Si Titanium Alloy Research Articles

Precipitation location of secondary phase and microstructural evolution during static recrystallization of as-cast Ti-25V-15Cr-0.3Si titanium alloy

Abstract The microstructural evolution and precipitation location of the secondary phase of an as-cast Ti-25V-15Cr-0.3Si titanium alloy were investigated via isothermal compression experiments and heat treatment. The average aspect (length-to-width) ratio, average area and size of the grains at different heat treatment temperatures and holding time were analyzed and the effects of deformation and annealing time on the grain area and size were considered. It was found that the grain size was strongly influenced by the height reduction and holding time. Grain growth was significant when annealing time increased from 10 min to 2 h at 950 °C and height reduction of 30%; however, grain growth was minimal at annealing time between 2 and 4 h. Many dispersion particles were observed to form in continuous chains; the precipitation location was confirmed to be along initial grain boundaries, and the dispersion particles were identified to be Ti5Si3 phase by TEM.

Fatigue crack tip strain evolution and crack growth prediction under single overload in laser melting deposited Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy

This paper focuses on the fatigue crack tip strain evolution in laser melting deposited Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy under constant amplitude loading plus single overload (OL). The digital image correlation (DIC) technique was used to capture and evaluate the crack tip strain fields and crack closure variation; the results showed that the strains near the crack tip become larger and the crack opening load increases after the application of the OL. The size of the retardation distance is consistent with the analysis results of the crack tip strain fields. The crack closure model based on the DIC analysis results can provide good predictions of the crack growth life for both the low and high overload ratios.

Characterization of deformation in primary α phase and crack initiation and propagation of TC21 alloy using in-situ SEM experiments

In present work, the localized deformation, crack initiation and propagation behavior of Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si (TC21) titanium alloy was investigated through the in-situ scanning electron microscope (SEM) test and electron backscatter diffraction (EBSD). The results indicate that deformation in α phase is mostly accommodated by the prismatic slip. The localized deformation is concentrated at the interface of primary α lath. Some parallel slip bands with prismatic and pyramidal slip are prevalent in primary α lath, while the multiple slip bands with prismatic slip are dominant in equiaxed primary α phase. Higher ability to coordinate local plastic strain (nearly to 79.5%) is induced in equiaxed α than the primary α lath (7.9%). Most of the cracks easily initiate at the shear bands in primary α lath, which is a fatal site for the tensile deformation. The main crack propagates quickly along the shear bands or interfaces of α lath and then penetrates into the βtrans matrix and also transfers across the equiaxed α phase along the slip bands. The results indicate that the high compatibility of deformation in equiaxed primary α phase delays the initial crack nucleation. Whereas, it does not show satisfied resistance against crack propagation in front of the main crack tip due to the abundant slip bands.

Effect of deformation of constituent phases on mechanical properties of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy

Deformation of constituent phases determines the mechanical properties of titanium alloys. To address this relationship, the deformation of primary α (αp), secondary α plates (αs) and residual β phase (βr) in Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy was investigated based on the microstructure near fracture surface of uniaxial tensile samples. It is found that αp is elongated and αs mainly undergoes kinking deformation during tensile tests. The deformation of αp is dominated by planar slipping, the deformation of αs in colony is dominated by planar slipping and in basket weave is controlled by twin, planar slipping and dislocation tangling and the deformation of βr is controlled by dislocation tangling. In the microstructure with high volume fraction of αp (about 60%), the deformation amount of both αp and αs increases with the thickness of αs, which increases the elongation of the alloy and changes the failure modes of αp from fracture to boundary separation. In the microstructure with low volume fraction of αp (19%) and long αs, αp has large deformation amount, while αs with relative small deformation amount has large kinking region which increases the strength of the alloy. However, due to low volume fraction of αp, its contribution to the ductility of alloy is limited and thus the elongation of the alloy is relatively small.

