Strength Optimization of Diffusion-Bonded Ti2AlNb Alloy by Post-Heat Treatment

Man J., Huang L., He J., Yang H., Lin X.

Li T., Fan W., Li X., Wu H., An D., Hu Q., Chen J.

Multilayered structures of dissimilar titanium alloys can achieve excellent fracture ductility and strength, while their fatigue characteristics especially dislocation networks and twin formation are rarely reported. Heterogeneous microstructures are observed in the multilayered TC4/TB8 alloys, including fine acicular α grains, continuous α layer at prior β grain boundaries (αGB) and β matrix on the TB8 layer, together with equiaxed α grains on the TC4 layer. High-cycle fatigue (HCF) and low-cycle fatigue (LCF) tests show that the initial fatigue damage appears at the αGB/β matrix interfaces on the TB8 layer instead of the boned TC4/TB8 interfaces. Since the stress concentration induced by dislocation pile-up is prone to micro-void formation and crack propagation at the αGB/β interfaces. For LCF, the αGB/β interfaces can not only act as impenetrable barriers and sources of lattice dislocations, but also allow the dislocations cross boundaries during cyclic tension and compression because of the high boundary energy. The formation characteristic of deformation twins that is beneficial for the plastic deformation of α grains in TC4 layer during cyclic strain is investigated. Furthermore, the hexagonal dislocation networks are also found within the equiaxed α grains of TC4 layer after LCF, and the role between interface barrier and slip direction in the formation mechanism is analyzed.

Cao X., An D., Liu Q., Chen G., Li X.

In this study, we reported the ultrafast precipitations of O phase in Ti2AlNb alloy caused by pulse current in the range of 450°C–750 °C, which results in an abnormal increase of strength with increasing temperature in electrically-assisted uniaxial tension. The morphology of precipitate phase is controlled by the increase in thermal and non-thermal free energy with increasing pulse current. The elongated needle-like O phase, caused by high temperature, would reduce flow stress, whereas nano-sized α2 precipitates, formed at specific pulse current density and temperature, could enhance the strength of Ti2AlNb. To describe the stress changes caused by precipitation hardening effect, S-curve has been introduced. The modified Z-A model reduces parameter amount but keeps accuracy. This model efficiently predicts the stress change caused by pulse currents, which combines with the linear model of pure electroplasticity and S-curve. Furthermore, the specific impacts of temperature, effective pulse current density and strain on the stress reduction are decoupled.

Fan J., Li X., Pan C., Zhu Z., Wang X., Qu S., Yang C., Hou J.

The development of lightweight high-temperature materials and their processing methods are receiving increasing attention as the need for improved performance and energy efficiency in aero-engines increase. In this study, the γ-TiAl alloy was successfully soundly bonded to Ti2AlNb alloy by pulsed high current (PHC) diffusion welding. The effect of different high level (HL)/low level (LL) of composite pulse current on the microstructure and mechanical properties of the joints have been studied. The representative interface microstructure of PHC-950 °C TiAl/Ti2AlNb joint was γ-TiAl substrate/diffusion bonding zone/Ti2AlNb impacted zone/Ti2AlNb substrate. The diffusion bonding zone was composed of continuous equiaxed α2 grains and a small amount of B2 and O phases. The α2 phase in the diffusion bonding zone was mainly transformed from the γ phase in γ-TiAl substrate and the B2 and O phase in Ti2AlNb substrate. The composite pulse current has a significant promoting effect on the diffusion of elements and joint formation. The diffusion coefficients of DAl and DNb in hot-pressing (HP, without current effect) diffusion welding joint at interface were 2 × 10−14 m2/s and 0.92 × 10−14 m2/s, respectively. In the representative PHC-950 °C joint with HL/LL = 12/2 cycles (single cycle duration 3.3 ms), diffusion coefficients of DAl-12/2, DNb-12/2 at interface were 19.62 × 10−14 m2/s, 13.34 × 10−14 m2/s, respectively. Increasing the duration of LL would weaken the current effect. The current waveform parameters were tuned to HL/LL = 12/4 and 12/6 cycles, the diffusion coefficients of DAl-12/4, DNb-12/4 DAl-12/6 and DNb-12/6 were reduced to 15.69 × 10−14 m2/s, 12.49 × 10−14 m2/s, 15.31 × 10−14 m2/s and 10.99 × 10−14 m2/s, respectively. The PHC-950 °C joints have the highest shear strength of 257.3 MPa at HL/LL = 12/2 cycles, reaching 83.3% of the base material strength. The HP-950 °C TiAl/Ti2AlNb joint has a shear strength of 230.5 MPa, reaching 74.6% of the base material strength. The shear strength of PHC-950 °C joints at HL/LL = 12/4 and 12/6 reduced to 205.3 MPa, 195.8 MPa due to insufficient element diffusion and damages the bonding quality. This work provides a strategy that contributes to reduce the thermal damage of base metal and improve the joint forming efficiency.

