Ti Al Nb Research Articles

Isothermal forging of titanium aluminides without beta-phase — Using non-equilibrium phases produced by spark plasma sintering for improved hot working behavior

This paper investigates the hot forgeability of a TNB-V5 (Ti–45Al–5Nb–0.2B–0.2C, at.%) alloy produced by spark plasma sintering (SPS) from pre-alloyed powder particles using the concept of processing maps. For the first time in this paper, to the best of our knowledge, we report the possibility of the isothermal forging of a TiAl alloy without the necessity to induce the ductile β-phase in the microstructure, by using non-equilibrium phases produced by SPS. At room temperature, the SPSed sample reveals a homogeneous equiaxed microstructure consisting of globular α2-and γ-grains with the α2-phase formed mainly at the necks between powder particles as confirmed by EBSD analyses. The necks contain a lower Al content and thus transform into α-phase upon heating to forging temperature. A stable plastic flow is observed for all ranges of the studied strain rates. The presence of a fine equiaxed microstructure at hot working temperature seems to ensure good workability and leads to a relatively wide processing window. The processing map reveals two safe processing windows for the studied alloy. The lower strain rate domain (10−3-10−2 s−1) corresponding to a strain rate sensitivity value of ≈0.8 represents superplasticity of the fine and uniform (α+γ) equiaxed microstructure. The higher strain rate domain (10−2-10−1 s−1, T > 1215 °C) with a strain rate sensitivity value of ≈0.45 represents dynamic recrystallization of the α-grains at elevated temperatures. The processing map also exhibits a domain of flow instability represented by the deformation conditions of T < 1215 °C and ε˙ > 10−2 s−1, which might result from the void formation and/or cracking. Although the TNB-V5 alloy was not designed for forging operations, the SPS process can create a homogeneous equiaxed microstructure consisting of globular α- and γ-grains decorating the interfaces and triple junctions between former powder particles leading to a good hot workability. The current study offers a new process design strategy by opening up the possibility of using local non-equilibrium conditions in the material during the forging process, which could stabilize the ductile phases locally.

Influence of Nb and Cu Elements on the Intermetallic Particles Morphology in Ti/Al Multilayer Composite Processed Through Hot Pressing and Rolling

Ti–Al–Nb composites were produced by solid state diffusion bonding through hot pressing and rolling followed by annealing at 700 °C for 0.5, 1, 1.5 and 2 h. The morphologies of TiAl3 intermetallics were investigated by Scanning Electron Microscopy combined with Energy-dispersive X-ray spectroscopy. Titanium tri-aluminide (TiAl3) particles with blocky morphology were dispersed into Aluminum matrix. In the presence of niobium and copper, TiAl3 particles were produced in different sizes and morphologies. The presence of Nb in the composite led to the formation of irregular angular morphology, while the copper resulted in cubic morphology of the intermetallic particles. The EDS results indicated that TiAl3, (Ti, Nb)Al3 and (Ti, Nb, Cu)Al3 intermetallic compounds appeared near Ti zone, Nb Zone and in the presence of Cu, respectively.

Physically-based constitutive model for flow behavior of a Ti-22Al-25Nb alloy at high strain rates

The objective of the study is to understand the dynamic flow behavior of Ti–22Al–25Nb alloy at high temperatures using a modified Zerilli–Armstrong (Z-A) model. To investigate the dynamic mechanical behavior of titanium alloy, the material was subjected to quasi static and dynamic compression testing. It is found out that the modified Z-A models possess better predictability in evaluating the flow behavior of the Ti–22Al–25Nb alloy at elevated strain rates and temperatures. The model well established the quasi-static and dynamic behavior of the titanium alloy, mainly the effects of strain hardening and thermal softening. The regimes of high strain rate deformation behavior represented by dynamic recrystallization (DRX), lamellar globularization and the instability flow have been discussed. Using transmission electron microscopy (TEM) studies, it is found that the dislocation density is directly associated with strain rate.

