Abstract:To investigate the effect of solution treatment and aging process parameters on the microstructure and mechanical properties of TB18 titanium alloy, process optimization research was conducted based on the mixed-level orthogonal experiment design of factor levels. Results show that through range analysis, the significance order of process parameters is determined as follows: solution cooling method>solution temperature>aging time>aging temperature>solution time. Considering the strength-ductility matching and engineering application requirements, the benchmark parameters are selected as solution time of 1 h, solution cooling method of air cooling (AC), aging temperature of 525 °C, and aging time of 4 h. Furthermore, the effects of solution temperature in the range of 790–870 °C on the impact toughness and micro-fracture characteristics of the alloy were studied. The results reveal that the larger the area of shear lip and fibrous zone, and the smaller the area of radiation zone, the better the toughness of the alloy. With the increase in solution temperature, the length of secondary cracks on the fracture surface increases, the number of dimples increases, and the toughness is enhanced. Based on the collaborative optimization of strength and toughness, the optimal heat treatment process for TB18 alloy is determined as 870 °C/1 h, AC+525 °C/4 h, AC.

Abstract:The combustion behavior of Ti-Al-Mo-Zr-Sn-W alloy (TC25G) was studied in a high-temperature and high-speed air flow environment using the laser ignition method combined with ultra-high temperature infrared thermometer, scanning electron microscope, X-ray diffractometer, and transmission electron microscope. The burn-resistant performance of TC25G and TC11 alloys was compared. Meanwhile, the microstructural characteristics, crystal structure, and formation mechanism of the combustion products of TC25G alloy were analyzed in detail. The results show that the high-temperature and high-speed air flow promotes combustion within the air flow temperature range of 200–400 °C and the air flow velocity range of 0–100 m/s. The combustion path advances along the direction of the air flow. The combustion of TC25G alloy mainly relies on the diffusion of the oxygen and the expansion of the combustion area caused by the movement of the melt. Based on the microstructure and composition of combustion product, it can be divided into the combustion zone, the melting zone, and the heat affected zone. During combustion, the formation of microstructures is closely correlated with the behavior of alloying elements and their selective combination with O. The major oxidation products of Ti are TiO and TiO₂. The oxides formed by Mo and W hinder the movement of the melt during the combustion. Al and Zr tend to undergo internal oxidation. Al₂O₃ precipitates on the surface of ZrO₂, forming a protective oxide layer that inhibits the inward diffusion of O. Moreover, the element enrichment at the interface between the melting zone and the heat affected zone increases the melting point on the solid side, hindering the migration of the solid-liquid interface.

Abstract:The effects of channel segregation on the macro- and micro-scale chemical composition, microstructure, hardness, and tensile deformation behavior of Ti45Nb wires were investigated. The results show that wires with severe channel segregation exhibit a macroscopic chemical composition identical to those without segregation, and 3D X-ray imaging result also reveals no abnormalities. After annealing, both types of wires exhibit an equiaxed single-phase microstructure with comparable grain sizes, suggesting that channel segregation has negligible influence on the macroscopic composition and grain size. Metallographic examination reveals that channel segregation manifests as spot-like features in the transverse section and band-like structures in the longitudinal section. EDS analysis identifies these regions as Ti-enriched segregations, with a Ti content higher than that of the surrounding matrix by approximately 4.42wt%. Compared to segregation-free wires, those containing extensive channel segregation demonstrate a 15.5% increase in ultimate tensile strength and a 12.3% increase in yield strength, but suffer a reduction in elongation and reduction of area by 19.8% and 18.9%, respectively. Furthermore, the mechanical properties of wires with segregation show significant fluctuations. Fractographic analysis reveals a larger fracture surface area in segregated wires. Severe dislocation pile-ups occur at the interfaces of these segregated regions, initiating microcrack nucleation. This promotes rapid crack propagation of the Ti45Nb wire, leading to a significant decrease in plasticity and reduction of area.

Abstract:To accurately predict the thermoforming process of TC4 titanium alloy, the high-temperature rheological behavior of TC4 titanium alloy was investigated, and a high-precision thermoforming phenomenological constitutive model was developed. Firstly, high-temperature tensile tests of TC4 titanium alloy were conducted at 973-1123 K with strain rates of 0.01-1 s^-1. Based on the experimental data, two constitutive models were established: an Arrhenius constitutive model with strain compensation and a modified Johnson-Cook constitutive model. Sparrow search algorithm (SSA) was employed to optimize the model parameters. Finally, the predictive abilities of the phenomenological constitutive models for TC4 titanium alloy were assessed using statistical analysis. The results indicate that the Arrhenius constitutive model achieves relatively high predictive accuracy despite limited experimental data. However, it has a restricted parameter optimization space. In contrast, the modified Johnson-Cook constitutive with lower predictive accuracy, offers a larger parameter optimization space. The SSA-optimized modified Johnson-Cook constitutive model provides a good fit with experimental results, serving as a solid foundation for high-precision numerical simulations of TC4 titanium alloy thermoforming.

Abstract:A systematical analysis of the macrostructure, microstructure, composition, and crystal orientation of the bright-band defect was conducted by OM, SEM, and EBSD methods, as well as Gleeble tests, and the formation mechanism of bright-band defect of forged TC18 alloy was studied. The results show that the bright-band defects in the center of TC18 alloy forging stocks correspond to β cube-grains with the size of around 100 mm. During the forging process, an inhomogeneous distribution of temperature and equivalent strain in the forging stocks is caused by adiabatic heating, which is an important reason for the microstructural heterogeneity. The large β cube-grains are formed due to the repeated compression along the orthogonal direction, which results in continuous strengthening of the <100> texture in the center of the forging stocks, and the merging of <100> grains with similar orientations. Through annealing treatment and compression along diagonal direction, it is possible to effectively reduce and avoid bright-band defects in TC18 alloy.

