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Abstract This study aims to determine an effective microstructure unit that controls the strength and fracture toughness of TC21 alloy with multi-level lamellar microstructure, including original β grain, α colony, and α plate. The multi-level lamellar microstructure of TC21 alloy, which was prepared by solution treatment at 980 °C and annealed temperature ranging from 720 °C to 820 °C, was characterized using scanning electron microscopy, electron backscatter diffraction, optical microscopy, and image analysis software. The relationship between multi-level microstructure and strength and fracture toughness was discussed using the Hall–Petch formula. Results show that the sizes of α colony (d c) and α plate (d α) increase gradually with the annealing temperature. Although the sizes of α colonies and α plates obey the Hall–Petch relationship with respect to strength, the distribution of points between d α − 1 / 2 and strength is rather dispersed than d c − 1 / 2. Thus, α colony is the effective controlling unit of strength of the alloy. Meanwhile, α plate is the effective microstructure unit for controlling fracture toughness. It is confirmed by EBSD that the fracture toughness depends on the bending, rotation and shearing of α plate during the crack propagation. Moreover, an intensity relationship exists for the crack growth resistance with the increasing size of α colony, which cause rough propagation path by shearing amount of α plates and result in high toughness with increasing annealing temperature, thereby providing the reasonable evidence that α plates is the effective control unit of fracture toughness. [ABSTRACT FROM AUTHOR]

Porous carbon electrode materials with appropriate proportion of hierarchical pores are critical for high performance supercapacitors. Here, a mesopore-dominated hierarchical porous carbon was reported. KOH, melamine (MA), and the carbonized vinasse were sequentially mixed using dry ball milling, the mixture was pyrolyzed to acquire mesopore-dominated porous carbon. The pore structure can be controlled by adjusting the addition amount of melamine. The optimal sample with high meso-porosity, large total pore volume, ultrahigh specific surface area (SSA) and hydrophilicity makes it performs quite well in different electrolytes. A high specific capacity of 413 F g−1 was achieved and retained 267 F g−1 at a high rate of 50 A g−1 with excellent capacitance retention (107 % after 10000 cycles). The symmetrical supercapacitor gained a superior energy density of 16.7 Wh kg−1, 21.2 Wh kg−1, 54.0 Wh kg−1 in 6 M KOH, 1 M Na 2 SO 4 , 1 M Et 4 NBF 4 /ACN electrolyte, respectively. This work opens a new strategy for the preparation and high-efficient application of biomass porous carbon materials in energy storage. • Mesopore-dominant vinasse-based porous carbon. • Innovatively using dry ball milling to add melamine for mixing. • NVCP-1 with high SSA of 3442.97 m2 g−1 and large pore volume of 2.41 cm3 g−1. • NVCP-1 shows excellent performance in three different electrolytes. [ABSTRACT FROM AUTHOR]

A pre-formed martensitic quenching plus inverted multi-step austempering process is performed on a medium-carbon alloy steel: after austenitization, the specimen is cooled to 175 °C below the martensite start temperature (Ms), then raised to the first austempering step at 220 °C for 5 h, followed by the second step at 300 ℃ and held for 3 h. The conventional inverted multi-step austempering process and the one-step austempering process are carried out to form a contrast test. Through pre-formed martensitic quenching plus an inverted multi-step austempering process, the martensite/austenite (M/A) and bainitic ferrite in the microstructure are refined. The total time used for heat treatment is reduced markedly due to accelerating bainitic ferrite formation. This study provides theoretical support for the process design of ultra-fine bainitic steel. [ABSTRACT FROM AUTHOR]