Electron beam welding of laser additive manufacturing Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy thick plate

Individually fabrication parts by laser additive manufacturing (LAM) and then jointing them together through electron beam welding (EBW) is a viable way for manufacturing large components with reduction of internal stress. For investigating the microstructure and mechanical property of EBW joint along longitudinal and transverse direction in LAMed component, two LAMed Ti–6.5Al–3.5Mo–1.5Zr–0.3Si plates were successfully welded without defects. Results show that the microstructure of base metal (BM) is a typical basket-weave morphology that exhibits lamellar α within β matrix. In heat affected zone (HAZ), the part of primary α transforms to β with the some very fine lamellar αs precipitates out. Due to the fast solidification rate, a large number of acicular α′ forms in fusion zone (FZ), leading to the highest microhardness. All tensile samples fail in BM region with the fracture type of intergranular dimpled fracture. Compared with the T-joint, the L-joint shows higher ultimate tensile strength and yield strength, but lower elongation and reduction of area due to the morphology of columnar grains and the strong texture of β<010> parallel to the deposition direction.

Simulation of microstructure evolution during forging and heat treatment of Ti-6Al-3.5Mo-1.5Zr-0.3Si Titanium Alloy

The model of dynamic recrystallization of Ti-6Al-3.5Mo-1.5Zr-0.3Sihas been developed based on experimental data and implemented in QForm FEM code. Kinetics of dynamic recrystallization was simulated by Johnson-Mehl-Avrami-Kolmogorov equation. Effect of aging time (1-6 h) and temperature (450-650 °C) on mechanical properties has been experimentally studied. The model of heat treatment of Ti-6Al-3.5Mo-1.5Zr-0.3Si has been developed and implemented in QForm FEM code. The model is capable to predict phase compositionand hardness during and after arbitrary heat treatment within studied range.It was found that the highest hardness of Ti-6Al-3.5Mo-1.5Zr-0.3Si can be obtained by aging during 4-6 hours at 550 °C after solution treatment at 960 °C.

Microtexture and Nanoindentation of α and β Phases in Ti–6Al–1.5Cr–2.5Mo–0.5Fe–0.3Si Titanium Alloy

Microstructure and nanoindentation of two-phase Ti-6Al-1.5Cr-2.5Mo-0.5Fe-0.3Si titanium alloy fabricated by drawing and subsequent annealing are systematically investigated. We find a strong fiber texture for alpha phase formed during drawing, which is characterized by crystallographic direction of parallel to the drawing direction, yet a weak texture for beta phase, which is mainly consisted of and aligned with the drawing direction. The major texture components for the both phases during recrystallization are similar to those of the drawn state, but the corresponding texture intensities increase. A large number of alpha grains maintain the Burgers relationship with the beta grains for the drawn and annealed states. In the case of the annealed specimen, the mean values of hardness and elastic modulus for alpha phase are found to be 5.79 GPa and 165.22 GPa, respectively, yet are 4.73 GPa and 152.06 GPa, respectively, for the beta phase. Moreover, we also discuss the relation between elemental distribution and nanoindentation for individual phases. The hardness of textured alpha grains undergoes significant change as a function of the crystal orientation, in good agreement with the Schmid factor analysis. In contrast to the hardness, the measured elastic modulus values of alpha phase are less sensitive to the crystal orientation. No obvious change is identified for hardness and elastic modulus for beta grains in that all the beta grains are prone to generate slips.

Microstructure, Texture Evolution and Mechanical Properties of VT3-1 Titanium Alloy Processed by Multi-Pass Drawing and Subsequent Isothermal Annealing

Microstructure, texture evolution, and mechanical properties of Ti–6Al–1.5Cr–2.5Mo–0.5Fe–0.3Si (VT3-1) titanium alloy processed by multi-pass drawing and subsequent isothermal annealing were systematically investigated. A fiber-like microstructure is formed after warm drawing at 760 °C with 60% area reduction. After isothermal annealing, the samples deformed to different amounts of area reduction show a similar volume fraction (80%) of α phase, while the sample deformed to 60% exhibits a homogeneous microstructure with a larger grain size (5.8 μm). The major texture component of α phase developed during warm drawing is centered at a position of {φ1 = 10°, φ = 65°, φ2 = 0°}. The textures for annealed samples are almost along the orientation of original deformation textures and show significant increases in orientation density and volume fraction compared with their deformed states. In addition, for the drawn samples, the ultimate tensile strength increases but the ductility decreases with increasing drawing deformation. A negative slope of yield strength of annealed samples versus grain size (d−1/2) is found due to the difference between texture softening for as-rolled + annealed state and texture hardening for drawn + annealed state. The mechanical properties of annealed samples are found to be strongly dependent on grain size and texture, resulting in the balance of the strength and ductility.

