Conventional Titanium Alloys Research Articles

Workpiece surface integrity considerations when finish turning gamma titanium aluminide

Gamma titanium aluminides (γ-TiAl) are currently being evaluated by aeroengine manufacturers as possible replacements for conventional titanium alloys and nickel based superalloys in gas turbine engines. Unfortunately, even when machining using operating parameters selected to minimise workpiece damage, turned surfaces contain cracks and microstructural alterations. The paper initially reviews prior work on the surface integrity of γ-TiAl produced by various machining processes. Following on from this, surface integrity data from a full factorial experiment are presented when turning Ti–45Al–2Mn–2Nb+0.8 vol.% TiB 2 employing extremely fine finishing cuts. Workpiece surfaces/subsurfaces were evaluated in relation to microstructural alterations, strain hardening/microhardness changes, and 2D surface roughness (Ra). All the machined surfaces contained cracks, cracked TiB 2 particles, deformed microstructure and microhardness increases regardless of the operating parameters. However, when comparing the surfaces with those obtained in previous work using more abusive parameters, a significant reduction in surface damage was obtained.

Oxidation of Orthorhombic Titanium Aluminide TI-22AL-25NB in Air between 650 and 1000 °C

The oxidation behavior of orthorhombic titanium aluminide alloy Ti-22Al-25Nb was studied in air between 650 and 1000 °C by isothermal thermogravimetry and postoxidation scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction. Microhardness measurements were performed after exposure to gage hardening due to nitrogen and oxygen ingress. The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys and Ti3Al-based titanium aluminides at and below 750 °C. Between 800 and 1000 °C, the oxidation resistance of Ti-22Al-25Nb was as good as that of γ-TiAl based aluminides; however, the growth rate changed from parabolic to linear after several tens of hours at 900 and 1000 °C. The mixed oxide scale consisted of TiO2, AlNbO4, and Al2O3, with TiO2 being the dominant oxide phase. Underneath the oxide scale, a nitride-containing layer formed in the temperature range investigated, and at 1000 °C, internal oxidation was observed below this layer. In all cases, oxygen diffused deeply into the subsurface zone and caused severe embrittlement. Microhardness measurements revealed that Ti-22Al-25Nb was hardened in a zone as far as 300 µm below the oxide scale when exposed to air at 900 °C for 500 h. The peak hardness depended on exposure time and reached five times the average hardness of the bulk material under the above conditions.

Rapidly Solidified Powder Metallurgy Mg<SUB>97</SUB>Zn<SUB>1</SUB>Y<SUB>2</SUB>Alloys with Excellent Tensile Yield Strength above 600 MPa

Nanocrystalline magnesium alloys having high tensile strength, high elevated-temperature tensile strength, high-strain-rate superplasticity and high thermal stability have been developed in Mg 97 Zn 1 Y 2 (at% I alloy by rapidly solidified powder metallurgy (RS P/M) processing. The tensile yield strength and elongation that were dependent on the consolidation temperature were in the ranges of 480 to 610MPa and 5 to 16%, respectively. Young's modulus of the RS P/M alloy was 45 GPa. The specific tensile yield strength was four times as high as that of a commercial AZ91-T6 alloy, and was higher than those of conventional titanium (Ti-6Al-4V) and aluminum (7075-T6) alloys. The RS P/M alloys exhibited excellent elevated-temperature yield strength that was 510 MPa at 423 K. The RS P/M alloy also exhibited high-strain-rate superplasticity at a wide strain-rate range from I × 10 - to I × 10 0 s -1 and at a low temperature of 623 K. It is expected that the Mg 97 Zn 1 Y 2 RS P/M alloy can he applied in some fields that requires simultaneously the high specific strength at ambient and elevated temperatures and high workability.

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ∼2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10-2 s-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.

