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The effect of TiB particles on the β recrystallization behavior of an as-cast Ti-2Al-9.2Mo-2Fe-0.1B (hereafter 2A2F10B) metastable β titanium alloy was investigated during hot rolling with a related β solution treatment. The microstructure evolution was analyzed based on the electron backscatter diffraction (EBSD). The particle-stimulated nucleation (PSN) behavior of the TiB particles occurred naturally in the dynamic recrystallization during hot rolling and in the static recrystallization during the β solution treatment, which effectively promoted the recrystallization of 2A2F10B under the studied conditions. The PSN mechanism caused recrystallized grains with different orientations to nucleate around single TiB particles by random lattice rotation, which led to a weak texture in the recrystallized microstructure. The complete recrystallization of the as-cast 2A2F10B alloy could be achieved, and it had a fine microstructure after single-heating hot rolling, with a reduction of at least 65%, along with the related β solution treatment.

A metastable β titanium alloy, Ti–2Al-9.2Mo–2Fe (wt.%), with athermal ω phase after quenching has been subjected to high strain rate (∼3000 s−1) compression at ambient temperature by Spilt Hopkinson Pressure Bar arrangement. Reverse β-to-ω transformation was investigated by transmission electron microscopy during deformation. Furthermore, the novel distribution of athermal ω phase in {332} twinning and stress-induced martensitic was observed. We reveal that such novel distribution is attributed to the influence of ω phase on {332} twinning and stress-induced martensite transformation (SIMT) based on the viewpoint of the instability of the lattice structure of metastable β phase. Furthermore, a new process of twinning and SIMT in current alloy with ω phase was proposed.

The hot deformation behavior of Ti-6Al-4V-0.11Ru titanium alloy was investigated by isothermal compression under the conditions of deformation temperature ranging from 800 °C to 1100 °C and strain rate ranging from 0.01s−1 to 10s−1. The results revealed that the peak stress of Ti-6Al-4V-0.11Ru titanium alloy increased with decreasing the deformation temperature and increasing the strain rate, the softening mechanism was dynamic recovery below Tβ and changed to dynamic recrystallization above Tβ. The constitutive equation of Ti-6Al-4V-0.11Rualloy was calculated by a linear regression analysis in two phase regions in the form of Arrhenius-type relationships. It is found that the apparent activation energies were calculated to be 926.608 kJmol−1 in the α+β phase region and 128.947 kJmol−1 in the β phase region, respectively. A series of processing maps considering strain were drawn according to dynamic materials mode and Kumar's stability criterion, it can be found from processing maps that the strain had significantly effect on the peak region of power dissipation efficiencies in the processing map of Ti–6Al–4V-0.11Ru alloy and the flow instability region was very close to the optimum hot deformation condition at high strain. Furthermore, optimized hot working regions were investigated and validated through microstructure observation. The optimum thermo mechanical process condition for hot working of Ti-6Al-4V-0.11Ru titanium alloy was suggested to be in the temperature range of 900–950 °C with strain rate of 0.1-1s−1.

The mechanical behavior of a metastable β titanium alloy, Ti-2Al-9.2Mo-2Fe (wt%), was studied through high compression strain rate (~3000 s−1) at ambient temperature by Spilt Hopkinson Pressure Bar (SHPB) tests after solution treatment at 850 ℃ for 0.5 h and 950 ℃ for 1 h. The mechanism of deformation and microstructure evolution was investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Multiple deformation mechanisms, including primary and secondary {332} twinning, stress-induced martensite, and stress-induced ω-phase and dislocation slips, were identified in specimens subjected to solution treatment at 850 ℃ for 0.5 h. In contrast, only primary and secondary {332} twinning and dislocation slips were detected in specimens subjected to solution treatment at 950 ℃ for 1 h. The athermal ω precipitation produced after solution treatment and elevated temperature generated during dynamic loading played a crucial role in the different deformation mechanisms observed in the two different specimens. Moreover, the deformation mechanisms active during plastic deformation in the different strain regimes affected the development of different textures in the Ti-2Al-9.2Mo-2Fe alloys from each regime.

