Near-α Titanium Alloy Research Articles

Evolution laws of microstructures and mechanical properties during heat treatments for near-α high-temperature titanium alloys

Evolution laws of microstructures and mechanical properties during heat treatments for near-α high-temperature titanium alloys

High-temperature mechanical performance of directed energy deposited Ti6242S alloy

Ti6242S is a near-α titanium alloy that is expected to be used in engineering below 500 °C. The high-temperature mechanical performance including the high-temperature tensile property and the high-temperature high-cycle fatigue property is well investigated by directed energy deposition using a laser beam (DED-LB) in the paper. It is confirmed that the high-temperature tensile mechanical property can highly satisfy the forging standard for heat treated DED-LB Ti6242S. And the heat-treated DED-LB Ti6242S is of reliable high-temperature high-cycle fatigue limit relying on an analysis on the coefficient of variation of the fatigue life. It is presented that the high-temperature high-cycle fatigue life of DED-LB Ti6242S alloy is highly related to the pore position and its size. Thus a new parameter is proposed to build a relationship with the high-temperature high-cycle fatigue life. Moreover, the fatigue deformation mechanism depends strongly on the α and α′ colony orientations that vary frequently and intricately in accordance with the fine microstructure morphology, leading to a reduced crack expansion rate for DED-LB Ti6242S.

Deformation modes investigation during ex-situ dwell fatigue testing in a bimodal near-α titanium alloy

In this paper, quasi-cleavage facet formation and deformation mechanisms in dwell fatigue have been studied on specimens from a Ti834 compressor disc alloy with a bimodal microstructure by ex-situ dwell fatigue testing at temperatures between 80 °C and 200 °C. Basal <a> slip was observed in grains with their c-axis near parallel the loading direction, while colonies similarly oriented accommodate deformation by tensile twins 101¯2<112¯0>. This type of slip in basal planes is the most critical damage mode leading to failure during dwell fatigue loading. A Rogue colony-grain combination is presented and a possible criterion for slip transfer in bimodal titanium alloys is introduced. The requirements cited for quasi-cleavage facet formation leading to dwell fatigue failure have been experimentally observed in agreement with the suggested hypothesis in previously presented crystal plasticity models.

Ductilization of fracture of a high strength interwoven α/βt structured PM titanium alloy by in-situ formed submicron Y2O3 particles

A method combining internal oxidation and thermomechanical consolidation of a composite powder with a trace amount of Y addition was used to fabricate powder metallurgy (PM) near-α Ti–6Al–2Sn–4Zr–2Mo−0.1Si−0.5Y (wt%) alloy, in which submicron Y2O3 particles were in-situ formed and distributed uniformly in the matrix to achieve a low oxygen content and an excellent combination of high tensile strength of 1242 MPa and excellent tensile ductility of 16.2%. The in-situ reaction of uniformly dispersed Y particles in the composite powders prepared by high-energy ball milling with the oxygen in the matrix to form submicron Y2O3 particles during powder consolidation can significantly scavenge oxygen in the matrix and eliminate the coarse grain boundary α layers. The Y2O3 particles also work as nucleants for heterogeneous nucleation of α grains in the prior β grains during cooling, thus promoting the formation of α plates with a larger number of orientation variations and a decrease of their aspect ratios in the interwoven α/βt microstructure consisting of fine α plates and β transformed structure (βt) domains, which is beneficial to suppress the initiation and rapid propagation of cracks during tensile deformation. It is demonstrated that with the high flow stress rendered by the ultrafine interwoven α/βt microstructure and oxygen scavenging effect of Y on the matrix, twinning across α/βt interfaces and decohesion between the submicron Y2O3 particles and matrix occur during tensile deformation. This further suppresses formation and propagation of microcracks often seen in the Y2O3 particle free PM near-α titanium alloy of the same composition in the same situation. It is believed that the twinning and decohesion of the interfaces between Y2O3 particles and matrix are responsible for the mitigation of strain localization needed for formation of microcracks, which is essential to maintain a good tensile ductility and allow the fracture to be totally ductilized.

