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In the present work, two β-solidifying γ-TiAl based intermetallic alloys doped with Nb, Zr, Hf (designated TNZ γ-TiAl alloy) and additionally Sn (designated TNZ-Sn) were investigated. Sn is known to be widely used as an alloying element in titanium alloys but almost has not been used for γ-TiAl alloys. In contrast to Nb, Zr and Hf atoms occupying only or preferentially the Ti sublattice, Sn atoms occupy the Al sublattice because Al and Sn belong to the same group of post-transition metals. Two microstructural conditions per alloy, duplex and fully or near lamellar, were obtained in the alloys by upset forging and heat treatments. Microstructure examination by SEM and XRD analysis revealed two basic phases in both alloys, γ-TiAl and α 2 -Ti 3 Al. The feature of the Sn-containing alloy was the presence of a small amount (0.5–1.5 vol%) of the phase enriched with Sn, Zr and Hf and located along grain/colony boundaries. After upset forging and heat treatment this phase had the average chemical composition 60(Ti + Nb + Zr + Hf)-40(Al + Sn) (at.%) and was identified as the α 2 phase enriched with Sn, Zr and Hf and depleted with Al. The creep tests were performed for the produced microstructural conditions of the alloys. It was revealed that doping with Sn led to a strong increase in the creep resistance at 800 and 850 °C. The obtained effect was attributed to solid solution hardening due to substitution of Al by Sn and probably to a more ionic interatomic bonding taking into consideration an appreciably higher electronegativity of tin as compared to that of other alloying elements. • Two multi-alloyed TiAl alloys free of Sn and doped with 2 at.% Sn were studied. • Forging and heat treatment provided duplex and lamellar structures in the alloys. • Doping of the TiAl alloy with Sn did not lead to formation of undesirable phases. • Doping of the TiAl alloy with Sn led to a strong increase in creep resistance. • Higher electronegativity of Sn apparently contributed to higher creep resistance. [ABSTRACT FROM AUTHOR]

Our previous works revealed that alloying of a β-solidifying intermetallic γ-TiAl alloy with Zr and Hf instead of Nb led to improvement in the strength and creep resistance due to effective solid solution hardening, a strong partitioning preference γ>α 2 for Zr and a lower lattice misfit of the γ-TiAl and α 2 -Ti 3 Al phases. On this basis, a new β-solidifying γ-TiAl alloy with a composition Ti-44Al-X(Nb,Zr,Hf)-0.15B (at.%) (designated as TNZ γ-alloy) has been designed. The as-cast ingot had a fine lamellar structure with a small amount of the metastable β(β o) phase. Compression tests performed at T = 950–1200 °C showed that the new alloy possessed good forgeability even at 950 °C without signs of strain localization. The alloy ingot was subjected to upset forging at T = 950 °C followed by heat treatments. The applied processing led to formation of refined duplex and lamellar type structures almost free of the β(β o) phase. The obtained microstructural conditions were examined and then the tensile and creep tests were performed. The mechanical tests showed that with increasing the lamellar constituent and decreasing the lamellar spacing the creep resistance expectedly increased and ductility decreased. The obtained results are discussed from the viewpoint of processability, mechanical properties and oxidation resistance of the new alloy. • A new β-solidifying γ-TiAl alloy doped with Nb, Zr and Hf was investigated. • Upset forging at 950 °C did not lead to strain localization and flow instabilities. • Various microstructural conditions were obtained by upsetting and heat treatment. • The onset of the brittle-ductile transition in the alloy occurred at T ≥ 850 °C. • The new alloy exhibited a good balance of tensile and creep properties. [ABSTRACT FROM AUTHOR]