Hot deformation behaviors of Ti–2Al–9.2Mo–2Fe alloy with boron were investigated in a hot compression test at temperatures ranging from 850 to 1000 °C and strain rates ranging from 0.01 to 10.00 s−1. With strain rate decreasing and deformation temperature increasing, dynamic recrystallization (DRX) was promoted in both alloys. The nucleation mechanism of DRX involved grain boundary bulging and subgrain rotation. In case of deformation at low temperatures and high strain rates, the boron-free alloy only showed dynamic recovery (DRV) phenomenon. However, due to the particle-stimulated nucleation mechanism of recrystallization by TiB particles, the boron-containing alloy shows DRX at that condition. These DRX grain nuclei are formed by lattice rotation and subgrain growth from deformation zone in the initial grains, which are close to those of TiB particle. Moreover, the flow stress of the boron-containing alloy is lower than that of the boron-free alloy due to the grain refinement during hot deformation process.

In order to study the aging hardening characteristics and isothermal aging process of Ti-6Cr-5V-5Mo-4Al-1Nb alloy the time-temperature-transformation (TTT) curve of the alloy was measured by aging hardness method, and the microstructure changes during isothermal process were analyzed by XRD and SEM. It was found that the alloy had a high aging response speed at other aging temperatures except for the slow aging at 300°C and 400°C. When the aging time is prolonged at medium and low temperatures, the alloy can also produce the same aging strengthening effect as that at medium temperature aging, indicating that the temperature window of the alloy aging is wide, the aging strengthening effect is good, and it has the operability of heat treatment strengthening, so it is an ideal structural material. The isothermal aging transformation curve of the alloy is C type, and it can be obtained from the figure that the nose temperature is about 545°C

In order to develop a new titanium alloy with a good combination of strength–ductility–toughness, a near-beta titanium alloy was designed based on the already widely used Ti-1023 alloy. To avoid beta fleck occurring in the microstructure, the new Ti–Al–Fe–V (Cr, Zr) alloy has been made through decreasing the content of Fe, based on molybdenum equivalency and Bo–Md molecular orbital method (a method for new alloy designing based on the molecular orbital calculating). After primary design computation, Ti–Al–Fe–V (Cr, Zr) alloy was optimized as Ti–3Al–4.5Cr–1Fe–4V–1Zr finally. The microstructure and tensile properties of this alloy subjected to several commonly used heat treatments were investigated. The results show that the tensile strength of the alloy after solution treated below the β-transus temperature comes between 850 and 1100 MPa, with elongation in the range of 12.5 %–17.0 %. In solution-treated and solution-aged samples, a low-temperature aging at 500 °C results in the precipitation of finer α phase. With the increase in aging temperature, the secondary α phase becomes coarser and decreases in amount. Thus, it will lead to the increase in tensile ductility, but reduction in strength. Eventually, after modulated aging treatment, the alloy can obtain high-strength level with acceptable ductility. The tensile strength of the alloy can achieve 1273 MPa, with an elongation of 11.0 %. At the same time, the fracture toughness (K IC) of the alloy achieves 83.8 MPa·m1/2. It is obvious that the newly designed alloy has achieved a good blend of strength–ductility–toughness.

The deposition rate of the wire+arc additive manufacturing (WAAM) technique is high and its equipment and operating costs are low, making it suitable for fabricating large-size components with special-shape. For improving the high-strain-rate mechanical properties, 0.1 wt% boron was added to the Ti-6Al-4 V alloy wire to modify the microstructure of the boron-added titanium alloy fabricated via WAAM. In this study, the α-phase morphology and texture of as-built and β-annealed WAAM Ti-6Al-4 V-0.1B alloy were investigated by optical microscopy and electron backscatter diffraction technique. The addition of 0.1 wt% boron caused the length of the α-phase lamellae to reduce significantly in both the as-built and the annealed WAAM Ti-6Al-4 V-0.1B specimens. This was attributed to the combined action of the fine prior β-grains and the growth-inhibiting effect of the TiB particles on the α-phase. The continuous-grain-boundary α-phases were broken and even completely absent in both the as-built and the annealed WAAM Ti-6Al-4 V-0.1B specimens. Moreover, in both specimens, the α-phases exhibited two distinct fiber textures, 〈11−20〉 α and α, parallel to the building direction. Further, the α-phases that had nucleated at the boundaries of the prior β-grains showed the appearance of the variant selection, which preferentially exhibited the former fiber texture.

