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In this study, high–cycle fatigue (HCF) damage behavior of Ti–55531 alloy with bimodal microstructure (BM) was studied at room temperature. Fatigue crack initiation and propagation mechanisms of the alloy were thoroughly investigated by studying fracture morphology, crack front profiles, polished microstructure and dislocation structures beneath fatigue main–crack initiation sites of HCF samples. The results indicate that this alloy presents an excellent HCF strength (10 7 cycles, R = −1) as equal to 656 MPa. Dislocation analyses exhibit that typical dislocation structures include straight prismatic slip lines, curved dislocation lines, dislocation tangles and { 1 ¯ 011 } α type twins in fatigued specimens. Primary α p particles and secondary α s lamellae accommodate more cyclic deformation than retained β r laths. Furthermore, the dislocation free zone can be observed in the α p /β trans (β transformed microstructure) boundary. As a result, microcracks primarily nucleate at the α p /β trans interface or at α p particles. Moreover, a few microcracks initiate at the α s /β r interface or at α s plates of β trans microstructure. These fatigue crack initiation behaviors promote the fracture of Ti–55531 alloy. [ABSTRACT FROM AUTHOR]

High–cycle fatigue (HCF) damage behavior of Ti–5Al–5Mo–5V–3Cr–1Zr (Ti–55531) titanium alloy with lamellar microstructure (LM) was systematically investigated at room temperature. Results indicate that both grain boundary (GB) α phase and small–scale (~10 µm) heterogeneous microstructure regions (SHMRs) in prior β grains are weak microstructures in LM. Microcracks nucleate at prior β GBs caused by fracturing of some GB α during HCF loading. Dislocations nucleate and annihilate at α/β interfaces, which promote strain concentration and microcracks initiation at α/β interfaces in SHMRs. A combination of slip and twinning predominate cyclic deformation in α lamellae, which leads to nucleation of microcracks at α lamellae in SHMRs. Small cracks grow along α/β interfaces or transfer across α lamellae, and then form relatively long cracks in β grain interiors. Such crack initiation and propagation behaviors promote the HCF fracture of the alloy. [ABSTRACT FROM AUTHOR]

High-cycle fatigue (HCF) behavior of Ti–5Al–5Mo–5V–3Cr–1Zr (Ti-55531) alloy with both lamellar microstructure (LM) and bimodal microstructure (BM) was studied at room temperature. The results indicate that BM presents much higher strength, lower ductility and slightly higher HCF strength (10 7 cycles, R = −1) than those of LM. Typical dislocation structures including straight prismatic slip lines, curved dislocation lines, dislocation tangles and twins can be discovered in fatigued specimens with two different microstructures. Primary α (α p ) particles and secondary α (α s ) lamellae accommodate more cyclic deformation than retained β (β r ) laths. Grain boundary (GB) α layers have more effect on promoting crack initiation in LM than that in BM. As a result, fatigue microcracks mainly initiate at the interface between GB α films and prior β grains or at the α s /β r interphase for LM. However, microcracks primarily nucleate at the α p /β trans (β transformed microstructure) interface or at α p particles in BM. The combination of transgranular and intergranular crack propagation could be observed in the two microstructures. Crack front profile of macrocrack in LM is rougher than that of BM during the stable propagation region. [ABSTRACT FROM AUTHOR]

The main objective of the present study was to understand the oxygen ingress in titanium alloys at high temperatures. Investigations reveal that the oxygen diffusion layer (ODL) caused by oxygen ingress significantly affects the mechanical properties of titanium alloys. In the present study, the high-temperature oxygen ingress behavior of TC21 alloy with a lamellar microstructure was investigated. Microstructural characterizations were analyzed through optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Obtained results demonstrate that oxygen-induced phase transformation not only enhances the precipitation of secondary α-phase (αs) and forms more primary α phase (αp), but also promotes the recrystallization of the ODL. It was found that as the temperature of oxygen uptake increases, the thickness of the ODL initially increases and then decreases. The maximum depth of the ODL was obtained for the oxygen uptake temperature of 960 °C. In addition, a gradient microstructure (αp + β + βtrans)/(αp + βtrans)/(αp + β) was observed in the experiment. Meanwhile, it was also found that the hardness and dislocation density in the ODL is higher than that that of the matrix. [ABSTRACT FROM AUTHOR]