The effects of cold pre-deformation on aging behavior and mechanical properties in Ti-1300 alloy are examined using optical microscopy, scanning electron microscopy, and transmission electron microscopy analyses. Results show that cold pre-deformation markedly influences the aging behavior and mechanical properties of Ti-1300 alloys. The cold deformation mechanism of Ti-1300 alloys is found to involve the dislocation slip and twinning. The microstructures of the pre-deformation specimens, which were aged at 550 °C, comprise α phases in β matrices regardless of cold deformation reduction. Fine/uniform and coarse/needle-like α precipitates are formed in the slightly and heavily deformed zones, respectively. The Vickers hardness of Ti-1300 alloys aged after cold pre-deformation respectively decreases and increases with increased aging temperature and cold pre-deformation reduction. [ABSTRACT FROM AUTHOR]

This paper aims to reveal the effect of aging temperature on strength, impact toughness and fracture toughness of TC21 alloy with multi-level lamellar microstructure. The multi-level lamellar microstructures of TC21 alloy were prepared by solid solution at 1000 °C, annealing at 800 °C and respectively aging at 550 °C, 600 °C and 650 °C. The multi-level structures include β grains, α colony (α c), primary α phase (α p) and aged α plates (α s). The microstructures were performed on using optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), laser scanning confocal microscope (LSCM) and image analysis software. Results show that as aging temperature goes up, the size of α s increases and the segregation of alloy elements increases, while the β grain, grain boundary α phase (G α) and α c almost remain unchanged. Hall-Petch formula is used to reveal that α s phase is the minimum control unit of strength. Fine grain strengthening, volume fraction of α phase (V α) and element segregation have great influence on mechanical properties. In the impact experiment, microstructure deformation and secondary cracks consume more energy, while cleavage fracture consumes less energy. The TEM analysis found that the coordination of microstructure deformation became worse after aging. Fracture analysis shows that the size of α c plays an important role in crack propagation, and the crack propagation path always tends to cut α c vertically and pass through it. Whether the size of α c is large or small, it is not conducive to the increase of crack propagation path. Crack propagation path length and the interface energy consumed by different fracture mechanisms should be taken into consideration when there are more crack propagation paths and the fracture toughness decreases in the multi-level lamellar microstructure of TC21 alloy. [ABSTRACT FROM AUTHOR]

With the speedy development and the continuous application of special alloy manufacturing nowadays, M2 high-speed (HSS) steel is difficult to meet industrial needs. For improving the red hardness and wear resistance of M2 HSS surface, 20 wt.% Co was added into NiCrCu alloy powder and preplaced it onto the surface of M2 HSS, and then, the coating was fabricated by plasma cladding. The organization structure and mechanical properties of coatings before and after Co addition were studied. The results show that the addition of Co strengthened the matrix in the form of substitutional solid solution. Co did not change the phase composition and microstructure characteristics of the coating, but it reduced the residual stress and undesirable residual austenite content of the coating. The surface quality and crack resistance of the coating were optimized. In addition, the red hardness and wear resistance of the coating were significantly improved after adding Co. The average hardness increased by 22.53%, the average friction coefficient decreased by 21.65%, and the volume wear decreased by 56.86% after the coating tempering at 550 °C for 4 h. Moreover, the average hardness of Co-containing coating decreased slowly after tempering for different times. The present work is of great significance in providing a potential guidance for broadening the HSS application fields. [ABSTRACT FROM AUTHOR]

This paper aims to reveal how multi-level lamellar microstructure influences strength and toughness of TC21 alloy after different solution treatment ranging from 980 °C to 1020 °C and annealed at 770 °C. Microstructural characterization was performed by using optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM), and image analysis software. Results show that the size of β grains (d β) grows significantly while α colony (d c) increases slowly with the solution temperature increases. Meanwhile, α plate (d α) presents an opposite tendency. In addition, the Hall-Petch formula was applied to reveal the relationship between multi-level microstructure (including prior β grain, α colony and α plate) and strength and toughness of the alloy; α colony could be a key microstructure unit correlated to strength of the alloy. Besides, α plate obeys the Hall-Petch relationship with toughness. Meanwhile, it is confirmed by EBSD that more α plate were cut with the increasing size of the multi-level lamellar microstructure. The larger α colony size is, the stronger the interaction between the crack and α plate. Thus, α plate was regarded as the key microstructure unit for influencing toughness because it strongly hinders the crack initiation and growth. In order to understand crack propagation path and propagation mechanism of TC21 alloy, the EBSD observation on fracture surface were investigated in detail, then the schematic illustration for the crack propagation was made according to experiment results. Unlabelled Image • Different mechanical properties depend on different level microstructure unit. • α colony could be a key microstructure unit correlated to strength. • α plate could be a key microstructure unit correlated to toughness. • Different mechanical variation tendency is attributed to different microstructure unit correlated to performance. [ABSTRACT FROM AUTHOR]

