Recent studies of self-diffusion and impurity diffusion in α- and β-titanium are reviewed. The transition metal elements and phosphorus exhibit fast diffusion in α-Ti, which are three to five orders of magnitude faster than the self-diffusion. The mechanism of the fast diffusion is discussed on the basis of the several features: the effect of α→β phase transformation, the diffusion anisotropy, the atomic size effect and the correlation between solubility and diffusivity. In β-Ti self- and impurity diffusion exhibit marked upward curvatures in the Arrhenius plots, which are known as anomalous diffusion, Various models proposed to explain the anomalous behaviour are briefly reviewed.

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The alloying element concentration in the grain boundary allotriomorphs, sideplates and intragranular plates of proeutectoid α isothermally formed from the β matrix between 500 and 700°C was measured by scanning-transmission electron microscope equipped with energy dispersive X-ray analyzer (STEM-EDX) in four ternary titanium alloys, i.e., in Ti-Al-Cr and Fe, and Ti-V-Cr and Fe alloys. The local equilibrium theory which incorporates the diffusional interaction between alloying elements was extended to analyze the partition behavior of alloying elements having diffusivity of similar order of magnitude. Fairly good agreement was obtained between calculation and theory above and even below T₀ (the upper limiting temperature at which the diffusionless β to α transformation can occur) where the formation of α is often considered to occur by massive or martensitic transformation.

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The solidification structures and segregation of the VAR cast ingots of alpha+beta titanium alloys were investigated by using a new etching technique and CMA analysis.
Main results were summarized as follows:
(1) The fine structures of the solidification were revealed by immersing the specimens in an advanced hydrofluoric acid etchant.
(2) The metallographic structure consist of the chilled layer, the columnar grain structure in its inner zone and the equi-axis grain structure in the central zone.
(3) The etch pit and shrinkage cavity lines intersect perpendicularly to the growing directions of the columnar grains. And these lines correspond to the interface of solid-liquid phases under solidification. This parallel lines show intermitted solidification.
(4) In Ti-6Al-6V-2Sn alloy with 0.7% Fe and 0.7% Cu, the segregation of these elements was observed in the final solidification zone, which is referred to so called beta flecks.
(5) To prevent segregation, the authors proposed a melting method in which faster solidification is maintained by making the edge diameter of the consumable electrodes smaller, as the hot-topping of VAR.

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This paper will summarize the strengthening and toughening behaviors of α+β and β alloys at room temperature. In addition, some considerations on the strengthening capacity and some relationships between microstructures and mechanical properties will be presented.
The strength of α+β alloys can be improved by increasing the volume fraction of the β phase and using an ultra refinement of an equiaxed α structure. On the other hand, the toughness can be improved by adjusting an acicular α structure. The strength of β alloys can be increased by the homogeneous and fine precipitation of the α phase and by the β grain refinement. The toughness of β alloys is usually superior to that of α+β alloys at given strength levels. This is mainly due to the disappearance of the coarse primary α phase in β alloys.
Increases in the strength of smooth, notched and precracked specimens are limited by ductility, notched tensile strength and fracture toughness, respectively. As a result, there is a critical value for strengthening for each specimen. Finally, some guidelines for microstructural modification for optimization of the strength-ductility and strength-toughness balance will be presented.

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The cold rolling effect of β single-phase on α precipitation behavior and the mechanical properties were studied using a Ti-15V-3Cr-3Sn-3Al metastable β alloy. The β single-phase material was prepared by heating the alloys above the β transus temperature followed by quenching in water, and subsequent aging at the (α+β) two-phase temperature after cold rolling. Cold rolling of the β single-phase had a considerable effect on the resulting (α+β) structures. Without cold rolling, preferential α precipitation occurred at the β grain boundaries, and then lath-shaped α precipitated in the β grains. When the amount of cold rolling was less than 50%, preferential precipitation of α occurred at dislocations introduced into the β grain by cold rolling, as well as the β grain boundaries. On the other hand, with an increase in the amount of cold rolling, α precipitation occurred predominantly at β subgrain boundary nodes which were formed through recovery of the deformed β single-phase during heating to the aging temperature or in the early stage of again. In heavily cold rolled specimens, an (α+β) microduplex structure, which consisted of very fine β subgrains and α particles, was obtained. Mechanical properties, such as tensile strength and elongation, were improved by the formation of the (α+β) microduplex structure.