Zhang Q., Li T., Han Y., Zheng W., Li X., Wu J.

Superplastic forming is a practical method to manufacture complex-shaped parts of titanium alloys with large deformation. Laminated parts of dissimilar titanium alloys fabricated by superplastic forming can achieve excellent performance by combining the advantages of components. This work displays the superplastic tension behavior and microstructural evolution of dissimilar TC4/SP700 laminate prepared by the diffusion bonding process. Two titanium alloys can achieve metallurgical bonding at parameters of 800 ℃/1 h/5 MPa. Except for dynamic recrystallization and grain growth behaviors upon superplastic tension, stress-induced phase transformation plays an important role in α to β phase transformation apart from the elevated temperature. The superplastic deformation can be attributed to the grain boundary sliding accommodated multiplex motion of dislocations. In addition, the retained strengths of all dissimilar TC4/SP700 laminates after superplastic deformation with different strain rates and temperatures range from 807 to 890 MPa.

Ayadh W., Denand B., Halkoum A., Boulet P., Sennour M., Delfosse J., Sallot P., Esin V.A.

To study the influence of prior α2 phase on O-phase formation kinetics in β matrix, different isothermal heat treatments were performed at 876 and 780 °C using an advanced Ti2AlNb alloy with varying fraction of α2 phase. The kinetics of phase transformations was followed by electric resistivity measurements coupled to multi-scale microstructure characterization using scanning and transmission electron microscopy, energy dispersive X-ray spectrometry (EDS) and quantitative X-ray diffraction. It is revealed that the effect of α2 phase on the formation of O-phase depends on transformation temperature: while at 876 °C prior α2 precipitates lead to significantly slower formation of O-phase, as compared to α2 free alloy, the kinetics of O-phase formation is faster at 780 °C in the presence of α2 phase. Using EDS data as well as accurate measurements of lattice parameter of β phase, the diffusion character of O-phase formation in β matrix with and without prior α2 is shown for 876 °C while at 780 °C the diffusionless character of the O-phase formation is revealed for the beginning of the transformation followed by the partitioning of the alloying elements for prolonged treatment durations.

Li T., Zhou F., Long L., Wang B., Tang Z., Li X.

Laminated TC4/TB8 titanium alloys with heterogeneous microstructures of lath and equiaxed shape are fabricated by diffusion bonding and heat treatment to enhance the fatigue crack growth (FCG) resistance with high tensile strength. It is worth noting that the forward direction of fatigue crack can be changed due to different alloy layers and bonded interfaces, which can reduce the FCG rate along the main direction. In contrast, heat treatment accompanied by active α precipitates in TB8 layer can remarkably improve the tensile strength of laminated alloys. The strain distribution of TC4 and TB8 layers upon tension is also characterized by the digital image correlation (DIC). In addition, a generalized Paris model describing steady state FCG is established based on the tensile properties of two monolithic alloys. This study can provide guidance that diffusion bonding process is available to fabricate the heterogeneous laminates with as many interfaces as possible for the improvement of FCG resistance and tensile strength.