The formation of mono-vacancy, vacancy clusters and hydrogen-vacancy complexes with 30 keV H ion-irradiated pure titanium at different doses and temperatures was measured using by Positron annihilation spectroscopy (PAS). Results show a large number of H m V n clusters and vacancy-like defects in the samples irradiated at for room temperature, and that the formation of H m V n (m > n) at the sample irradiated at a high dose inhibits the increase of the S parameter. At increased irradiation temperature, the shrinkage of vacancy clusters and the effective open volume of defects decrease the S parameters. The high-temperature irradiation results in decreased vacancy-type defect concentration, and some hydrogen atoms diffuse from the cascade region to the track region, forming a large number of hydrogen-vacancy complexes in the track region. The coincidence Doppler broadening spectroscopy, an element analysis method, used to detect hydrogen in the ion-irradiated pure titanium sample, and results show hydrogen-related peaks in the high-momentum region, which may be due to the information of positron annihilation in the covalent bond formed by the H and the Ti elements. The increased radiation dose and temperature contribute to the formation of the hydrogen vacancy-complex, and the positron annihilation in high-momentum regions easily obtain hydrogen-related information. • The increased radiation dose and temperature contribute to the formation of the hydrogen vacancy-complex (H m V n). • The formation of H m V n (m > n) at the sample irradiated at a high dose inhibits the increase of the S parameter. • Hydrogen-vacancy complexes (H m V n) formed as hydrogen atoms deposited in the material. [ABSTRACT FROM AUTHOR]

Mechanism of Hydrogen-induced defects and cracking in Ti and Ti–Mo alloy was systematically analyzed from the perspective of microstructure. Results show that more hydrogen atoms can be dissolved in Ti–Mo alloy due to the existence of β phase, and the precipitation amount of hydride is reduced under the same hydrogen charging conditions. The analysis of positron annihilation results show that the solid solution of Mo element reduces the hydrogen induced defect concentration in the alloy and inhibit the combination of hydrogen and defects. At the same time, Mo atom can affect the electronic structure of Titanium hydrides, and then reduce the bonding between titanium atoms and hydrogen atoms, thereby reducing the formation of brittle titanium hydride. The reduction of hydride and hydrogen induced defect concentration significantly decrease the hardening rate of Ti–Mo alloy, thereby effectively reducing the hydrogen embrittlement sensitivity of titanium and inhibiting its hydrogen absorption cracking tendency. • The formation of β phase in Ti–Mo alloy improves the solubility of hydrogen, thereby reducing the precipitation of hydride. • The solid solution of Mo can affect the electronic structure of titanium hydride and weaken the bonding between Ti–H. • The solid solution of Mo can reduce the concentration of hydrogen-induced defects in Ti–Mo alloy. • Reducing the concentration of hydride and hydrogen induced defect can inhibit hydrogen absorption cracking tendency. [ABSTRACT FROM AUTHOR]