The effect of Al addition on ω precipitation and age-hardening behavior of Ti-9.2Mo-2Fe and Ti-2Al-9.2Mo-2Fe alloy during aging treatment was investigated. The results showed that athermal and isothermal ω phase formation in Ti-2Al-9.2Mo-2Fe alloy was suppressed to a certain extent due to Al addition. In addition, a small amount of athermal ω phase was observed in the β matrix with a size of about ~5 nm during water quenching from above the β transus temperature for both alloys. Isothermal ω formation was also found during aging at temperatures ranging from 573 K to 773 K (300 °C to 500 °C) in both alloys, although it had a limited time of stability at 773 K (500 °C). The hardening due to isothermal ω precipitation exhibited no over-aging as long as ω phase existed in both alloys, and ω phase played a more important role in hardening than α phase. And the ω phase in 50 to 100 nm size exhibited the best hardening effect in Ti-9.2Mo-2Fe alloy. Similarly, α phase with 100 to 200 nm in length showed better hardening effects in Ti-2Al-9.2Mo-2Fe alloy. Both the alloys showed stronger age hardening at an intermediate temperature of 673 K (400 °C) and in the first aging stage at a higher temperature of 773 K (500 °C) due to the sufficiently fine size (50 nm), while they exhibited weaker age hardening at a lower temperature of 573 K (300 °C) and long period aging at a higher temperature of 773 K (500 °C) due to incomplete ω formation and/or coarsening of α phase. No over or peak aging stage was found at 573 K and 673 K (300 °C and 400 °C) during the aging for 72 hours, while the peak hardness values of both alloys aged at 773 K (500 °C) were obtained in the first stage of aging. The hardness of the alloys was very sensitive to size and volume fraction of ω phase, which depends on aging temperature, time, and composition of the involved alloys.

A study on microstructural evolution and age hardening behavior of a new metastable beta titanium alloy Ti–2Al–9.2Mo–2Fe was undertaken by microscopic observation, Vickers hardness and tensile tests in this research. The result showed that the athermal ω phase was formed in the beta matrix after solution treatment and followed by water quench, but it seemed that the athermal ω phase did not result in a considerable hardening (300 HV in hardness and 670 MPa in YS in ST condition). However, the isothermal ω phases with 10–40 nm and nano-scaled α platelets with 30–100 nm were observed in the alloy aged at lower temperatures (300–450 °C). The nano-scaled ω and α phase led to an attractive hardening effect (400–500 HV in hardness and above 1500 MPa in YS). However, micro-scaled α phase with 0.5–3 μm obtained in the samples aged at temperatures (500–600 °C) showed a moderate hardening (350–450 HV in hardness and 1100–1500 MPa in YS). The hardening went through an under-aging, peak-aging and over-aging due to the continuation of nucleation and growth of the α phases and subsequent coarsening. The coarsen α phases (3–5 μm) and grain boundary α layers (0.1–0.5 μm in thickness) obtained at high temperatures aging (650–750 °C) showed a poor hardening or even a softening (around 300 HV in hardness and below 1000 MPa in YS). The Ti–2Al–9.2Mo–2Fe alloy had a fast aging response, and can be age hardened to high hardness above 400 HV within 30 min at a broad aging temperature region. The age hardening curves can also provide a basic criterion for selecting aging treatment for a given stress level.

