Hongzhen Guo

Microstructure and mechanical properties of TC21 titanium alloy after heat treatment

Microstructure evolutions during different heat treatments and influence of microstructure on mechanical properties of TC21 titanium alloy were investigated. The results indicate that the excellent mechanical properties can be obtained by adopting air cooling after forging followed by heat treatment of (900 °C, 1 h, AC)+(590 °C, 4 h, AC). Deformation in single β field produces pan-like prior β grains, while annealing in single β field produces equiaxed prior β grains. Cooling rate after forging or annealing in single β field and the subsequent annealing on the top of α+β field determine the content and morphology of coarse α plates. During aging or the third annealing, fine secondary α plates precipitate. Both ultimate strength and yield strength decrease with the content increase of coarse α plates. Decreasing effective slip length and high crack propagation resistance increase the plasticity. The crisscross coarse α plates with large thickness are helpful to enhance the fracture toughness.

Hot deformation behavior of TC11/Ti–22Al–25Nb dual-alloy in isothermal compression

Abstract The high-temperature flow behavior of TC11/Ti–22Al–25Nb electron beam (EB) weldments was investigated by the isothermal compression tests at the temperature of 900–1060 °C and the strain rate of 0.001–10 s −1 . Based on the experimental data, the constitutive equation that describes the flow stress as a function of strain rate and deformation temperature is obtained. The apparent activation energy of deformation is calculated, which decreases with increasing the strain and the value is 334 kJ/mol at strain of 0.90. The efficiency of power dissipation η changes obviously with the variation of deformation conditions. Under the strain rates of 0.01, 0.1 and 1 s −1 , the value of η increases with increasing the true strain for different deformation temperatures. While the value of η decreases with increasing the strain under the strain rates of 0.001 and 10 s −1 . The optimum processing condition is ( t opi =1060 °C, opi =0.1 s −1 ) with the peak efficiency of 0.51. Under this deformation, dynamic recrystallization (DRX) is observed obviously in the microstructure of welding zone. Under the condition of 1060 °C and 0.001 s −1 , the deformation mechanism is dominated by dynamic recovery (DRV) and the value of η decreases sharply ( η =0.02). The flow instability is predicted to occur since the instability parameter ξ () becomes negative. The hot working process can be carried out safely in the domain with the strain rate of 0.001–0.6 s −1 and the temperature of 900–1060 °C.

Microstructure characterization and mechanical properties of TC4-DT titanium alloy after thermomechanical treatment

Influence of thermomechanical treatments (mill annealing, duplex annealing, solution treatment plus aging and triple annealing) on microstructures and mechanical properties of TC4-DT titanium alloy was investigated. Results showed that thermomechanical treatments had a significant influence on the microstructure parameters and higher annealing and aging temperature and lower cooling rate led to the decrease of the volume fraction of primary α and the size of prior-β and the increase of the width of grain boundary α and secondary α. The highest strength was obtained by solution treatment and aging due to a large amount of transformed β and finer grain boundary α and secondary α at the expense of slight decrease of elongation and the ultimate strength, yield strength, elongation, reduction of area were 1100 MPa, 1030 MPa, 13% and 53% separately. A good combination of strength and ductility has been obtained by duplex annealing with the above values 940 MPa, 887.5 MPa, 15% and 51% respectively. Analysis between microstructure parameters and tensile properties showed that with the volume fraction of transformed β phase and the prior-β grain size increasing, the ultimate strength, yield strength and reduction of area increased, but the elongation decreased. While the width of grain boundary α and secondary α showed a contrary effect on the tensile properties. Elimination of grain boundary α as well as small prior-β grain size can also improve ductility.

Microstructure and mechanical properties of TC21 titanium alloy by near-isothermal forging

Microstructure and tensile properties of TC21 titanium alloy after near-isothermal forging with different parameters plus solution treatment and aging were investigated. It is found that the residual β matrix, which was strengthened by fine secondary α platelets forming during aging, exists in all the samples; while primary equiaxed α phase, bent lamellar α phase and α plates are simultaneously or individually present in one sample. The strength of alloy increases proportionally with increasing the content of residual β matrix, which is the result of increasing α/β interphase boundary. The plasticity of alloy has a downward trend as the content of residual β matrix increases. This attributes to the increase of fine secondary α platelets, which are cut by dislocations during the deformation. Additionally, coarse α plates with long axis parallel to the maximum resolved shear stress (MRSS) also reduce the plasticity of TC21 alloy.

