Characterization of hot deformation and microstructure evolution of a new metastable β titanium alloy

The hot deformation characteristics of as-forged Ti?3.5Al?5Mo?6V?3Cr?2Sn?0.5Fe?0.1B?0.1C alloy within a temperature range from 750 to 910 °C and a strain rate range from 0.001 to 1 s^?1 were investigated by hot compression tests. The stress?strain curves show that the flow stress decreases with the increase of temperature and the decrease of strain rate. The microstructure is sensitive to deformation parameters. The dynamic recrystallization (DRX) grains appear while the temperature reaches 790 °C at a constant strain rate of 0.001 s^?1 and strain rate is not higher than 0.1 s^?1 at a constant temperature of 910 °C. The work-hardening rate θ is calculated and it is found that DRX prefers to happen at high temperature and low strain rate. The constitutive equation and processing map were obtained. The average activation energy of the alloy is 242.78 kJ/mol and there are few unstable regions on the processing map, which indicates excellent hot workability. At the strain rate of 0.1 s^?1, the stress?strain curves show an abnormal shape where there are two stress peaks simultaneously. This can be attributed to the alternation of hardening effect, which results from the continuous dynamic recrystallization (CDRX) and the rotation of DRX grains, and dynamic softening mechanism.     

Microstructure and mechanical properties of new metastable β type titanium alloy

A new metastable β type titanium alloy called TB-13 with the combination of excellent strength and ductility was developed successfully.In order to develop a perspective on this new alloy,the influence of several commonly used heat treatments on the microstructure and properties was studied.In solution-treated and quenched samples,a low-temperature aging at 480°C results in the precipitation of finerαphase.The precipitation of coarserαphase plate at higher aging temperature(560°C)leads to the increase of tensile ductility but reduction of strength.During low-temperature aging at 300°C,quite homogeneous distribution of fine isothermalωphase particles was found.The isothermalωphase provides nucleation sites forαphase during two-step aging process and makesαphase extremely fine and disperse uniformly in β matrix.Thus,TB-13 alloy is strengthened and its mechanical properties are improved.     

Hot deformation mechanism and microstructure evolution of TC 11 titanium alloy in β field

Hot deformation behaviors of TC11 alloy with β-annealed larnellar structure and forged equiaxed structure were investigated in the β field in the temperature range of 1 090-1 030 ℃ and strain rate of 0.001-0.1 s-1. By means of isothermal compression tests. Hot deformation characteristics and microstructure evolutions of the two starting structures were analyzed. And hot processing power dissipation efficiency maps were established. EBSD technique was used for testing grain boundary characteristic of deformation structures. The results indicate that hot deformation mechanism of TC11 alloy in β field is dynamic recovery accompanied by geometric dynamic recrystallization at large strains, or discontinuous dynamic reerystallization based on the starting structure states and deformation parameters. Accordingly, there are two different grain refining patterns. One is characteristic of new fine grains in the interior of elongated prior β grains that have serrated grain boundaries; and the other is that of new fine grains along elongated prior β grain boundaries.     

Hot deformation behavior and microstructural evolution of beta C titanium alloy in β phase field

The hot deformation behavior of beta C titanium alloy in β phase field was investigated by isothermal compression tests on a Gleeble–3800 thermomechanical simulator. The constitutive equation describing the hot deformation behavior was obtained and a processing map was established at the true strain of 0.7. The microstructure was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) technique. The results show that the flow stress increases with increasing strain rates, and decreases with increasing experimental temperatures. The calculated apparent activation energy (167 kJ/mol) is close to that of self-diffusion in β titanium. The processing map and microstructure observation exhibit a dynamic recrystallization domain in the temperature range of 900–1000 °C and strain rate range of 0.1–1 s⁻¹. An instability region exists when the strain rate is higher than 1.7 s⁻¹. The microstructure of beta C titanium alloy can be optimized by proper heat treatments after the deformation in the dynamic recrystallization domain.     