Abstract:The hot deformation response and dynamic recrystallization behavior of two representative initial microstructures (a fully lamellar microstructure and an equiaxed-lamellar bi-modal microstructure) were systematically investigated in a wide-width hot-rolled bloom TC4 alloy using a Gleeble thermal simulation testing system at deformation temperature of 1173 K and strain rates of 10 and 0.01 s?1. Meanwhile, a coupled phase-field and crystal plasticity model was developed to simulate the stress-strain distribution and dislocation density evolution in the α/β phases under different initial microstructural conditions. This model was used to examine how initial microstructure configurations influence the dynamic recrystallization behavior of the α phase. The results indicate that under a high strain rate of 10 s?1 and the deformation of 60%, the fully lamellar microstructure undergoes significant dynamic recrystallization in the α phase, resulting in a uniform fine-grained structure with an average grain size of 0.58 μm. In contrast, in the bi-modal structure, only part of the lamellar α phase exhibits localized recrystallization, while the equiaxed α phase primarily undergoes dynamic recovery. Compared with the fully lamellar structure, the bi-modal microstructure requires greater deformation to activate dynamic recrystallization in both the equiaxed and lamellar α phases. This discontinuous recrystallization behavior is attributed to differences in stress-strain distribution between the equiaxed and lamellar α phases during concurrent deformation. These differences influence dislocation accumulation and subgrain formation, ultimately altering the driving force conditions for dynamic recrystallization.

Abstract:The diffusion behavior of hydrogen in lamellar and bi-modal TC4 alloys was investigated through electrochemical hydrogenation combined with multi-scale characterization techniques. The results show that after electrochemical hydrogen charging, the diffusion surfaces of lamellar and bi-modal samples present a gradient distribution of hydrogen concentration, and the thickness of the hydrogen diffusion layer of two samples is similar. The volume fraction of hydride in the diffusion surface of the lamellar sample is larger, hydrides preferentially form at the α/β interface and grow in the form of twin pairs into the α phase. In the case of the bi-modal sample, due to the relatively large equiaxed α grain size, hydrides cannot fill the entire α grain. Different hydride variants alternate in nucleation and growth near the α/β interface. TEM analysis results indicate that the hydrogenation nucleation in both microstructure samples presents a multi-level structural transformation mechanism regulated by stacking faults.

Abstract:Manganese, serving as a cost-effective and potent stabilizer of the β-phase, plays a pivotal role in the development of economically viable and easily deformable β-type γ-TiAl alloys. In this investigation, we focused on a low-cost and easily deformable Ti-44Al-3Mn-0.4Mo-0.4W-0.1B-0.1C alloy (at%), which was rolled into 12 mm-diameter bars by vacuum induction melting and conventional hot rolling techniques. The effects of high-temperature treatments at 1270, 1220, and 1170 °C on the microstructure and mechanical properties of the alloy bars were studied by EPMA, TEM, and EBSD. The results show that the microstructure of the alloy contains γ, α₂, and β_o phases after heat treatment. Decreasing the temperature of high-temperature treatment under identical aging conditions significantly reduces the α₂/γ lamellar content within the alloy. Moreover, both the size of the lamellar colonies and the spacing between lamellae exhibit pronounced reductions as the treatment temperature decreases. The tensile performance tests demonstrate that as the temperature of high-temperature treatment decreases, the tensile strength at room temperature and 800 °C of the alloys with different microstructures declines. At room temperature, the elongation of the heat-treated alloys shows a trend of first increasing and then decreasing, and the values are all within the range of 0.5%–1.0%. However, at 800 °C, significant variations in elongation are observed in the alloys. Specifically, an increase in equiaxed γ phase content correlates with enhanced alloy elongation. Compared to samples treated at 1270 °C, those treated at 1220 °C exhibit a 280% increase in elongation, while those treated at 1170 °C show a 480% increase. This enhancement is attributed to the improved deformability of the equiaxed γ phase at elevated temperatures. Additionally, greater activation of dislocations within the β_o phase occurs, while the γ/γ and α₂/γ interfaces impede the movement of twins and dislocations. This study provides a comprehensive discussion on the evolution behavior and patterns of different heat-treated alloys, emphasizing their correlation with mechanical properties.

Abstract:By applying different current frequencies during the tensile process of the aerospace TC4 titanium alloy, the flow stress of the material is increased and its maximum yield strength is reduced. The microstructural evolution of the material after electrification and the fracture morphology of the samples were observed. The influence of the electric-assisted forming process on the tensile process was analyzed in combination with the tensile test results. The experimental results show that with the increase in pulse current density, the content of the α phase decreases significantly, while the β phase content increases substantially, and the grain size begins to increase. A small amount of martensitic phase suffers transformation during cooling, resulting in fine acicular α′ phase. As the current density further increases, the primary α phase disappears completely, the β phase grows further, and the size of the transformed α′ phase increases. During tensile deformation, the sample temperature rises sharply at the moment when current is applied. It continues to increase during the tensile process, with rising increment until it reaches a peak value at the moment of fracture. The peak temperature increases with the rise in current density and pulse frequency. As the current density increases, the flow stress of TC4 titanium alloy gradually decreases, and its ductility improves. SEM and TEM results show that with the increase in current density, the dimples in the tensile fracture surface of TC4 titanium alloy sheets become significantly deeper, p