Shao B., Tang W., Guo S., Zong Y., Shan D., Guo B.

The Ti–22Al–25Nb alloy, which possesses a complex microstructure, is a new type of lightweight high-temperature structural material. Revealing the effects of microstructure and temperature on the mechanical properties of this alloy is crucial for applications in the aerospace industry. In this study, we designed three types of microstructures of the Ti–22Al–25Nb alloy, namely, single-phase B2, dual-phase α 2 + B2, and triple-phase O + α 2 + B2, and investigated their deformation and fracture behavior with increasing temperature (25, 750, and 930°C). The O + α 2 + B2 sample possessed the best comprehensive mechanical properties, and the softening of the O phase at elevated temperatures was mainly due to massive {001} O basal slip and twinning. The evolution of O phase twins with increasing temperature can be described as follows: (021) → (021) + (110) → (110). At 750°C, (021) twins formed in the strip-shaped O phase in B2 grains, while (110) twins formed in the spheroidized O phase at B2 grain boundaries. In addition, B2 grain boundary embrittlement was observed at elevated temperatures, and its mechanism can be summarized as follows. Elevated temperatures induced element segregation at the grain boundaries between hypersaturated-state B2 grains, enriching Ti and Al and diffusing Nb into the B2 grains on the nanometer scale, leading to a thin-shell O phase with a thickness of tens of nanometers precipitating at the B2 grain boundaries. During elevated-temperature fracturing, cracks propagated quickly along the interface between the thin-shell O phase and B2 phase, leading to embrittlement of the B2 grain boundary. The precipitation of many fine granular O or α 2 phases at the B2 grain boundaries can inhibit the precipitation of the thin-shell O phase and hinder the propagation of cracks along the grain boundaries. Therefore, the microstructure of this alloy should be controlled to precipitate many fine granular O or α 2 phases to occupy the B2 grain boundaries and inhibit its elevated-temperature embrittlement.

Zhang C., Bao X., Hao M., Chen W., Zhang D., Wang D., Zhang J., Liu G., Sun J.

Due to the low thermal stability of crystallographic boundaries, the grain boundary engineering (GBE) manifests some limits to the fineness and types of microstructures achievable, while unique chemical boundary engineering (CBE) enables us to create a metallic material with an ultrafine hierarchically heterogeneous microstructure for enhancing the mechanical properties of materials. Here, using a low cost metastable Ti-2.8Cr-4.5Zr-5.2Al (wt.%) alloy as a model material, we create a high density of chemical boundaries (CBs) through the significant diffusion mismatch between Cr and Al alloying elements to architecture hierarchical nano-martensites with an average thickness of ~20 nm. For this metastable titanium alloy, the significantly enhanced yield strength originates from dense nano-martensitic interface strengthening, meanwhile the large ductility is attributed to the multi-stage strain hardening of hierarchical 3D α'/β lamellae assisted by equiaxed primary α (αp) nodules. The hierarchical nano-martensite engineering strategy confers our alloy a desired combination of strength and ductility, which can potentially be applied to many transformable alloys, and reveal a new target in microstructural design for ultrastrong-yet-ductile structural materials. It is challenging to obtain Ti alloys with ultrafine microstructure owing to the low thermal stability of crystallographic boundaries. Here the authors demonstrate a chemical boundary-based strategy to produce a hierarchical Ti alloy with nano-martensites that has excellent strength and ductility.

Li T., Zhong L., Wu H., An D., Li X., Chen J.

