The cast ingots of Ti–46Al–6Nb–0.25Si–0.2B and Ti–46Al–6Nb–0.5W–0.25Si–0.2B (at%) were made by induction skull melting (ISM) technique. A series of heat treatments (HTs) were conducted to research the microstructure evolution of both alloys. Microstructure and tensile property were examined by scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), and tensile testing machine. The results show that microsegregation (liquid segregation and solid segregation) is exacerbated by the addition of 0.5 at% W; the addition of Nb, W in TiAl alloy makes the phase transition difficultly take place; then, the microstructures and tensile properties of both alloys are improved after certain HT processes; finally, the thicknesses of the γ/α2 lamellae after a certain HT process are significantly affected by the number of residual γ phases before the furnace-cooling moment.

The effects of Ni addition on solidification microstructure and tensile properties of Ti-48Al-2Cr-2Nb alloy were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy-dispersive spectroscope (EDS) and transmission electron microscopy (TEM). Results show that with 3 at% Ni addition, the as-cast microstructure is mainly composed of fine lamellar colonies (~50 μm), γ grains and Ni-riched τ3 phase. After heat treatment at 1380 °C, the Ni-containing alloy is characterized by fine fully lamellar microstructure (~90 μm). The heat-treated Ni-containing specimen exhibits superior room temperature tensile properties than other specimens. The tensile properties are discussed in light of the microstructure evolution and role of Ni-riched τ3 phase.

•After forging, the microstructure was significantly refined and uniform.•The presence of carbide and boride also led to uniform and finer precipitation of α during aging as compared to the matrix alloy.•The size of secondary α phase increased with the increase of aging temperature. This trend leads to the decrease of strength and the increase of ductility.A hybrid of (TiB+TiC) reinforced beta titanium matrix (Ti-B20) were produced by non-consumable arc-melting technology and hot-forging. Microstructures of the composites were observed by optical microscopy (OM), transmission electron microscope (TEM) and scanning electron microscopy (SEM). The results show that both the TiB whiskers and TiC particles tend to segregate at β boundaries. The β grain size and secondary α lath width are refined by reinforcements and aging treatment. Evolution of tensile properties shows that enhancement in yield strength and ultimate tensile strength with the addition of reinforcements, as well as the remarkable increase in the ductility can be attributed to aging treatment at 600 °C and 650 °C. The size of secondary α phase increased with the increase of aging temperature. This trend leads to the decrease of strength and the increase of ductility to get good balance of properties. The fracture mechanism of the composite can be attributed to the cracking of the reinforcements.

Effects of TiB2 addition on microstructure and mechanical properties of Ti–48Al–2Cr–2Nb+(0.72,1.62)wt% TiB2 alloys fabricated by the induction skull melting (ISM) process were investigated. Results showed that the TiB2-induced microstructure was characterized by randomly orientated fully lamellar colonies and both the colony size and lamellae spacing were refined (100 μm and ~185 nm, respectively) by TiB2 addition. The borides were identified to be TiB2 with plate, needle and block morphologies, determined by different growth stages during solidification. At room temperature and 700 °C, the TiB2-containing alloys exhibit non-deteriorated fracture toughness and superior tensile properties than that of the as-cast and heat-treated matrix alloys. Furthermore, the fracture toughness anisotropy was eliminated due to the randomly orientated lamellar microstructure induced by TiB2 addition. The fine TiB2 particles with special morphology (plate and needle) and the easy-to-deform ligament bridges induced by the refined microstructure can account for the notable fracture toughness of the studied TiB2-containing alloys. The main toughening mechanism was analyzed and discussed in light of the microstructure characterization, size and morphology of borides and the deformed ligament bridges.

•A new β high strength titanium alloy is designed.•A high yield strength of 1614MPa is obtained with elongation of 6.2%.•The size and volume fraction of secondary α are sensitive to aging temperature.•The strength is relative to volume fraction and size of secondary α.Microstructure and mechanical properties of a new β high strength Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe titanium alloy were investigated in this paper. Both the α/β and β solution treatment and subsequent aging at temperatures ranging from 440 °C to 560 °C for 8 h were introduced to investigate the relationship between microstructures and properties. Microstructure observation of α/β solution treatment plus aging condition shows that the grain size is only few microns due to the pinning effect of primary α phase. The β solution treatment leads to coarser β grain size and the least stable matrix. The size and volume fraction of secondary α are very sensitive to temperature and strongly affected the strength of the alloy. When solution treated at 775 °C plus aged at 440 °C, the smallest size (0.028 μm in width) of secondary α and greatest volume fraction (61%) of α resulted in the highest yield strength (1624 MPa). And the yield strength decreased by an average of 103 MPa with every increase of 40 °C due to the increase of volume fraction and decrease of the size of secondary α. In β solution treatment plus aging condition, tensile results shows that the strength if the alloy dramatically decreased by an average of 143 MPa for every increase of 40 °C because of larger size of secondary α phase than α/β solution treated plus aged condition.

