Texture Evolution in Boron Modified Ti-6Al-4V Alloy

Owing to their high strength-to-weight ratio, excellent mechanical properties and corrosion resistance, titanium (Ti) and its alloys, especially (alpha+beta) alloys like Ti-6Al-4V is the backbone materials for aerospace, energy, and chemical industries. Trace boron addition (similar to 0.1 wt. %) to the alloy Ti-6Al-4V produces a reduction in as-cast grain size by roughly an order of magnitude resulting in enhanced ductility, higher stiffness, strength and good fracture resistance. Boron addition could also affect the evolution of texture and microstructure in the material. The solidification microstructures of Boron free as well as Boron containing Ti-6Al-4V are found to be almost homogeneous from periphery towards the center of as-cast ingot in terms of both alpha-colony size and distribution. Boron addition substantially reduces alpha-colony size (similar to 50-80 mu m). A gradual change in alpha texture from periphery towards the center has been observed with orientations close to specific texture components suggesting the formation of texture zones. The mechanism of texture evolution can be visualized as a result of variant selection during solidification through (alpha+beta) phase field.

Evolution of Transformation Texture in a Metastable B - Titanium Alloy

Texture evolution in h. c. p. (alpha) phase derived from aging of a differently processed metastable b.c.c. (beta) titanium alloy was investigated. The study was aimed at examining (i) the effect of different b. c. c. cold rolling textures and (ii) the effect of different defect structures on the h. c. p transformation texture. The alloy metastable beta alloy Ti-10V-4.5Fe-1.5Al was rolled at room temperature by unidirectional (UDR) and multi-step cross rolling (MSCR). A piece of the as-rolled materials were subjected to aging in order to derive the h. c. p. (alpha) phase. In the other route, the as-rolled materials were recrystallized and then aged. Textures were measured using X-ray as well as Electron Back Scatter Diffraction. Rolling texture of beta phase, as characterized by the presence of a strong gamma fibre, was found stronger in M S C R compared to UDR, although they were qualitatively similar. The stronger texture of MSCR sample could be attributed to the inhomogeneous deformation taking place in the sample that might contribute to weakening of texture. Upon recrystallization in beta phase field close to beta-transus. the textures qualitatively resembled the corresponding beta deformation textures; however, they got strengthed. The aging of differently beta rolled samples resulted in the product alpha-phase with different textures. The (UDR + Aged) sample had a stronger texture than (MSCR + Aged) sample, which could be due to continuation of defect accumulation in UDR sample, thus providing more potential sites for the nucleation of alpha phase. The trend was reversed in samples recrystallized prior to aging. The (MSCR + Recrystallized + Aged) sample showed stronger texture of alpha phase than the (UDR + Recrystallized + Aged) sample. This could be attributed to extensive defect annihilation in the UDR sample on recrystallization prior to aging. The (MSCR + Aged) sample exhibited more alpha variants when compared to (MSCR + Recrystallized + Aged) sample. This has been attributed to the availability of more potential sites for nucleation of alpha phase in the former. It could be concluded that alpha transformation texture depends mainly on the defect structure of the parent phase.

Thickness-dependent fcc-hcp phase transformation in polycrystalline titanium thin films

Polycrystalline Ti thin films are shown to gradually transform from face-centered cubic (fcc) to hexagonal close-packed structure (hcp) with increasing film thickness. Diffraction stress analysis revealed that the fcc phase is formed in a highly compressive hcp matrix (>= 2 GPa), the magnitude of which decreases with increasing film thickness. A correlation between stress and crystallographic texture vis-a-vis the fcc-hcp phase transformation has been established. The total free energy change of the system upon phase transformation calculated using the experimental results shows that the fcc-hcp transformation is theoretically possible in the investigated film thickness regime (144-720 nm) and the hcp structure is stable for films thicker than 720 nm, whereas the fcc structure can be stabilized in Ti films much thinner than 144 nm. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Catalytic current polarographic determination of traces of titanium in water after preconcentration by coflotation

A new method is reported for the determination of trace levels of Ti(IV) in water by coflotation and polargraphy. Ti(IV) is preconcentrated and separated by coflotation using aluminium hydroxide as coprecipitant and sodium oleate as surfactant. Polarographic determination of titanium content in the froth is based on the catalytic wave of Ti(IV) in the presence of chlorate and oxalate. The effect of various cations and anions on the flotation and determination of titanium has been investigated. The method has been applied to estimation of titanium in natural fresh water samples.

