Ti-5553 Alloy Research Articles

TEM observation of precipitates and their role to mechanical properties in Ti-5553 alloy heated to some temperatures up to 923 K

A Ti-5553 specimen was continuously heated to 923 K and simultaneously in-situ HEXRD profiles were taken. In addition, specimens heated at the same rate to several temperatures up to 923 K and further quenched were observed by transmission electron microscopy. Based on both results obtained, transformation sequence was clarified, precipitations of ω-, α”iso- and α-phases were confirmed, and size and density of these precipitates were measured. Hardness values of those specimens were also measured. The hardening mechanism was considered as shearing-mechanism for specimens aged at lower temperatures and by-pass one for specimens aged at higher temperature. An attempt of distinction between α”iso - and α-precipitates was also tried. Both precipitates were in needle-like shape and a possibility was suggested by measuring angles between two needle-shape precipitates on {110} of the matrix and comparing with each other.

Decomposition of the β phase at intermediate temperature in β-metastable Ti-5553 alloy

The present contribution focuses on the nature of the β → α transformation in Ti-5333 alloy at intermediate temperatures. It is indeed still unclear whether this transformation is only controlled by bulk diffusion or whether interfacial kinetics may play a role. To address this issue, we have combined SEM and STEM-EDX to measure the concentrations of Al, Cr, Mo, V and Fe in α intragranular precipitates as well as in the abutting β matrix, paying a particular attention to the concentrations at flat interfaces corresponding to the precipitates habit planes. The comparison with Calphad calculations suggests that interfaces are not at equilibrium during the thickening of the plates.

MICROSTRUCTURAL EVOLUTIONS AND MECHANICAL PROPERTIES DURING LONG-TERM AGEING OF TITANIUM ALLOY Ti-17

Microstructural evolutions and resulting mechanical properties have been investigated in the near-β Ti-17 alloy following long- term ageing heat-treatment up to 6000 h at 450 °C. The initial microstructure was bimodal lamellar, consisting of two populations of α grains (αlam-primary and αsecondary) in a β phase matrix. Two microstructures were obtained either via controlled heat- treatments from the β phase field - in order to generate significant differences in the grain fraction, size, density and spatial distribution - or sampled from a part submitted to an industrial processing route. High energy XRD reveals that whatever the initial microstructure, the amount of α phase increases significantly after 1000 h long-term ageing. Complementary SEM and image analysis characterizations enable to deduce that this evolution is the consequence of αsecondary growth and/or coarsening. Also, TEM observations and EDX analysis show that the Mo and Cr contents of the β phase increase and that α2 nano-precipitates form within the αlam-primary grains. Considering the mechanical properties, long-term ageing leads to an increase in the yield and ultimate tensile strength, as well as a decrease in the elongation at failure, at an extent which depends on the ageing time.

Effect of microstructure on tensile properties of Ti-17 alloys forged using a 1500-ton forging simulator

Microstructure dependence on mechanical properties were investigated for Ti-17 forged at temperatures between 700 and 850 ˚C with deformation ratio from 33 to 80 %, and solutiontreated at 800˚C for 4 hours and aged at 620 ˚C for 8 hours. The microstructure was observed after solution and aging treatments. The volume fraction and the size of the primary alpha phase was controlled by solution treatment temperature, not forging temperature and deformation ratio. Forging temperature affected the morphology of grain boundary (GB) alpha phase. Deformation ratio affected the grain size and the aspect ratio of the horizontal and vertical grain size of the prior beta phase. The tensile strength was investigated at room temperature, 450, and 600 ˚C. Forging temperature and deformation ratio did not affect the tensile strength because there is no large difference of the volume fraction of the alphaphase. On the other hand, the elongation and the reduction of area increased with increase of the aspect ratio of the prior beta grains; that means, increase of the deformation ratio. Raising of forging temperature also increased elongation and reduction of area due to the film-like GB alphaphase.