Variant selection of nanocrystalline a phase in metastable β TB8 titanium alloy via electric pulse treatment

In this paper,the metastable β TB8 titanium alloy with nanocrystalline α phase is achieved by electric pulse treatment.The morphology evolution and variant selection of nanocrystalline α phase in metastable β TB8titanium alloy were investigated by using scanning electron microscope(SEM),electron backscattered diffraction(EBSD) and transmission electron microscope(TEM)analysis.The results indicated that the morphologies of the nanocrystalline α phase were mainly triangular clusters and needle-lik...     

Influence of transformation temperature on microtexture formation associated with α precipitation at β grain boundaries in a β metastable titanium alloy

The influence of transformation temperature on microtexture development associated with α precipitation at β/β grain boundaries (GB) in the near-β Ti17 alloy was studied using electron backscatter diffraction and considering isothermal treatments. For the alloy studied and the temperature range considered, decreasing the transformation temperature decreased the local microtexture strength within each prior β grain because of a larger number of α_WGB colonies (standing for α Widmanstätten GB) formed per β grain, each colony increasing by one the number of α orientations inside each prior β grain. This larger number of α_WGB colonies was a consequence of faster formation along β/β GB of their precursors, the allotriomorphic α_GB grains (standing for α-GB) at lower transformation temperatures, as evidenced by detailed examination of the first stages of α_GB formation. α_GB crystallographic orientations frequently followed a variant selection (VS) criterion based on the alignment of (0 1 1)β//(0 0 0 1)α_GB//(0 1 1)β. From a statistically relevant number of observations, VS was found to be more frequent at a lower transformation duration and a lower temperature, but the effect was not significant enough to influence the final α microtexture, considered at the scale of one prior β grain. α_GB grains that followed the VS criterion emitted two α_WGB colonies on either side of the β/β GB more frequently than those with no particular orientation.     

Room temperature deformation and mechanisms of slip transmission in oriented single-colony crystals of an α/β titanium alloy

Primary creep at low homologous temperatures and low stresses has been widely reported in α/β Ti alloys. Creep in these alloys is dependent on microstructure, with the colony microstructure showing the least creep resistance. There exists a Burgers orientation relationship between the α and the β phase, which has been assumed to allow for easy slip transmission across the α/β interfaces. Constant strain rate and creep tests were performed on single-colony crystals of a near-α alloy oriented for slip along different prismatic slip systems in the α phase. A distinct anisotropy in the deformation behavior of different colony crystals is found. The origin of this anisotropy is due to the relative misalignment of the slip systems between the α and the β phases which results in the formation of residual dislocations during slip transmission. A simple model is presented based on the accumulation of residual dislocations at both the α/β interface and the α matrix, which provides insight into the mechanism of slip transmission, strain hardening and primary creep of these colony structures.     

Hot deformation behavior and microstructure evolution of titanium alloy Ti-Al-Zr-Sn-Mo-Si-Y

Samples of Ti-Al-Zr-Sn-Mo-Si-Y alloy were compressed on the Gleeble-1500 heat stimulation machine. The compression test was carried out in the temperature range from 800 ℃ to 1 100℃ and strain rate range from 0.001 s^-1 to 10 s^-1. Stress-strain behavior and variation of microstructure of the alloy during hot compression were investigated. The experimental results show that the alloy is sensitive to temperature and strain rate, and the flow softening behavior is more obvious with the decrease of deformation temperature. At higher strain rate, discontinuous yielding is observed in β phase region. When deformed in α＋β phase region, with the increment of deformation temperature, the lamellar a structures globularization is more quick and more uniform. When deformed in β phase region, coarse β grains can be got because of high deformation temperature.     

Evolution of microstructure,phase composition,and tensile properties of severely cold deformed titanium metastable β alloy in rapid continuous heating

Influence of severe cold deformation of titanium alloy Ti–1.5%Al–6.8%Mo–4.5%Fe in metastable β condition on the evolution of phase composition, microstructure, and tensile properties during continuous rapid heating was studied. As-deformed alloy was characterized by quasi-amorphous single-phase β condition with an abnormal temperature dependence of electric resistance that was normalized after 48 h exposure at room temperature as a result of isothermal ω phase precipitation. Subsequent rapid heating with a rate of 5 °C/s caused recovery and recrystallization. Tensile properties of the alloy after different treatments were determined and discussed.     