Effect of Rare-earth Element Addition on Structure of Heat-resistant Ti–6.5Al–4Zr–2.5Sn–2.4V–1Nb–0.5Mo–0.2Si Titanium Alloy

To guarantee the requirements for modern technology it is necessary to create new alloys with enhanced characteristics or upgrade existing ones. Especially it concerns high-tech industries, particularly aviation and rocket engineering. When designing and creating modern aircraft engines, a special place is devoted to heat-resistant titanium alloys. High strength characteristics and relatively low density make it possible to create alloys with outstanding specific parameters. However creation of new alloying systems is quite expensive and implies parallel development of technological process as well as various processing modes. There are several ways to solve this problem; one of them is modification of already existing alloy. Due to the unique combination of physical-chemical properties of rare earth metals adding them to existing alloying systems makes it possible to create alloys with increased characteristics. In particular, microalloying of titanium alloys with gadolinium leads to transformation of their microstructure and increase of some mechanical characteristics. Thus, microalloying of the high-temperature Ti–6.5Al–4Zr–2.5Sn–2.4V–1Nb–0.5Mo–0.2Si titanium alloy in as-cast state results in a change in the intragranular structure, but does not significantly affect the macrostructure of the ingots. At the same time, in deformed state increase of gadolinium content leads to refinement of the initial β-grain, growth and formation rates of the primary and secondary phases change, which in its turn affects the mechanical characteristics of the alloying system as a whole. The Rockwell hardness measurements carried out using the BUEHLER Macromet 5100T device show an increase of parameters in the alloy with gadolinium.

Experimental Optimization of Process Parameters for Turning TC11 Titanium Alloy Using Taguchi Methodology Design

This paper presents a study of the influence of cutting conditions (cutting velocity, feed, cutting depth and lubrication) on turning TC11 (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si) titanium alloy. Taguchi methodology design was adopt for carrying out experiments. Turning process parameters such as cutting speed, feed rate and depth of cut were varied to study their effect on process responses such as cutting force (Ft), surface roughness (Ra) and temperature on cutting zones (T). Minimum quantity lubrication (MQL) technology was adopt to increase the lubricating and cooling effect. Meanwhile, CVD diamond coating was deposited on the cemented carbide insert to reduce its friction with workpiece and increase its wear resistance. From the analysis of orthogonal tests, depth of cut contributes the most for the main cutting force and cutting temperature, while feed rate had the most significant effect on surface roughness on the workpiece. MQL can reduce the cutting temperature at the cutting zones, especially for the uncoated cutting inserts whose temperature decreases by an average of 60~80°C. The cutting force, surface roughness and cutting temperature of CVD diamond coated inserts were all higher than those of uncoated tools, especially with MQL lubrication. Considering the cutting efficiency and cost, the optimal parameters in the turning process of TC11 for minimizing the cutting force, surface roughness and cutting temperature are obtained as Vc=115m/min, f=0.08mm, ap=0.5mm under MQL lubricating with uncoated cemented carbide as the cutting tool.

Flow Behavior and Constitutive Equation of Ti-6.5Al-2Sn-4Zr-4Mo-1W-0.2Si Titanium Alloy

In order to get a reliable constitutive equation for the finite element simulation, flow behavior of Ti-6.5Al-2Sn-4Zr-4Mo-1W-0.2Si alloy under high temperature was investigated by carrying a series of isothermal compression tests at temperatures of 1153-1293 K and strain rates of 0.01-10.0 s−1 on the Gleeble-1500 simulator. Results showed that the true stress-strain curves exhibited peaks at small strains, after which the flow stress decreased monotonically. Ultimately, the flow curves reached steady state at the strain of 0.6, showing a dynamic flow softening phenomenon. The effects of strain rate, temperature, and strain on the flow behavior were researched by establishing a constitutive equation. The relations among stress exponent, deformation activation energy, and strain were preliminarily discussed by using strain rate sensitivity exponent and dynamic recrystallization kinetics curve. Stress values predicted by the modified constitutive equation showed a good agreement with the experimental ones. The correlation coefficient (R) and average absolute relative error (AARE) were 98.2% and 4.88%, respectively, which confirmed that the modified constitutive equation could give an accurate estimation of the flow stress for BT25y titanium alloy.