In order to reduce the weak area caused by traditional connection method and improve the structural efficiency of aircraft, the design concept of an integral component with dual microstructure is proposed. The electropulsing treatment (EPT) was used to produce an axial gradient microstructure (AGM) in TC21 titanium alloy, followed by the step-quenching treatment (SQT) to tailor the precipitation behavior of the α phase. Results demonstrate that the AGM produced by the combination of EPT and SQT achieves a simultaneous enhancement in both strength and plasticity of the alloy. Compared to EPT alone, the SQT results in the dissolution of the bimodal microstructure region with small-sized α phase, and an increased density of secondary α phase precipitation in both the transition and lamellar microstructure regions. During room temperature tensile deformation, coordinated distribution of deformation occurs sequentially in various regions, the 10 1 ̅ 0 11 2 ̅ 0 type of prismatic slip and deformation nanotwins exist in the equiaxed and lamellar α phase, respectively. Moreover, the deformation gradually declines from the bimodal region to the lamellar region, leading to the fracture of AGM in the bimodal region and an overall augmentation of the strength and plasticity of TC21 alloy. Hence, the results of this paper can provide research basis for the strength and plasticity improvement and deformation damage mechanism of high strength and toughness titanium alloy. • SQT significantly improved the nucleation rate of the lamellar α phase and formed discontinuous grain boundary α phase. • TC21 alloy with AGM manufactured by EPT and SQT achieved a simultaneous enhancement in strength and ductility. • Dislocation slip and TWIP effect in different regions form the coordinated distribution of deformation. • The result is significant importance in designing and researching for gradient structural materials. [ABSTRACT FROM AUTHOR]

The purpose of the present work is to reveal the effect of annealing temperature on the microstructure and mechanical properties of multilevel lamellar microstructure of TC21 titanium alloy in triple heat treatments, where the annealing temperatures are 760, 800 and 840 °C, respectively. The multilevel microstructure consists of β grain, α colony (α c), α plates (α p) and aged α plates (α s). The strength, plasticity, impact toughness and fracture toughness of the material were tested. The microstructure was characterized by using optical microscopy (OM), scanning electron microscopy (SEM), laser scanning confocal microscope (LSCM) and electron backscatter diffraction (EBSD). Results show that the size of α c increases and the widths of α p and β t increase gradually with the increase in annealing temperature. The number of α s precipitated on β matrix increases sharply and their size becomes smaller after aging treatment. The strength of TC21 titanium alloy increases as increasing annealing temperature while plasticity decreases. The α s is a critical unit which plays an important role in controlling strength of the alloy through Hall-Petch fitting. Furthermore, increasing the size of α c and decreasing the size of α p and α s are beneficial to increase the strength of the alloy. The impact toughness and fracture toughness of the alloy increase firstly then decrease with the increase in annealing temperature. The synergistic effect originating from different level α phase determine the toughness of the alloy. Massive cleavage and intergranular fractures in the fracture contribute to the increased length of crack propagation path so as to the decrease of toughness of the alloy. Meanwhile, it is confirmed found the toughness is also related to the change of size of α c after annealing treatment. And too large or too small size of α c is not conducive to the improvement of the toughness of the alloy. [ABSTRACT FROM AUTHOR]

[Display omitted] • A gradient microstructure was successfully fabricated by high-frequency induction quenching treatment (HFIQT). • Such a gradient microstructure is a result of the temperature gradient developed during induction heating. • The Ti-6Al-4 V alloy with gradient microstructure produces an attractive strength-ductility synergy. • The mechanical properties are noticeably better than those obtained with conventional treatments. A gradient microstructure was successfully fabricated by high-frequency induction quenching treatment (HFIQT) to improve the mechanical behavior of Ti-6Al-4V alloy in this work. Microstructural evolution was systematically characterized by electron backscattered diffraction and transmission electron microscopy. The results showed that the gradient microstructure of the alloy after HFIQT and aging varied from a fine α s lamellae decomposed from α′-martensite at the surface layer to a bimodal microstructure at the center. Moreover, the alloy with gradient microstructure presents an optimal strength-ductility synergy. The tensile test results showed that a best strength plastic product U T of 12433.1 MPa·%, an ultimate strength of 1231 MPa and an elongation of 10.1 % were obtained after HFIQT for 5.2 s and aging at 400 °C for 6 h. The tensile strength of the alloy aged at 400 °C markedly increased with the increase in soaking time from 5.1 to 5.3 s during HFIQT. Meanwhile, the tensile strength of the alloy slightly increased with the decrease in aging temperature from 550 to 400 °C during HFIQT. Surprisingly, the elongation of the alloy also increased with decreasing aging temperature. The effects of the gradient microstructure characteristic on strength and ductility of the Ti-6Al-4V alloy were discussed. [ABSTRACT FROM AUTHOR]

