Effect of hydrogen on phase and structural transformations in titanium alloys of different classes

We study the laws of phase and structural transformations in titanium alloys under the action of dissolved hydrogen and in the course of subsequent degassing. We show the possibility to control structure formation in them with the help of alloying with hydrogen and to obtain structures that cannot be obtained by conventional technological methods. It has been established that the alloying system determines the character of interaction of these alloys with hydrogen in the course of hydrogenation. After additional alloying with hydrogen, intermediate hydrides are formed in titanium alloys with β-isomorphic stabilizers (V, Nb), Ti Cr₂ intermetallic compound appears in alloys with β-eutectoid stabilizer (Cr), and Ti₃Al compound is formed in alloys with high aluminum content. After vacuum annealing of hydrogen-containing specimens at 600°C, a composite heterophase (α + β + α₂) or (α + β + TiCr₂) structure is formed, and intermetallic particles in it have an incoherent boundary. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 33–39, May–June, 2006.     

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.     

Interstitial Hardening of the Surface of Titanium Alloys by Vacuum Heat Treatment

We have determined the kinetic parameters of interaction of VT1-0, VT5, OT4-1, and VT16 alloys with a rarefied gaseous medium. We have established the interrelation between the parameters of chemical heat treatment (T, τ, and p, the interstitial hardening of surface layers of the metal, and its phase and structural state. The conditions of saturation (T, τ, and p) being equal, hardened layers differing in the microhardness and thickness of the layer H and l are formed on titanium alloys of different structural classes. Single-phase VT1-0 and VT5 α-titanium alloys as well as OT4-1 pseudo-α-alloy are the most sensitive to the conditions of gas saturation: the hardness and its gradient in the hardened layer grow significantly. As the β-phase content increases (VT16), changes in the parameters of chemical heat treatment affect less the hardness of the surface layer, but the depth of the hardened zone grows with temperature T and time τ. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 2, pp. 63–70, March–April, 2005.     

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.     

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.     

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.     

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.     

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.     

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.     

Crack resistance of the near-weld region of welded joints of titanium and aluminum alloys

We studied the crack resistance K_c (K_Ic) of the near-weld region of welded joints of VT3-1 and VT14 titanium alloys and AMg6NPP, 12401, and 1420 high-strength aluminum alloys under fatigue and static loading. By the methods of fracture mechanics and according to the results of testing cylindrical specimens with external circular cracks initiated in the weld metal, fusion zone, zone of thermal influence, and parent material, we established that the minimum crack resistance is observed in the fusion zone and the maximum crack resistance in the parent material. The weld metal and the zone of thermal influence are characterized by intermediate values of crack resistance both under fatigue and static loading. By using 1402 high-strength aluminum alloy as an example, we studied the influence of the texture, procedure of welding, modes of thermal hardening, and scaling factor on the character of brittle fracture of the metal in the near-weld region. To enhance the characteristics of crack resistance of the near-weld region of welded joints in its weakest zones (weld and fusion zone), we propose efficient methods for welding these alloys, e.g., electron-beam welding together with the efficient choice of the modes of thermal hardening instead of argon-arc welding. This enables one to improve the purity and homogeneity of the structure of the metal in the molten-metal bath in the process of welding by decreasing its porosity and the number of inclusions and cracks appearing in these zones. “L’vivs’ka Politeknika” State University, L’viv, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 89–99, September–October, 1998.     

Damages to conic specimens of titanium and aluminum alloys under impact loading

We present the results of experimental determination of the character of damage to specimens having the shape of truncated cones made of titanium VT14 and aluminum AMg6 alloys under impact loading and perform a numerical analysis of longitudinal stresses at the sites of formation of rear cleavage cracks. Translated from Problemy Prochnosti, No. 5, pp. 110–112, September–October, 1997.     

Effect of thermomechanical treatment on damping capacity of titanium alloys

This paper gives the results of experimental studies of the influence of thermomechanical treatment and post-annealing on the level of damping capacity of titanium alloys VT3-1 and VT8. We subjected VT3-1 alloy to both treatment modes, and VT8 alloy to only one. We evaluated the efficiency of the thermomechanical treatment modes by the vibration strength criterion (σ_aδ). Translated from Problemy Prochnosti, No. 1, pp. 95–101, January–February, 1999.     