The microstructures and tensile properties of Ti–6Al–4V–0.1B alloys with lamellar structures after direct rolling were investigated systematically. When sheet is rolled in the β phase region, boron-containing alloys with fine grains show good ductility. No crack is found in the macrostructure when examined. By scanning electron microscopy, the observation shows that TiB whiskers are mainly at the grain boundary in as-cast microstructure, while in as-rolled microstructure the whiskers become smaller and arrange along the rolling direction. Electron backscatter diffraction analysis shows that the matrix microstructure of the as-rolled alloys has a low degree of distortion, and remains lamellar (α lath width is reduced to 1.73±0.42 μm). The microstructure refinement causes the change of mechanical properties that both the room temperature tensile properties and the high temperature strength of as-rolled alloys are obviously higher than those of as-cast alloys. TiB whiskers debond with matrix alloys when the temperature is high, which causes the high temperature ductility of as-rolled alloys are relatively low.

A new beta gamma TiAl alloy Ti-43Al-4Nb-2Mo-0.5B (at %) was fabricated by ISM method. The as-cast microstructure consisted of fine lamellar colonies and mixtures of small γ and B2 grains around lamellar colony boundaries. By canned hot forging, the cast microstructure was further refined and homogenized. Microstructure evolution during hot forging was characterized by means of SEM, TEM and EBSD in detail. DRX and phase transformation during forging are also discussed; Based on nano-indentation tests, B2 was found much harder than γ and α2 phases. The tensile properties of the forged alloy were investigated and compared with as-cast condition; this forged alloy maintains high tensile strength over 900 MPa up to 750 °C, and tensile superplasticity appears above 800 °C. B2 phase is proved to be detrimental to room-temperature ductility and to reduce tensile strength sharply above 800 °C, mainly because of its hard and brittle nature at low temperature and its soft feature at high temperature.Graphical abstractHighlights► Hot forging further refines the cast microstructure of current beta gamma alloy. ► This forged alloy maintains high tensile strength over 900 MPa up to 750 °C. ► B2 is the hardest phase in TiAl alloy, with nanohardness of 8.5 GPa. ► B2 phase is detrimental to room-temperature ductility of γ-TiAl alloy.

A novel high Nb, low Al contained TiAl alloy Ti-43Al-6Nb-1B (at %) was fabricated by induction skull melting (ISM) technique. The as-cast alloy exhibits fully lamellar (FL) microstructure with mean colony size of 90 μm. By means of thermophysical simulation, the hot deformation mechanism at 1250 °C is concluded as dislocation slip and twinning in γ phase; bending, rotating and elongation of lamellar colonies are the primary deformation modes. A large-size TiAl pancake was successfully produced by triple-step canned forging at 1250 °C. The forged alloy displays duplex (DP) microstructure consisting of elongated lamellar colonies and γ grains. Typical DP and FL microstructures were further obtained by different heat treatments. The tensile strength increases rapidly from 670 MPa in as-cast state to 975 MPa in as-forged condition at room temperature, with elongation increasing from 0.4% to 1.5%. The as-forged alloy maintains strength higher than 950 MPa until 750 °C; also the DP and FL materials hold excellent strength more than 830 MPa and 770 MPa respectively. The superior tensile properties after forging are mainly ascribed to the microstructure refinement and homogenization, the lamellar spacing reducing, as well as the low Al and relatively low Nb additions in this novel high Nb containing TiAl alloy.Highlights► Large-size Ti-43Al-6Nb-1B pancake is produced by triple-step canned forging. ► The high-temperature deformation behavior of this alloy is discussed. ► The microstructure evolution during forging and heat treatments is studied. ► This alloy exhibits superior tensile strength.

The influences of rare earth metal (Y) on microstructure, mechanical properties and deformability of Ti–43A1–9V alloy are reported. The results show that Ti–43Al–9V–0.3Y alloy is composed of γ, α2, B2 and YAl2 phases. Adding 0.3at.% Y refines the grain size, decreases α2/γ/B2 lamellar thickness of Ti–43Al–9V alloy, and promotes the formation of fine α2/γ lamellae. Mechanical property tests show that adding 0.3at.% Y can apparently enhance strength and plastic property of Ti–43Al–9V alloy. The hot deformation experiment at 1200 °C indicates that adding 0.3at.% Y can improve deformability of Ti–43Al–9V alloy, because Ti–43Al–9V–0.3Y alloy has smaller grain size, lower resistance to deformation, and fine and uniform recrystallized grains.

•The new phase L12-Ti3Al in TiAl alloys is observed via TEM.•The precipitation method of L12 phase is analyzed in detail.•The orientation relationships between γ, α2 and L12 phases are confirmed.•Phase transitions of TiAl alloys at application temperature are discussed thermodynamically.A new phase L12-Ti3Al was found in the Ti-46Al-6Nb alloy after rapid-cooling and then annealing at 850 °C for 5 h. The transformation kinetics involving the L12 phase were observed by transmission electron microscopy (TEM): The plate-like L12 phase precipitates in the massive γ (γm) phase and then partially transforms into α2 or B2 phase; the transformations of bulk α2 (αb) → L12/L12 → γ were also observed to form a fine lamellar structure with an average interlamellar spacing < 20 nm. The orientation relationships between γ, α2 and L12 phases were confirmed by TEM observation and crystallographic calculation, which are (001)γ // (001)L12, [010]γ // [010]L12, [100]γ // [100]L12 and (0001)α2 // {111}L12, 〈112¯0〉α2 // 〈11¯0〉L12. The L12 phase was also observed in Ti-the 47Al alloy after the quenching-annealing process. Thermodynamic analysis of the transformations indicates that the γ → L12 transformation is extremely susceptible to temperature changes while transformations between the α2 and L12 phases are not.Download high-res image (220KB)Download full-size image