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We have investigated the behavior of the recrystallization and the grain growth of a cold-rolled Ti-15V-3Cr-3Sn-3Al and proposed a method for estimating the grain size after recrystallization during a continuous annealing process. The estimation of the grain size recrystallized under continuous annealing has been carried out by sequentially integrating, step by step, each increment of the isothermal grain growth at each temperature.
The observation of the microstructure reveals that the grain size recrystallized under isothermal annealing, D(T, τ)/μm, can be indicated by D(T, τ)=0.80×10⁴τ^0.24 exp(–1.50×10⁴/RT), where T and τ are the annealing temperature and time, and R is the gas constant. The estimate of the grain size recrystallized under a continuous heating process is in good agreement with the actual grain size. In addition, the grain size of the Ti alloy produced on the production line also agrees well with the calculated results using the temperature and passing time period of the continuous annealing line. This estimation provides good information for controlling the grain size after continuous annealing and leads to the production of finer-grained Ti-15V-3Cr-3Sn-3Al coils than have ever been obtained before.

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The relation between deformation mode and β phase stability was investigated using Ti-16V and Ti-7Cr base alloys doped with Sn, Zr and Al as ternary or quaternary additions. Sn and Al additions change the dominant deformation mode from (332) twinning in the binary alloys to deformation induced martensitic transformation, which is related to the suppression of athermal ω phase transformation by the addition of these elements. The deformation induced martensite has orthorhombic structure (α") or hexagonal structure (α') depending on the alloying element. On the other hand, Zr addition has no significant influence on deformation mode and athermal ω phase transformation, and thus the dominant deformation mode of Zr added alloys is (332) twinning identical to the binary alloys. Discrepancy in deformation mode observed in metastable β titanium alloys can be explained reasonably by considering the present results on the effect of Sn, Zr and Al on the deformation mode.

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Deformation characteristics of β titanium alloy Ti-15V-3Cr-3Sn-3Al at elevated temperature were examined. Analysis by constitutive equation shows the deformation can be controlled by a single activated rate process, and activation energy for deformation is (253±10) kJ·mol^–1, which is close to the value of self diffusion of titanium in Ti-V binary alloys.
The grain refinement due to recrystallization occurred above 1 373 K and partial recrystallization did between 1 273 and 1 373 K. While, recrystallization did not occur and grain boundary fracture was resulted below 1 173 K.

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Commercially pure Ti sheets having a strong recrystallization texture were subjected to cylindrical cup drawing tests, and strain distributions, hardening behavior and microstructural changes developed in various parts of the cups were investigated in detail.
It was found that, due to the presence of the strong texture, deformation in the parts of cup flange was highly anisotropic. In the drawn cups, appreciable thickening and work hardening were observed in the transverse direction of the textured sheets, accompanied by extensive twin formation. On the other hand, in the rolling direction, in which only a little twinning took place, thickening and hardening were much smaller. Since constraints of the blank holder were preferentially imposed on the thicker part in the transverse direction, deformation in cup drawing might be most severe in the transverse direction. However, failure of the cup wall did not occur in this direction, but in the rolling direction. It was suggested that the plane strain fracture strength in the wall part in the initially transverse direction was significantly increased by enhanced twinning due to the particular stress state in cup drawing.