The reverse transformation temperature, recovery strain and pull-out force of TiNiFe alloy pipe couplings under different diameter expansion ratios (7%, 9%, 11%, 13%, 15%) were studied. The results show that the martensite reorientation occurs and the elastic strain energy released decreases with the increase of the expansion ratio, which leads to the increase of martensite reverse transformation temperature as’ and Af’. The recovery strain increases and the recovery rate decreases with the increase of expansion ratio. The expansion ratio of pipe couplings has a great influence on the pull-out force, which is related to the fit clearance and the diameter of the connected rod. When the fit clearance is the same, the larger the expansion ratio is, the greater the pull-out force is, and when the diameter of the connected rod is the same, the pull-out force decreases with the increase of the expansion ratio.

The influence of solution and aging parameters on the microstructures and mechanical properties of Ti–8V–1.5Mo–2Fe–3Al alloy was studied through microstructural observation and mechanical property test. It is a newly designed near beta titanium alloy for high strength and toughness on the basis of Ti–10V–2Fe–3Al alloy. According to microstructural observation, the shape and volume fraction of primary α phase is found to be determined by the solution treatment, while aging parameters control the precipitation behavior of secondary α platelets. Moreover, the effect of aging time on tensile properties is slighter compared with that of aging temperature. Tensile test reveals the dependence of mechanical properties on the heat treatment parameters, according to which the tensile strength and elongation can be controlled in a range of 1230–1435 MPa and 9–13%. It is a little superior to the Ti-1023 alloy, the alloy solution treated at 800–820 °C for 1 h and aged at 550–600 °C for 2–8 h has well balanced combination of strength and ductility. At the same strength level, the Ti–8V–1.5Mo–2Fe–3Al alloy has higher toughness and higher ductility than the Ti-1023 alloy, the reason is unknown and should be studied further. Morphological features of the fracture surfaces are consistent with the tensile test result and provide further information on the intragranular fracture behavior when solution treated above the β-transus.

Ti-62421S (Ti–6Al–2Sn–4Zr–2Nb–1Mo–0.2Si) is a novel short-time using high-temperature titanium alloy. The effects of annealing on microstructure and tensile properties of Ti-62421S alloy plate were studied through optical microscopy (OM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and tensile tests. The results show that, with annealing temperature increasing, the volume fraction of primary α (αp)-phase decreases while that of transformed β (βt)-structure and secondary α (αs)-phase increases. The room-temperature strength and plasticity are insensitive to annealing temperature. However, with annealing temperature increasing, the tensile strength decreases at 550 °C, while increases at 600 and 650 °C instead. It is suggested that, at 550 °C, the strengthening mechanism is mainly boundary strengthening and the biggest contributor is αp-phase by providing αp/β-boundary area. Above 600 °C, the strengthening mechanism is grain strengthening, where αs-phase strengthens the β-phase.

Grain growth behaviors of a beta titanium alloy (Ti–2Al–9.2Mo–2Fe) have been systematically investigated for various temperatures. Quantitative evaluations of the grain growth kinetics over a wide range of temperatures from 820 °C to 1000 °C have been performed. The results showed that the isothermal grain growth kinetics followed the normal growth behavior. The solution-treated alloy at lower temperatures (820–900 °C) exhibited a good uniformity of the grain size. It showed a poor uniformity of the grain size due to the abnormal grain growth at higher temperatures (950–1000 °C). The grain growth exponent values ( n ) were at the range of 0.1–0.2, about 0.1 at lower temperatures and about 0.2 at higher temperatures. They were lower than other beta titanium alloys. The lower exponent values ( n ) and the higher activation energy ( Q ) can be attributed to the solute drag effect and the high Mo content with low diffusivity in Ti matrix.

Split Hopkinson pressure bar test system was used to investigate the plastic deformation behavior and dynamic response character of α-type Ti–5Al–2.5Sn ELI and near α-type Ti–8Al–1Mo–1V titanium alloy when subjected to dynamic loading. In the present work, stress–strain curves at strain rate from 1.5 × 103 to 5.0 × 103 s−1 were analyzed, and optical microscope (OM) was used to reveal adiabatic shearing behavior of recovered samples. Results show that both the two alloys manifest significant strain hardening effects. Critical damage strain rate of the two alloys is about 4.3 × 103 s−1, under which the impact absorbs energy of Ti–5Al–2.5Sn ELI and Ti–8Al–1Mo–1V are 560 and 470 MJ·m−3, respectively. Both of them fracture along the maximum shearing strength orientation, an angle of 45° to the compression axis. No adiabatic shear band (ASB) is found in Ti–5Al–2.5Sn ELI alloy, whereas several ASBs with different widths exist without regular direction in Ti–8Al–1Mo–1V alloy.

