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.     

Development of titanium alloys by the method of complex alloying

We study the effect of alloying on the mechanical properties of welded joints and base metal of titanium alloys of the system Ti-Al-Mo-V-Cr-Fe. We also formulate the theoretical aspects and principles of complex alloying of titanium alloys and the theory of alloying of additive materials for the welding of α-, (α + β), and β-alloys. It has been shown that (α + β)-(VT23) and β-(VT19) structural titanium alloys, developed on the basis of the proposed theory of complex alloying, provide high weight efficiency of construction of present-day flying vehicles. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 5, pp. 45–50, September–October, 2006.     

Structure and physicomechanical properties of eutectic Ti-Si-X alloys

We study the structure and physicomechanical properties of various eutectic alloys of Ti-Si-Zr, Ti-Si-B, and Ti-Si-Ga systems. It is shown that Ti-Si-Zr alloys with elevated concentrations of Zr reveal, due to the presence of (Ti, Zr)₂ Si dispersed silicides with sizes of about several hundred nanometers, improved mechanical properties as compared with the properties of alloys based on Ti₅Si₃ silicides. The cast eutectic alloy of the Ti-Si-Zr-Sn system with a plasticity of ∼ 1.7% is obtained for the first time. The formation of superfine eutectics based on the Ti₆Si₂B ternary compound in alloys of the Ti-Si-B system enables one to obtain titanium composites with improved refractory properties and elevated moduli of elasticity (of about 150 GPa or, after additional alloying, as high as 165 GPa). This can be promising for the development of new refractory titanium composite materials with elevated stiffness. The analysis of the combined effect of gallium and silicon in Ti-Si-Ga alloys reveals the possibility of getting titanium materials with high heat resistance, i.e., materials based on the (α-Ti(α₂-Ti₃Ga) + Ti₅ (Si, Ga)₃ binary eutectics. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 35–42, May–June, 2008.     

Effect of titanium on the structure and mechanical properties of Cu-Al-Ti alloys

The structure and mechanical properties of Cu10 wt% Al base alloys with 0–2.5 wt% Ti additions were investigated using transmission electron microscopy, optical microscopy and tensile tests. Addition of titanium has a decreasing effect on the grain size after quenching fromα + β region and causes significant strengthening of alloys. Alloy containing 1 wt%Ti quenched from 900° C shows mixture ofα, retainedβ (DO₃), disorderedβ′ (3R) and orderedβ′ ₁ (18R) martensites. Alloy with 2.5 wt% Ti addition after quenching containsα, retainedβ (DO₃), ordered T₁ phase of L2₁ superlattice and orderedβ′ ₁ martensite with either R18 or L1₀ structure indicating different stacking of ordered planes as the effect of titanium addition.     

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.     

Influence of N and Fe on α-Ti precipitation in the in situ TiC–titanium alloy composites

High strength with high ductility can be achieved in the titanium alloys by using metal precipitated ceramic particle as reinforcement. In this work, α + β or β-Ti alloy composites were prepared with α-Ti precipitated TiC particles. A series of Ti–Fe–C–N alloys were prepared and a constitutional diagram was constructed as a function of N and Fe contents. Two criteria were identified for the formation of α-Ti precipitation. One is the existence of Ti₂C phase and the other is the presence of α-Ti phase in the matrix. The mechanism of α-Ti formation from the Ti₂C phase is discussed.     

Corrosion of titanium alloys in aqueous solutions of hydrochloric acid

We investigate the kinetics of corrosion of α-, (α+β)-, and β-titanium alloys in aqueous solutions of hydrochloric acid. It is shown that the process of dissolution obeys a parabolic law and is accompanied by the formation of an oxide (TiO₂ rutile) film on the metal surface. We demonstrate that corrosion processes are intensified as the amount of the β-phase in titanium alloy increases. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 112–115, May–June, 1997.     

Mechanical alloying synthesis and structural characterization of ternary Ni-Al-Fe alloys

Ternary Ni-Al-Fe alloys with different Fe additions have been synthesized by mechanical alloying of elemental powder mixtures. The effects of Fe-substitution for Al in an equi-atomic NiAl alloy on the mechanical alloying process and on the final Ni-Al-Fe alloys were investigated experimentally. Lower Fe additions have been found to prolong the milling time prior to explosive formation of the NiAl(Fe) compound, while ≥ 15 at % Fe addition has been found not only to eliminate the explosive reaction during milling but also to produce a Ni-based supersaturated solid solution (the 15 at % Fe addition results in the formation of an amorphous-like phase). The addition of Fe improves the plastic deformation ability of the alloys and hinders the tendency of fracture during milling, resulting in the formation of larger alloy particles. Upon heating, the as-milled samples with lower Fe addition remain as NiAl(Fe) compound, whereas the Ni-based supersaturated solid solutions decompose into a mixture of compounds of β-(Ni, Fe) (Al, Fe) + γ′-(Ni, Fe)₃Al. It is suggested that the ternary addition into the binary intermetallic compound might be a possible route to improve ductility by forming the dual phase of β + γ′. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of arsenic, antimony and phosphorous on the microstructure and corrosion behavior of brasses

