Microstructural control of Ti-Al-Mo-Nb alloy system with respect to variation of B

In the current study, phase stability of Ti-Al-Mo-Nb alloys was investigated, and the effect of B addition was examined for cast alloys. The fabricated cast alloys were mainly composed of α₂ / γ lamellar with a β phase, when they were heat treated at 1100 °C followed by air cooling, the alloy was composed of α2 / γ lamellar with γ+β necklace phase at the colony boundary for the Ti-45Al-3Mo-2Nb-1B alloy, and the colony size was refined to ~ 20 μm. In order to identify the effect of the microstructures on mechanical strength, compressive tests were performed on the fabricated alloys of Ti-45Al-3Mo-2Nb and Ti-45Al-3Mo-2Nb-1B at room temperature and at 800 °C. The microstructural variations and phase stability were discussed in terms of pseudo-binary phase diagram calculated by Pandat software?.     

Mechanical properties of the cast intermetallic alloy Ti-43Al-7(Nb,Mo)-0.2B (at %) after heat treatment

A new approach to obtaining fine-grained structure in intermetallic-compound alloys such as γ-TiAl + α₂-Ti₃Al has been suggested. This approach is based on the use of alloys that solidify as the β phase, which contain β-stabilizing additives such as Nb and Mo and are characterized by the small size of crystallites already in the cast state; in these alloys, a simple heat treatment makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase. It is shown on the example of the Ti-43Al-7(Nb,Mo)-0.2B (at %) alloy that this heat treatment ensures superplastic properties in the material in the temperature range of T = 1050–1130°C at a deformation rate $ \dot \varepsilon A new approach to obtaining fine-grained structure in intermetallic-compound alloys such as γ-TiAl + α₂-Ti₃Al has been suggested. This approach is based on the use of alloys that solidify as the β phase, which contain β-stabilizing additives such as Nb and Mo and are characterized by the small size of crystallites already in the cast state; in these alloys, a simple heat treatment makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase. It is shown on the example of the Ti-43Al-7(Nb,Mo)-0.2B (at %) alloy that this heat treatment ensures superplastic properties in the material in the temperature range of T = 1050–1130°C at a deformation rate = 1.7 × 10⁻⁴ K⁻¹. Under these temperature-strain-rate conditions, relative elongations such as δ = 160–230% and low flow stresses such as σ = 36–100 MPa characteristic of superplastic flow have been obtained. It has been shown for the first time for the intermetallic γ-TiAl + ga₂-Ti₃Al alloy that a sheet semifinished product cut out from an ingot subjected only to heat treatment can have plasticity acceptable for press forming. Original Russian Text ? V.M. Imayev, R.M. Imayev, T.G. Khismatullin, 2008, published in Fizika Metallov i Metallovedenie, 2008, Vol. 105, No. 5, pp. 516–522. The author is also known by the name Imayev. The name used here is a transliteration under the BSI/ANSI scheme adopted by this journal.—Ed.     

Effect of the annealing temperature on the long-range order in the B2 phase of the Ti-Al-Nb(Zr,Mo) alloy

Investigation has been performed of changes in the degree of long-range order (LRO) upon annealing in a temperature range of 1000–1150°C for 1 h in the cubic B2 phase of the Ti-28.2% Al-13.2% Nb-1.2% Zr-0.86% Mo alloy. The calculation carried out using X-ray diffraction data showed that the degree of LRO in the B2 phase in the three-phase field (O + α₂ + B ₂, T = 1000°C) is 0.36. Structure of the B2-phase regions with a low degree of LRO was studied using transmission electron microscopy. Temperature ranges suitable for thermomechanical treatment of the alloy have been determined.     