The effect of lamellar α phase ( α L ) thickness on tensile behavior at 700 °C of BTi-6431S titanium alloy is investigated. Two types of bi-modal microstructures are introduced through different heat treatments. Both of them have similar volume fraction of equiaxed α phases ( α e ). One has thick α lamella named BTL, and another has fine α lamella named BFL. The results show that dynamic recrystallization (DRX) happens in the α phase during tensile process at 700 °C. After being deformed at 700 °C, the shape of α L in BFL and BTL transforms into sphere. BFL possesses higher tensile strength, and the elongation is twice as large as BTL at 700 °C. Compared to the thick α L , the fine α L has more excellent accommodative deformation capability. There are cracks found to be of wedge type located at the grain triple junctions in both BFL and BTL. In addition, some small voids stay at the lamellar boundaries in BTL. In order to acquire a good combination of tensile strength and elongation at 700 °C for BTi-6431S alloy, the thickness of α L should be reduced.

A series of Ti–Al–V titanium alloy bars with nominal composition Ti–7Al–5V ELI, Ti–5Al–3V ELI, commercial Ti–6Al–4V ELI and commercial Ti–6Al–4V were prepared. These alloys were then heat treated to obtain bimodal or equiaxed microstructures with various contents of primary α phase. Dynamic compression properties of the alloys above were studied by split Hopkinson pressure bar system at strain rates from 2,000 to 4,000 s−1. The results show that Ti–6Al–4V alloy with equiaxed primary α (αp) volume fraction of 45 vol% or 67 vol% exhibits good dynamic properties with high dynamic strength and absorbed energy, as well as an acceptable dynamic plasticity. However, all the Ti53ELI specimens and Ti64ELI specimens with αp of 65 vol% were not fractured at a strain rate of 4,000 s−1. It appears that the undamaged specimens still have load-bearing capability. Dynamic strength of Ti–Al–V alloy can be improved as the contents of elements Al, V, Fe, and O increase, while dynamic strain is not sensitive to the composition in the appropriate range. The effects of primary alpha volume fraction on the dynamic properties are dependent on the compositions of Ti–Al–V alloys.

An investigation is given on the influence of heat treatment by the microstructural characteristics and tensile properties of a new high strength alloy Ti–2Al–9.2Mo–2Fe. Both of the α/β and β solution treatment (α/β-ST and β-ST), then aged at temperatures ranging from 400 °C to 600 °C, were prepared. The primary α phases having 2–5 μm are formed during the α/β-ST, and restrain the size of β grains below 10 μm. As a result of the fine β grains, the α/β-ST contributes a higher strength than the β-ST. The co-existence of α″ and athermal ω phase is found in the β-ST and water quenched samples. However, these have little influence on the alloy hardening. After aging, the alloys in the α/β-ST and β-ST condition reveal the phase transformation of β to isothermal ω, and β to α depending on the aging temperature. Although the primary α phase formed during the α/β-ST increases the stability in the β matrix, and the isothermal ω phase also appears to occur during aging at 400 °C and 450 °C for 2 h. These phenomena are less common in beta titanium alloys, when treated in the α/β-ST and aged at lower temperatures. The isothermal ω phase formed in both conditions results in high strength levels (1600 MPa of ultimate strength) with much ductility loss (2.5–4.5% of elongation) as a result of the superior hardening effect and brittleness of ω phase. However, the secondary α phase with the size of 1–3 μm leads to attractive combinations of strength and ductility (1200–1400 MPa of ultimate strength with 7.5–12.5% of elongation). The reason for that is too fine α phase below 1 μm tends to result in ultra-high strength with much ductility loss. As a whole, the alloy can be heat treated to obtain excellent balances of strength and ductility, and provided abundant stress levels with optional ductility as a usable material.