The effect of minor additions of As, Sb and P on phase distribution and corrosion behavior has been studied in brasses. The alloys investigated were 60Cu–39Zn–1Pb, 48.95Cu–45Zn–5Pb–1Sn–0.05As, 48.90Cu–45Zn–5Pb–1Sn–0.05As–0.05Sb and 48.85Cu–45Zn–5Pb–1Sn–0.05As–0.05Sb–0.05P. Immersion tests in 1% CuCl₂ solution indicated that the addition of As improved corrosion resistance while the combined addition of As + Sb and As + Sb + P was not beneficial. The hardness increased significantly with the addition of As, Sb and P. Microstructural observations indicated an increase in β phase fraction in the As, Sb and P containing alloys. X-ray diffraction studies confirmed the formation of intermetallic compounds in As, Sb and P containing alloys. Based on the microstructural observations, the intermetallic compounds appear to be primarily precipitated in the β phase with As + Sb and As + Sb + P additions. The lower corrosion resistance of the alloys 48.90Cu–45Zn–5Pb–1Sn–0.05As–0.05Sb and 48.85Cu–45Zn–5Pb–1Sn–0.05As–0.05Sb–0.05P has been related to increase in β phase volume fraction and precipitation of intermetallic compounds in the β phase.     

The effect of metastability on room temperature deformation behavior of β and α + β titanium alloys

The deformation behavior of single-phase metastable β-titanium alloys and two-phase α+metastable-β alloys strongly depends on the degree of stability of the β-phase. Recently, it has been shown that the tensile deformation behavior, as well as the creep deformation behavior at low temperatures (<0.25T _m), is strongly influenced by the degree of metastability. For example, the titanium β-alloy Ti–13.0wt%Mn, which has higher stability than the titanium β-alloy Ti–14.8wt%V, deforms by slip only; whereas the latter deforms by slip and twinning. In addition to the mechanical properties, the deformation mechanisms also depend on the degree of metastability. Further, the deformation mechanisms of a given metastable β-alloy depend on whether the β-phase is present by itself as a single-phase alloy, or in the presence of α-phase in the form of a two-phase alloy. For example, it was found that a metastable Ti–V alloy deforms by slip and twinning when it is in the form of a single-phase alloy, but deforms by slip and martensitic transformation when the same metastable β-phase is present in a two-phase α + β alloy. The mechanical properties of the metastable β alloys in turn depend on these deformation mechanisms. These recent developments are reviewed in this article.     

Properties and structural-phase state of the surface layers of titanium after combined nitriding

We study the simultaneous nitriding of titanium alloys by two methods: thermodiffusion saturation and ion implantation. Prior to nitrogen implantation, a thin oxynitride film and a thick nitride one were formed on the surface of VT6 titanium alloy of the (α + β)-class (Ti-6Al-4V). We show that nitrogen implantation changes the state of the surface of titanium and increases the surface microhardness of nitride and oxynitride coatings. An increase in the hardness of the surface without loss of its quality is possible in the case of nitrogen implantation into a thin oxynitride film. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 65–70, May–June, 2007.     

Effect of copper addition on the β-phase formation rate in FeSi2 thermoelectric materials

The transformation kinetics of the β-phase from an as-solidified structure composed of α and ε in the Fe–Si system was investigated by using rapidly, unidirectionally or conventionally solidified FeSi₂ alloys containing a small amount of Cu (0.1–1 at%). The addition of Cu decreased the size of primary ε and slightly changed the solidified eutectic morphology. The solubility of Cu in the α-Fe₂Si₅ phase was estimated to be less than 0.2 at%. A needle-like Cu enriched phase was newly formed in the conventionally solidified alloys containing more than 0.2 at % Cu. Microdifferential thermal analysis (DTA) clearly showed that the addition of Cu drastically accelerated β-phase formation. X-ray diffraction analysis and microstructural observation of the isothermally heat-treated specimens showed that Cu addition was effective in increasing the rate of eutectoid decomposition (α → β + Si) and the initial stage of the peritectoid reaction (α + ε → β). For complete β formation, heat treatment for a long time was still required because it took a long time for the coarse ε-phase in the slowly solidified alloy to be eliminated by peritectoid reaction. The effect of Cu depended on the annealing temperature. The decomposition rate of α in the Cu-added cast specimen was about 15 times higher at 1073 K than that of the binary cast specimen and exceeded more than 30 times at 873 K. This revised version was published online in November 2006 with corrections to the Cover Date.     