Chemical Composition, Microstructure, Tensile and Creep Behavior of Ti60 Alloy Fabricated via Electron Beam Directed Energy Deposition.

Electron beam directed energy deposition (EB-DED) is a promising manufacturing process for the fabrication of large-scale, fully dense and near net shape metallic components. However, limited knowledge is available on the EB-DED process of titanium alloys. In this study, a near-α high-temperature titanium alloy Ti60 (Ti-5.8Al-4Sn-4Zr-0.7Nb-1.5Ta-0.4Si) was fabricated via EB-DED. The chemical composition, microstructure, tensile property (at room temperature and 600 °C), and creep behavior of the fabricated alloy were investigated and compared with those of the conventional wrought lamellar and bimodal counterparts. Results indicated that the average evaporation loss of Al and Sn was 10.28% and 5.01%, respectively. The microstructure of the as-built alloy was characterized by coarse columnar grains, lamellar α, and the precipitated elliptical silicides at the α/β interfaces. In terms of tensile properties, the vertical specimens exhibited lower strength but higher ductility than the horizontal specimens at both room temperature and 600 °C. Furthermore, the tensile creep strain of the EB-DED Ti60 alloy measured at 600 °C and 150 MPa for 100 h under as-built and post-deposition STA conditions was less than 0.15%, which meets the standard requirements for the wrought Ti60 alloy. The creep resistance of the EB-DED Ti60 alloy was superior to that of its wrought bimodal counterpart.

Multi-dimensional study of the effect of early slip activity on fatigue crack initiation in a near-α titanium alloy

During service of gas turbine engines, high cycle fatigue of titanium is a leading cause of component failure highlighting the need for better understanding of the crack initiation mechanism to predict initiation sites. In this study, the relationship between plastic slip activity and fatigue crack initiation was investigated in a near-α titanium alloy using cyclic four-point bending at up to 90% of the proof stress. The finding from surface characterization show that plasticity at such low stress levels was dominated by the basal slip and two types of cracking were seen parallel to basal slip traces. Detailed 3D analysis of both crack types highlighted out-of-plane Burgers vector activity for the observed basal slip associated with crack initiation, consistent with the classic surface roughening mechanism. The transgranular crack initiation was accompanied by the formation of crystallographic facet which was identified to be 6° away from the basal plane due to additional prismatic slip activation during multi-step crack formation. The intergranular crack facet along the boundary between primary α grain pairs, which have their c-axes aligned nearly parallel to each other but with mis-aligned prismatic planes, was formed by an easy cleavage in one step along the basal plane. Statistical evaluation demonstrated that grains combining a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being oriented strongly out-of-surface plane favoured transgranular crack initiation. Based on those observations a new parameter involving these three geometrical factors was developed to predict transgranular surface crack initiation sites.

Investigation of Electrochemical Dissolution Behavior of Near-α TA15 Titanium Alloy in NaCl Solution with Low-Frequency Pulse Current

TA15 material is a typical near-α titanium alloy and widely used for the aircraft key load bearing components. Electrochemical machining (ECM) is a cost-effective method to machine difficult-to-cut TA15. Due to the high chemical reactivity of titanium, titanium alloy is prone to passivation, which increases the difficulty of ECM, especially for some common ECM methods with low-frequency pulse currents. To investigate the change of surface characteristics of titanium alloy in pulse ECM, the dissolution behavior of TA15 in NaCl solution under the low frequency pulse current was analyzed by scanning electronic microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). The results showed that even after removal of initial air-formed oxide film, TA15 would be passivated to form a new passive film during long pulse-off time. Under short pulse-on time, the passive film formed in pulse-off time could not be effectively removed in one pulse-on time, and the passivation effect would be accumulated, resulting in poor machined surface with humps and pits. When the pulse-on time was long enough to completely remove the newly formed passive film, TA15 was dissolved normally to obtain excellent surface with clear crystal structure.