Study of the Ti-Al-Nb phase diagram

Differential thermal and X-ray diffraction analyses as well as optical and transmission electron microscopy were used to study orthorhombic Ti₂AlNb-based alloys, such as Ti-22 at. % Al-26.6 at. % Nb, Ti-23.5 at. % Al-21 at. % Nb, Ti-24.6 at. % Al-22 at. % Nb. These alloys in the initial state contain an orthorhombic O phase (Ti₂AlNb) ordered in terms of all three elements. The differential thermal analysis was performed at different heating and cooling rates, namely, 40 and 80 K/min. The solidus and liquidus temperatures of the Ti-22 at. % Al-26.6 at. % Nb, Ti-23.5 at. % Al-21 at. % Nb, Ti-24.6 at. % Al-22 at. % Nb alloys have been measured; these are T _S = 1707 and T _L = 1737°C, T _S = 1703 and T _L = 1730°C, and T _S = 1694 and T _L = 1728°C, respectively. The temperature boundaries of phase transformations occurring in these alloys have been determined. The activation energy of phase transformations has been calculated using the Johnson-Mehl-Avrami equation. The results obtained are compared with the available literature data. Causes of the discrepancy between equilibrium phase diagrams of the Ti-Al-Nb system reported by different authors are analyzed.     

Microstructure and fracture toughness of a β phase containing TiAl alloy

Microstructures and fracture toughness of Ti-45Al-2Nb-1.5V-1Mo-0.3Y alloy have been investigated. The alloy exhibits fine nearly lamellar microstructures, consisting mainly of fine lamellar grains, together with mixtures of γ and residual β phases along lamellar colony boundaries. Precipitation of both β and γ phases from α₂ lamellae was found after aging at 950 °C for 48 h. Phase transformations involving β phase both in α₂ laths and along colony boundaries are discussed. This TiAl alloy possesses a higher K_IC value up to 23.5 MPam^1/2 at room temperature, compared with fully lamellar Ti-45Al-5Nb-0.3Y alloy. The toughening mechanism for current alloy is concluded as trans-lamellar fracture, ligament bridges and crack deflection, together with precipitation of β and γ phases. The precipitation of fine β and γ particles is considered as intrinsic toughening mechanism, because α₂/β and α₂/γ interfaces generating due to precipitation can restrict dislocation motion effectively.     

添加晶粒细化剂硅对Ti-6Al-4V合金组织和性能的影响(英文)

采用真空感应凝壳熔炼工艺在石墨模中制备Ti-6Al-4V和Ti6Al4V0.5Si两种钛合金。将硅作为一种晶粒细化剂加入到Ti-6Al-4V合金中,考察添加硅对铸态和模锻态Ti-6Al-4V合金组织和性能的影响。铸态合金先在900°C下进行热模锻处理,然后分别进行两种不同的热处理。一种是将模锻样品在1050°C下保温30min,然后水淬以获得细小的层片状组织;另一种是将模锻件在1050°C下保温30min,然后再在800°C下保温30min,以获得粗大的层片状组织。Ti6-Al-4V合金中添加0.5%Si后,铸态合金的晶粒尺寸从627μm减小到337μm,其极限抗拉强度增加约25MPa。具有细小、层片状组织的Ti-6Al-4V0.5Si合金的最大极限抗拉强度为1380MPa,在Hank溶液和NaCl溶液中的腐蚀速度分别为1.35×106和5.78×104mm/a。Ti-6Al-4V合金中添加0.5%Si后,在低滑动速度下的磨损率降低50%,在高滑动速度下的磨损率降低约73%。     

Microstructure and mechanical properties of large size Ti-43Al-9V-0.2Y alloy pancake produced by pack-forging

A pancake of Ti-43Al-9V-0.2Y (at.%) alloy with dimensions of ϕ480 mm × 46 mm was fabricated by pack-forging with a thick reduction of 80%. The as-forged Ti-43Al-9V-0.2Y alloy pancake has a duplex (DP) microstructure, which is composed of B2/α₂/γ lamellar colonies and massive B2 and γ phase regions distributed along the boundaries between the lamellar colonies. Different microstructures were obtained by heat treatment of samples cut from the as-forged Ti-43Al-9V-0.2Y alloy pancake. A fully lamellar (FL) structure consisting of B2/α₂/γ lamellar colonies was obtained after the heat treatment of 1350 °C/8 h. Tensile test results exhibited that the yield strength (YS) and ultimate tensile strength (UTS) of the alloy with DP microstructure were decreased from 680.7 MPa to 834.3 MPa at room temperature to 589.5 MPa and 693.1 MPa at 700 °C, respectively, and the elongation (δ) of the alloy with DP microstructure was increased from 1.99% at room temperature to 12.12% at 700 °C; the elongation (δ) of the alloy with FL microstructure was increased from 1.52% at room temperature to 85.84% at 800 °C.     