Phase precipitation behavior and tensile property of a Ti–Al–Sn–Zr–Mo–Nb–W–Si titanium alloy

The characteristic of precipitation behavior of α2 phase and silicide, and the tensile properties at room temperature and 650 °C after heat treatments in a novel Ti–Al–Sn–Zr–Mo–Nb–W–Si titanium alloy (BTi-6431S) were investigated by microstructure analysis and mechanics performance testing. The results show that no second phase precipitates after solution treatment (980 °C/2 h, air cooling (AC)). However, when the solution-treated specimens are aged at 600 °C (600 °C/2 h, AC), α2 phase precipitates in the primary α phase, and the size of α2 phase increases with the aging temperature increasing to 750 °C. Meanwhile, 50–100-nm S2-type silicide particles precipitate along lamellar phase boundaries of transformed β phase after aging at 750 °C. BTi-6431S alloy shows the best 650 °C ultimate tensile strength (UTS) and yield strength (YS) when treated in solution treatment. However, aging treatment results in a decline in 650 °C ultimate tensile strength. This may be attributed to the loss of solution strengthening due to the depletion of Al, Si and Zr of the matrix caused by the precipitation of Ti3Al and (TiZr)6Si3. Silicide is a brittle phase; therefore, its precipitation causes a sharp decrease in the room-temperature ductility of BTi-6431S alloy.

Microstructural evolution and mechanical properties of laser melting deposited Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy

A rectangular plate of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy was fabricated by laser melting deposition (LMD) technology. Macrostructure and microstructure were characterized by optical microscope (OM) and scanning electron microscope (SEM). Room temperature tensile properties were evaluated. Results indicate that the macro-morphology is dominated by large columnar grains traversing multiple deposited layers. Two kinds of bands, named the wide bands and the narrow bands, are observed. The wide band consists of crab-like α lath and Widmanstätten α colony. The narrow band consists of α lath and transformed β. The formation mechanism of the two bands was explored. The influence of heat effect caused by subsequent deposition layers on microstructural evolution during deposition process was discussed. The room temperature tensile test demonstrates that the strength of laser deposited Ti–6.5Al–3.5Mo–1.5Zr–0.3Si is comparable to that of wrought bars.

The anisotropy of laser melting deposition additive manufacturing Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy

The anisotropy of laser melting deposition additive manufacturing Ti–6.5Al–3.5Mo–1.5Zr–0.3Si was studied by microstructure observation and room tensile properties testing. The as-deposited sample exhibits a strong tensile anisotropy due to the columnar grains morphology and the strong texture of β 〈001〉 parallel to the deposition direction. Annealing in the upper α+β region has little effect on the grain morphology and the anisotropy. However, annealing in the single β region transforms the grain morphology from columnar to equiaxed which substantially reduces the tensile anisotropy to a very low level.

Microstructure evolution and layer bands of laser melting deposition Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy

Laser melting deposition (LMD) is a newly developed additive manufacturing technique which offers a potential of time and cost savings for fabricating aerospace titanium alloys components. In this paper, the grain morphology, microstructure and micro-hardness of LMD Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy were investigated. A variety of microstructures and two kinds of distinct layer bands were found in the as-deposited sample. The interlayer bands (ILB) spaced uniformly in the whole sample. However, the dark heat affected bands (HAB) with a special bimodal microstructure were absent in the last three layers. In order to obtain a deep understanding on the microstructure evolution and bands structure, much attention was paid on the microstructures of the last deposited several layers. The microstructure gradient between two HAB was due to the re-heating effect in the α+β region. The formation of interlayer bands was due to the chemical composition homogenization difference within and between ILB, which resulted from the superheating effect.

Microstructure and mechanical properties of hybrid fabricated Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy by laser additive manufacturing

The hybrid fabricating technique by laser additive manufacturing provides an attractive potential for manufacturing titanium alloy components. Microstructure, micro-hardness and room tensile mechanical properties of hybrid fabricated TC11 titanium alloy sample were examined. Results show that the hybrid manufactured sample consists of three typical zones: the laser additive manufactured zone (LAMZ), the wrought substrate zone (WSZ), and the bonding zone without any metallurgical defects. Superfine basket-wave microstructure forms in LAMZ and heat affected zone (HAZ) due to the rapid cooling rate. No obvious grain growth or recrystallization occurs in the HAZ. A special bimodal microstructure consisting of coarse fork-like primary α and fine β transformed microstructure is found in the transition zone due to the heat effect in α+β region. The hybrid fabricated TC11 sample has good mechanical properties with tensile strength of 1033±13MPa and elongation of 6.8±0.2%. The fracture of hybrid sample occurs in the substrate in tensile testing, meaning that the bonding zone has better mechanical properties than the substrate.