The effect of solution treatment on microstructural characteristics and tensile properties of a new high strength Ti–6Cr–5Mo–5V–4Al alloy was studied. The microstructure–mechanical property relationship is also discussed. The results show that, the Ti–6Cr–5Mo–5V–4Al alloy in solution treated condition has moderate strength levels (800–1000 MPa) with an excellent ductility (−22% of the elongation and 50–60% of the reduction of area). The alloy in the α/β-solution and aged (α/β-STA) condition shows a more attractive combination of strength and ductility than the alloy in the β-solution and aged condition (β-STA). The α/β-STA results in the occurrence of mixed α precipitates at nano-scale (−100 nm) and at micron-scale (1 μm), which contributes to the high ultimate strength levels (1400–1600 MPa). However, the fine β grains below 10 μm and the primary α phase contribute to good ductility (6–13.5% of the elongation and 22–52% of the reduction of area). The β-ST leads to equiaxed and coarsen β grains (40–100 μm), and subsequent aging contributes to the α phase with the size of 1–5 μm. The β-STA microstructure contributes to 1200–1400 MPa of the ultimate strength with 7–10% of the elongation and 22–28% of the reduction of area, which shows less attractive behavior on the properties than that of the α/β-STA.

The effect of aging treatment on the microstructures and hardening behavior of a metastable beta Ti–9.2Mo–2Fe alloy, which is new designed on the basis of Bo–Md molecular orbital method (Bo, the bond order; Md, the metal d-orbital energy level), has been investigated using hardness measurement, X-ray diffraction, optical microscopy and TEM characterization. The results reveal that, the athermal ω phase with the size of 2–5 nm is found in Ti–9.2Mo–2Fe alloy when solution-treated at 860 °C followed by water quench. The isothermal ω phase is also found when Ti–9.2Mo–2Fe alloy aged at temperatures from 300 °C to 500 °C, although the ω phase has a limited time of stability at 400 °C and 500 °C. The athermal ω phase does not lead to attractive hardening due to its too small size and volume fraction, so the hardness of the solution-treated alloy is about 370 Hv. However, the isothermal ω phase with the size of 5–10 nm shows a moderate hardening (470 Hv), while the ω phase with the size of 50–100 nm contributes the best hardening (∼500 Hv). As well, the hardening due to isothermal ω phase precipitation hardly exhibits over-aging response as long as ω phase exists.

A study on the microstructural characteristics and tensile properties of a new high strength Ti–6Cr–5Mo–5V–4Al alloy were investigated. Both the α/β and β solution treatment and subsequent aging at temperatures ranging from 460 °C to 560 °C were introduced to investigate the microstructural characteristics and microstructure–property relationships. The results show that the primary α phase formed during the α/β solution treatment can increase the stability of the β matrix and then restrain the size of secondary α phase during subsequent aging. In the α/β solution plus aging condition, the secondary α phase with the size of about 1 μm results in a high strength above 1500 MPa with −6% of elongation. The β solution treatment leads to the least stable β matrix and the greatest driving force for secondary α phase, so coarser α phases tend to form during aging. The β solution plus aging does not lead to attractive high strength because of the coarser and non-uniform α phase. And it also leads to a poorer combination of strength and ductility than the α/β solution plus aging. Direct aging exhibits the best strengthening. The ultimate strength of the alloy is about 1600 MPa with acceptable elongations of 6.5–7.5% when direct aged at 500 °C for 2–8 h. It benefits from the mixed microstructure with nano-scale and micron-scale α phases. However, more detailed investigations are needed to improve the ductility. As a whole, the alloy can be heat treatable to obtain an excellent balance of strength and ductility, and provide abundant stress levels with optional ductility (900–1600 MPa in ultimate strength and 6–20% in elongation).

