 titanium alloy

We investigate the wear resistance of VT14 titanium alloy after oxynitriding realized by modifying nonstoichiometric titanium nitride by oxygen. It is shown that the oxide component in an oxynitride coating improves the wear resistance of the alloy in a friction pair with BrAZh9-4l brass. It is established that the wear rate of a “VT14 titanium alloy with oxynitride coating–U8 steel” tribopair is an order smaller than that of a “VT14 titanium alloy with oxynitride coating–BrAZh9-4l brass” tribopair. It is also established that the tribologic behavior of the former tribopair improves after replacing AMH-10 lubricant by I-40A lubricant.     

Microstructure and mechanical properties of brazed titanium/steel joints

Microstructure and fracture behavior of brazed joint between commercially pure titanium and low carbon steel using silver (Ag–34Cu–2Ti) and copper (Cu–12Mn–2Ni) based alloys have been characterized to determine the effect of brazing parameters and chemical composition on the strength of brazed joints. It is found that the shear strength of brazed joints strongly depends on the lap width. Furthermore, the fracture path and the value of shear strength significantly changed with the type of filler alloy. The two filler metals showed metallurgical interaction with steel and titanium forming different kinds of intermetallic compounds such as CuTi, Cu2Ti, and FeTi with silver based filler and Ti2Cu, FeTi and TiCuFe with copper based filler.     

Preparation of titanium diboride nanopowder

We have studied the reaction between NaBH₄ and TiCl₄ at elevated temperatures in the range 570–1020 K and pressures of up to 10 MPa, with no solvent. The results indicate that nanoparticulate titanium diboride forms at temperatures above 820 K. According to electron microscopy data, the titanium diboride powder obtained at 1020 K consists of spherical particles 35–50 nm in diameter, in reasonable agreement with the equivalent particle diameter of ≃45 nm evaluated from the specific surface area of the TiB₂ and with the crystallite size D _hkl ≃ 30 nm evaluated from X-ray diffraction data.     

Brazing of titanium to steel with different filler metals: analysis and comparison

Evaluations of vacuum brazed commercially pure titanium and low-carbon steel joints using one copper-based alloy (Cu–12Mn–2Ni) and two silver-based braze alloys (Ag–34Cu–2Ti, Ag–27.25Cu–12.5In–1.25Ti) have been studied. Both the interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate the joint quality. The optical and scanning electron microscopic results showed that all the filler metals interact metallurgically with steel and titanium, forming different kinds of intermetallic compounds (IMC) such as CuTi, Cu2Ti, Cu4Ti3, and FeTi. The presence of IMC (interfacial reaction layers) at the interfacial regions strongly affects the shear strength of the joints. Furthermore, it was found that the shear strength of brazed joints and the fracture path strongly depend on the thickness of the IMC. The maximum shear strength of the joints was 113 MPa for the specimen brazed at 750 °C using an Ag–27.25Cu–12.5In–1.25Ti filler alloy.     

Regularities of the oxidation of niobium with increased titanium content

We have studied the processes of oxidation of unalloyed niobium and titanium and also an alloy of the niobium-titanium system, high-alloyed with titanium (up to 50%), in air at temperatures of 100–1100°C. Data on the structure, composition, and physicomechanical properties of gassaturated layers as well as on the crystallographic features of oxides forming them have been obtained. As a result, we conclude that the mechanism of alloy oxidation is multistage and includes the formation from lower and higher oxides to a compound with a structure close to rutile. We have also shown that the alloying of niobium with titanium enables one to suppress the catastrophic high-temperature oxidation of niobium and to enhance substantially its refractoriness. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 4, pp. 102–108, July–August, 2008.     

Effect of the composition and porosity of sintered titanium nanolaminates on their mechanical properties at high temperatures

By the methods of micro-and macroindentation, uniaxial compression, and scanning electron microscopy, we study the behavior of the structure and mechanical properties of mono-and biphase Ti₃SiC₂, Ti₃AlC₂, and Ti₄AlN₃ titanium nanolaminates (in-situ composites) prepared by the method of reaction sintering in compact and porous states. The regularities, specific features, and mechanisms of deformation and fracture processes are established for each material within the temperature range 20–1300°C. The temperature-strain and force boundaries of their existence in the plastic state are determined. The comparative analysis of the mechanical properties of nanolaminates is performed. It is shown that the strength characteristics of nanolaminates and their strain and creep resistance at medium and high temperatures increase in the following sequence: Ti₃AlC₂-Ti₄AlN₃-Ti₃SiC₂. The obtained picture of high-temperature properties of these materials is explained. It is shown that the procedure of prestraining of a porous material by ε = 4–8% may result in a significant increase in its specific high-temperature strength up to values higher than those typical of the compact material. __________ Translated from Problemy Prochnosti, No. 6, pp. 79–94, November–December, 2006.     