Effect of grain size on the yield stress and microscopic mechanism of a near-α titanium alloy during non-superplastic hot deformation

In this paper, the effect of grain size on the yield stress of a near-α TA15 titanium alloy during the non-superplastic hot deformation was modelled, and the microscopic mechanism was analyzed through the crystal plastic finite element simulation. The effect of grain size on the yield stress changes from the refinement strengthening to refinement softening with the increase in temperature. To model this transformative effect, the grain boundary region is separated to consider the deformation mechanism of grain boundary sliding, and a concise relationship was obtained with a max prediction error of 3.7%. The distribution of microscopic strain is determined by the critical grain size Dc, when the strength of grain boundary regions is equal to α phase. When the grain size is greater than Dc, there are two high strain regions in the α phase, which are the intragranular deformation band caused by the prismatic slipping and the deformation band near grain boundaries caused by the multiple slipping. The continuous and discontinuous dynamic recrystallization appears simultaneously. When the grain size is less than Dc, the strain in the α phase is concentrated near the grain boundary due to the stimulant grain boundary sliding, and the discontinuous dynamic recrystallization is promoted.

Three-dimensional morphology of tri-modal microstructure and evolution mechanisms of constitute phases in dual heat treated near-α titanium alloy

Dual heat treatment, the combination of near-β heat treatment and two-phase field heat treatment, provides an effective approach for near-α titanium alloy to obtain a tri-modal microstructure with excellent comprehensive performance. In this paper, the three-dimensional morphology of a tri-modal microstructure in TA15 Ti-alloy and the evolution mechanisms of equiaxed α and lamellar α were revealed by means of interrupted quenching experiments and three-dimensional reconstruction experiments. It is found that the equiaxed α and the lamellar α showed irregular shapes in three-dimensional space, and the α phases were in contact with each other except for some small and fine lamellar α. During the near-β heat treatment, the decomposition of equiaxed α was caused by β phase wedging at grooves. Grooves formed on the surface of the equiaxed α, extending inwards and further the equiaxed α being separated. In addition, a prediction model of equiaxed α decomposition was proposed in this paper. During two-phase field heat treatment, the increase of equiaxed α size was caused by the merging of adjacent α grains or the adhering growth of fine lamellar α. Some lamellar α decomposed through the mechanism of deflection fracture proposed in this paper, i.e., grooves formed at the substructures of the lamellar α and expanded inwards. The lamellar α on both sides of the grooves deflected at small angles to reduce the interface energy and finally the fracture occurred.

Slip activity during low-stress cold creep deformation in a near-α titanium alloy

Near-α titanium alloys are known to be susceptible to cold dwell fatigue (CDF) debit, which has been linked to the occurrence of cold creep during high-load dwell times superimposed onto low cycle fatigue loading. In order to shed new light on the deformation mechanisms during cold dwell and to understand better the role of the microstructure, two different bimodal microstructures (fine and coarse transformation product) of TIMETAL®834 were investigated at stress levels below the 0.2% proof stress using a combination of grain orientation mapping and in-situ electron microscopy imaging. This enabled in-depth analysis of 2D slip patterns and slip system activity using High-Resolution Digital Image Correlation (HRDIC), showing that in both microstructures basal slip is initially the dominant slip mode before prismatic slip activity increases approaching the 0.2% proof stress. Comparing the two constituents in the bimodal microstructure, first slip bands are localised predominantly in primary α grains, indicating higher strength of secondary α colonies, particularly for finer transformation products. During 10 min load holds at stresses below 0.2% proof stress, more plastic strain and longer connected slip traces across several grains were observed in the sample with coarse transformation product, indicating higher susceptibility to cold creep deformation.Full-field crystal deformation modelling was used to determine local stresses in individual grains at the onset of plasticity and test the hypothesis that the dominance of basal slip at low-stress levels can be explained by the elastic anisotropy in Ti alloys. While consideration of elastic anisotropy increased resolved shear stress (RSS) values for basal slip relative to prismatic slip, it did not unambiguously explain the early activation of basal slip. Furthermore, thermal residual stresses at the crystal level, due to the anisotropy of coefficients of thermal expansion (CTE), were included in the simulation, which created a wider spread of the RSS data but did not preferentially promote high RSS values for grains well aligned for basal slip. In the absence of an unambiguous conclusion, it is hypothesised that basal slip might display lower critical resolved shear stress values than typically reported but high work hardening rates compared to prismatic slip.