Phase equilibria in Ti3Al-Nb alloys at 1000 °C

Titanium aluminides are considered to be the future high-temperature structural materials for turbine applications. Major focus is on α₂Ti₃Al based and γTiAl based alloys. Niobium additions to Ti₃Al alloys is found to improve the room-temperature ductility. Thus phase equilibria in Ti-Al-Nb system is of practical significance with regard to their processing and high-temperature phase stability characteristics. In the present research, four alloys with compositions Ti-22Al-12Nb, Ti-21A1-16Nb, Ti-20Al-20Nb, and Ti-25Al-25Nb (all in atom percent) were equilibrated at 1000 °C for 225 hours and then quenched in water. The quenched alloys were characterized for phase relations by optical microscopy, X-ray diffraction (XRD), and electron probe microanalysis (EPMA). Based on the phase analysis, the ternary isotherm of the Ti-Al-Nb system at 1000 °C was constructed on the Ti₃Al-rich side. The orthorhombic Ti₂AlNb phase was observed in the sample with Ti-25Al-25Nb composition signifying the presence of this phase at 1000 °C.     

Improvement of mechanical properties of the Ti–45Al–5Nb–1Mo–0.2B (аt %) intermetallic alloy by means of microstructure controlling

The effect of heat and thermomechanical treatments conditions on the microstructure and main mechanical characteristics (obtained by tensile, high-temperature long-term strength, fracture toughness, and high-cycle fatigue tests) of the Ti–45Al–5Nb–1Mo–0.2B (аt %) alloy was studied. Before the treatments, the sequence of phase transformations in the alloy after its solidification was determined by testquenching method. The obtained data were used to develop conditions for the heat and thermomechanical treatments. It was found that a small but stable increase in the plasticity and strength of the cast alloy is observed after three-stage annealing at temperatures that correspond to the (α + γ)- and (α₂ + β(В₂) + γ)-phase region. The thermomechanical treatment at temperatures corresponding to the (α(α₂) + β(В₂) + γ)-phase region and subsequent two-stage annealing at temperatures that correspond to the (α + β(В₂) + γ)- and (α₂ + β(В₂) + γ)-phase region lead to the formation of fine-grained duplex structure. This determined the substantial improvement of the low-temperature plasticity and strength (δ = 3.1% and σ_u = 860 MPa at 20°C, respectively) and retained high creep resistance to 700°C.     

Allotropic phase transformation and high-temperature tensile deformation behaviour of powder metallurgy Ti-5553 alloy

Ti-5Al-5V-5Mo-3Cr metastable beta titanium alloy was prepared by rapid thermomechanical powder consolidation approach from blended elemental powder mixture. Allotropic phase transformation and high-temperature tensile behaviour of the consolidated powder metallurgy Ti-5553 alloy were investigated in this work. The studied alloy has a high β phase transformation temperature of 975 °C±5 °C, which is higher than other conventional ingot metallurgy Ti-5553 alloys. The β grains in the microstructure of the alloy are coarsened significantly with increasing the heating temperature from 890 °C to 1050 °C, however, the grain coarsening tendency is mitigated when the heat treatment temperature reach to the range of 1080 °C–1100 °C. The high-temperature tensile mechanical properties of the alloy are sensitive to both the deformation temperature and strain rate, and superplastic deformation of the alloy was achieved at the condition of 850 °C/0.001 s⁻¹ with the tensile elongation of 103.5%. The microstructural evolution characteristics and the fracture mechanisms of the alloy are varied with changing the deformation variables, which are revealed by the microstructure observation of the fractured specimens from different sampling positions.     

The effects of long-term air exposure on the stability of lamellar TiAl alloys

The thermal stability of fine-grained and coarse-grained lamellar TiAl alloys was studied at 700°C in air. Both α₂ and γ lamellae were altered markedly after 1000- and 3000-h exposure. Three types of decomposition were found: (a) parallel decomposition of coarse α₂ into bunches of very fine α₂+γ lamellae, (b) perpendicular decomposition of fine γ (or γ+α₂) lamellae into short-ranged γ domains or (γ+α₂) grains and (c) perpendicular decomposition of fine α₂ lamellae into short-ranged α₂/γ grains. All three types are still at an intermediate stage after 3000-h exposure and neither break-up of lamellae nor globularisation of segments was observed. The thermal stability of microstructural constituents in increasing order was found to be: thick α₂, thin α₂, thin γ, and thick γ lamellae. Tensile properties were affected by these changes and the extent was found to vary from alloy to alloy.     