• Stacking faults would promote LCF microcracks nucleation of Ti-55531 alloy. • LCF microcracks nucleation and growth are dependent on both strain level and microstructure. • Fatigue life of BM is lower than that of LM at high strain levels (>0.866%). • LCF damage of Ti-55531 alloy is dramatically affected by α phase features. Low cycle fatigue (LCF) crack initiation and propagation behavior of Ti-55531 alloy with lamellar and bimodal microstructures were comparatively investigated at room temperature. Results indicated that the two microstructures displayed different sensitivity to the change of cyclic strain, which resulted in significantly different cyclic deformation, fatigue crack nucleation, and crack propagation behavior of two microstructures. Finally, it was found that cyclic lives of two microstructures were entirely different. Moreover, besides slips and deformation twins, stacking faults promoting cracking of α s /β interface may be another important LCF microcrack nucleation mechanism of Ti alloys, which hasn't been revealed by other researchers. [ABSTRACT FROM AUTHOR]

Hydrogen behavior and its related property degradation have been long-standing problems for structural materials used in hydrogen energy. The hydrogen atoms can easily interact with vacancy defects, forming hydrogen vacancy complexes which play an important role in the hydrogen-induced structural damage. However, the interaction mechanisms and its evolutions are still unclear. In this work, the hydrogen behavior and the interaction between hydrogen with defects in pure titanium implanted by 30 keV and 50 keV hydrogen ions were studied by positron annihilation spectroscopy. The implantation doses were 5 × 1016 H/cm2, 1 × 1017 H/cm2 and 5 × 1017 H/cm2, respectively. The results show that the structural damage of pure titanium is positively correlated with the ion implantation energy. For the implantation of 50 keV hydrogen ions, a large number of hydrogen atoms are deposited in the samples. With the increase of implantation dose, the formation of hydrogen vacancy complexes (H m V n) reduces the effective open-volume of defects and changes the structural features of defects in implanted samples, thus suppressing the formation of vacancy defects and causing the damage range shifting from the peak damage (PD) region to the near surface (NS) region. Eventually, the movement of hydrogen atoms intensifies, and the "hydrogen peak" becomes more obvious. The chemical information related to deposited hydrogen atoms can be easily identified in the processing and analysis of positron annihilation results. • The formation rate of vacancy type defects increases in direct proportion to the implantation energy. • Implantation dose promoted the formation of hydrogen vacancy complex. • The increase of the proportion of hydrogen vacancy complex leads to the inhibition of vacancy formation. • With the increase of hydrogen concentration, the "hydrogen peak" signal is more obvious. [ABSTRACT FROM AUTHOR]

[Display omitted] • High density dislocations inhibit hydrogen atoms diffusion. • Different types of hydrides were identified by CDB spectrum and theoretical calculations. • Hydrogen concentration decreases with increasing dislocation density. This work aims to elucidate the interaction between hydrogen atoms and pre-introduced dislocations in commercially pure titanium (CP-Ti). Positron annihilation spectroscopy supplemented by the first-principle calculations was utilized to reveal the effects of dislocation densities on the formation of hydride types and the concentration of hydrogen-induced defects in CP-Ti. Results show that, in the hydrogen-charged sample with 40 % deformation, hydrogen-induced damage gradually decreases when the hydrogen enters the sample at a depth of about 350 nm. This finding suggests that the high-density dislocations present in deformed specimens effectively trap hydrogen atoms and inhibit their diffusion, which ultimately mitigate damage within the samples. γ-TiH and δ-TiH 2 hydrides are observed in the hydrogen-charged sample. The thermal desorption spectroscopy results show that the desorption amount of deuterium decreases from 1.14 × 1018 D/cm2 to 5.10 × 1017 D/cm2 with an increase in dislocation density because dislocations inhibit the diffusion of deuterium. In samples with higher deformation, the high dislocation density inhibits the formation of δ-TiH 2 hydrides and promotes the formation of γ-TiH hydrides, which significantly reduce the hardening of samples. These results provide insight into the interaction of hydrogen with pre-introduced defects and provide strong theoretical support for the development and improvement of more damage-resistant materials. [ABSTRACT FROM AUTHOR]