O Phase Precipitation and Variant Selection in Ti–22Al–25Nb Alloy during the Hot Shear Spinning

O phase precipitation and variant selection in Ti–22Al–25Nb alloy during the hot shear spinning are investigated using scanning electron microscope (SEM), electron backscattered second diffraction (EBSD), and transmission electron microscope (TEM). The results show that the random O phase precipitates from the B2 matrix during the hot shear spinning. With the increase of reduction ratio, the average thickness of O precipitates increases from ≈0.18 to ≈0.50 μm. Meanwhile, the O precipitates show various morphologies, including lamella, short rod, and sphere. Twelve O variants simultaneously form in the parent B2 phase and the orientation relationship between the B2–O phases is confirmed as [−111] B2//[1–10] O and (110) B2//(001) O. Different patterns of dislocation are also found in the B2 matrix, such as dislocation tangles, dislocation nets, and dislocation arrays. They play an important role in the variant selection of O precipitates, resulting in the different angled combinations of O variants. In addition, massive dislocation piles up along the O/B2 interface and some dislocation wall generates within the O phase, which are benefit for the globularization of O precipitates.

Dynamic spheroidisation behaviour of the lamellar Ti–22Al–25Nb alloy during hot compression

ABSTRACTThe dynamic spheroidisation behaviour of the lamellar Ti–22Al–25Nb alloy during hot compression at 970°C/0.01 s−1 and strain of 30%, 50% and 70% was investigated. The results showed the lamellar-O phase that participated in the B2 matrix could be obtained by hot pressed sintering from pre-alloy powders. The corresponding scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy observations indicated that the dynamic spheroidisation process can be divided into three stages, the first is the primary interphase high-energy defect stage, introduced by dislocation migration twinning and localised shearing. The second is the separation, and the last is the spheroidisation stages, which could be explained by boundary splitting and termination migration.

Evaluation of precipitation hardening in TiC-reinforced Ti2AlNb-based alloys

Ti2AlNb-based alloys with 0.0wt%, 0.6wt%, and 2.0wt% carbon nanotube (CNT) addition were fabricated from spherical Ti–22Al–25Nb powder by sintering in the B2 single-phase region. Phase identification and microstructural examination were performed to evaluate the effect of carbon addition on the hardness of the alloys. Carbon was either in a soluble state or in carbide form depending on its concentration. The acicular carbides formed around 1050°C were identified as TiC and facilitated the transformation of α2 + B2 → O. The TiC was located within the acicular O phase. The surrounding O phase was distributed in certain orientations with angles of 65° or 90° O phase particles. The obtained alloy was composed of acicular O, Widmanstatten B2 + O, and acicular TiC. As a result of the precipitation of carbides as well as the O phase, the hardness of the alloy with 2.0wt% CNT addition increased to HV 429 ± 9.

SELECTIVE LASER MELTING OF INTERMETALLIC TITANIUM ALLOY

The in-situ synthesis of the Ti2AlNb-based intermetallic alloy using selective laser melting of powder materials was studied. The object of research is the Ti–22Al–25Nb alloy (at.%), the main phase of which is the Ti2AlNb intermetallic compound with an ordered orthorhombic lattice (O phase). The Ti–22Al–25Nb alloy has good mechanical properties at room and elevated temperatures, low specific weight, and is considered as a promising material for aerospace industry applications. Experiments used a mechanical mixture of pure titanium, aluminum and niobium powders in a ratio required for Ti–22Al–25Nb alloy synthesis. Selective laser melting as an additive technology is the most promising way for additive layer manufacturing of parts. This technology allows manufacturing complex-shaped items based on CAD model data. Selective laser melting was used to make compact samples for investigations. Their microstructure, density, phase composition and microhardness were studied. In addition, the effect of heat treatment homogenization at 1250 °C for 2,5 h and then aging at 900 °C for 24 h on the microstructure and chemical homogeneity of samples were studied. It was shown that the compact material obtained by selective laser melting contains unmelted niobium particles. Homogenization annealing makes it possible to dissolve these particles completely in the alloy. As a result, the material microstructure consists of B2 phase grains of different sizes and needle-like precipitates of the orthorhombic phase.