Sintering of beta titanium: Role of dislocation pipe diffusion

Sintering of titanium in its high temperature beta phase was studied by isothermal dilatometry. The sintering shrinkage y did not follow the normal time exponent type of behaviour, instead being described by the equation y = Kt(m)/[1-(A+Bt)(2)], where m = 1.93 +/- 0.07, with an activation energy of 62-90 kJ mol(-1). A detailed analysis of these results, based on the 'anomalous' diffusion behaviour reported for beta titanium, is carried out. It is shown that the generation of a high density of dislocations during the alpha --> beta phase transformation, coupled with sluggish recovery at the sintering necks, enables sintering mass transport by pipe diffusion through dislocation cores from sources of matter within the particles to become dominant.

The influence of pH on the corrosion of medium strength aerospace alloys 8090, 2091 and 2014

The influence of pH on the corrosion behaviour of two aluminium-lithium-copper-magnesium-zirconium (8090 and 2091) alloys was studied and compared with a standard aircraft alloy, 2014 (Al-4.4% Cu) and 99.9% pure Al. In constant exposure and potentiodynamic polarization studies conducted in 3.5% NaCl solution having different pH values, all the alloys exhibited high corrosion rates in acidic and alkaline environments, with a minimum in less hostile environments close to neutral pH. The pitting potentials for aluminium-lithium alloys were slightly lower than those for 2014 and pure Al. The effect of pH on the passive current density was also less for aluminium-lithium alloys.

On the decomposition of ? phase in some rapidly quenched titanium-eutectoid alloys

The decomposition of the beta phase in rapidly quenched Ti-2.8 at. pet Co, Ti-5.4 at. pet Ni, Ti-4.5 at. pet, and 5.5 at. pet Cu alloys has been investigated by electron microscopy. During rapid quenching, two competitive phase transformations, namely martensitic and eutectoid transformation, have occurred, and the region of eutectoid transformation is extended due to the high cooling rates involved. The beta phase decomposed into nonlamellar eutectoid product (bainite) having a globular morphology in Ti-2.8 pet Co and Ti-4.5 pet Cu (hypoeutectoid) alloys. In the near-eutectoid Ti-5.5 pet Cu alloy, the decomposition occurred by a lamellar (pearlite) type, whereas in Ti-5.4 pct Ni (hypereutectoid), both morphologies were observed. The interfaces between the proeutectoid alpha and the intermetallic compound in the nonlamellar type as well as between the proeutectoid alpha and the pearlite were often found to be partially coherent. These findings are in agreement with the Lee and Aaronson model proposed recently for the evolution of bainite and pearlite structures during the solid-state transformations of some titanium-eutectoid alloys. The evolution of the Ti2Cu phase during rapid quenching involved the formation of a metastable phase closely related to an ''omega-type'' phase before the equilibrium phase formed. Further, the lamellar intermetallic compound Ti2Cu was found to evolve by a sympathetic nucleation process. Evidence is established for the sympathetic nucleation of the proeutectoid alpha crystals formed during rapid quenching.

Retarding effect of surface hydroxylation on titanium(IV) oxide photocatalyst in the degradation of phenol

Nanosized powders of TiO2 (anatase) were prepared by the hydrothermal method, acid-medium hydrolysis or by vacuum freeze-drying of sols, and annealing at temperatures <700-degrees-C. Photocatalytic activities of these powders in the mineralization of phenol, were evaluated in comparison to that of Degussa P25. Kinetic data indicated that surface hydroxylation had a retarding effect on the degradation of phenol. Formation of stable peroxotitanium species were observed on hydroxylated powders, whereas only V(Ti)-O- hole trap centres were detected by EPR on the heat treated samples. The data supports direct hole oxidation of the substrate preadsorbed on the photocatalyst, which is otherwise blocked by surface hydroxyls.