Flow behavior and microstructure evolution of PM Ti-5553 alloy during 0.1 s−1 hot deformation

The hot deformation behavior of powder metallurgy Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy was investigated by isothermal compression test at the wide temperature range of [Formula: see text]–[Formula: see text]C, under an intermediate strain rate of 0.1 s[Formula: see text], and with the sample deformation degree of 30%, 50% and 70%. Results suggested that the flow stress was very sensitive to the deformation variables and it decreased with increasing the deformation temperature. Flow localization occurred when the temperature was lower than [Formula: see text]C, accompanied by the dynamic morphology changing of [Formula: see text] phase. Dynamic recovery (DRV) features appeared when the temperature was over [Formula: see text]C, while dynamic recrystallization (DRX) took a more important role than DRV for the alloy compressed at [Formula: see text]–[Formula: see text]C.

Coupling effect of second phase and phase interface on deformation behaviours in microscale Ti-55531 pillars

The metastable-β Ti-55531 alloys with different second phases were chosen to investigate coupling effect of second phase and interface as well as the dislocation interaction details under uniaxial compression. In Ti-55531 alloy, nanoscale ω phase was acquired by solution treatment, which was coherent with the β matrix. The orientation relationship between hexagonal ω and bcc β lattice was (0001)ω//(111)β and [112‾0]ω//[1‾10]β. Sub-microscale α phase was acquired by ageing process, following the Burgers orientation relationship (BOR), {110}β//{0001}α and <11‾1>β//<112‾0>α. The characters of mechanical behaviors and deformation microstructures in this alloy have been analyzed by combining the results from uniaxial compression, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM. Ti-55531 alloy micropillars with deformable α phase show great plastic stability and high strength, which are attributed to the deformable α phase and the interaction between the dislocations and the high-density α/β interface. However, Ti-55531 alloy micropillars with nanoscale ω phase exhibit size effect and poor plastic stability. The deformation mechanism of the Ti-55531 pillars with ω phase is dislocation shearing the ω phase, which will cause the ω→β phase transformation. The ω-free channels result in the collective movement of dislocations, performed as the strain bursts in the stress-strain curves. But the dislocation slip between the α phase and the β matrix follows a harmony slip mode, which is similar to “slip relay” behaviour. The results reveal that tuning the morphology of phase interface and the density of second phase to govern the activation, multiplication and movement of dislocation is an effective method to tailor the strength and plastic deformation of materials. The deformation model between the dislocations and the phase interface has great significance to the design of the high performance materials by optimizing the microstructures.

Experimental investigation on dynamic phase transformation and texture evolution of Ti55531 high strength titanium alloy during hot compression in the α+β region

This work aims to study the flow behavior, dynamic phase transformation (DPT) and texture evolution of Ti55531 (Ti–5Al–5Mo–5V–3Cr–1Zr) alloy in the α+β region. Hot compression tests are carried out at temperatures from 730 to 820 °C and strain rates from 0.001 to 10 s−1 on the Gleeble-3800D simulator. Scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) techniques are used to analyze the morphology, volume fraction and texture evolution of each phase (primary αp and β phase). The measured stress is corrected for deformation heat, and an Arrhenius-type constitutive model is developed for the alloy in a whole strain range. The mechanisms of the DPT and texture evolution are explained by the grain boundary maps (GBMs), local orientation maps (LOMs), inverse pole figures (IPFs) and Schimid factor (SF). The softening fractions of deformation heat, DPT, and texture to the flow softening are determined according to the mixture law and Taylor theory. It is found that the maximum intensity of texture decreases with strain during initial stages, but it increases sharply with further straining. Orientation distribution function (ODF) shows that some texture components vanish and other new ones appear during deformation, but the texture intensity keeps unchangeable. The softening fraction due to the deformation is significant at strain rates higher than 1.0 s−1, it is in a range from 5.9% to 17.8%. The DPT plays a main role in the softening in strain rates lower than 0.1 s−1, its softening fraction is in a range from 14.7% to 20%.