Production of titanium particulate metal matrix composite by mechanical milling

Mechanical milling is an established production method for aluminium particulate metal matrix composites (MMCs). There are examples of its use for high performance automotive applications and within the aerospace industry. The production of a titanium particulate MMC is still in the developmental stage. However, compared to conventional titanium alloys such materials offer improvements in stiffness, strength, fatigue and creep properties, high temperature capability, and wear resistance. This paper describes the use of mechanical milling for the production of titanium particulate MMCs with the addition of 10 vol.-%TiB. Gas atomised titanium powders with additions of either boron or TiB2 were milled in a high purity argon atmosphere to avoid contamination of the powders by oxygen or nitrogen. The distribution of the boron or TiB2 with increasing milling time is discussed along with the effect of the alloy composition. Gas atomised, hydride dehydride, and sponge fine powder blends are also compared. The powders were subsequently hot isostatically pressed at 500°C for 2 h at 150 MPa followed by 900°C for 2 h at 150 MPa. During this consolidation process TiB was formed by an in situ reaction between either the TiB2 or boron and the titanium matrix.

Oxidation-Resistant Coatings for Application on High-Temperature Titanium Alloys in Aeroengines

High-temperature application of titanium alloys in aeroengines is often limited by their insufficient resistance to the aggressive environment. Magnetron-sputtered Ti–Al based coatings were developed in order to increase the maximum service temperature of conventional titanium alloys from the present 520–600 °C, the temperature limit set by the mechanical capabilities of most advanced alloys. The coatings not only demonstrated excellent oxidation resistance but also demonstrated beneficial effects on mechanical properties. Most importantly, the fatigue behavior of the substrate alloys was not degraded, a major hurdle for coating application on titanium alloys so far. Initial results on Cr-containing Ti–Al based coatings indicated significant potential for application on titanium aluminides.

Oxidation-Resistant Coatings for Application on High-Temperature Titanium Alloys in Aeroengines

High-temperature application of titanium alloys in aeroengines is often limited by their insufficient resistance to the aggressive environment. Magnetron-sputtered Ti-Al based coatings were developed in order to increase the maximum service temperature of conventional titanium alloys from the present 520-600 °C, the temperature limit set by the mechanical capabilities of most advanced alloys. The coatings not only demonstrated excellent oxidation resistance but also demonstrated beneficial effects on mechanical properties. Most importantly, the fatigue behavior of the substrate alloys was not degraded, a major hurdle for coating application on titanium alloys so far. Initial results on Cr-containing Ti-Al based coatings indicated significant potential for application on titanium aluminides.

Titanium Particulate Metal Matrix Composites - Reinforcement, Production Methods, and Mechanical Properties

Titanium metal matrix composites (MMCs) offer potential advantages for structural applications, where they combine the high strength, high temperature capability, and oxidation resistance of titanium with an increase in stiffness provided by the ceramic reinforcement. They have the advantage of being isotropic in behaviour, cheaper to manufacture and more amenable to subsequent processing and component forming operations. Of potential reinforcing phases for titanium, including TiB, TiB2, SiC, Al2O3, and TiC, TiB offers the best balance of stiffness, stability, and similarity of thermal expansion coefficients. The methods used to produce these Ti–TiB MMCs, such as arc melting, gas atomization, rapid solidification, and powder blending have been assessed and the benefits these composites offer over conventional titanium alloys including increased stiffness, good creep performance, fatigue resistance, and wear resistance are highlighted.

Long-term oxidation of orthorhombic alloy Ti-22Al-25Nb in air between 650 and 800°C

Titanium alloys based on the orthorhombic phase Ti{sub 2}AlNb are currently under investigation for those aeroengine applications where the maximum service temperatures exceed the capabilities of conventional titanium alloys but are still lower than recommended for {gamma}-TiAl based aluminides. For some orthorhombic alloys, a better fracture toughness, higher ductility, higher specific yield and lower coefficient of thermal expansion for {gamma}-TiAl based alloys as well as for alpha-2 alloys has been reported and creep and fatigue behavior are similar to that of alpha-2 alloys. However, oxidation and embrittlement of orthorhombic alloys are a major concern when applications in the oxidizing atmosphere of an aeroengine and operating temperatures between 650 and 700 C are considered. The goal of the present study was to assess the long-term environmental resistance of high niobium-containing Ti-22Al-25Nb, an alloy presently under consideration for titanium matrix fiber reinforced composites (TMCs). TMCs are needed to introduce blisk (= bladed disk) and bling (= bladed ring) technology into modern aeroengine design. Oxidation tests on cast Ti-22Al-25Nb matrix material in laboratory air were conducted between 650 and 800 C up to 4,000 hr.