Fabrication of a high strength and ductility Ti‒22Al‒25Nb alloy from high energy ball-milled powder by spark plasma sintering

A high strength and ductility Ti‒22Al‒25Nb alloy was fabricated from high energy ball-milled powder by spark plasma sintering. The pre-alloyed powder with a normal composition (at %) of 53 Ti, 22 Al and 25 Nb was firstly ball-milled for different milling times, and subsequently consolidated by spark plasma sintering. The effects of ball-milling time on the microstructure and mechanical properties of the ball-milled powder and sintered compact were investigated. The Ti‒22Al‒25Nb alloy sintered by SPS at 950 °C for 10 min under a pressure of 50 MPa from the ball-milled powder showed the microstructure with a large amount of ultrafine/nano O-phase. The best balance of tensile properties was exhibited in the Ti‒22Al‒25Nb alloy sintered from the ball-milled powder for 20 h; the yield strength, the tensile strength and the elongation to failure were 1092 MPa, 1105 MPa and 9.4% at room temperature respectively, exceeding other coarse grain Ti‒22Al‒25Nb alloys fabricated by other powder metallurgy methods in the comprehensive tensile property. The precipitation of secondary phase, the grain refinement and the change of residual dislocation density in the sintered alloy directly affected the tensile properties of the sintered alloy.

Microstructure and Mechanical Properties of Ti2AlNb‐Based Alloys Synthesized by Spark Plasma Sintering from Pre‐Alloyed and Ball‐Milled Powder

Ti2AlNb‐based alloys are synthesized by spark plasma sintering from pre‐alloyed and ball‐milled Ti–22Al–25Nb powder, and the structure of the alloys is regulated by aging at 800 °C for 0.5, 1, 2, and 3 h. Comparison of phase composition, microstructure, and mechanical properties are made in this study. Aging in the B2 + O phase region refers to the reversible transformation of B2 ↔ O. Complete B2 + O Widmanstätten structure is obtained in the aged alloys synthesized from pre‐alloyed powder, and the ones from milled powder contain acicular O and B2 + O Widmanstätten structure. Ball milling eliminates the B2 + O colonies, facilitates the continuous transformation of O → B2, and induces acicular O in the alloys; however, the hardness of these alloys is decreased in contrast with that of the ones synthesized from pre‐alloyed powder. Owing to the precipitation strengthening based on a large amount of fine O‐phase precipitates, the 3 h aged alloy from pre‐alloyed powder exhibits comprehensive mechanical properties with Vickers hardness of 448 ± 9 HV, ultimate tensile strength of 730 MPa, and elongation of 0.43%.

Investigation of the effect of aluminum on the phase composition of Ti–Al–Nb–Mo gamma alloys

A quantitative analysis of the influence of aluminum concentration on the phase composition of TNM-type Ti–Al–Nb–Mo γ-alloys has been carried out using the Thermo-Calc software and experimental methods. Isothermal and polythermal sections of the corresponding phase diagram have been calculated; the critical temperatures of phase transformations in the alloys of the system, and the chemical compositions of phases formed in them (β, α, α2, γ) have been determined. The influence of the annealing temperature on the microstructure and phase composition of the alloys containing 43 and 40% Al has been studied.

Hot Deformation Behavior, Dynamic Recrystallization, and Texture Evolution of Ti–22Al–25Nb Alloy

The hot deformation behavior, dynamic recrystallization, and texture evolution of Ti–22Al–25Nb alloy in the temperature range of 950–1050 °C and strain rate range of 0.001–1 s−1 is investigated by plane‐strain compression testing on the Gleeble‐3500 thermo‐mechanical simulator. The results show that the flow stress decreases with the increase of temperature and decrease of strain rate. Besides, the flow curves appear a serrate oscillation at a strain rate of 0.1 s−1 for all the temperature ranges, which may result from instability such as flow localization or micro‐cracking. The flow behavior can be expressed by the conventional hyperbolic sine constitutive equation and the calculated deformation activation energy Q in the (α2 + B2) and B2 regions are 631.367 and 304.812 kJ mol−1, respectively. The microstructure evolution is strongly dependent on the deformation parameters, and dynamic recrystallization (DRX) is the dominant softening mechanism in the (α2 + B2) region, including discontinuous dynamic recrystallization (DDRX), and continuous dynamic recrystallization (CDRX). In addition, the ηbcc‐fiber of {110} &lt;001&gt; is the dominant texture component in deformed Ti–22Al–25Nb alloy. It is observed that the weakening of the deformation texture is accompanied by the occurrence of DRX, which can be attributed to the large misorientation between DRX grains and neighboring B2 matrix induced by the rotation of DRX grains toward the preferred slip systems.