Effect of initial microstructure on the micromechanical behavior of Ti-55531 titanium alloy investigated by in-situ high-energy X-ray diffraction

Microstructure features including the morphology and texture of constituent phases play an important role in the mechanical properties of Ti-55531 high strength titanium alloy. However, the underlying understanding from the micromechanical aspect is still lacking. In the present work, the effect of microstructure on the micromechanical behavior of Ti-55531 alloy was investigated by high-energy X-ray diffraction. For two typical microstructures of Ti-55531 alloy, the micromechanical behavior including lattice strains of oriented grains are determined. Furthermore, the stress partitioning between α and β phase and internal microstress evolution in the specimens with different microstructures are discussed. In combination with the microstructure characterization, the corresponding mechanism for microstructure influence on the mechanical properties is discussed. It is found that, for the specimens with different types of microstructures, stress partitioning between constituent phases is obviously different. The results show that the α phase stress is higher than that of β phase in the specimen with the bimodal microstructure (BM), while the β phase is subjected to much higher stress than α phase in the specimen with the lamellar microstructure (LM). For the specimen with BM, the morphology of α phase leads to a remarkable difference in the microscopic deformation behavior between αp (primary α phase) and αs (secondary α phase). It is suggested that the acicular αs precipitates bear higher stress than equiaxed αp. The intergranular and interphase microstresses in both specimens have been quantitatively evaluated. The results show that the <200>//LD fiber texture of β phase leads to higher intergranular microstress in the specimen with BM. The interphase microstress is more remarkable in the specimen with LM. Combining the in-situ HEXRD and microstructure characterization, the present study provides a fundamental understanding of the relationship between microstructure and mechanical properties from the perspective of micromechanical behavior.

Designing novel lightweight, high-strength and high-plasticity Tix(AlCrNb)100-x medium-entropy alloys

Due to the demand for reduced energy consumption by transportation vehicles, lighter weight high-entropy alloys (with density comparable to that of commercial Ti alloys around 4.5–5 g/cm3) are attracting more attention in terms of alloy design and application as structural materials. In this study, a nonequiatomic quaternary alloy system, Tix(AlCrNb)100-x, was designed through the calculation of phase diagrams. Simulation results reveal that a single body-centered cubic (BCC) phase can be formed and can be stable at temperatures above 950 °C. Accordingly, a series of Tix(AlCrNb)100-x (x = 45–80) alloys containing a BCC structure were prepared through vacuum arc melting and rapid cooling. The designed alloys can exhibit desirable mechanical properties with high compression yield strength about 1500 MPa, high compression fracture strength about 1800 MPa and high compression plasticity more than 30% at room temperature. Moreover, the Ti65 alloy can demonstrate a tensile strength of 1200 MPa with a tensile elongation of 32%, after a homogenization treatment for 24 h. The specific compression and tensile strength can reach 0.36 and 0.24 GPa cm3/g.

Formation of omega phase induced by laser shock peening in Ti-17 alloy

Synchrotron-based high-energy X-ray diffraction and transmission electron microscopy were used to study the in-depth compressive residual strains and microstructure evolution in Ti-17 alloy subjected to two-sided laser shock peening (LSP). A noticeable difference was observed in the lattice strains and extents of peak broadening on the two sides of the sample. The compressive residual strain was smaller and the peak broadening was more pronounced on the bottom surface in comparison to the top surface. This is due to the β → ω phase transformation induced by LSP on the bottom surface. It was determined that the ω phase with fine particle morphology and incommensurate structure is located mainly in the 15 μm-thick layer under the bottom surface. The formation of the LSP-induced ω phase on the bottom surface can be attributed to the adiabatic increase in temperature due to the higher shock peak pressure on the bottom surface, and can result in the relaxation of the accumulated residual compressive strain in the original phases.