Role of Li+ ions in corrosion behaviour of 8090 Al-Li alloy and aluminium in pH 12 aqueous solutions

The influence of Li+ ions on the corrosion behaviour of the Al-Li alloy 8090-T851 and of commercially pure aluminium in aqueous solutions at pH 12 was studied by weight loss and electrochemical polarisation methods. The inhibiting role of Li+ was concentration dependent, corrosion rate decreasing lineally with log[Li+] in the concentration range 10(-4)-10(-1) mol L(-1). A change from general to pitting corrosion was evident from scanning election microscopy studies. Polarisation studies revealed that Li+ primarily acts as an anodic inhibitor (passivator). Passive film formation and stability also become more feasible with increasing Li+ concentration. Fitting potential was dependent on the Cl- ion concentration in the solution. Both materials were affected similarly by the presence of Li+ ions, the corrosion rate of the alloy being slightly lower. This is attributed to the lithium in the alloy acting as a source of lithium for passive film formation. (C) 1995 The Institute of Materials.

Hot deformation and microstructural evolution in an alpha(2)/O titanium aluminide alloy Ti-25Al-15Nb

Deformation processing and microstructural development of an alpha(2)/O aluminide alloy Ti-25Al-15Nb (at.%) was studied in the temperature range of 950 to 1200 degrees C and strain rate range of 10(-3) to 100 s(-1). Regions of processing and instability were identified using dynamic materials model. Dynamic recrystallization (DRX) of alpha(2)/O phase and p phase were seen to occur in the region of 950 to 1050 degrees C/0.001 to 0.05 s(-1) and 1125 to 1175 degrees C/0.001 to 0.1 s(-1), respectively. Unstable flow was seen to occur in the region of 1050 to 1190 degrees C/10 to 100 s(-1). Thermal activation analysis showed that DRX of alpha(2)/O and beta was controlled by cross-slip.

Deformation behaviour of commercially pure titanium at extreme strain rates

The evolution of microstructure and texture during room temperature compression of commercially pure Ti with four different initial orientations were studied under quasi-static and dynamic loading conditions. At a low strain rate (epsilon)over dot = 3 x 10(-4) s(-1) the different initial textures yielded the same end texture, despite different microstructural evolution in terms of twin boundaries. High strain rate deformation at (epsilon)over dot = 1.5 x 10(3) s(-1) was characterized by extensive twinning and evolution of a texture that was similar to that at low strain rate with minor differences. However, there was a significant difference in the strength of the texture for different orientations that was absent for low strain rate deformed samples at high strain rate. A viscoplastic self-consistent model with a secant approach was used to corroborate the experimental results by simulation. (C) 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

Processing map for the hot working of near-alpha titanium alloy 685

Using a dynamic materials model, processing and instability maps have been developed for near-alpha titanium alloy 685 in the temperature range 775-1025 degrees C and strain-rate range of 0.001-10 s(-1) to optimise its hot workability. The alloy's beta-transus temperature lies at about 1020 degrees C. The material undergoes superplasticity with a peak efficiency of 80% at 975 degrees C and 0.001 s(-1), which are the optimum parameters for alpha-beta working. The occurrence of superplasticity is attributed to two-phase microduplex structure, higher strain-rate sensitivity, low flow stress and sigmoidal variation between log flow stress and log strain rate. The material also exhibits how localisation due to adiabatic shear-band formation up to its beta-transus temperature with strain rates greater than 0.02 s(-1) and thus cracking along these regions. (C) 1997 Published by Elsevier Science S.A.

Synthesis of nanostructured titanium alloys

The synthesis of nanostructured materials is a critical step in the development elf these novel materials. The basic principles involved in the production of nanocrystals and nanocomposites by rapid solidification are dealt with. An analysis of the various factors influencing the final grain size of the nanocrystals achieved during mechanical alloying has been presented. The devitrification of amorphous phase formed during rapid solidification processing and mechanical alloying provides an alternative and attractive route. Examples of the synthesis of nanostructured materials using these three different routes are drawn from our work on titanium alloys.

Synthesis and structure of a novel terminal imido complex of titanium

The reaction of eta(5)-Cp*TiCl3 and (LiNHBu)-Bu-t in hexanes yields a novel [eta(5)-Cp*Ti(=(NBu)-Bu-t)((NHBu)-Bu-t)(2)]Li . (NH2Bu)-Bu-t complex with a terminal tert-butylimido moiety. The synthesis and X-ray structural characterization are described. (C) 1999 Elsevier Science S.A. All rights reserved.