Forging-microstructure-tensile properties correlation in a new near β high-strength titanium alloy

Microstructures and tensile properties of a new β high-strength titanium alloy Ti-5321 (Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe) were investigated in this study.Four micr...     

Hot deformation behavior and constitutive equations of titanium alloy Ti26

Samples of Ti26 （Ti-V-Sn-Cr-AI-Zr-Nb） alloy were compressed on the Gleeble-1500 heat stimulation machine. The compression test was carried out at 900-1 150 ℃ and strain rates from 0.001 s^-1 to 10 s^-1. Flow stress data at various temperatures and strain rate were obtained; and the compressive true stress vs. true strain curves were measured and studied. The deformation activation energy was calculated. The results show that the flow stress of Ti26 alloy decreases with the increase of temperature and the decrease of strain rate, and the deformation activation energy is 278.11 kJ/mol in β phase region. The flow stress curves and deformation activation energy reveal that the main softening mechanism is dynamic recovery in β phase region. Constitutive equations were formulated to describe the temperature dependence of the flow stress over a wide range of strain rates.     

Hot deformation behaviors and dynamic recrystallization mechanism of Ti-35421 alloy in β single field

Hot deformation behaviors and microstructure evolution of Ti?3Al?5Mo?4Cr?2Zr?1Fe (Ti-35421) alloy in the β single field are investigated by isothermal compression tests on a Gleeble?3500 simulator at temperatures of 820?900 °C and strain rates of 0.001?1 s^?1. The research results show that discontinuous yield phenomenon and rheological softening are affected by the strain rates and deformation temperatures. The critical conditions for dynamic recrystallization and kinetic model of Ti-35421 alloy are determined, and the Arrhenius constitutive model is constructed. The rheological behaviors of Ti-35421 alloys above β phase transformation temperature are predicted by the constitutive model accurately. The EBSD analysis proves that the deformation softening is controlled by dynamic recovery and dynamic recrystallization. In addition, continuous dynamic recrystallization is determined during hot deformation, and the calculation model for recrystallization grain sizes is established. Good linear dependency between the experimental and simulated values of recrystallized grain sizes indicates that the present model can be used for the prediction of recrystallized grain size with high accuracy.     

Influence of deformation parameters on microstructure and mechanical properties of TA 15 titanium alloy

The influence of deformation parameters including deformation temperature, degree and speed on the microstructure and mechanical properties of TA15 titanium alloy was studied. The volume fraction, size and aspect ratio of primary a phase in deformed TAI 5 titanium alloy were quantitatively characterized. With increasing deformation temperature, the volume fraction and size of primary α phase decrease. The aspect ratio of primary a phase decreases with increasing deformation temperature. The tensile strength of TA15 alloy deformed at 0.1 mm/s is higher than that at 0.3 mm/s. After therlnoforming at 960℃, 0.1 mrn/s, the tensile strength is the highest up to 1 035 MPa and the elongation is 13.5%. When the thermoforming temperature is 1 050℃, the strength and elongation are both relative low, because the microstructure is Widmanstaten structure.     

Microstructure evolution model based on deformation mechanism of titanium alloy in hot forming

The microstructure evolution in hot forming will affect the mechanical properties of the formed product. However, the microstructure is sensitive to the process variables in deformation process of metals and alloys. A microstructure evolution model of a titanium alloy in hot forming, which included dislocation density rate and primary a phase grain size, was presented according to the deformation mechanism and driving forces, in which the effect of the dislocation density rate on the grain growth was studied firstly. Applying the model to the high temperature deformation process of a TC6 alloy with deformation temperature of 1 133 - 1 223 K, strain rate of 0.01 -50 s^-1 and height reduction of 30%, 40% and 50%, the material constants in the present model were calculated by the genetic algorithm（GA） based objective optimization techniques. The calculated results of a TC6 alloy are in good agreement with the experimental ones.     