Microstructure evolution and fatigue crack growth (FCG) behaviors of diffusion bonded Ti-6Al-4 V titanium alloy with synchronously changed temperature and pressure are systematically investigated. Ti-6Al-4 V alloy plates can be metallurgically bonded under suitable process parameters. Meanwhile, bonded plates displayed different FCG resistance and excellent tensile properties at room temperature. FCG life of the 780–18 (bonded temperature-pressure) sample from a crack length of 0.5 mm to fracture is 108147 cycles, which is 1.6 and 1.84 times than that of the 830–12 and 900–6 samples, respectively. According to the observation of crack paths and α grain crystallography before and after fatigue tests, fatigue crack propagation is mainly affected by the α crystallographic orientation, α grain size, and β phase. By choosing higher pressure (18 MPa) to promote bonding and lower temperature (780 ºC) to optimize the microstructures, diffusion bonded Ti-6Al-4 V alloy can be achieved outstanding FCG resistance with uncompromised tensile properties in this study. ● Ti-6Al-4 V is diffusion bonded with synchronously changed temperature and pressure. ● Excellent FCG resistance with uncompromised tensile property is obtained. ● { 0001 } 〈 10 1 ¯ 0 〉 texture and fine α grains improve FCG resistance. ● β phase can hinder crack motion and change forward direction.

Panov D., Naumov S., Stepanov N., Sokolovsky V., Volokitina E., Kashaev N., Ventzke V., Dinse R., Riekehr S., Povolyaeva E., Nochovnaya N., Alekseev E., Zherebtsov S., Salishchev G.

The effect of pre-heating and post-weld heat treatment on microstructure and mechanical properties of laser-welded joints in a Ti–23Al–23Nb-1.4V-0.8Zr-0.4Mo-0.4Si (at.%) alloy was studied. Laser beam-welding was carried out at room temperature as well as after pre-heating up to 800°С. The post-weld heat treatment comprised either air quenching from 920°С followed by aging at 800°С or only aging at 800°С. The microstructure of the fusion zone consisted of columnar β-grains after welding at room temperature and 400 °C or both the columnar and large equiaxed crystals at 600 and 800 °C. An increase in the pre-heating temperature caused the columnar β-crystals growth as well as an increase in the fusion zone and heat-affected zone widths. Meanwhile, a decrease in the Al and Ti content, as well as an increase in both the porosity and gaseous elements content (O and N) after welding at 600–800 °C were found. The microhardness of each joint obtained after welding with pre-heating temperatures up to 600 °C was lower than that of the base material. All the welded joints showed the yield strength and ultimate tensile strength levels between 1070 and 1110 MPa, which correspond to approximately 80% of the base metal level. Reasonable total elongation of the joint was achieved after welding at 400 °C (4.3%). The post-weld heat treatment involving air quenching from 920 °C with subsequent aging at 800 °C for 6 h demonstrated the best results. The heat treatment resulted in the precipitation of the O- and α 2 -phases and an increase in total elongation to 6.5%. • Ti–23Al–23Nb-1.4V-0.8Zr-0.4Mo-0.4Si alloy was laser beam-welded at 20–800 °C. • Width of the fusion and heat-affected zones increased with pre-heating temperature. • Dissolution of the O- and α 2 -phases in the weld was observed. • Reasonable ductility (A f = 4.3%) was achieved after welding at 400 °C. • Quenching from 920 °C with subsequent aging at 800 °C improved ductility further.

Zhao Q., Lv M., Cui Z.

To achieve grain refinement, the recrystallization behavior of B2 matrix phase in a Ti 2 AlNb-based alloy was investigated by performing a series of thermal compression tests at temperatures in the range of 950 ~ 1025 °C with strain rates from 0.001 s − 1 to 1 s − 1 . Instead of water quenching immediately after deformation, samples were further held at deformation temperature for a certain time, in order to make the heat-affecting time at various strain rates consistent. Electron back scattered diffraction (EBSD) technique was employed for microstructure analysis, in combination with grain orientation spread (GOS) method to distinguish recrystallized grains from non-recrystallized grains. The results reveal that: (i) dynamic recrystallization (DRX) is not easy to occur in this alloy, and increasing strain rate leads to a decrease of DRX fraction; (ii) when evaluated on the same time scale, the sample at strain rate of ε ˙ = 0.1 s − 1 has the highest recrystallized fraction among all applied strain rates, which provides sufficient nucleation rate of DRX as well as notable grain growth during post-dynamic recrystallization (PDRX); and (iii) the recrystallization evolution with temperature changes is closely linked to the phase transformation in Ti 2 AlNb. The presence of O and α 2 phases affects recrystallization process via pinning effect and particle stimulated nucleation (PSN) mechanism. This work therefore provides a new understanding of recrystallization behavior during and post deformation of Ti 2 AlNb-based alloy, which is meaningful to refine grains by combining the contributions of DRX and PDRX. • The recrystallization behavior of Ti 2 AlNb alloy was first evaluated on the same heat-affecting time scale. • DRX process is severely inhibited at high strain rates. • The contribution of PDRX to the final recrystallized fraction is greater than that of DRX. • The effect of O and α 2 phases on recrystallization behavior was investigated.