Exploring the Ti-5553 phase transformations utilizing in-situ high-temperature laser-scanning confocal microscopy

Recently, the β metastable Ti-5553 alloy has attracted significant interest owing to the possibility of achieving excellent mechanical properties. In this study, for the first time, the Ti-5553 phase transformations were investigated using high-temperature laser-scanning confocal microscopy (HT-LSCM). During the continuous heating of an aged sample, the α phase dissolution was not clearly visible owing to the remaining grooves of former grain and interphase boundaries. However, the continuous cooling experiments (from the β field) showed that the α phase precipitation started preferentially at grain boundaries. With the increase in cooling rate, the α precipitates became more refined and the supercooling necessary to promote allotriomorphic precipitation increased. At a cooling rate of 100 °C min−1, no α phase was observed in the analyzed field of view. A higher oxygen content was suggested to contribute to the microstructural differences between the free surface and bulk. The volume fraction of the α phase was lower than that obtained on the original surface at cooling rates of 5, 25, and 50 °C min−1, while only minor differences were observed at cooling rates of 100 and 250 °C min−1. The α phase precipitation was analyzed in detail. The HT-LSCM technique can be a useful method for the analyses of solid-state phase transformations in Ti alloys.

Microplasticity behavior study of equiaxed near-β titanium alloy under high-cycle fatigue loading: crystal plasticity simulations and experiments

A crystal plasticity finite element (CPFE) model has been developed to study the microplasticity behavior of Ti-5553 alloy with equiaxed dual-phase microstructure under high-cycle fatigue (HCF) loading process. The microstructure and density distributions of geometrically necessary dislocations (GNDs) after HCF loading were characterized by electron backscattered diffraction (EBSD). Experimental results show that the microplasticity accumulation behavior happens in α phase during the HCF loading process. The developed CPFE model captures the essential physics of microplasticity accumulation in the primary α phase and stress concentration at the softer α/β interface. Besides, the effect of stress levels and volume fraction of primary α phase on microplasticity behavior was discussed. Results indicate that the stress concentration can be taken as fatigue indicator parameter (FIP) for high stress level HCF loading conditions, while the cumulative shear strain can be taken as FIP for low volume fraction of primary α phase conditions. This work provides a strategy not only contributing to understand the microplasticity behavior during HCF loading process but also choosing a suitable FIP for different loading conditions and microstructure states.

Tuning the morphology of Ti–5Al–5Mo–5V–3Cr–1Zr alloy: From brittle to ductile fracture

The tensile properties and crack initiation of Ti-55531 alloy were studied by tuning the microstructure from lamellar microstructure (LM) to bimodal microstructure (BM). The results indicate that the mechanical properties change from ultimate tensile strength of 1287 MPa with zero plasticity to ultimate tensile strength of 1310 MPa and ductility of ∼10%. Further transmission electron microscopy and scanning electron microscopy characterization show that the bimodal microstructure have shown a coarsen primary α surrounded by ultra-fine α precipitation, which could be the origin of combination of strength and ductility. TEM images after loading indicate that multiple slip systems have been activated in primary α of BM. Plastic strain localized within primary α due to the low strength, which results about 83.2% voids initiation within or at boundary of primary α. The cracks propagate not only along the interface between primary α and matrix, but also along slip bands in primary α in BM, which results in transgranular fracture. The results show that the micron-scale α phase is necessary to accommodate plastic strain and bring about ductility for β alloys with ultra-fine α precipitation.

Effect of streaming water vapor on the corrosion behavior of Ti60 alloy under a solid NaCl deposit in water vapor at 600 °C

The corrosion behavior of Ti60 alloy coated with solid NaCl layer in dry O2 and humid O2 (30.8 vol.% H2O) environments at 600 °C have been investigated. The isotopic tracing of 18O showed involvement of H2O in both the outer corrosion layer (OCL) and the inner corrosion layer (ICL). Theoretical calculations revealed that H2O provided 14% and 21% of the oxygen required for the formation of the outer and inner corrosion layers, respectively. Detailed analysis of corrosion products revealed that H2O enhanced the reaction of solid NaCl and TiO2 to form Na4Ti5O12 and promoted the crystallinity of corrosion products.