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The isothermal forging process has been developed to produce turbine blades made of near β Ti-alloy Ti-10V-2Fe-3Al. It is important to set the preform at the optimum position of the die in order to get a high precision product. The deformation analysis by using FEM is effective to determine the optimum position. And also it is necessary to avoid buckling induced by the restriction of axial elongation of the material.
As a result, Ti-10V-2Fe-3Al blades could be formed precisely by using only one stage of forging, and machining was needed only at the root. The thickness of the oxide layer induced on the surface of the forged blade was only 70 μm. The mechanical properties of Ti-10V-2Fe-3Al blades after forging and annealing were superior to those of Ti-6Al-4V blades and were nearly uniform across the length of the blades.

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Mechanical properties of beta titanium alloys strongly depend on the precipitation behavior of alpha, which is controlled by thermo-mechanical treatments. To obtain high strength beta titanium alloys, uniform distribution of fine alpha precipitates is required, and various thermo-mechanical treatments have been developed for this purpose.
This paper discusses strengthening by fine alpha precipitation using a thermo-mechanical process which combinates cold rolling and un-recrystallized heat treatments.
High tensile strength of over 1 700 Mpa can be attained by the thermo-mechanical process combining oveer-aging, cold rolling and aging for 86.4 ks at 723 K. Higher strength can be attained after aging by alpha-beta solution treatment than by beta solution treatment because fine alpha particles precipitate in the aging after the former treatment. It seems that un-recrystallized beta matrix supplies the effective nucleation sites of alpha particles.
Ultra high tensile strength of 1 940 Mpa can be attained through the aging after repeated applications of the cold rolling and recovering process. The microstructure after 723 K aging displayed uniform distribution with very fine alpha precipitates 2-5 nm in thickness.

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There are many ongoing U.S. aerospace programs which have identified requirements for higher temperature capability from conventional titanium alloys. In order to meet these requirements, two new titanium alloys have been developed. A near alpha alloy, designated "Ti-1100" (Ti-6Al-2.7Sn-4Zr-0.4Mo-0.45Si), is a modification of the well known Ti-6242S alloy (Ti-6Al-2Sn-4Zr-2Mo-0.1Si). It offers roughly a 55°C (100°F) creep advantage over the older alloy and is currently being considered for sheet metal and forging applications in both turbine engine and airframe applications. On the other hand, a metastable beta alloy designated "Beta-215S" (Ti-15Mo-2.7Nb-3Al-0.2Si) has shown exceptional promise as a matrix in metal matrix composites for high temperature applications. Its key attributes are its ease of processing in foil production and its excellent oxidation resistance. This paper will review the development of these alloys and some of their more important mechanical properties.

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Strength, toughness and microstructure relations in α, α+β and β titanium alloys are reviewed. Quantitative fracture mechanisms of α+β titanium alloys are described from the view point of strain-controlled ductile fracture models based on fracture mechanics. Various strengthening and toughening processes of titanium alloys are described in relation to 1) special heat treatments for producing desirable microstructures, 2) thermomechanical processing, 3) thermochemical processing incorporating cold work, and 4) deformation induced transformation.

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Mechanical properties were studied by tensile test of a Ti-15V-3Cr-3Sn-3Al alloy obtained by duplex-aging after cold-swaging, i.e. aging at high temperatures followed by re-aging at a lower temperature. Combinations of short-time aging at 823, 873 and 923 K and re-aging at 673 K have greatly improved the strength-ductility balance of this alloy, realizing high strength of about 1.8 GPa with tensile elongation of 5%. This duplex-aging process further diminishes the influence of the amount of reduction by cold-swaging on the mechanical properties after aging.

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Web-buckling, sometimes called pocket wave or oil canning, is one of the typical defects which take place in a cold roll-forming process of wide profiles. The relations between web-buckling appearance and several properties of commercially pure titanium sheets were experimentally investigated. Results showed that the grain size is most correlative to web-buckling, and that the magnitude of pocket wave tends to decrease with decreases in the grain size. The mechanism of inhibiting web-bucking in fine-grained titanium sheets is considered to be the fact that deformation must be localized at bend corners of formed profiles, as those titanium sheets reveal clear yield drop at the start of plastic flow.