Li T., Yan S., Liu X.

• γ growth of medium-Mn steel relies on Mn atoms diffusion related to crystal defect. • The volume fraction of γ increases from 29% to 38.2% with decreasing cold-rolling. • The work first proposes cold-rolling and continuous annealing to improve γ content. • It is a feasible method to improve γ content by controlling crystal defect. The austenite (γ) reversed transformation of medium-Mn steel during intercritical annealing (IA) process was investigated based on different initial martensite (α') microstructure. The results reveal that the volume fraction of γ phase gradually increases from 29% to 38.2% with the decreasing degree of the α recrystallization. This is because the γ growth is controlled by Mn atoms diffusion from α to adjacent γ grains, and Mn atoms diffusion preferentially depends on the passageway of crystal defects such as dislocations. This method sheds light on intensifying initial crystal defects and inhibiting subsequent recrystallization to improve γ content of medium-Mn steels treated by cold-rolling and continuous annealing process.

Zhang H., Yan N., Liang H., Liu Y.

In recent years, the Ti 2 AlNb-based alloys are selected as potential alloys for elevated temperature applications to replace conventional Ni-based superalloys owing to their good creep resistance and oxidation resistance which are related to the O precipitates. In this paper, the precipitation mechanisms of O phase, phase transformation and microstructure control of Ti 2 AlNb-based alloys are reviewed. Ti 2 AlNb-based alloys generally consist of B2/β, α 2 , and O phase with different morphologies which are derived from the various heat treatment processes, including equiaxed α 2 /O particles, bimodal microstructure, and Widmannstätten B2/β + O structures etc. As a newly developed strengthening phase, O precipitates can be precipitated from the B2/β matrix or α 2 phase directly as well as generated by means of peritectoid reaction of α 2 phase and bcc matrix. Microstructural control of the Ti 2 AlNb-based alloys can be implemented by refining the original B2/β grain size and regulating the O precipitates. Multidirectional isothermal forging (MIF) and powder metallurgy technique are two effective methods to refine the original B2/β grains and the morphology and size of O precipitates can be regulated by adding alloying components and pre-deformation process. Moreover, the phase diagram as well as coarsening behavior of Ti 2 AlNb-based alloys in ageing process is also reviewed. For the further application of these alloys, more emphasis should be paid on the deep interpolation of microstructure-property relationship and the adoption of advanced manufacturing technology.

Goyal K., Sardana N.

Ti2AlNb intermetallics are promising next-generation aerospace materials. Advancement in non-conventional manufacturing methods has made the fabrication of these intermetallics economical. However, post-heat treatments are required to obtain desired mechanical properties, which further depend on the microstructural features of the intermetallics. Extensive studies have been conducted to understand the relation between mechanical behavior and microstructural features of this intermetallic. This review presents the effect of various microstructural features of (a) fully B2 and α2 + B2, (b) fully lamellar, (c) bimodal lamellar, (d) equiaxed and duplex, and (e) plate-like O phase on room temperature properties, namely strength, ductility, and fracture mechanisms. Moreover, the review emphasizes the special microstructural features that are required to enhance the mechanical properties of the alloy.