Impact of the initial microstructure and the loading conditions on the deformation behavior of the Ti17 titanium alloy

In this work, the impact of the microstructure and the loading conditions on the mechanical behavior of a β-rich Ti17 titanium alloy is investigated. For this purpose, two different initial microstructures are considered : (i) a two-phase lamellar α + β microstructure and (ii) a single-phase equiaxed β-treated microstructure. First, compression tests are performed at different strain rates (from $$10^{-1}$$ to 10 s−1) and different temperatures (from 25 to $$900\,^\circ $$C) for both microstructures. Then, optical microscopy, scanning electron microscopy, EBSD and X-ray diffraction analyses of deformed specimens are carried out. Whatever the loading conditions are, the flow stress of the as-received α + β Ti17 is higher than that of the β-treated Ti17. Also, because of a higher strain-rate sensitivity, the β-treated Ti17 is less prone to shear banding. At low temperatures (i.e., $$T \le 450\,^\circ $$C), the deformation behavior of both the as-received α + β and the β-treated Ti17 is controlled by strain hardening. For the β-treated Ti17 alloy, martensitic transformation is systematically detected in this temperature range. The softening behavior of the as-received α + β Ti17 observed at high temperatures is due to the joint effect of dynamic recrystallization, dynamic transformation, adiabatic heating and morphological texture evolution. For the β-treated Ti17 alloy, when the temperature exceeds $$700\,^\circ $$C, stress–strain curves display a yield drop phenomenon, which is explained by dynamic recrystallization.

The analysis of fracture toughness and fracture mechanism of Ti60 alloy under different temperatures

Fracture toughness and fracture mechanism of Ti60 alloy with duplex microstructure under different temperature conditions were investigated in the present work. The experimental result shows that the fracture toughness of CT samples increases from the room temperature (40 MPa m1/2) to 400 °C (71.45 MPa m1/2) but declines at 600 °C (62.55 MPa m1/2). It is observed from fracture surface through SEM that the predominant fracture mechanism has changed from quasi-cleavage fracture at room temperature to ductile fracture at higher temperatures. In addition, the tortuosity of crack propagation path has a limited impact on the fracture toughness. Path selections for crack propagation are obtained through SEM observation which can be summarized as: cut through lamellar α, parallel to lamellar α, bypass the equiaxed αp and cut through the equiaxed αp. Moreover, it is found that the intrinsic contribution is the primary reason leading to the change of the fracture toughness of Ti60 alloy under different temperatures. Meanwhile, it is noteworthy that the area of the crack tip plastic zone increases from RT to 400 °C but decreases at 600 °C, which is seen as the main impact of temperature on fracture toughness. To be exact, the CT sample with a larger area of the plastic zone could provide a higher K1C value. Furthermore, a prediction model of K1C based on tensile properties is established, which has a good accuracy with experimental results. The model is useful in predicting the fracture toughness of Ti60 alloy at different test temperatures.

Microstructure and mechanical properties of as-deposited and heat treated Ti–5Al–5Mo–5V–3Cr–1Zr (Ti-55531) alloy fabricated by laser melting deposition

In this work, the first successful attempt of additive manufacturing (AM) Ti-55531 near-β alloy fabricated by laser melting deposition (LMD) is reported. Optimized processing parameters of LMD were used to manufacture high relative density (99.8%) Ti-55531 alloy. It was found that the microstructure of as-deposited sample is mainly composed of β phase with heterogeneous distribution of α phase. The heterogeneity of microstructure of as-deposited sample was caused by the ubiquitous temperature gradient during LMD process. Thus, the as-deposited samples have a significant anisotropic tensile property which may not meet the aeronautical criterion. To overcome the issue of inhomogeneous microstructure and minimize the anisotropy, the microstructure was tailored by two kinds of heat treatment, i.e., β solution treatment subsequent aging (β-STA) and triplex annealing (α/β-STA). A transformed β microstructure was obtained in β-SAT condition, which resulted in the high ultimate tensile strength (UTS) (>1350 MPa) and low ductility (<6.5%). Besides, the bimodal structure was formed in α/β-STA condition and the associated sample shows an excellent balance of strength (UTS>1200 MPa) and ductility (>8%). Furthermore, the formation mechanism of microstructure and the subsequent impact on mechanical properties was systematically discussed. This work demonstrates the potential of fabricating Ti-55531 alloy parts by LMD and paves the way for enhancing LMD processed near β Ti-alloys through heat treatment.