Hydride formation under cathodic charging of titanium and TiAl-based alloys in alkaline solutions

The electrochemical methods and the data of X-ray diffraction analysis are used to determine the parameters of cathodic polarization for the hydrogenation of α-Ti and alloys based on the TiAl intermetallic phase without formation of the hydride phase or with formation of hydrides. In α-Ti, the increasing cathodic polarization in a 0.1 M NaOH solution leads to the dissolution of hydrogen in the metal lattice and its modification and to the increase in the amount of hydrides. The hydride phase is not recorded TiAl-based alloys even for much higher levels of absorption of hydrogen as compared with pure titanium. However, hydrogen affects the phase composition of alloys and the lattice parameters of the phases. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 103–106, May–June, 2008.     

In situ nitridation of titanium–molybdenum alloys during laser deposition

In situ nitridation during laser deposition of titanium–molybdenum alloys from elemental powder blends has been achieved by introducing the reactive nitrogen gas during the deposition process. Thus, Ti–Mo–N alloys have been deposited using the laser engineered net shaping (LENS^TM) process and resulted in the formation of a hard α(Ti,N) phase, exhibiting a dendritic morphology, distributed within a β(Ti–Mo) matrix with fine scale transformed α precipitates. Varying the composition of the Ar + N₂ gas employed during laser deposition permits a systematic increase in the nitrogen content of the as-deposited Ti–Mo–N alloy. Interestingly, the addition of nitrogen, which stabilizes the α phase in Ti, changes the solidification pathway and the consequent sequence of phase evolution in these alloys. The nitrogen-enriched hcp α(Ti,N) phase has higher c/a ratio, exhibits an equiaxed morphology, and tends to form in clusters separated by ribs of the Mo-rich β phase. The Ti–Mo–N alloys also exhibit a substantial enhancement in microhardness due to the formation of this α(Ti,N) phase, combining it with the desirable properties of the β-Ti matrix, such as excellent ductility, toughness, and formability.     

Microstructure evolution and alloying elements distribution between the phases in powder near-β titanium alloys during thermo-mechanical processing

In the present study, two powders near-β Ti alloys having a nominal composition of Ti-5Al-5Mo-5V-XCr-1Fe (X = 1–2, wt%) were studied. The alloys were produced via the blended elemental powder metallurgy technique using hydrogenated Ti powder. Microstructure evolution and the distribution of the alloying elements between the phases were investigated after each step of thermo-mechanical processing (TMP). Microstructures were refined through the TMP in both alloys. Porosity was reduced with deformation at 1173 K (900 °C) in the β phase field. The β → α phase transformation occurred during soaking at 1023 K (750 °C) in the α + β phase field. Fragmentation of the continuous grain boundary α occurred because of the 40 % deformation at 1023 K (750 °C). Variation in the concentration of the alloying elements in each phase took place through the diffusion during soaking in the α + β phase field, e.g. exit of β-stabilisers from the α-phase. However, the α phase remained supersaturated with β stabilisers. Deformation had no influence on the distribution of the alloying elements. An addition of 1 % Cr content slightly affects the amount of the α phase formed and β grain size, but it has no noticeable effect on the distribution of the alloying elements between the phases.     

High-temperature thermal barrier coating formation by laser alloying of CP-Ti with pre-placed Ni-SiC coating

High-temperature thermal barrier coating was created on CP-Ti using a pre-placed Ni-SiC layer by laser alloying technique. The coating was developed using 80% Ni + 20% SiC, 50% Ni + 50% SiC and 60% Ni + 40% SiC, and the latter two compositions are found to be efficient in producing a uniform layer. The 100% SiC pre-placement was also used. A flaw-less coating of 0.4–0.6 mm thickness was produced at a lower power density of 1.3 to 1.9 × 10⁵ W cm^–2. Very high power density of 2.5–3.0 × 10⁵ W cm^–2 is inefficient to produce uniform coating. The laser alloyed coating consists of dendrites and intermetallic precipitates. The degree of dendrite population depends upon the coating composition and laser processing conditions. The coating hardness was 600–1200 HV, which is three to six times higher than the base titanium. Uniform hardness was obtained for the coatings produced at a laser power density of 1.3 × 10⁵ W cm^–2. The titanium silicide (TiNiSi, Ti₅Si₃, TiSi) and nickelide (NiTi₂) phases formed on the laser-alloyed coating surface was confirmed by X-ray analysis. These intermetallic phases can improve high-temperature properties of titanium and its alloys. The effect of laser power density and coating composition on the alloying depth alloying width, hardness and microstructure are discussed. The present work investigated the microstructure evolution, hardness and compound phases by means of optical and scanning electron microscopy, Vickers hardness testing, EDXRD and SIMS analysis. A 5 kW CW CO₂ laser was used for laser alloying experiments.     