Effect of vacuum plasma surface treatment on the titanium alloy resistance to static and cyclic loading

We present the results of the experimental investigations on the static and cyclic strength of titanium alloy VT20 with a TiN vacuum plasma coating. This coating has been found to increase the static strength characteristics and to reduce the plasticity characteristics. Under conditions of low-cycle loading, the coating application results in the improvement in the cyclic strength characteristics in quasistatic fracture and in their deterioration in fatigue fracture. __________ Translated from Problemy Prochnosti, No. 1, pp. 100–106, January–February, 2007.     

Calcium and titanium release in simulated body fluid from plasma electrolytically oxidized titanium

The release of titanium and calcium species to a simulated body fluid (SBF) at 37°C has been investigated for titanium treated by dc plasma electrolytic oxidation (PEO) in three different electrolytes, namely phosphate, silicate and calcium- and phosphorus-containing. The average rate of release of titanium over a 30 day period in immersion tests, determined by solution analysis, was in the range ~1.5–2.0 pg cm⁻² s⁻¹. Calcium was released at an average rate of ~11 pg cm⁻² s⁻¹. The passive current densities, determined from potentiodynamic polarization measurements, suggested titanium losses of a similar order to those determined from immersion tests. However, the possibility of film formation does not allow for discrimination between the metal releases due to electrochemical oxidation of titanium and chemical dissolution of the coating.     

Structure and properties of diffusion bonded transition joints between commercially pure titanium and type 304 stainless steel using a nickel interlayer

The solid-state diffusion bonding was carried out between commercially pure titanium and Type 304 stainless steel using nickel as an interlayer in the temperature range of 800–900 °C for 9 ks under 3 MPa load in vacuum. The transition joints thus formed were characterized in the optical and scanning electron microscopes. The inter-diffusion of the chemical species across the diffusion interfaces were evaluated by electron probe microanalysis. TiNi₃, TiNi and Ti₂Ni are formed at the nickel–titanium (Ni–Ti) interface; however, the stainless steel–nickel (SS–Ni) diffusion interface is free from intermetallic compounds up to 850 °C temperature. At 900 °C, the Ni–Ti interface exhibits the presence of α-β Ti discrete islands in the matrix of Ti₂Ni and λ + χ + α-Fe, λ + FeTi and λ + FeTi + β-Ti phase mixtures occur at the SS–Ni interface. The occurrence of different intermetallics are confirmed by the x-ray diffraction technique. The maximum tensile strength of ∼276 MPa and shear strength of ∼209 MPa along with 7.3% elongation were obtained for the diffusion couple processed at 850 °C. At the 900 °C joining temperature, the formation of Fe–Ti base intermetallics reduces the bond strength. Evaluation of the fracture surfaces using scanning electron microscopy and energy dispersive spectroscopy demonstrates that failure takes place through Ni–Ti interface up to 850 °C and through the SS–Ni interface of the joint when processed at 900 °C.     

In situ boron carbide–titanium diboride composites prepared by mechanical milling and subsequent Spark Plasma Sintering

Boron carbide–titanium diboride composites were synthesized and consolidated by Spark Plasma Sintering (SPS) of mechanically milled elemental powder mixtures. The phase and microstructure evolution of the composites during sintering in the 1,200–1,700 °C temperature range was studied. With increasing sintering temperature, the phase formation of the samples was completed well before full density was achieved. The distribution of titanium diboride in the sintered samples was significantly improved with increasing milling time of the Ti–B–C powder mixtures. A bulk composite material of nearly full density, fine uniform microstructure, and increased fracture toughness was obtained by SPS at 1,700 °C. The grain size of boron carbide and titanium diboride in this material was 5–7 and 1–2 μm, respectively.     

Anatase formation on titanium by two-step thermal oxidation

Two-step thermal oxidation of commercially pure Ti was investigated with a focus on the formation of anatase. A first-step treatment was conducted in Ar–(0.1–20)%CO atmosphere at a temperature of 773–1173 K for a holding time of 0 or 86.4 ks, and a subsequent second-step treatment was conducted in air at 473–873 K for 0–86.4 ks. Titanium oxides and titanium oxycarbide were obtained in the first step, with relative amounts depending on heating temperature, holding time, and CO partial pressure. An anatase-rich layer on Ti was obtained after second-step treatment in air at 573–773 K in cases where single-phase titanium oxycarbide formed in the first step. Thus, the formation of single-phase titanium oxycarbide in the first step and temperature control in the second step were required for the formation of an anatase-rich layer. The bonding strength of an anatase-rich layer with a thickness of 0.5 μm was calculated to be around 90 MPa. This study reveals the conditions under which an anatase-rich layer with excellent adherence to Ti can be prepared by thermal oxidation.     