Ti-Al-Nb Material for High-Temperature Applications: Combustion Synthesis from Oxide Raw Materials

This work is focused on preparation of Nb-doped Ti–Al material (Ti–29.3Al–17.6Nb wt %) by centrifugal SHS. Results of studying of combustion and synthesis regularities of intermetallic material on γ-TiAl base from oxide materials under high gravity are presented in the article. For the first time is used a reduction mixture on Al/Ca base which markedly increases the yield of Ti–Al–Nb and decreases the amount of non-metallic impurities (oxygen, nitrogen, carbon) in target product.

Phase selection and solidification path transition of Ti–48Al–xNb alloys with different cooling rates

Ti–48Al–xNb alloys were solidified by containerless electromagnetic levitation with quenching system of the conical copper mold. The influence of cooling rates on phase selection of Ti–48Al–xNb alloys was investigated. In near-equilibrium solidification condition, the dendrite β phase is observed as the leading phase. No other metastable phase (e.g., α phase) is observed. In contrast, in rapid solidification condition, the metastable α phase is observed in as-quenched Ti–48Al–2Nb alloy. Furthermore, the metastable α phase is replaced by the primary β phase with Nb addition increasing. For Ti–48Al–(x = 4, 6, 8)Nb alloys, increasing cooling rate results in a solidification path transition. The peritectic reaction (L + β → α) is therefore significantly suppressed. The relationships between primary dendrite arm spacing (λ1) and cooling rate (τ) can be described.

Effects of Solution Treatment on the Microstructure and Mechanical Properties of Ti–22Al–25Nb Alloys

The effects of solution treatment on the microstructure and mechanical properties of Ti‐22Al‐25Nb alloy are investigated. Numerous equiaxed α2 particles distribute in a B2 matrix after solution treatment in the α2 + B2 region. The growth rate of the B2 grain increases with solution temperature and decreases with increasing holding time. When the solution temperature is lower than B2‐transus, it is seen that both the strength and ductility decrease with increasing solution temperature and holding time. After being solution‐treated at a super‐transus temperature, the alloy with a fully B2 microstructure shows an ultralow elongation (∼5%) and a typical brittle fracture characteristic.

Alloying Effects on the Phase Transformation Behaviors of the Orthorhombic and Ordered ω Phases in High Nb–TiAl Alloys

In this work, alloying effects of Mn, Mo, and Cr on the phase transformation behaviors of the orthorhombic and ordered ω phases in high Nb–TiAl are investigated. Modulated structures are observed in the Mn and Mo‐containing alloys indicating that Mn and Mo cannot hinder the formation of the orthorhombic phase while Cr hinders the formation of the orthorhombic phase completely. For the ordered ω phases, small ordered ω particles are observed in the Mn‐containing alloy. Though Mn cannot hinder the formation of the ordered ω phases, the formation temperature of which is decreased to 600 °C. No ordered ω phases are observed in the Mo and Cr‐containing alloys. Two percent of Mo and Cr hinder the formation of the ordered ω phases completely. The stabilization effect of Mn, Mo, and Cr on βo phase is mainly a thermodynamic effect.

Effects of Nb content in Ti–Ni–Nb brazing alloys on the microstructure and mechanical properties of Ti–22Al–25Nb alloy brazed joints

Vacuum brazing was successfully used to join Ti–22Al–25Nb alloy using Ti–Ni–Nb brazing alloys prepared by arc-melting. The influence of Nb content in the Ti–Ni–Nb brazing alloys on the interfacial microstructure and mechanical properties of the brazed joints was investigated. The results showed that the interfacial microstructure of brazed joint consisted of B2, O, τ3, and Ti2Ni phase, while the width of brazing seams varied at different Nb contents. The room temperature shear strength reached 359MPa when the joints were brazed with eutectic Ti40Ni40Nb20 alloy at 1180°C for 20min, and it was 321, 308 and 256MPa at 500, 650 and 800°C, respectively. Cracks primarily initiated and propagated in τ3 compounds, and partially traversed B2+O region. Moreover, the fracture surface displayed typical ductile dimples when cracks propagated through B2+O region, which was favorable for the mechanical properties of the brazed joint.