Phase and structural transformations in the alloy on the basis of the orthorhombic titanium aluminide

Phase and structural transformations in the Ti-24.3 Al-24.8 Nb-1.0 Zr-1.4 V-0.6 Mo-0.3 Si (at %) alloy that take place during heating in the temperature range of 700–1050°C have been investigated. The temperature ranges of existence of the O + β, O + β + α₂, β + α₂, and β phase fields have been established. A scheme of the relationships between the volume fractions of the O, β, and α₂ phases depending on the temperature of heating of the alloy have been investigated. The formation of an ordered incommensurate ω (V _ω) phase has been revealed in the alloy during quenching from 900°C. The existence of a correlation between the hardness properties and changes in the phase composition and morphology of particles precipitating in the alloy has been shown.     

Martensitic transition and shape memory effect of Ti-Zr-Mo series alloys

Development of shape memory alloys is always one of the most important directions for functional Ti alloys. The Ti-Zr-Mo series alloys with various Mo contents were prepared. The main aim of the current work is to investigate the effects of Mo on martensitic transition and shape memory effect of Ti-Zr alloy. The X-ray diffraction and transmission electron microscope results indicate that the phase constitution of the examined alloys is greatly dependent on Mo content. The Ti-Zr-Mo alloy with 2 wt% Mo is composed mainly of α′ martensite and a few β phase. As the Mo content increased to 4 wt%, the Ti-50Zr-4Mo alloy consists of α″ martensite and β phase. As the Mo content further increased to 8 wt%, the alloy consists mainly β phase and a barely detectable amount of α″ martensite. Thermal analysis shows that the reverse martensitic transition temperature of the examined alloys decreases with the increasing of Mo. The reverse martensitic transition start, A_s, temperature is approximately 584 °C for Ti-50Zr-2Mo alloy and 519 °C for Ti-50Zr-4Mo, respectively. And the martensitic transition start, M_s, temperature is approximately 553 °C and 501 °C for that two alloys, respectively. But no obvious exothermic and/or endothermic peak can be observed in DSC curve of Ti-50Zr-8Mo alloy. Furthermore, the effect of Mo content on shape memory recovery ratio, η, of the examined alloys was also investigated. Results show that the η first increases and then decreases with the increasing of Mo. The alloy with 4 wt% Mo has the maximum η approximately 13.8%. The influencing mechanism of Mo content on shape memory effect of the examined alloys was also discussed. This findings not only supplied a series of shape memory TiZr-based alloys, but also enriched and deepened the theory of shape memory effect.     

In-situ multi-layer formation in the oxidation of Ti3Al-Nb

The effect of niobium on the oxidation of a Ti₃Al alloy was studied in pure oxygen in the range of 850-1,100°C. The oxidation products for the Ti-30Al-2.7 Nb alloy were mainly TiO₂ (rutile) mixed with A1₂O₃ (alumina) and small amounts of niobium oxide. The oxidation resistance of Ti₃Al was improved by the addition of niobium. An in-situ multiple-layer structure comprising a mixture of rutile and alumina formed on the oxide scale of the alloy at temperatures 1,000°C and above. The number of layers increased as the temperature increased but the individual layer thickness decreased.     

High-strength Ti-Al-V-Zr cast alloys designed using α and β cluster formulas

Ti-Al-V-Zr quaternary titanium alloys were designed following α-{[Al-Ti₁₂](AlTi₂)}_17−n+β-{[Al-Ti₁₂Zr₂](V₃)}_n, where n=1–7 (the number of β units), on the basis of the dual-cluster formula of popular Ti-6Al-4V alloy. Such an alloying strategy aims at strengthening the alloy via Zr and V co-alloying in the β-Ti unit, based on the original β formula [Al-Ti₁₄](V₂Ti) of Ti-6Al-4V alloy. The microstructures of the as-cast alloys by copper-mold suction-casting change from pure α (n=1) to α+α′ martensite (n=7). When n is 6, Ti-5.6Al-6.8V-8.1Zr alloy reaches the highest ultimate tensile strength of 1,293 MPa and yield strength of 1,097 MPa, at the expense of a low elongation of 2%, mainly due to the presence of a large amount of acicular α′ martensite. Its specific strength far exceeds that of Ti-6Al-4V alloy by 35%.