Titanium and titanium alloys have several advantages, but the cost of titanium alloys is very expensive compared with the traditional metal materials. This article introduces two new low-cost titanium alloys Ti-2.1Cr-1.3Fe (TCF alloy) and Ti-3Al-2.1Cr-1.3Fe (TACF alloy). In this study, we used Cr-Fe master alloy as one of the raw materials to develop the two new alloys. We introduce the microstructure and tensile properties of the two new alloys from β solution treated with different cooling methods. Optical microscopy (OM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM) were employed to analyze the phase constitution, and scanning electron microscopy (SEM) was used to observe the fracture surfaces. The results indicate that the microstructures consist of β grain boundary and α′ martensite after water quenching (WQ), β matrix and α phase after air cooling (AC) and furnace cooling (FC), respectively. Also, the microstructure is the typical basketweave structures after FC. Of course, athermal ω is also observed by TEM after WQ. The strength increases with decreasing cooling rates and the plasticity is reversed. Because of the athermal ω, the strength and ductility are highest and lowest when the cooling method is WQ. The strength of TACF alloy is higher than the TCF alloy, but the plasticity is lower. The fracture surfaces are almost entirely covered with dimples under the cooling methods of AC and FC. Also, we observe an intergranular fracture area that is generated by athermal ω, although some dimples are observed after WQ.

The addition of 3%∼9% Zr on the martensitic transformation of Ti-18Nb(at.%) alloy was investigated. The results of microstructure and X-ray diffraction (XRD) analysis show that the phase constitution of as-quenched Ti-18Nb-9Zr(at.%) alloy consists of the retained β matrix and α″ martensite, while that of the other three alloys is single α″ martensite. No trace of athermal ω phase was found in any of the as-quenched alloys. Unlike the effect of Nb addition on the martensitic transformation start temperature Ms of Ti-18Nb(at.%) alloy, Ms decreased nonlinearly as increasing the Zr addition from 3% to 9% and Ms decreased much more sharply as increasing the Zr addition. The Ms of as-quenched Ti-18Nb-9Zr alloy was around room temperature. The effect of Zr addition on the β phase stabilizing in the Ti-18Nb(at.%) alloy was briefly discussed.

The dynamic fracture toughness of TA15ELI alloy with two types of microstructures was studied by instrumented impact test. Charpy specimens with both the 0.2 mm U-notch and the a/W = 0.2 pre-crack were adopted to compare notch sensitivity in the two microstructures. The result shows that the specimen with Widmanstatten microstructure exhibits a better dynamic fracture toughness and lower notch sensitivity than that with lath-like microstructure. Fracture surfaces in the case of the two microstructures are analyzed to have a ductile and brittle mixed feature under dynamic loading. The fracture surface of lath-like microstructure is composed of dimples and tear ridges, while that of Widmanstatten microstructure is covered with rough block-like facets and dimples and tear ridges. The α phase boundaries and α/β interfaces act as locations for void nucleation and crack arrest and deviation. The decrease in width of α phase lamellae leads to the increase in the amount of boundaries and interfaces, which causes the increase in the consumption of impact energy and results in the improvement in dynamic fracture toughness.

The Ti-Al-Sn-Zr-Cr-Mo-V-Si (Ti-62A) alloy, an alpha-beta alloy with high strength and fracture toughness, is currently used as an advanced structural material in aerospace and non-aerospace applications. Thermo-mechanical processes can be used to optimize the relationship between its strength and fracture toughness. A Ti-62A alloy bar can be machined through a transus β-forged plus α+β solution treated and aged specimen with a lamellar alpha microstructure. The effects of heat treatment on the mechanical properties were discussed. Heat treatment provided a practical balance of strength, fracture toughness, and fatigue crack growth resistance. A comparison of the Ti-62A alloy with the Ti-62222S alloy under the same thermo-mechanical processing conditions showed that their properties are at the same level.

The present study compares the high cycle fatigue (HCF) properties of the Ti–6Al–2Zr–1Mo–1V alloy with lamellar and bimodal microstructure. This comparison study includes the high cycle fatigue (resistance against crack initiation) with different ratio of stress ( R = 0.1 and −1) and different notch concentration factor ( K t = 1 and 3), and the fatigue crack propagation behavior. The comparison principally showed much lower high cycle fatigue strength for bimodal microstructure as compared with lamellar microstructure for smooth and notched specimens at positive load ratio. Whereas, the difference of the high cycle fatigue strength result at negative load ratio is small for the two microstructures. The results of fatigue crack propagation test show that the lamellae structure also provides higher crack growth resistance than the bimodal structure, and the lamellar microstructure displayed a more tortuous and deflected crack path.