Enhanced Ductility of a Bimodal Grain Structure Ti–22Al–25Nb Alloy Fabricated by Spark Plasma Sintering

A bimodal grain structure Ti–22Al–25Nb alloy with enhanced ductility is fabricated from the blended powder mixture of argon gas atomized powder and mechanically alloyed powder by spark plasma sintering. The microstructure of the bimodal grain structure Ti–22Al–25Nb alloy and the effects of the argon gas atomized powder mass fraction on the compression properties are investigated. This alloy shows the bimodal microstructure, which the coarse grain regions are surrounded by the ultrafine grained matrix. Compared with the Ti–22Al–25Nb alloy sintered from 100% mechanically alloyed powder, this alloy sintered from the blended powder mixture containing 20% argon gas atomized powder exhibits the fracture elongation to improve 20.5%.

Formation of titanium and niobium aluminides induced by hydrogen in a hydride cycle

The results of our study of formation of aluminides by the hydride cycle (HC) method in Ti–Al and Ti–Al–Nb systems and of their hydrides under the conditions of self-propagating high-temperature synthesis are presented. The HC method was developed at the High-Temperature Synthesis Laboratory of the Nalbandyan Institute of Chemical Physics, Armenian Academy of Sciences. The effect of various parameters: the ratio of titanium and niobium hydrides and aluminum powders in the reaction mixture, hydride powder grain size (micro and nano size), compacting pressure during compaction of hydrides, and conditions of dehydrogenation and sintering (heating temperature and rate) on the characteristics of the obtained aluminides: crystal structure, density, etc. was studied. As a result, single-phase aluminides α2-Ti3Al, Ti2Al, γ-TiAl, TiAl3, Ti0.33Al0.34Nb0.33, Ti0.5Al0.23Nb0.27, Ti0.52Al0.15Nb0.33, TiAl6Nb, etc., and hydrides were synthesized. The HC method has significant advantages over conventional procedures: low temperature (1000–1100°C) and processing time (30–60 min), one-stage formation of single-phase aluminides, etc.

Effect of synthesis parameters on structural and mechanical properties of Ti3Al–Nb–Mo intermetallic compound obtained by powder metallurgy techniques

The influence of microstructure, heat treatment and alloying addition on mechanical and fracture properties of Ti3Al-based intermetallic at room and elevated temperatures was studied. Ti3Al–11Nb–1Mo (mole fraction, %) alloy was consolidated via powder metallurgy processing by mechanical alloying (MA) and hot pressing (HP). MA powders were characterized using XRD and SEM-EDS. Optimum MA duration was 25 h and HP conditions of 1350 °C, 2 h, 35 MPa. After HP, solution treatment at 1050 °C for 1 h and water quenching α2+β Widmanstätten microstructure is present, while subsequent aging at 800 °C during 24 h induces small content of O-phase. High fraction of β-phase is a direct consequence of Mo. Compression tests were performed from room temperature to 750 °C in vacuum. The yield strength of compacts increases with temperature up to 250 °C (pyramidal slip systems activation), after which it decreases. Ductility increases throughout the whole temperature range. The presence of O phase contributed to ductility increase in aged alloys, while negligibly lowering yield strength. Registered drop in the yield strength of aged alloys compared with non-aged ones was mostly influenced by precipitation of α″2 particles. Mixed fracture modes are operative at all temperatures.

Glass Ceramic Composite Protective Technological Coatings for Thermomechanical Processing of Intermetallic Alloys

Glass ceramic composite protective technological coatings were developed in the systems Na2O–B2O3–SiO2 and BaO–Al2O3–SiO2 for deformation of intermetallic alloys in the systems Ti–Al–Nb and Ni–Al–Co under isothermal conditions in air. The physical-chemical and technological properties of the coatings were investigated and the coatings were shown to be effective in application.