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Under the constant potential fatigue crack was propagated in pure titanium polycrystal in NaCl aqueous solution which had very small effect on the fatigue crack propagation rate. The polarization current during the fatigue crack propagation was measured and the electric charge passed during one fatigue cycle was calculated. The amount of fresh bare surface produced at the crack tip during one fatigue loading was quantified by the charge, and this related linearly to the crack propagation rate. This suggests that the shapes of the crack tip show the similar profile through the tested ΔK range. By comparing with the plastic zone size and the crack tip opening displacement, it is also concluded that the fatigue crack propagation mechanism directly relates to the creation of fresh bare surface at the crack tip, not directly to the plastic zone size or the crack tip opening displacement.

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Effects of the microstructural and compositional factors on creep properties of α+α₂ titanium alloys were examined using ten Ti-Al-Sn-Zr alloys designed by the thermodynamic calculations based on the sublattice model. From creep tests for these alloys, it was made clear that the α₂ precipitates would be very effective for improving high temperature strength of the α phase titanium alloys. Regression equations to predict creep properties were obtained by multiple regression analysis.
In the furnace cooled (FC) condition, a remarkable decrease in the elongation was observed at around the calculated V_α2 of 0.1 (at 873 K) in room temperature tensile tests. On the other hand, the 0.2% proof stress and the tensile strength increased with increasing V_α2 and good ductility was observed in the solution treated (ST) condition showing applicability of this type of alloys in the ST condition. From these results, the V_α2 of about 0.1 (at 873 K) was proposed as the best design condition for α+α₂ high temperature titanium alloys.

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Tensile, fracture toughness, and high cycle fatigue tests were done at 293, 77, and 4 K for Ti-6Al-4V alloys with three levels of oxygen content. The alloys were investigated both in as-forged condition and in the rolled condition. Rolling did not necessarily make α grains finer, but changed the shape from plate-like to globular. Strengths depended mainly on the oxygen content; the lower content produced lower strengths. The alloy with lowest oxygen content showed the best ductility at 4 K. The fracture toughness at cryogenic temperature was also enhanced by the reduction of oxygen. In the lowest oxygen alloy, no drop in the fracture toughness was observed between 293 and 4 K. Fatigue properties were influenced by the forming process. The rolled materials had higher fatigue strength than the forged materials. The difference was accentuated at 4 K. This is believed to be due to the difference in the morphology of α grains. The lowest oxygen alloy showed the highest fatigue strength at 4 K.

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In order to understand alloying effects on the corrosion resistance of titanium, polarization curve were measured at 343 K in both 10 mass% HCl and 10 mass% H₂SO₄ solutions with various Ti-M binary alloys (M=Al, Nb, Ta, Zr, Hf, Fe, V, Cr, Mo, Co and Ni). The results were interpreted by using the bond order (Bo) obtained by a moleculr orbital calculation. The bond order (Bo) is a measure of the strength of the covalent bond between titanium and alloying elements. It was found that alloys containing the elements with higher Bo values showed the lower critical anodic current density in the polarization curve and hence the higher corrosion resistance. It is likely that the Bo is a convenient parameter for describing the corrosion rate of titanium alloys in acid environments such as HCl and H₂SO₄ solution.

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The effect of single alloying additon (Pd, Ni, Co, Mo, W, V, Nb, Si) and double alloying addition (Ni-(Mo, W, V, Ta, Zr, Nb, Si), Co-(Mo, W, V) or Pd-(Co, Ni, Mo, W, V)) on crevice corrosion resistance of titanium were investigated by autoclave test up to 473 K. Weight loss measurements of those materials in boiling HCl solutions were also conducted. It was found that small amount of Pd addition greater than 0.05% was remarkably effective and prevent from the crevice corrosion occurrence as same as ASTM Grade 7 (0.15 Pd). Ni and Co additions also improved the crevice corrosion resistance but their effect was not complete. In double alloying, synergistic improvement of crevice corrosion resistance was observed in the case of Pd-Co. Simultaneous Ni, W and Co addition with Pd also improved acid corrosion resistance. It was considered that the effect of Co addition was to reduce anodic current density in passive state, and reduce hydrogen overpotential to stabilize the passivity more than the material added Pd alone.