This paper aims to unveil effects of α colony and α plate parameters on tensile properties and impact toughness of Ti-55531 alloy with multilevel lamellar microstructures. Microstructures with different α colony and α plate sizes were prepared using varied heat treatments and analyzed by X-ray diffraction, scanning electron microscopy, electron back-scattered diffraction, transmission electron microscopy. Relationships of microstructure and strength, plasticity and impact toughness were discussed. Results indicate that tensile strength increases, while plasticity and impact toughness of the alloy decline with decreasing effective sizes (width/length ratio) of α colony (dc) and α plate (dα) and increasing of volume fraction of α phase. Both dc and dα follow the Hall–Petch principle with strength, and dc was determined to be the critical controlling parameter of strength owing to higher interfacial strengthening effect induced by α colony. However, plasticity is primarily related to dα. Moreover, impact toughness for both dc and dα follows the linear relation, but the coefficient of dα is higher. Furthermore, impact toughness is more dependent on plasticity than strength. Larger α plates lead to more mechanical twins during impacting and result in higher plasticity and impact toughness. Therefore, the impact toughness should depend more on the size of α plates. In order to unveil the influence of α plate size on impact deformation and microcrack initiation behavior of Ti-55531 alloy, the microstructural deformation and crack nucleation characteristics in the impact crack nucleation region were detected by TEM in detail; then, the schematic illustration for microstructural deformation and crack nucleation characteristics was made according to experiment results. [ABSTRACT FROM AUTHOR]

The evolution of defects and irradiation damage in Ti and Ti-Mo alloy under hydrogen ion irradiation were investigated by slow positron-beam doppler broadening spectroscopy (DBS) and coincidence doppler broadening spectroscopy (CDB), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD results indicate that the existence of the β phase enhances the solid solubility of hydrogen in Ti-Mo alloy. The metastable γ-TiH hydrides are formed in pure titanium after hydrogen ion irradiation, while no hydride is found in Ti-Mo alloy under the same irradiation conditions. Moreover, DBS and CDB results show that hydrogen vacancy-complexes are formed in Ti and Ti-Mo alloys after hydrogen ion irradiation. Calculated doppler curves trend qualitatively agrees well with the experimental results. The TEM results show lower defect concentration and smaller defect sizes in Ti-Mo alloy with (α + β) than pure titanium. The addition of molybdenum significantly refines the grains and increases the density of the interface, which plays an important role in the management of irradiation-induced defects. • Molybdenum alloying increases the solid solubility of titanium alloy to hydrogen. • Molybdenum alloying can reduce hydrogen-induced defects and irradiation damage. • The hydrogen-vacancy complexes were identified by calculated Doppler spectra. [ABSTRACT FROM AUTHOR]

The “titanium fire” as produced during high pressure and friction is the major failure scenario for aero-engines. To alleviate this issue, Ti-V-Cr and Ti-Cu-Al series burn resistant titanium alloys have been developed. However, which burn resistant alloys exhibit better property with reasonable cost needs to be evaluated. This work unveils the burning mechanisms of these alloys and discusses whether burn resistance of Cr and V can be replaced by Cu, on which thorough exploration is lacking. Two representative burn resistant alloys are considered, including Ti14 (Ti-13Cu-1Al-0.2Si) and Ti40(Ti-25V-15Cr-0.2Si) alloys. Compared with the commercial non-burn resistant titanium alloy, i.e., TC4 (Ti-6Al-4V) alloy, it has been found that both Ti14 and Ti40 alloys form “protective” shields during the burning process. Specifically, for Ti14 alloy, a clear Cu-rich layer is formed at the interface between burning product zone and heat affected zone, which consumes oxygen by producing Cu-O compounds and impedes the reaction with Ti-matrix. This work has established a fundamental understanding of burning resistant mechanisms for titanium alloys. Importantly, it is found that Cu could endow titanium alloys with similar burn resistant capability as that of V or Cr, which opens a cost-effective avenue to design burn resistant titanium alloys. [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]