Electric-Spark Alloying of Steels by Materials Based on WK Alloy with Admixtures of Chromium Carbide

The electric-spark alloying of 45 steel with electrodes made of WC + Co + (2–4)% Cr₃C₂ hard alloys obtained by sintering on the “coating-substrate” boundary leads to the formation of an intermediate zone. This zone is formed as a result of cavitation mixing of the materials of the electrodes in the course of spark discharge and its composition corresponds to the composition of high-alloy high-speed steel. The intermediate zone increases the cohesion strength between the coating and the substrate, the depth of the obtained layer, and its wear resistance. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 2, pp. 105–108, March–April, 2005.     

Interaction of intermetallic compounds of rare-earth metals, nickel, and aluminum with hydrogen

AtT=293 K andp _H2≤10 MPa. we synthesized hydridesR _3Ni8Al-H_7.2–15.2 (R=Nd. Sm. Gd. Tb. Dy, Ho, Er, Tm, and Lu). The X-ray analysis revealed an isotropic deformation of the original crystal lattice due to hydrogen penetration. For the hydrides obtained, the lattice constanta exceeds that of the original intermetallide by 3.9–6.0% and the constantc and the cell volumeV are increased by 6.0–12.6% and 15.6–26.4%, respectively. The volume expansion per absorbed hydrogen atom is (2.4–3.6)·10⁻³ nm⁻³. By construction of the isotherms of hydrogen desorption for the systemsR _3Ni>8Al-H, we established the existence of the α-solid hydrogen solution and the region of its coexistence with β-hydride, the region of coexistence of β- and γ-hydrides, and the β- and γ-hydride phases. Heats of the phase transitions were determined as follows: ΔH=31.4±0.8 kJ/(mole H₂) in the system Gd₃Ni₈Al-H for the transition γ→β, ΔH=39.8±1.1 kJ/(mole H₂) in the system Tb₃Ni₈Al−H for the transition β→α. and ΔH=37.1±5.1 kJ/(mole H₂) in the system Tm₃Ni₈Al−H for the transition γ→β. Among the products of hydrogenation, intermetallic compounds Lu₃Ni8Al and Tm₃Ni₈Al. we revealed the β′- and γ-phases. We also found that, atp _H2≤10 MPa andT=400–430 K, the phases of Sm₃Ni₈Al and Gd₃Ni₈Al decay into two hydridesRH_2–3 andR(Ni.Al)₅H _x . Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 1, pp. 76–82, January–February, 2000.     

Foundations of the development of economically alloyed widely applicable titanium alloys

We consider the prospects of using Ti-Fe-O-N compositions for the development of economically alloyed (α + β)-titanium alloys with a strength of 700–1000 MPa and high technological plasticity. We determined the limiting concentrations of alloying elements guaranteeing the acceptable complex of characteristics of strength and manufacturability and proposed the optimal composition of a new alloy: Ti-1.5Fe-0.49O-0.05N. We also analyzed the technological and operating characteristics of the new alloy and conclude that its use has good prospects in many branches of industry. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 81–84, May–June, 2008.     

Structural, electrical and thermodynamical aspects of hydrogenated La-Ni-Si alloy

The structural, electrical and thermodynamic properties of a La-Ni-Si [La = 28.9%, Ni = 67.5%, Si = 3.6%] alloy have been investigated. Powder XRD results show that the lattice constants and unit cell volume of the alloy increase after hydrogen storage. It was also found that the resistance of the alloy increased with dissolved hydrogen concentration. Hydrogen absorption pressure composition isotherms have also been investigated which show the presence of two single a and β regions and one mixed (α + β) phase. The thermo-dynamic parameters viz. the relative partial molar enthalpy (ΔH) and relative partial molar entropy (ΔS) of dissolved hydrogen, are found to be in the range 8–18 kJ (mol H)^-1 and 25–63 JK^-1 (mol H)^-1. From the dependence of ΔH on the hydrogen concentration,X, the different phases [α, α + β, β] and phase boundaries of the alloy-H system are identified. Thermal conductivity and diffusivity of La-Ni-Si and its hydride have been measured at room temperature by using TPS technique. Thermal conductivity was found to decrease due to absorbed hydrogen in the alloy.