Corrosion wear fracture of new β type biomedical titanium alloys

Abstract Metallic materials such as stainless steel, Co–Cr alloy, pure titanium and titanium alloys have been used for surgical implant materials. The α+β type titanium alloy such as Ti-6Al-4V ELI has been most widely used as an implant material for artificial hip joint and dental implant because of its high strength and excellent corrosion resistance. Toxicity of alloying elements in conventional biomedical titanium alloys like Al and V, and the high modulus of elasticity of these alloy as compared to that of bone have been, however, pointed out [1] , [2] . New β type titanium alloys composed of non-toxic elements like Nb, Ta, Zr, Mo and Sn with lower moduli of elasticity, greater strength and greater corrosion resistance were, therefore, designed in this study. The friction wear properties of titanium alloys are, however, low as compared to those of other conventional metallic implant materials such as stainless steels and Co–Cr alloy. Tensile tests and friction wear tests in Ringer’s solution were conducted in order to investigate the mechanical properties of designed alloys. The friction wear characteristics of designed alloys and typical conventional biomedical titanium alloys were evaluated using a pin-on-disk type friction wear testing system and measuring the weight loss and width of groove of the specimen.

Strain ageing and yield plateau phenomena in γ-TiAl based alloys containing boron

There has been considerable interest over the past few years in {gamma}-TiAl based alloys since they offer a combination of low density and useful mechanical properties at temperatures higher than those possible with conventional titanium alloys. However, there are still serious limitations to their use in engineering components due to their limited ductility and fracture toughness. Much of the recent work has been focused on improving the room temperature ductility of these materials, and a significant part of the work has been involved with studying the effects of thermo-mechanical processing (TMP) and alloying. One of the alloying additions which has received much attention is boron. Addition of boron ({ge}0.5 at.%) leads to refined as-cast grain structures and can increase the strength and ductility of these alloys. If boron does segregate to grain boundaries, it would be expected that segregation would also occur at dislocations, which can result in solute locking and yield point phenomena. Nakano and Umakoshi`s results show some signs of this, with regions of distinct upward curvature in stress-strain curves for boron-containing material, although the flow stress was always increasing with strain. Evidence of strain ageing in TiAl alloys containing boron has also been reported by Wheeler etmore » al., and the work reported here also suggests that boron can act to produce solute locking of glide dislocations in a different class of near {gamma}-TiAl alloys.« less

Environmental Effects on Orthorhombic Alloy Ti-22Al-25Nb in Air Between 650 and 1000°C

The environmental behavior of an orthorhombictitanium-aluminide alloy, Ti-22Al-25Nb, was studied indry and humid air between 650 and 1000°Cby scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction.Microhardness measurements were performed after exposureto gage hardening due to nitrogen and oxygen ingress.The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys andTi3 Al-base titanium aluminides at and below750°C. Between 800 and1000°C, the oxidation resistance ofTi-22Al-25Nb was as good as γ-TiAl base aluminides;however, the growth rate changed from parabolic tolinear after several tens of hours at 900 and1000°C. The mixed oxide scale consistedof TiO2, AlNbO4, andAl2O3 with TiO2 beingthe dominant oxide phase. Underneath the oxide scale, anitride layer formed in the temperature rangeinvestigated and, at 1000°C, internal oxidation was observed below thislayer. In all cases, oxygen diffused deeply into thesubsurface zone and caused severe embrittlement.Microhardness measurements revealed that Ti-22Al-25Nbwas hardened in a zone as far as 300 μm belowthe oxide scale when exposed to air at900°C for 500 hr. The peak hardnessdepended on exposure time and reached five times theaverage hardness of the bulk material under the aboveconditions.

Hot workability of titanium and titanium aluminide alloys—an overview

The hot workability of conventional titanium alloys and titanium aluminides is reviewed. For both alloy classes, the influence of hot working variables and microstructure on failure via fracture or flow-localization controlled processes is summarized. The occurrence of wedge cracking and cavitation during bulk forming of α/ β alloys with Widmanstatten microstructures or γ titanium aluminides with lamellar or equiaxed structures, is examined. In particular, the effects of grain size, grain boundary second phases and process variables on failure are presented. Observations and models of flow localization and cavitation processes which lead to failure during low strain rate, superplastic, tensile-type deformation of titanium and titanium aluminide alloys with fine equiaxed structures, are also described. In the area of flow-localization-controlled failure during bulk forming, the occurrence of shear bands and other flow nonuniformities during both conventional and isothermal hot working processes is reviewed. The influence of material properties, such as flow softening rate and strain rate sensitivity and process variables, which lead to temperature and hence flow nonuniformities, is examined. The flow localization concepts are illustrated for several hot working processes.