A continuous dynamic recrystallization model to describe the hot deformation behaviour of a Ti5553 alloy

A physical based model is developed to describe recrystallization phenomena of titanium alloys during hot working of the β phase. Continuous dynamic recrystallization is attributed as the restoration mechanism based on the progressive transformation of low angle boundaries (subgrains) into high angle boundaries (grains). The model describes both, the microstructure, and the flow stress evolutions during hot deformation with large strains. The microstructure is conceived as been formed by three different populations of dislocations, and high as well as low angle grain boundaries. Evolution rates of all microstructural features are determined based on the effects of generation, interaction and annihilation of dislocations during deformation due to dynamic recovery, continuous dynamic recrystallization and static recovery. Continuous dynamic recrystallization is modelled and is considered to occur homogeneously within the microstructure. The model is able to predict the formation of subgrains from a fully annealed microstructure and the progressive formation of high angle grain boundaries. The subgrain and grain sizes are also obtained as output of the model. The model was applied to describe the hot compression behaviour of a Ti5553 alloy deformed between 880°C to 920°C and strain rates from 0.001 s−1 up to 10 s−1. The model is validated with flow curves and microstructural characterisation of hot deformed, and with microstructural information of non-deformed samples. The critical strain rate increases with increasing strain rate and decreasing temperature, similar to the discontinuous dynamic recrystallization phenomenon. The model can be implemented to simulate the microstructure and predict flow stresses of titanium alloys in industrial processes.

Microstructural evolution, mechanical properties and fracture toughness of near β titanium alloy during different solution plus aging heat treatments

Ti-55531 (Ti-5Al-5Mo-5V-3Cr-1Zr) is near β titanium alloy, which plays a significant role in manufacturing landing gears and flap tracks in the aerospace industry. This study aims to find out an optimal heat treatment to obtain an excellent balance of strength, ductility and fracture toughness. For this purpose, the Ti-55531 alloy was subjected to solution plus aging treatments and mechanical properties tests. Optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) were adopted to observe the microstructural evolution including morphology, distribution and size. Standard tensile and compact tension (CT) tests were carried out to obtain the yielding/tensile strength (YS, TS), elongation (EL) and fracture toughness (KIC) of Ti-55531 alloy. The effects of solution temperature, aging temperature and time on the microstructural evolution and fractography were analyzed. The relationships between the mechanical properties and microstructures were qualitatively described. The influencing mechanism of plastic deformation and crack propagation path on the KIC were discussed. Simultaneously, the values of YS and KIC were predicted based on mechanical property models, the predicted values were compared with tested data. Finally, an optimal heat treatment was proposed for the Ti-55531 alloy, to obtain a good balance of strength, ductility and fracture toughness.

Precipitation response and hardening behaviors of Fe-modified Ti5553 alloy

Abstract Ti5553−xFe (x=0.4, 1.2, 2.0, wt.%) alloys have been designed and fabricated through BE (blended element) sintering to investigate the effect of Fe-addition on athermal ω-phase transformation, α-phase evolution and age hardening behavior. The results show that the formation of athermal ω-phase is fully suppressed in water-quenched specimens when Fe-addition is up to 2 wt.%. The relevant timescales of α formation during initial stages of aging indicate that incubation time increases with Fe-addition. Further aging results in continuous nucleation and growth of α-phase but finer intragranular α lamellae exhibit in Ti5553−2Fe alloy. In addition, the width and extent of grain boundary α-film increase slightly with incremental Fe-addition, especially in furnace cooling condition. Result of Vickers hardness manifests that Fe-addition leads to a strong hardening effect in both solution and aging treatment. The solid solution strengthening is quantitatively estimated by ab initio calculation based on the Labusch−Nabarro model. The evolution of α-precipitate is rationalized by Gibbs free energy. The prominent hardening effect of Ti5553−2Fe alloy is attributed to both large lattice misfit of β-matrix and fine α-precipitate distribution.