Optimization of the Post-Process Heat Treatment Strategy for a Near-α Titanium Base Alloy Produced by Laser Powder Bed Fusion.

During the last decades, titanium alloys have been of great interest for lightweight applications due to their high strength in combination with a low material density. Current research activities focus on the investigation of near-α titanium alloys produced by laser powder bed fusion (LPBF). These alloys are known for their superior tensile strength and high creep resistance. This study focuses on the optimization of post-process heat treatments and the impact on tensile and creep strength of a LPBF produced Ti6242S alloy. Therefore, a variety of annealing steps were conducted to gain knowledge about the decomposition process of the non-equilibrium as-built microstructure and the arising influence on the mechanical properties. Components made of Ti6242S and produced by LPBF reveal an extraordinarily high ultimate tensile strength of about 1530 MPa at room temperature, but show a low elongation at fracture (A5 = 4.3%). Based on microstructure-property relationships, this study recommends precise heat treatments on how to improve the desired mechanical properties in terms of strength, ductility as well as creep resistance. Moreover, this study shows a triplex heat treatment, which enhances the elongation at fracture (A5) to 16.5%, while the ultimate tensile strength is still at 1100 MPa.

Wire-fed electron beam directed energy deposition of Ti–6Al–2Zr–1Mo–1V alloy and the effect of annealing on the microstructure, texture, and anisotropy of tensile properties

Wire-fed electron beam directed energy deposition (EB-DED) is gaining increasing attention owing to its significant advantage of producing high-quality, large-scale metallic components. In this study, EB-DED of a near-α titanium alloy (Ti–6Al–2Zr–1Mo–1V) was conducted, and the effect of post-deposition single annealing treatment on the microstructure, texture, and anisotropy of the tensile properties was investigated. Optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy were employed to study the microstructure and texture characteristics. The tensile properties along the vertical (Z axis) and horizontal (Y axis) directions were evaluated at both room temperature and 500 °C. The results indicated that the prior β grains had a strong < 001 >β texture along the grain growth direction. Although the α variant selection occurred in some micro-regions, the overall α phase texture was weak. In terms of tensile properties, the vertical specimens exhibited lower strength but higher ductility than the horizontal specimens at both room and elevated temperatures. The anisotropic elongation results from the directional columnar prior β grains and continuous grain boundary α phase, which facilitated the intergranular cracking. The tensile strength and ductility were simultaneously enhanced by annealing at 950 °C for 2 h to meet the standard requirements for wrought counterparts. Moreover, the anisotropy of the tensile properties was decreased significantly. The enhanced and isotropic mechanical properties can be attributed to the combined effect of the bi-lamellar microstructure, discontinuous grain boundary α phase, and the weak α texture.