粉末冶金法Ti<SUB>3</SUB>Al-Nb金属間化合物の金属組織制御による高サイクル疲労強度の改善

Ti3Al-based alloys are attractive materials for aircraft and automobile parts because of their higher strength to weight ratio and higher Young’s modulus compared to those of conventional titanium alloys. In the present study, Ti3Al alloys containing large amounts of Nb were produced using a blended elemental (BE) powder metallurgy (P/M). Since high cycle fatigue strength is particularly important for industrial applications, special attention was paid to improve this property by the modification of microstructure.Extra low chlorine titanium powder and Niobium-Aluminum master alloy powder were used as starting powder materials. Blended powders were cold pressed using mechanical pressing or cold isostatic pressing, vacuum sintered at 1573 K for 10.8 ks and finally hot isostatic pressed (HIP’ed) at 1373 K and 200 MPa for 10.8 ks. The compacts produced by this conventional method showed a coarse colony-like microstructure. A refined equiaxed α2 microstructure was obtained by the innovative BE P/M method, in which a water-quenching step is added prior to HIP’ing. Mechanism of the formation of an equiaxed microstructure was analyzed based on the observation of microstructural evolution during the HIP thermal cycle. Remarkable improvement of smooth high cycle fatigue strength was attained by this microstructural modification. For example, high cycle fatigue strength at 107 cycles increased to 440 MPa, from 340 MPa for conventionally processed materials.

Sputtered intermetallic Ti–Al–X coatings: phase formation and oxidation behavior

Intermetallic Ti–Al–X (X=Cr, Nb) coatings were deposited on conventional near-α titanium alloy TIMETAL 1100 and on γ-TiAl based Ti–48Al–2Cr–2Nb by magnetron sputtering for oxidation protection. Low temperature deposition process leads to a metastable coating structure which transforms into a two-phase microstructure during high temperature exposure. Oxidation behavior was tested in interrupted weight gain tests in laboratory air at 750°C for TIMETAL 1100 and at 900°C for Ti–48Al–2Cr–2Nb. Oxidation behavior of the coatings shows the same tendency for both substrates and oxidation temperatures. Nb-containing coatings exhibited only poor protection due to partial spallation of the coatings during cooling. Maximum oxidation resistance was achieved by Ti–63Al–7Cr coatings on both substrates. For the γ-TiAl based Ti–Al–Cr coatings oxidation resistance is improved with increasing Cr content. Despite high oxidation resistance of the Cr-containing coatings rutile was always found in addition to protective alumina in the oxide scale.

Investigation of the effects of microstructure on creep deformation in α2-based titanium aluminide intermetallics

The results of a systematic investigation of the effects of microstructure/substructure on the secondary creep behaviour of α2-based titanium aluminide alloys are presented. This includes a study of the effects of heat treatment on the steady-state creep behaviour of α2+β-processed Ti-24Al-11Nb and Ti-25Al-10Nb-3V-1Mo at 540, 650 and 760°C, and an investigation of the effects of creep deformation on dislocation substructures in Ti-24Al-11Nb. The parameters that control secondary creep deformation are identified for both alloys, and the results are compared with data obtained for conventional high-temperature near-α titanium alloys and β-forged Ti-24Al-10Nb-3V-1Mo.

Microstructure development and fracture of in-situ reinforced Ti-8.5Al-1B-1Si

The conventional titanium alloys like Ti-6Al-4V are not suitable for application at high temperatures due to their metastable microstructure, high oxidation rates and loss of strength. In comparison, the {alpha} + {alpha}{sub 2} alloys have been studied for elevated temperature properties. Research on titanium alloys have of late included synthesis through the rapid solidification processing (RSP) route. The use of RSP allows the incorporation of B and Si in Ti alloys which allow for the in-situ nucleation of needle shaped borides and spherical silicides as dispersoids. This contrasts with the heterogeneous distribution of coarse dispersoids with conventional melting techniques. It has also been shown that these reinforcements lead to improvements in modulus, high temperature strength and creep resistance. Very limited information is presently available on the elevated temperature fracture properties of in-situ formed titanium composites. It is the object of this study to elucidate the micromechanisms of fracture under monotonic loading.