• The ageing treatment promoted the precipitation of more α s phase. • The deformation of β trans phase in ageing samples was more difficult. • More α s /β phase interfaces led to higher strengths. In the present study, the strength of TC21 titanium alloy was tuned by systematically controlling the density of α s precipitations via a two-step heat treatment. The first step consists of hot rolling at 940 °C to form precursor metastable β phase and precipitate a small number of second α phases (α s), followed by subsequent ageing treatment in the second step at a range from 500° to 600°C/4H resulting in more α s precipitations. The obtained results showed that the ageing temperature greatly affects the number density of α s , and thus this factor can be applied to control the ultimate tensile strength to 1.4 Gpa, while retaining a reasonable ductility. It is found that combination of the hindrance of more α s /β phase interfaces to dislocation motion and the smaller deformation of β transus (β trans) microstructure led to higher strengths. [ABSTRACT FROM AUTHOR]

• A kind of heterogeneous martensitic microstructure was successfully fabricated in Ti-6Al-4V. • The alloy shows an excellent ductility and a considerable tensile strength. • It's the first time to achieve excellent ductility in Ti-6Al-4V by two types of phase transformations under tensile load. • Dislocation hardening and FCC phase transformations lead to excellent ductility. In this work, a kind of heterogeneous martensitic microstructure was successfully fabricated within the fine β grains of Ti-6Al-4V through a rapid heating and cooling processing (RHCP) method, resulting in excellent ductility during tensile deformation. A total elongation was reached at 25.1%, while a considerable tensile strength of 1118 MPa was maintained. The main reasons for achieving this good ductility were the occurrence of dislocation hardening and deformation-induced phase transformation during tensile deformation. We used transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) to characterize and analyze the dislocation configurations and new face-centered cubic (FCC) phase during tensile deformation. We found that, to accommodate plastic deformation between the microcrystalline soft lamellae and ultrafine hard lamellae, many geometrically necessary dislocations (GNDs) were generated, which inevitably led to dislocation hardening, thereby greatly improving the ductility of the alloy. As a result of dislocation hardening, the improved ductility induced two types of FCC phase transformations, and the FCC phase quantity increased significantly during further tensile deformation. Plastic deformation can be effectively accommodated by the formation of a new FCC phase so that the ductility of the material can be further improved. These accommodating plastic deformation mechanisms reveal the nature of the good ductility of the material under a tensile load and provide a new strategy for fabricating strong and ductile titanium alloys. [ABSTRACT FROM AUTHOR]

The main objective of this study was to optimize mechanical properties of TC21 alloys by combining hot rolling with single ageing. The microstructure was characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and image analysis software. Results showed that hot-rolled and 520 °C aged samples exhibited more excellent comprehensive properties. It was observed that the rolling process not only refined primary α phase (α p), but also resulted in the precipitation of fine secondary α phases (α s) in the β transformed microstructure (β trans). The ageing treatment enhanced the precipitation of more α s , but had little effect on the α p size. Hot-rolled sample exhibited the higher strength than the as-received sample, which was attributed to an increase in the number of α p /β trans interfaces and the refinement of α s precipitations. The aged samples had the higher strength than the hot-rolled sample due to an increase in the number of α s /β interfaces. The as-received and hot-rolled samples exhibited higher ductility than the aged sample, because the β trans underwent a larger deformation and had a better coordinated deformation between α p and β trans of the as-received and hot-rolled samples. Both strength and ductility of TC21 alloy decreased with increasing ageing temperature, the decrease in the strength was attributed to the reduction of the number density of α s precipitates, whereas the decrease in the ductility was attributed to the reduction in the α p volume fraction and the coarsening of α s. [ABSTRACT FROM AUTHOR]