Effect of hot working on microstructure and mechanical properties of TC11/Ti2AlNb dual-alloy joint welded by electron beam welding process

Abstract The influence of hot working on the microstructures of TC11/Ti2AlNb dual-alloy joints welded by electron beam welding (EBW) process was investigated. The tensile tests were performed at room temperature for specimens before and after thermal exposure. The results show that the fusion zone of TC11/Ti2AlNb dual-alloy joint welded by EBW is mainly composed of β phase. After deformation and heat treatment, the grain boundaries of the as-cast alloy are broken and the fusion zone mainly consists of β, α2 and α phases. The fusion zone performs poor property in the tensile test. Specimens before and after thermal exposure all fail in this area under different deformation conditions. The ultimate tensile strength of specimens after heat treatment is up to 1190 MPa at room temperature. The joints by water quenching after deformation have better plasticity with an elongation up to 4.4%. After thermal exposure at 500 °C for 100 h, the tensile strength of the specimen slightly rises while the ductility changes a little. SEM observation shows that the fracture mechanism is predominantly transgranular under different deformation conditions.

Dependence of α-phase size on flow stress during dynamic recrystallization steady state in Ti60 alloy

The dependence of α-phase size on flow stress was characterized by a proposed kinetic model during dynamic recrystallization (DRX) steady state in Ti60 alloy. According to the isothermal compression tests, the influence of deformation parameters on the steady-state flow stress was analyzed and the constitutive equation was established to predict the steady-state flow stress under different deformation temperatures and strain rates. A power-law relationship between the DRX average grain size and steady-state flow stress with an exponent of −2 is obtained from the dynamic balance during DRX steady state. The effect of deformation parameters on α-phase size was observed through the microstructure after deformation, and the applicability of the model for Ti60 alloy was verified by the comparison between predicted and experimental data.

Isothermal Beta Grain Growth Kinetics of TC4-DT Titanium Alloy under Two Different Prior Processing Conditions: Deformed vs. Undeformed

Abstract β grain growth kinetics of TC4-DT titanium alloy under two different prior processing conditions (deformed vs. undeformed) was investigated during isothermal heat treatments. Deformation was carried out in α+β two-phase field with deformation degree of 60% and air cooling. Heat treatment scheme with the temperatures above β transus temperature 10, 20, 30 °C and soaking time of 2, 5, 10, 30, 60, 120 min was used. The grain size parameters have been obtained by means of the image analysis software. Growth time exponent (n) and activation energy (Q) have been established for both conditions. For the undeformed samples, n varied in the range of 0.34∼0.35 and Q in the range of 86.8∼130 kJ·mol−1, while after deformation, the values of n and Q changed in the domain of 0.36∼0.39 and 76.6∼110 kJ·mol−1, respectively. Results show that the samples subjected to deformation exhibit higher growth time exponent and lower activation energy compared to the samples without deformation under the same heat treatment conditions. The variations of the growth time exponent with temperature and the activation energy with soaking time are ascribed to the interaction between diffusion of solute atoms and migration of grain boundaries both of which can be promoted by deformation. The influences of heating temperature, time and deformation on the uniformity of grain size were also studied and the intrinsic mechanism is attributed to the combined effect of β phase nucleation rate and β phase growth velocity under the above factors.

Flow Behavior and Dynamic Recrystallization of BT25y Titanium Alloy During Hot Deformation

Abstract The high-temperature plastic deformation and dynamic recrystallization behavior of BT25y alloy were investigated within the deformation temperatures of 1,213–1,293 K and strain rates of 0.001–1.0 s–1 on a Gleeble-1500 thermo-mechanical simulator. Results showed that the dynamic recrystallization (DRX) mechanism played an important role in the hot deformation of BT25y alloy. Based on the regression analysis of the true stress–strain data, the stress exponent and deformation activation energy of BT25y alloy were calculated to be 3.4912 and 288.0435 kJ/mol, respectively. The θ-σ and dθ/dσ–σ curves were plotted to further obtain the critical stress and critical strain for the occurrence of DRX. Based on the analysis results, the DRX kinetic model was established. The model was validated by the comparison between predicted and experimental volume fraction of DRX. As the DRX evolution was sensitive to deformation temperature and strain rate, quantities of dynamically recrystallized grains appeared at higher temperatures and lower strain rates.