Short-time beta grain growth kinetics for a conventional titanium alloy

The kinetics of beta grain growth during short-time, supertransus heat treatment of Ti 6Al 4V were determined using a salt-pot technique. The finite-time, subtransus temperature transient during salt-pot heating was quantified through measurements of the heat transfer coefficient characterizing conduction across the salt-titanium interface and a simple heat conduction analysis which incorporated this heat transfer coefficient. Grain size versus time data adjusted to account for the subtransus temperature transient were successfully fit to the parabolic grain growth law d n − d n 0 = ktexp(− Q/RT) using an exponent n equal to 2.0. Comparison of the present results to rapid, continuous heat treatment data in the literature for a similar titanium alloy revealed a number of semi-quantitative similarities.

Development and Applications of Titanium Alloy SP-700 with High Formability

Abstract A new β-rich α + β titanium alloy, SP-700, was designed to improve hot workability and mechanical properties over Ti-6A1-4V alloy. The chemical composition of the alloy is Ti-4.5A1-3V-2Mo-2Fe, and particularly enhanced properties include superplasticity, hardenability, and fatigue strength. Owing to its extremely fine microstructure and low β-transus temperature, SP-700 is superplastically formable at temperatures below 1073 K without significant increase in flow stress. More advantageously, diffusion bonding is also accomplished around this temperature. The low temperature SPF/DB process not only saves die material life and process costs, but also reduces alloy degradation from exposure at elevated temperatures, e.g., grain growth and oxidation. Isothermal and conventional hot forging are also performed better with SP-700 than ordinary titanium alloys like Ti6A1-4V. The alloy is heat treatable and one of its many mechanical properties is that high strength over 1200 MPa is attained in solution treatment and aging condition. The advantage to this is that the moderately stable β phase tolerates air cooling for solution treatment. Thermal distortion in work pieces, which would be caused by rapid cooling, is consequently avoided. In addition, the alloy is cold workable in such cases as rolling and blanking, when annealed around 1073 K. Furthermore, fatigue life and resistance to hydrogen-assisted stress corrosion cracking are improved due to reduced aluminum content. The superior properties of SP-700 over conventional titanium alloys make new applications possible in non-aerospace fields including sporting goods, hand tools, and wrist watches. Golf drivers, with club heads made of SP-700, are sold commercially. Crampons, a new product of SP-700, withstand cold blanking and bending which reduces production costs and extends life. Due to its high forgeability, SP-700 is expected to extend its application to the production of steam turbine blades. Details of practical applications have been demonstrated together with advantages of properties and fabrication for the blades.

Monotonic behavior of “neat” high temperature titanium alloys for use as composite matrices

The materials examined in this study included three conventional titanium alloys: Timetal[reg sign]21S (Ti-8Mo-1.5Nb-5.7Al-0.4Si), Ti-6242S (Ti-10.6Al-0.8Sn-2.1Zr-1Mo-0.2Si), Ti-1100 (Ti-10.5Al-1.1Sn-2.1Zr-0.2Mo-0.8Si) and four intermetallic compositions: Ti-24Al-11Nb, Ti-24.5Al-17Nb, Ti-24.5Al-17Nb-1Mo and Ti-22Al-23Nb (a%). Ultimate tensile strength of neat [alpha] + [beta] matrices (Timetal[reg sign]21S and Ti-6242S) are high at low temperature, but decrease above 650 C. In addition, they exhibit poor creep performance at 650 C and 760 C. Matrix alloys with continuous alpha-2 phase constituency (Ti-24.5-17 and Ti-24-11) exhibited poor tensile performance across the temperature interval examined (i.e. 23--760 C). In addition, they exhibited poor primary creep resistance, but reasonable creep rupture lives. Significant improvements in both elevated temperature (>650 C) tensile strength and creep performance were observed with increased levels of silicon (Ti-1100) and orthorhombic phase/molybdenum content (Ti-22-23 and Ti-24.5-17-1). Compositional and/or microstructural modification will be required to reach acceptable levels of primary creep performance.