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.     

Investigation of Thermodiffusion Carbonitride Coatings on Titanium Alloys

We study specific features of the processes of phase formation and gas saturation in VT1-0, VT14, OT-4, and VT20 titanium alloys held in a carbon-and-nitrogen-containing medium for 5 h at 1100°C. Depending on the type of alloy, the TiC _x N _y carbonitride phases of different compositions are formed on the metal surface under the indicated conditions. The analysis of the corrosion and electrochemical behavior of alloys shows that the best protective properties are exhibited by the carbonitrides formed on the surface of VT20 alloy (whose composition is close to equiatomic and the surface roughness is minimum). __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 4, pp. 59–65, July–August, 2005.     

Influence of fretting corrosion on the fatigue life of structural elements made of VT6 alloy

We present results of an experimental investigation of fatigue resistance characteristics of VT6 titanium alloy plates with washers made of D16AT aluminum alloy, VT3-1 titanium alloy, and 30KhGSA steel and pressed to them. It is established that, under operating loads, the fatigue life of titanium specimens decreases 400 – 750 times as a result of fretting corrosion.     

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.     

Influence of microstructure on corrosion behavior of Ti–5%Ta–1.8%Nb alloy

This paper presents the results of a study on the influence of microstructure on the corrosion behavior of a α–β Ti–5%Ta–1.8%Nb alloy—a candidate material for use in high concentrations of boiling nitric acid. The “as cast” alloy had a lamellar structure and showed a corrosion rate of about 1.5 mpy. Thermo-mechanical processing of the cast alloy resulted in a structure of predominantly of equiaxed α with random distribution of fine β particles. This “reference” structure was further modified employing different heat treatments similar to that for commercial titanium alloys such as mill annealing, solution treatment and aging or over aging treatments. Corrosion rates evaluated in boiling nitric acid in the liquid, vapor and condensate phases, showed low values ∼1 mpy. Of these, the lowest corrosion rate (∼0.03 mpy) was exhibited by the structure with minimum amount of β phase, distributed in an equiaxed α matrix. This structure was obtained by aging of the solution treated “reference” alloy. Hence, solution treatment high in the α + β phase field followed by aging at a temperature low in the α + β phase field has been identified as the optimum treatment to obtain a microstructure with superior corrosion resistance.     

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.     

Formation of a lamellar 14H-type long period stacking ordered structure in an as-cast Mg–Gd–Zn–Zr alloy

A novel as-cast Mg_96.82Gd₂Zn₁Zr_0.18 alloy was produced by conventional ingot metallurgy. By scanning electron microscope and transmission electron microscope observations, its as-cast microstructure is mainly composed of the α′-Mg solid solution, the coherent fine-lamellae and the eutectic. The β-phase ((Mg,Zn)₃Gd) as the second phase in eutectic has a face-center cubic structure. While, the coherent lamellae consist of the 2H–Mg and the 14H-type long period stacking ordered (LPSO) structure. At present, the lamellar 14H-type LPSO structure has first been observed within α′-Mg matrix in the as-cast Mg–Gd–Zn–Zr alloys. It can be concluded that the lamellar 14H-type LPSO structure within α′-Mg matrix is a special structure obviously different from α-Mg matrix (2H-type structure) in structure, composition, and formation condition.     

Polymorphic transformation of ZrO<Subscript>2</Subscript> in the 75 wt % Al<Subscript>2</Subscript>O<Subscript>3</Subscript> + 25 wt % ZrO<Subscript>2</Subscript> alloy solidified under nonequilibrium conditions

This paper examines the α-ZrO₂ ↔ β-ZrO₂ polymorphic transformation in 75 wt % Al₂O₃ + 25 wt % ZrO₂ alloys solidified under nonequilibrium conditions and then heat-treated under various conditions. The strength of the alloys is shown to be more sensitive to β-ZrO₂ content than to the size of their microstructural constituents.