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Fundamental investigations were done on electron beam (EB) melting of sponge titanium by using 80 kW EB melting furnace. Results obtained are as follows:
(1) To increase the melting yield of titanium in EB melting of sponge titanium, it is important to recover splashed metal by installation of water-cooled copper wall around the hearth and to decrease evaporation loss of titanium by keeping the surface temperature of molten metal just above the melting temperature of titanium without local heating.
(2) Specific power consumption of drip melting of pressed sponge titanium bar and hearth melting of sponge titanium are approximately 0.9 kWh/kg-Ti and 0.5–0.7 kWh/kg-Ti, respectively.
(3) Ratios of the heat conducted to water-cooled mould in the drip melting and to water-cooled hearth in the hearth melting to the electron beam input power are 50-65% and 60-65%, respectively.
(4) Surface defects of EB-melted ingots include rap which occurs when the EB output is excessively great, and transverse cracks when the EB output is excessively small. To prevent surface defects, the up-down withdrawal method is effective.

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Powder metallurgy of ordered alloys is not only a method to obtain net-shape products but is also an effective approach to microstructural refinement for improvement of some properties such as tensile ductility and fatigue strength. In the present study, prealloyed plasma rotating electrode process (PREP) powder was used to produce compacts with an ultrafine grain structure by employing conventional and hydrogen thermochemical processing routes. These processes are capable of producing a wide range of alpha-2 (α₂) structures, including an ultrafine microstructure with a grain size on the order of a micron. This paper will discuss some of the mechanisms leading to microstructure refinement with an emphasis on the role of hydride formation.

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Attempts were made to produce fatigue tolerant α-β titanium alloys by the blended elemental (BE) P/M method using extra low chlorine (ELCL) titanium powder. The study was conducted in two stages. In the first stage, the effect of microstructural modification on the high cycle fatigue strength of ELCL BE Ti-6A1-4V was investigated. Several different microstructural conditions were generated through combinations of processing and heat treatment and these were fatigue tested to find out the optimum microstructure. In a later stage, various kinds of ELCL BE α-β titanium alloys were produced by both conventional and microstructure-controllable new BE methods with emphasis on relating composition/processing/microstructure to fatigue properties. The precision sectioning method which enables the concurrent observation of the initiation facets and the underlying microstructure was utilized to analyze fatigue crack initiation mechanisms.

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Influences of hot deformation condition and alloy composition on the stress-strain relation, the workability and the microstructure have been studied in a temperature range between 1 200 and 1 600 K with strain rate of 10^-4 to 10^-1 sec^-1, for binary intermetallics of Ti-4450 mol%Al melted by a plasma beam furnace. A peak stress due to dynamic recrystallization is observed in all stress-strain relations. The strain at the peak stress in the γ+α₂ two phase alloys I smaller than that of the γ single phase alloy. The deformation stress and the deformability of these alloys are represented by a hot workability map with axes of temperature and strain rate. In the map, the area where the Ti-rich alloy can be hot worked soundly locates on the higher temperature range or the lower strain rate range compared with those of the γ single phase alloy. In the Ti-rich alloy with fully lamellar structure consisting of layers of the γ and α₂ thin plates, it is difficult to obtain an uniform fine recrystallized structure because of stronger anisotropy in plasticity and much slower diffusion rate in comparison with the γ single phase alloy. In order to obtain the fine recrystallized structure, the alloy must be deformed repeatedly at a sufficiently high temperature over the (γ+α₂)/(γ+α) eutectoid transformation temperature and with various direction of straining.

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