Effect of cyclic heat treatment on microstructure and mechanical properties of laser aided additive manufacturing Ti–6Al–2Sn–4Zr–2Mo alloy

Globular α phases can significantly improve the ductility of titanium alloys. Cyclic heat treatment (CHT) has been proved to be an effective way to induce the formation of globular α phases in α+β titanium alloy Ti–6Al–4V fabricated by laser aided additive manufacturing (LAAM). However, there is no prior research reporting methods for obtaining globular α phases in LAAM-built near-α titanium alloys. This work investigated the cyclic heat treatment (CHT) procedures suitable for the LAAM-built near-α titanium alloy Ti–6Al–2Sn–4Zr–2Mo (Ti6242) to attain the globular α phases. The results show that 980 ​°C is the most suitable upper temperature limit for CHT. However, it is difficult to achieve a high volume fraction of the globular α phases in the LAAM-built Ti6242 alloys through CHT, which is ascribed to the low composition gradient caused by more α-stabilizing elements and fewer β-stabilizing elements. The as-built sample demonstrated elongation of 6.3%, which is lower than the AMS 4919J standard (elongation ≥10%). After 980 ​°C CHT and 980 ​°C CHT with solution heat-treatment, the formation of the globular α phases significantly increased the elongation to 13.5% and 12.9%, respectively. Although the mechanical strength is reduced after heat-treatment, the room-temperature tensile properties still exceed the AMS 4919J standard. Fractography examination showed that the as-built sample exhibited a mixed brittle and ductile fracture behavior, while the 980 ​°C CHT and 980 ​°C CHT with solution heat-treated samples displayed ductile fracture.

Analysing the Interaction between Microscopic Deformation, Microstructure and Void Evolution of Near-α Titanium Alloys during Non-Superplastic Hot Deformation by an Integrated Crystal Plasticity Finite Element Model.

High-efficiency and low-cost hot forming technologies for titanium alloys have been developed for producing complex-shaped, thin-walled tubular components under non-superplastic forming conditions. Under these forming conditions, there exist complex and highly integrated material evolution processes including microscopic heterogeneous deformation, microstructure evolution and damage behaviour. This paper presents an integrated crystal plasticity finite element model of near-α titanium alloys during non-superplastic hot deformation conditions considering grain boundary sliding (GBS), dynamic recrystallisation (DRX), as well as void evolution. The polycrystalline model of a near-α TA15 titanium alloy was established, containing α phase, β phase and grain boundary (GB) regions, in which the GB region was a visualised representation of GBS. The quantitative strength ratio between the GB regions and α phase was calculated according to the Zener–Holloman parameter Z and grain size, which determined the microscopic deformation behaviour. There were found to be two high microscopic strain regions in the α phase: intragranular deformation bands through the most favourable slipping and near the GBs through multiple slipping, which promoted continuous and discontinuous DRX, respectively. With the decrease in parameter Z or grain size, the activated dislocations accommodating GBS were found to no longer pile up inside the grain, but instead travel across the grain interior. Finally, methods to improve the macroscopic plastic formability were proposed for the difficult-to-form titanium alloys experiencing non-superplastic hot deformation.

Correlation between Laser-Ultrasound and Microstructural Properties of Laser Melting Deposited Ti6Al4V/B4C Composites

The presence of large microtextured clusters (MTC) composed of small α-phase crystallites with preferred crystallographic orientations in 3D printed near-α titanium alloys leads to poor mechanical and fatigue properties. It is therefore crucial to characterize the size of MTCs nondestructively. Ti6Al4V/B4C composite materials are manufactured using Laser Melting Deposition (LMD) technology by adding an amount of nano-sized B4C particles to the original Ti6Al4V powder. TiB and TiC reinforcements precipitating at grain boundaries stimulate the elongated α crystallites and coarse columnar MTCs to equiaxed transition, and microstructures composed of approximately equiaxed MTCs with different mean sizes of 11–50 μm are obtained. Theoretical models for scattering-induced attenuation and centroid frequency downshift of ultrasonic waves propagating in such a polycrystalline medium are presented. It is indicated that, the studied composite material has an extremely narrow crystallographic orientation distribution width, i.e., a strong degree of anisotropy in MTCs. Therefore, MTCs make a dominant contribution to the total scattering-induced attenuation and spectral centroid frequency downshift, while the contribution of fine α-phase crystallites is insignificant. Laser ultrasonic inspection is performed, and the correlation between laser-generated ultrasonic wave properties and microstructural properties of the Ti6Al4V/B4C composites is analyzed. Results have shown that the deviation between the experimentally measured ultrasonic velocity and the theoretical result determined by the Voigt-averaged velocity in each crystallite is no more than 2.23%, which is in good agreement with the degree of macroscopically anisotropy in the composite specimens. The ultrasonic velocity seems to be insensitive to the size of MTCs, while the spectral centroid frequency downshift is approximately linear to the mean size of MTCs with a goodness-of-fit (R2) up to 0.99. Actually, for a macroscopically untextured near-α titanium alloy with a relatively narrow crystallographic orientation distribution, the ultrasonic velocity is not correlated with the properties of MTCs, by contrast, the central frequency downshift is dominated by the size and morphology of MTCs, showing great potentials in grain size evaluation.