As a crucial branch for titanium industry, high-strength titanium alloys (HS-TAs, with UTS ≥ 1100 MPa) are indispensable structural materials for advanced engineering applications such as aerospace and marine fields. Along with the expansion of HS-TAs' market, achieving satisfying synergies of high strength, high ductility (elongation ≥ 6%) and high toughness (K IC ≥ 50 MPa⋅m1/2) has been identified as the uppermost technical bottleneck for their research and development. To overcome the challenge, two primary strategies have been initiated by the titanium community, developing novel alloys and innovating processing technologies. For the former, a dozen of newly-developed alloys were reported to exhibit excellent strength-ductility-toughness combinations, including Ti-5553, BT22, TC21 and Ti-1300, for which the ideal mechanical performances were based on specific microstructures realized by low impurity rate (e.g. oxygen content ≤ 0.15 wt%), complicated processing and complex heat treatment. For the latter, several innovatory forging and heat treatment technologies were originated for the mature alloys to optimize their balanced property by extraordinary microstructural characteristics. In this review, we provide a comprehensive overview over the research status, processing and heat treatment technologies, phase transformation, processing-microstructure-property correlation and strengthening-toughening mechanism of HS-TAs for aerospace engineering applications manufactured via melting-forging process. Finally, the prospects and recommendations for further investigation and development are proposed based on this review. [ABSTRACT FROM AUTHOR]

• Long crack growth is dependent on both microstructure and load. • Long crack growth of Ti-55531 alloy is greatly influenced by α phase characteristics. • The area ratio (A real /A macro) is more accurate than the crack length ratio (L ε /L 0) to calculate K IC. • The da/dN of BM is higher than that of LM at high Δ K (>26.47 MPa·m1/2) in Ti-55531 alloy. This work aims to reveal the synergistic effect mechanism of microstructure type and loading mode on the long crack propagation in the Ti-55531 alloy. The fracture toughness (K IC) and fatigue crack propagation rate (da/dN) of the Ti-55531 alloy with lamellar microstructure (LM) and bimodal microstructure (BM) were investigated and compared at room temperature. The results show that both microstructural factors and the loading mode significantly affect the long crack propagation behavior of the alloy. Under static loading, the K IC of the LM is higher than that of the BM. Under cyclic loading, the da/dN of the BM is lower than that of the LM in the lower stress field intensity factor range (Δ K < 26.47 MPa·m1/2). However, when Δ K > 26.47 MPa·m1/2, the da/dN of the BM is higher than that of the LM. The results of the fracture surface morphology and the crack path analysis show that the difference in microstructure characteristics leads to various behaviors of the long crack propagation under both static and cyclic loadings. The key microstructural factors controlling the long crack propagation in the two microstructures under static and cyclic loadings are discussed. The influence of microstructure characteristics on the long crack propagation in different stages of crack growth is also explained. Moreover, a new calculation model of K IC is proposed. These results could provide support for the reliability design of engineering structures and promote the wider application of the alloy in engineering. [ABSTRACT FROM AUTHOR]

• Two types of titanium hydrides form after electrochemical hydrogen charging. • The formation of hydride leads to lattice expansion. • Hydrogen content affects the concentration and type of hydrogen induced defects. • The titanium hydride and hydrogen-vacancy complex were identified by CDB spectrum. The influence of hydrogen on defect structure in pure titanium was systemically investigated using X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS). The results showed that hydrogen damage was related to hydrogen concentration, and PAS provided a new idea for the study in the formation of hydride and open volume defects. XRD results showed that two types of titanium hydrides were observed in hydrogen-charged samples, i.e., δ-TiH x and ε-TiH 2 , and the degree of lattice expansion and the content of hydride increased with hydrogen content. The variation of the S parameter and W parameter indicated that a great deal of vacancy-type defects formed during electrochemical hydrogen charging, and hydrogen atoms could combine with vacancy to form a certain amount of hydrogen-vacancy complexes (H n V m), meanwhile, the hydrogen damage varied significantly with depth. The effective open volume of vacancy-type defect increased with hydrogen charging concentration, and hydrogen-vacancy cluster transformed into H n V m (n < m) cluster, vacancy defects were gradually dominated. In addition, According to the different annihilation environment of positron in titanium hydrides and hydrogen-vacancy complexes, the existence of titanium hydrides and the existing form of hydrogen in defect could be identified in Doppler broadening spectroscopy (CDB). This provided a new way to identify the formation of hydrides and to detect the existing form of low concentration hydrogen in samples. [ABSTRACT FROM AUTHOR]