     

粉末冶金态与铸态Ti-5553合金高温不连续屈服行为和绝热温升效应对比研究

本文系统地研究了粉末冶金态与铸态Ti-5553合金在温度为700 ℃~1100 ℃、应变速率为0.001 s_^-1~10 s_^-1条件下的高温不连续屈服行为和绝热温升效应,并对这两种同名义成分不同制备工艺的钛合金进行了对比研究。结果表明：两种合金不连续屈服的幅度均与应变速率呈正相关关系,并与温度呈近似负相关关系, 两种合金中出现的不连续屈服现象符合动态理论。在相同变形条件下,铸态合金中不连续屈服的幅度更大,其原因是相对于粉末冶金态合金,铸态合金中的起始位错密度低,这更有利于晶界处可动位错的突然增殖与扩展。两种合金在热变形中绝热温升的大小均随应变速率的升高而逐渐增大,并随着变形温度的升高而逐渐降低。在相同变形条件下,粉末冶金态合金的绝热温升效应相比与铸态合金较弱,这是因为粉末冶金态合金具有较低的变形抗力和较高的协调变形能力。     

The oxidation behavior of high Cr and Al containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250 °C

The oxidation behavior of two alloys containing different content of Al and Cr from the Nb-Si-Ti-Hf-Al-Cr system has been evaluated at 1200 and 1250 °C. The alloy compositions in atomic percent are Nb-24Ti-16Si-2Hf-2Al-10Cr (B1), and Nb-24Ti-16Si-2Hf-6Al-17Cr (B2). The oxidation kinetic of B1 alloy at 1200 and 1250 °C followed a mixed parabolic-linear law, while the oxidation kinetic of B2 alloy at 1200 and 1250 °C followed a parabolic law. The weight gain of B2 alloy was 18.9 mg/cm² after oxidation at 1200 °C for 100 h, which was a seventh of the value of that of B1 alloy. Besides, oxidation became more severe as temperature increased to 1250 °C. The oxide scales of B2 alloy consisted of CrNbO₄, TiNb₂O₇ and SiO₂, which were relatively compact and protective. In addition, the oxidation mechanism of Nb-Si based alloys were also discussed.     

Microstructural instability in surface layer of a high Nb-TiAl alloy processed by shot peening following high temperature exposure

Microstructural instability induced by shot peening was investigated in a Ti-45Al-8.5Nb-(W, B, Y) alloy following high temperature exposure. After shot peening and thermal exposure at 1000 °C for 300 h, fine grains are formed in the outermost (FG layer). Underneath, coarse grains (CG layer) are formed. The FG layer is composed of completely recrystallized γ grains. The CG layer is composed of incompletely recrystallized γ grains, where the critical strain for recrystallization is not reached. During long term thermal exposure, α₂ lamellae can undergo dissolution, precipitation and growth. After thermal exposure for 300 h, large α₂ grains precipitated at the γ/γ interface or inside the γ grains in both FG and CG layers. The precipitated α₂ particles almost have the same orientation with primary α₂ lamellae, indicating that nearly no recrystallization phenomena occur for α₂ phase. So the γ lamella is easier to recrystallize than that of α₂ lamella at the same temperature and residual strain.     

Refining of the microstructure of cast intermetallic alloy Ti-43% Al-X (Nb, Mo, B) with the help of heat treatment

The microstructure and high-temperature mechanical properties of cast alloy Ti-43% Al-X (Nb, Mo, B) belonging to the new class of β-hardening γ-TiAl + α₂-Ti₃Al alloys is investigated. The alloy in the cast state and after a heat treatment promoting structure refinement is studied by the methods of light and scanning electron microscopy, microscopic x-ray spectrum analysis, and differential thermal analysis. The mechanical characteristics are evaluated in tensile tests in air at 1000 and 1100°C. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 38–41, February, 2006.