Dynamic strain aging in the intermediate temperature regime of near-[formula omitted] titanium alloy, IMI 834: Experimental and modeling

A dynamic strain aging (DSA) regime is established for a near-α titanium alloy with a microstructure consisting of 80% equiaxed α, 15% lamellar α and 5% β with silicon in solid solution through a series of constant strain rate tests carried out in tension from 5 × 10−2 to 5 × 10−6 s−1 in the temperature range of 623–823 K. Transmission electron microscopy shows jogged screw dislocations within slip bands dominating the dislocation structure in this domain. Strain accumulation occurs by conservative jog glide along the length of screw dislocations due to line tension forces. We estimate the solute concentration accumulating at these edge jogs on arrest during their thermally activated glide over static solute obstacles. The domain of DSA is then predicted using Friedel's model to obtain the stress required to break the jogs free from the solute atmosphere. There is a good agreement between model and experimental data showing a DSA peak in the temperature range of 673–723 K. The solute species responsible for DSA from the model are C and Si, but dominated by C in this temperature and strain rate regime. Atom probe tomography demonstrates C and Si segregation at the dislocations, strongly supporting the model.

Effect of nano-Y2O3 addition on the creep behavior of an as-cast near-α titanium alloy

In this paper, the influent mechanism of nano-Y2O3 addition on the creep properties and fracture behavior of an as-cast near-α titanium alloy at 650 °C/150 MPa and 700 °C/150 MPa was discussed. The results showed that the steady state creep rate decreased and creep life increased by Y2O3 addition. The creep rupture life of Y2O3 addition alloy increased to three times at 700 °C/150 MPa. Due to the Y2O3 addition, on one hand, the refinement of β grains and α/β colonies can make effective slip distance of dislocations reduced and make the applied creep stress undertook by more grains, which makes the stress concentration of α/β boundaries reduced. On the other hand, the Y2O3 in grain interior played an important role in dislocation strengthening and load transfer strengthening effect, which demonstrates the positive effect of Y2O3 on the creep resistance. The fracture of Y2O3 particles, interface debonding and grain boundaries deformation all can make voids nucleated and cracks formed, therefore, the propagation and connection of voids and cracks caused the fracture of Y2O3/α-Ti alloy. The β phase dissolution and silicide precipitation indicated microstructure instability phenomenon presented within two alloys during creep exposure.

Microstructural characterization and mechanical properties of TIG weld joint made by forged Ti–4Al–2V alloy