Cyclic deformation response and micromechanics in strain controlled low cycle fatigue of Ti–5Al–5Mo–5V–3Cr–1Zr (Ti–55531) alloy with bimodal microstructure (BM) and lamellar microstructure (LM) were comparatively investigated at ambient temperature. Results showed that cyclic hardening/softening behavior of Ti–55531 alloy was found to be significantly dependent on both the strain amplitude and microstructure. The cyclic softening rate of LM was always larger than that of BM at strain level below 1.0%; however, the opposite result was produced for strain amplitude above 1.0%. Dislocation features revealed that cyclic deformation of the alloy primarily relied on primary α (α p) and secondary α (α s) phases. At low strain levels, lasting hardening of BM was due to the interaction of multiple prismatic and basal slips, and pyramidal slip in α p particles, while slight softening of LM was owing to the activation of a few twins in lamellar α s. However, at high strain levels, dislocation annihilation and numerous twins were detected in both α p and α s phases in BM, which resulted in the larger cyclic softening rate of BM than that of LM. Evidently, besides dislocation annihilation, twinning was found to be another important softening mechanism, which was not revealed by other studies on Ti alloys. • Cyclic deformation response is dependent on both strain amplitude and microstructure. • Cyclic softening rate of BM is faster than that of LM at high strain levels. • Cyclic deformation is primarily attributed to the α phase in Ti–55531 alloy. • Twinning is an important cyclic softening mechanism of Ti–55531 alloy. [ABSTRACT FROM AUTHOR]

The mechanical performance of Ti-13Cu-Al alloy is highly determined by its microstructure, which is significantly affected by the cooling process from semi-solid state as it induces peritectic solidification and the subsequent eutectoid transformation of the alloy. This work investigated the effects of cooling rates on the morphology of Ti 2 Cu phase and mechanical properties of Ti-13Cu-Al alloy after cooling from semi-solid state (β+L phase). Different cooling mediums have been considered, including furnace cooling (0.5 °C/s), air cooling (50 °C/s) and water cooling (150 °C/s), corresponding to a slow, normal and rapid cooling process, respectively. It is found that α+Ti 2 Cu phases are uniformly distributed in the alloy after different cooling processes, and the morphology of Ti 2 Cu phase is controlled by the peritectic solidification and eutectoid transformation. Specifically, the thickness and the volume fraction of primary Ti 2 Cu phase are larger than the sample cooling from β phase field when the cooling rate is the same. The morphology of Ti 2 Cu phase varies from "laths + blocky + sheaves" to "short laths + blocky + rodlike" as the cooling rate gradually increases. It is further revealed that the β/L interface provides a fast channel for solute atom diffusion, and the peritectic Ti 2 Cu phase provides a favorable nucleation site for the precipitation of eutectoid Ti 2 Cu. Based on the tensile property and hardness examinations, the mechanical properties of the Ti-13Cu-Al alloy show a clear dependency with the precipitation of Ti 2 Cu phase after cooling from semi-solid state, indicating the importance of cooling mode from semi-solid state on the microstructure and mechanical properties of the alloy. Image 1 • The peritectic Ti 2 Cu will be produced when cooling from semi-solid state (β+L phases). • The peritectic Ti 2 Cu/β interface provides rich nucleation sites for eutectoid transformation. • Active eutectoid Ti 2 Cu will more effectively improve the mechanical properties of the Ti-13Cu-Al alloy. [ABSTRACT FROM AUTHOR]