The work was devoted to study of microstructure and mechanical properties of discontinuously reinforced composite materials based on near-α Ti/TiB and near-α Ti/(TiB+TiC) fabricated in situ by casting. A near-α titanium alloy VT18U (Ti-6.8Al-4Zr-2.5Sn-1Nb-0.7Mo-0.15Si) was used as a matrix material. The boron addition corresponding to 6.5 vol.% of TiB and the carbon addition corresponding to 1.9 vol.% of TiC were used as additives to the titanium alloy. The as-cast materials were subjected to two-stage 3D forging in the α+β temperature field at T = 950 and 800 °C. This resulted in refinement of randomly oriented TiB whiskers while retaining near the same size of TiC particles. The forged workpieces were subjected to heat treatment, which included high temperature anneal in the β or upper part of the α+β phase field and provided similar matrix microstructures in the composites and the base alloy. The forged and heat treated composites demonstrated appreciably higher strength, creep resistance and lower ductility as compared with the base alloy. The load-bearing capacity of the reinforcements mainly contributed to the enhancement in strength and creep resistance. The carbon addition gave appreciable strengthening effect in VT18U/(TiB+TiC) at RT but at elevated temperatures it was not the case and only negligible positive influence on creep resistance was detected. At the same time, the carbon addition led to a strong decrease of the RT ductility. It was revealed that refined and randomly oriented TiB whiskers in VT18U/TiB provided the strengthening contribution and improvement in creep resistance comparable with those obtained in the same composite with aligned TiB whiskers having a higher aspect ratio. Presumably, more uniform distribution and shorter spacings between TiB whiskers in the case of refined borides promoted an increase in strength and creep resistance. Microstructure examination showed high adhesion strength of interfacial boundaries between the matrix and the reinforcements, which was retained up to T  = 600–700 °C. The main failure mechanism of the composite materials was fracture of the reinforcements followed by ductile failure of the matrix. [ABSTRACT FROM AUTHOR]

A novel two-phase titanium alloy based on Ti-6.5Al-3.3Mo-0.3Si (VT8) (wt%) having the composition VT8-20Zr-0.1B has been studied in the present paper. Modifying with boron and alloying with zirconium led to refinement of prior β grains and α/β colonies by a factor of about ten. Alloying with 20 wt% of zirconium resulted in significant decreasing the β-transus temperature that along with the refinement of the as-cast structure facilitated ingot breakdown and formation of fine grained structure during forging. After multidirectional hot forging and hardening heat treatment the strength of the VT8-20Zr-0.1B alloy at 20–500 °C was found to be higher by 30–40% as compared with that of the VT8 alloy after similar treatment while retaining near the same ductility of the alloys. For the VT8-20Zr-0.1B alloy with the duplex structure the following tensile properties have been attained: σ UTS /σ 0.2 =1560/1400 MPa and δ=4.2% at room temperature, σ UTS /σ 0.2 =1230/1090 MPa and δ=14% at Т =500 °С. The density normalized strength reached at Т =500 °С is the highest even obtained at this temperature for currently known titanium alloys. The novel alloy in the fine lamellar condition showed quite reasonable creep resistance at temperatures near the maximum service temperature. Solution treatment in the β or the α+β temperature field followed by water quenching was revealed to result in the martensitic β→α′′ transformation with retained β phase. Subsequent ageing led to formation of fine lamellar or duplex structure with nanosized lamellar spacings that in turn promoted achieving extraordinary high strength properties. [ABSTRACT FROM AUTHOR]

The present work has been devoted to study of β-solidifying γ-TiAl alloys based on Ti–44Al-0.2B and doped with Nb, Zr and Zr + Hf. The phase transformation sequences were established for the alloys. On this basis, upset forging followed by heat treatment was performed that resulted in formation of near the same duplex type microstructures in the alloys. Tensile tests in the temperature range of 20–900 °C and creep tests at 700 °C were carried out for the alloys in the duplex conditions. The alloys doped with Zr and Zr + Hf showed appreciably higher strength, higher temperatures of the brittle-ductile transition (BDT) and enhanced creep resistance while retaining near the same ductility below the BDT temperatures as compared to the Nb-containing alloy. Enhanced creep resistance and higher temperatures of the BDT in the alloys doped with Zr and Zr + Hf in contrast to the Nb-containing alloy were attributed to higher solid solution hardening due to larger atomic radii of Zr and Hf versus Nb and to different partitioning behaviors of Zr and Zr + Hf versus Nb leading to higher concentrations in the γ phase of Zr and Zr + Hf in the Zr- and (Zr + Hf)-containing alloys in contrast to that of Nb in the Nb-containing alloy. Both of these factors contributed to lower diffusivity (especially in γ grains) in the alloys doped with Zr and Zr + Hf. In particular, alloying with Zr and Zr + Hf shifted the development of dynamic recrystallization processes towards high temperatures that slowed down the increase of ductility with increasing the test temperature in the BDT range and led to higher BDT temperatures. • Four alloys based on Ti–44Al-0.2B and doped with Nb, Zr, and Zr + Hf were studied. • Upset forging and heat treatment provided similar duplex structures in the alloys. • Alloying with Zr and Zr + Hf led to higher solution hardening than alloying with Nb. • Alloying with Zr and Zr + Hf resulted in higher creep resistance than alloying with Nb. • Alloying with Zr and Zr + Hf led to higher BDT temperatures than alloying with Nb. [ABSTRACT FROM AUTHOR]