Surface modifications to improve Ti–porcelain bonding

Investigations of surface modifications on cast titanium surfaces and titanium-ceramic adhesion were performed. Cast pure titanium was subjected to surface modification by preoxidation and introduction of an intermediate layer of SnO _x by sol–gel process. Surfaces only sandblasted with alumina were used as controls. Specimen surfaces were characterized by XRD and SEM/EDS. The adhesion between the titanium and porcelain was evaluated by three-point flexure bond test. Failure of the titanium–porcelain with preoxidation treatment predominantly occurred at the titanium-oxide interface. Preoxidation treatment did not affect the fracture mode of the titanium–ceramic system and did not increase the bonding strength of Ti–porcelain. However, a thin and coherent SnO _x film with small spherical pores obtained at 300 °C served as an effective oxygen diffusion barrier and improved titanium–ceramic adhesion. The SnO _x film changed the fracture mode of the titanium–ceramic system and improved the mechanical and chemical bonding between porcelain and titanium, resulting in the increased bonding strength of titanium–porcelain.     

Control of the structure of titanium alloys by the method of thermohydrogen treatment

We show the possibility of controlling the structure of titanium alloys of different classes by means of their thermohydrogen treatment and consider the process of structure formation in shaped castings of VT20L alloy under additional alloying with hydrogen. Depending on the hydrogen content in the course of thermohydrogen treatment, the transformation of a course-lamellar structure into a fine-lamellar one with α-particle sizes from several micrometers to nanometers is possible. We describe the mechanism of phase and structural transformations in titanium alloys with a heightened content of β-eutectoid stabilizers (Ti-12Cr) or aluminum (VT6, VT23) under the action of hydrogen. We also show the possibility of producing a composite structure of the β-phase and TiCr₂ intermetallic compound or noncoherent particles of the α₂-and α-phases, depleted with aluminum. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 28–34, May–June, 2008.     

No evidence to indicate topographic dependency on bone formation around cp titanium implants under masticatory loading

In vitro studies have proved the topographic dependency upon osteogenesis on titanium plate by investigating the cell-adhesion, -shape, -proliferation, -differentiation, ALP activity and osteocalcin production of osteogenic stem cells, MG36, MC3T3-E1 and wild strains of bone formative cells from animal and human. However, this in vivo study on bone growth around cp titanium dental implants under masticatory loading did not demonstrate significant difference among the different surface roughness in the range of Ra 0.4–1.9 μm, Rz 2.8–11.2 μm, Rmax 3.6–28.1 μm and Sm 2.9–41.0 μm, which was estimated by measuring the bone contacts, bone occupancies and bone bonding strengths at the implant/bone marrow interface. It is revealed that the topographic dependency on the osteogenetic activity is apt to be covered with wide variation in bone healing potential under the clinical condition with functional biting load.     

Influence of the degree of oxygen rarefaction in a medium on the interaction of boron carbide with titanium alloys

We describe the mechanism of processes at the metal-saturating medium interface in the course of thermodiffusion saturation of titanium in boron carbide at temperatures that do not exceed the point of polymorphic transformation of titanium alloys. We have established the influence of oxygen partial pressure over a layer of boron carbide on the character of interaction between the saturating medium and titanium. We show the possibility to form stable boride coatings on a titanium matrix. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 80–84, January–February, 2007.     

Plasma coatings and their ability to protect titanium alloys against corrosion fretting-fatigue fracture

We study the influence of plasma-sprayed coatings with various chemical compositions and structures on the serviceability of Ti−5.0 Al−2.0 Zr−3.0 Sn−2.0V titanium alloy under the conditions of fretting-fatigue fracture for console specimens. It was discovered that, under the conditions of fretting fatigue, coatings made of powders of titanium and chromium carbides slightly decrease the fretting fatigue limit. Coatings made of a powder of TS-8 (α+β)-alloy, a mechanical mixture of self-fluxing alloys based on nickel (NiCrBSi), and tungsten carbides (WC) increase the endurance limit by 10–15%. At the same time, spraying with plated (Ni)−(Al₂O₃) powders increases this quantity by 40% and even more. Voltammetric investigations does not reveal any correlation between the corrosion electrochemical properties of coatings (corrosion currents and potentials) at 20°C and fretting fatigue durability in a 3% aqueous solution of sodium chloride. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 72–76, May–June, 1997.     

Effect of heat treatment on the structure of titanium dioxide films

The development of nanocrystalline phases during isothermal annealing of titanium dioxide films deposited by reactive magnetron sputtering at various rates onto silica glass substrates has been studied. It is established that the heat treatment at temperatures within 500–700°C in air or in vacuum leads to significantly different results, depending on the initial crystalline structure of as-deposited films.