A study on effect of boron addition on microstructure and property of low cost beta Ti-Al-Cr-Fe-B alloys are undertaken in the present. The Ti-Al-Cr-Fe alloys were developed as low cost beta Ti alloys for automotive springs, based on Molybdenum equivalency. Low priced Cr-Fe master alloys were introduced as beta stabilizing alloying elements for lowering cost, and elastic modulus. The boron addition is introduced in to refine the ingot microstructure and thus simplify hot working, which should lead to a cost reduction. On the other hand, the Ti-B compositions are able to restrict beta grain growth during heat treatments. So it can increase strength and microstructural stability. The mechanical tests show that the boron modified alloy has good tensile properties in solution condition. Moreover, the alloy can obtain well-balanced high strength levels with acceptable ductility after modulated aging treatment.

Ti-1023 alloy has been widely used in aerospace field as a typical titanium alloy with high strength and toughness. The relatively high content of Fe causes beta fleck which will make uneven microstructure and less plasticity. The new Ti-Al-Fe-V (Cr, Zr) alloys have been designed, based on Molybdenum equivalency and Bo-Md molecular orbital method, to aim at developing a new type of titanium alloy with high strength and fracture toughness. After primary design computation, Ti-Al-Fe-V (Cr, Zr) alloy was optimized finally. A large scale ingot was made by vacuum arc re-melting (VAR) for further property evaluation. Resultantly, it shows that athermal ω phase forms in Ti-Al-Cr-Fe-V-Zr alloy when solution treated at a temperature above the β-transus. Micro-hardness of alloy conducted to different aging conditions decreases with the aging temperature and time increasing respectively. Moreover, the new alloy has got a more sluggish age hardening response relating to the aging time. Additionally, after modulated aging treatment, the alloy can obtain high strength levels with acceptable ductility. When the alloy solution treated at 770 °C for 1h, followed by aging at 500 °C for 2h, the tensile strength of the alloy can achieve 1268 MPa, with an elongation of 11.5%, at the same time, the reduction of area has surpassed 30%. As a result, the newly designed alloy can achieve a good combination of tensile strength and plasticity through appropriate heat treatment.

Ti-Mo-Fe alloy was developed as low cost beta Ti alloys for automotive springs, and designed based on Molybdenum equivalency and Bo-Md molecular orbital method. Low priced Mo-Fe master alloys were introduced as alloying elements for the cost and elastic modulus reduction. A laboratory scale ingot was melted by ISM (Induction Skull Melting). Then, microstructure characterization and hardening behavior of a new designed Ti-12.1Mo-1Fe alloy during solution and aging treatment were investigated in the present study by microscopy, X-ray diffraction and hardness test. The results showed that ω phase played a more important role than a phase in hardening. The hardening due to ω phase can lead to high hardness about 470 Hv. However, the coarse a phase resulted in hardness below 300 Hv. On the other hand, the alloy exhibited fast aging response due to high diffusion rate of Fe element in the Ti matrix.

The effects of 0.1wt% boron addition on microstructures and properties of as-cast Ti-6Al-4V alloy were studied. The results show that TiB phase forms in the original β grain boundaries of Ti-6Al-4V alloy due to 0.1% boron addition. Compared with boron-free Ti-6Al-4V alloy, boron added alloy is refined with uniform β grains. In addition, boron addition results in an improved strength, ductility and a changed fracture mechanism. The fracture mechanism of Ti-6Al-4V alloy is as follows: dislocation slip and pile up in the alpha colonies results in stress concentration and then voids growing until cracks forming. However, the fracture of the boron added Ti-6Al-4V alloy is as follows: a stress concentration occurs around the TiB phase and then micro-voids are formed. TiB phase breaks and is separated with the matrix and then cracks nucleate.