In situ neutron diffraction and polycrystal plasticity modeling of a Mg–Y–Nd–Zr alloy: Effects of precipitation on individual deformation mechanisms

In situ neutron diffraction compression tests were performed on Mg–Y–Nd–Zr alloy WE43, in the solution heat-treated, peak- and over-aged conditions. The flow curves and internal strain evolutions were modeled using polycrystal plasticity simulation, with the inclusion of an elastic phase to account for the presence of precipitates. The results reveal that prismatic plate-shaped precipitates strongly impede basal slip; the critical resolved shear strength (CRSS) of basal slip increases from 12 to 37 MPa, an increase of over 200%. However, hard deformation modes such as non-basal slip of 〈a〉 dislocations are required for macroscopic yielding. These hard modes are not as strongly affected by aging, with CRSS values which increase from 78 to 92 MPa, an increase of only 18%. The results of the study are consistent with recent modeling of the relative Orowan strengthening of individual deformation modes and the superposition of various strengthening effects (solid solution and precipitation). This finding helps to explain why the age-hardening response of Mg–Y–Nd–Zr alloys is not exceptional. It is concluded that future precipitation-strengthened alloy and process design strategies should focus on promoting high number densities of particles. The effect of aging upon twinning is surprising. The most age-hardened material exhibits more twinning than the solutionized material. To model this behavior using polycrystal plasticity, the critical stress to activate twinning (especially the strain hardening thereof) must be decreased.     

Effect of spinning deformation on microstructure evolution and mechanical property of TA 15 titanium alloy

Hot spinning of tubular workpiece of TAI5 alloy was conducted on a CNC spinning machine, and the microstructure evolution during hot spinning and annealing was observed and mechanical properties of spun tubes were tested. The results show that with the increase of spinning pass, the fiber microstructure comes into being gradually in axial direction and the circumferential microstructure also stretches obviously along circumferential direction. At the same time, the tensile strength increases and elongation decreases not only in axial direction but also in circumferential direction. When the reduction ratio of wall thickness rises close to or over 40%, tensile strength increases and elongation decreases more rapidly, which means that tubular workpiece of titanium alloy can be strengthened bi-directionally by power spinning. The ductility of spun workpiece of TAI5 alloy could be improved by annealing at the temperature no higher than recrystallization temperature with slight decrease of tensile strength.     

Nucleation and variant selection of secondary α plates in a β Ti alloy

The microtexture of secondary α plates in Ti–4.5Fe–6.8Mo–1.5Al has been investigated by electron backscatter diffraction (EBSD) to obtain more insight in the nucleation and variant selection of these α plates. A statistical analysis of the EBSD data shows that for most β grain boundaries the variant selection of the α plates is in agreement with a commonly used variant selection criterion yielding that the α-{0 0 0 1} pole is nearly parallel to the closest β-{1 1 0} poles of the two adjacent β grains. For a small angle between the β-{1 1 0} poles nucleation is predominantly observed at both sides of the grain boundary, while with increasing angle some β grain boundaries exhibit nucleation of α plates at only one side. In the β grain interior many so-called Type 2 α–α grain boundaries are observed which are thought to originate from autocatalytic nucleation when a new α plate is formed at an existing α–β interface.     

Development and application of titanium alloy casting technology in China

1 Introduction Titanium, which is named as the third metal after iron and aluminum, is an idea structure alloy for aerospace because of its unique properties such as high specific strength and high corrosion resistance [1-4].Using titanium alloy products, the weight of aerocraft can be greatly lightened and the performance of flight can be dramatically improved. In recent years titanium alloys are not only used widely in aerocraft but also in other industries in the world [6-9]. Due to its hig…     

Effects of thermohydrogen treatment on microstructures of α＋β and β titanium alloy as-cast

The effects of thermohydrogen treatment on the microstructures of TC21 and Ti40 alloys as-cast were researched. The results show that the β phase content increases after charged hydrogen. Compound TixHy appears if H content reaches a certain content, which perfectly gathers on the grain-boundaries and/or dislocations and then diffuses into the grains. The microstructure of TC21 alloy after thermohydrogen treatment becomes fine and the best H content is 0.4% （mass fraction）. However, the influence of thermohydrogen treatment on Ti40 microstructure is not obvious.