The weld of titanium alloy promotes the complex components to be widely utilized in various fields. It is necessary to investigate the microstructure and mechanical properties to ensure the reliability of weld joint. In the present study, the microstructure, texture and mechanical properties of a joint made of the near-α titanium alloy Ti–4Al–2V, prepared by multi-layer and multi-pass TIG welding process, were investigated. The results show that the heat affected zone (HAZ) and fusion zone (FZ) are composed of α′ martensite, acicular α and lath like widmanstätten α + β, which is complex than that of the base metal (BM). Meanwhile, the texture in HAZ and FZ, including {0001} <11–20> plate texture, {11–20} <10-10> plate texture, and {11–20} <0001> plate texture, are also complicated than that of BM ({0001} <11–20> plate texture), and the texture intensity is also higher. Besides, the mechanical properties of the FZ are inferior, due to the microstructure and mechanical properties evolution. Specifically, the tensile strength of the BM, upper part and bottom part of FZ are 695 MPa, 644 MPa, and 679 MPa and the impact energy of the BM and FZ are calculated as 201.7 J/cm2 and 70.7 J/cm2, respectively. The hardness of the FZ (200–220 HV1) is lower than that of the BM (230-250 HV1). This study first comes to attention to systematically characterize and analyze the weldability, as well as the microstructure evolution and mechanical properties of the weld joint, of Ti–4Al–2V alloy, which will lay a solid foundation for the engineering application of this alloy in the future.

Superplastic deformation mechanical behavior and constitutive modelling of a near-α titanium alloy TNW700 sheet

TNW700 is a newly developed near-α high temperature titanium alloy and has great application potential in the aerospace industry owing to excellent creep strength and short-term service temperature up to 700 °C. The superplastic tensile deformation behavior and constitutive model of TNW700 alloy in the temperature range of 900–975 °C and strain rate range of 0.0005–0.01s−1 were investigated. Results indicated that TNW700 alloy exhibits significant work hardening behavior, which may be related to dynamic growth of β grains, dislocation evolution, and silicide precipitates. In addition, work hardening coefficient (n) as well as the critical strain between flow hardening and softening increases with decreasing strain rate, and first increases and then decreases with increasing deformation temperature. TNW700 alloy exhibits excellent superplasticity at 900 °C–950 °C corresponding to strain rate sensitivity exponent (m) greater than 0.3, and expresses poor superplasticity at 975 °C with lower m value. The m value decreases monotonously with strain owing to decrease of grain boundary sliding contribution caused by dynamic grain growth of β grains. The deformation activation energy increases with decreasing strain rate due to the change of deformation mechanism. A phenomenological constitutive model considering strain hardening, strain rate hardening and temperature softening was proposed and material constants were calibrated by true stress-strain data. The constitutive model was implemented into Abaqus code by UHARD subroutine to simulate the superplastic forming of cone parts, and thickness distribution and bulging heights under different forming time were used to verify the validity of constitutive model.

Hot Deformation Behavior and Hot Rolling Properties of a Nano-Y2O3 Addition Near-α Titanium Alloy

The hot deformation behavior and hot rolling based on the hot processing map of a nano-Y2O3 addition near-α titanium alloy were investigated. The isothermal compression tests were conducted at various deformation temperatures (950⁠–1070 °C) and strain rates (0.001–1 s−1), up to a true strain of 1.2. The flow stress was strongly dependent on deformation temperature and strain rate, decreasing with increased temperature and decreased strain rate. The average activation energy was 657.8 kJ/mol and 405.9 kJ/mol in (α + β) and β region, respectively. The high activation energy and peak stress were contributed to the Y2O3 particles and refractory elements comparing with other alloys and composites. The deformation mechanisms in the (α + β) region were dynamic recovery and spheroidization of α phase, while the β phase field was mainly controlled by the dynamic recrystallization and dynamic recovery of β grains. Moreover, the constitutive equation based on Norton–Hoff equation and hot processing map were also obtained. Through the optimal processing window determined by the hot processing map at true strains of 0.2, 0.4 and 0.6, the alloy sheet with multi-pass hot rolling (1050 °C/0.03–1 s−1) was received directly from the as-cast alloy. The ultimate tensile strength and yield strength of the alloy sheet were 1168 MPa and 1091 MPa at room temperature, and 642 MPa and 535 MPa at 650 °C, respectively, which performs some advantages in current research.