Abnormal microstructure in the weld zone of linear friction welded Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy joint and its influence on joint properties

A detailed investigation on an unexpected abnormal microstructure formed near the weld line in the linear friction welded Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy joint had been performed. Microstructure observations with the help of optical microscope, electron backscatter diffraction and transmission electron microscope with an energy dispersive X-ray spectroscopy were conducted to determine the compositions and phases near the weld line. The results indicate that the abnormal microstructure may be obtained at a low friction pressure and consists of α phase in the form of spherical particles. Tensile strength and fracture characteristics were also examined to clarify the influence of α grains. It is found that the tensile strength is only about 49% of the parent material. The explanation to the formation of spherical α is that lamellar α breaks up, spheroidizes and coalesces to form bigger particles by squeezing out the softer intergranular β phase. The effect of post-weld heat treatment (PWHT) was also investigated to optimize the joint microstructure and mechanical properties. The results suggest that the defects still exist after PWHT, and consequently the appropriate process parameters should be used to achieve a good weld.

Characterization of microstructure and mechanical properties of laser melting deposited Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy

A plate of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) titanium alloy is fabricated by laser melting deposition process. Grain morphology and microstructure characteristics and the formation mechanism have been systematically studied. It is shown that the three dimensional macrostructure is a specially alternately arrayed grain morphology of columnar grains and equiaxed grains. This structure is generated by the high temperature gradient in the overlap zone and relatively low temperature gradient in the body zone of the molten pool. Combining the results of the X-ray diffraction (XRD) and the solidification process, it can be inferred that a preferential orientation with the β〈001〉along the deposition direction exists in the as deposited TC11 plate. An ultrafine basket-weave microstructure within the columnar and equiaxed grains is formed due to the rapid solidification process. The lamellar α within the columnar grains is much uniform than that of the equiaxed grains. Room tensile test data have shown that the LMDed material was characteristic of high strength and low ductility compared with the conventional forged materials.

Monte Carlo simulations of microstructure and texture evolution during annealing of a two-phase titanium alloy

Nucleation and grain growth are important phenomena during static recrystallization of metallic materials and both processes have significant influences on the material properties. The Monte Carlo (MC) method has been widely used to simulate static recrystallization behavior during annealing of metallic materials. In this study, an MC model for static recrystallization of two-phase alloys is proposed by extending an existing MC model, through the introduction of the nucleation stage to account for the grain growth by both consuming deformed grains and competing with other recrystallized grains. The two-phase MC model is used to simulate the evolution of microstructure and texture during annealing of a TC11 (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si) titanium alloy, accounting for initial grain morphology, phase compositions, crystallographic orientations, and relative values of strain stored energy determined by electron back-scattered diffraction. The results show that the model can reproduce satisfactorily the recrystallization and grain growth behavior in annealing. Compared with the β phase, the α phase depicts a lower recrystallization rate but a higher grain growth rate: the former difference can be mainly attributed to the lower strain stored energy in the α phase before annealing, whereas the latter suggests that the grain growth in the system is significantly influenced by the grain morphology, distribution of grains, and relative volume fractions of the two phases in the initial condition. Due to the influence of heterogeneous nucleation accounted for in the model, the simulated recrystallization rate deviates considerably from that described by the Johnson-Mehl-Avrami-Kolmogorov equation. The simulation also indicates that for both phases the textures strengthen with little changes in their basic features during annealing.

Effect of additional hydrogen alloying on the formation of the phase composition and structure of a Ti-8.3Al-2.1Mo-2.2Zr-0.2Si titanium alloy

The effect of additional hydrogen alloying on the formation of the phase composition and structure of a titanium alloy with a high aluminum content is studied. It is shown that an α2 + β structure can be developed under certain temperature-concentration conditions. The phase composition-hydrogen concentration-hydrogenation annealing temperature diagram has been constructed. It shows a change in the phase composition of a Ti-8.3Al-2.1Mo-2.2Zr-0.2Si alloy as a function of the hydrogenation annealing temperature and the hydrogen content after cooling to a normal temperature at a rate higher than the first critical rate.