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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 HmVn clusters and vacancy-like defects in the samples irradiated at for room temperature, and that the formation of HmVn (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.

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.

A new type of low melting point Al-Si-Cu (Ni) filler metal for brazed 6063 aluminum alloy was designed, and the microstructure and properties of the filler metal were systematically studied. The results show that when the content of Cu in the Al-Si-Cu filler metal increased from 10 wt.% to 20 wt.%, the liquidus temperature of the filler metal decreased from 587.8 °C to 533.4 °C. Its microstructures were mainly composed of the α-Al phase, a primary Si phase, and a θ(Al2Cu) phase. After a proper amount of Ni was added to the Al-Si-20Cu filler metal, its melting range was narrowed, the spreading wettability was improved, and the microstructure was refined. Its microstructure mainly includes α-Al solid solution, Si particles, and θ(Al2Cu) and δ(Al3Ni2) intermetallic compounds. The results of the shear strength test indicate that the shear strength of the brazed joint with Al-6.5Si-20Cu-2.0Ni filler metal was 150.4 MPa, which was 28.32% higher than that of the brazed joint with Al-6.5Si-20Cu filler metal.

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 (dc) and α plate (dα) 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.

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 alloy 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., 6 figures

The microstructural evolution and alloying element partitioning in the &alpha, + &beta, &harr, &beta, phase transformation of Ti-17 alloy were explored under continuous heating and cooling conditions using the dilatometric method. Scanning electron microscopy and transmission electron microscopy were used to evaluate microstructural characteristics and trace alloying element partitioning behaviors occurring at different temperatures during heating and cooling. Results showed that the finer needle-like &alpha, phase first dissolved into the &beta, phase in the matrix with increasing temperature, while the grain boundary &alpha, phase first coarsened and then transformed gradually into &beta, phase during continuous heating. The dissolution of &alpha, phase of the alloy with the alloying element partitioning during continuous heating was observed. On the contrary, &alpha, GB formed at the prior &beta, grain of the alloy during continuous cooling, which might be the nuclei of &alpha, colony, thus resulting in the formation of &alpha, colony in the matrix. As the temperature decreased, the elements&rsquo, concentrations in the &alpha, and &beta, phases became increasingly varied due to element partition. Moreover, Al and Cr, which had higher diffusion coefficients than Mo, easily reached the concentration equilibrium of alloying elements in the &alpha, phases, respectively. The shrinkage of dilatometric curves during heating in the Ti-17 alloy are mainly attributed to the change of &alpha, HCP (hexagonal close-packed) lattice to &beta, BCC (body-centered cubic) lattice, while the element partitioning during the &beta, &rarr, &alpha, transformation plays an important role in the shrinkage of the dilatometric curves of the Ti-17 alloy during cooling.

To improve the cutting performance, the red hardness and wear resistance of M2 high-speed steel, as well as expand the application field, in this work, a coating was fabricated via plasma cladding on M2 high-speed steel using Ni, Cr and Cu alloy elements as precursor materials. The distribution and composition of alloying elements, microhardness and wear resistance of the coating were studied. The results show that the NiCrCu cladding layer contains many types of carbides. The secondary hardening caused by the dispersion of carbides can significantly improve the hardness, red hardness and wear resistance of material. The hardness of cladding layer is above 950 HV, after holding at 600 &deg, C for 4 h, the hardness is above 932 HV. The alloy elements are evenly distributed, but, due to the rapid solidification after the cladding, there are composition fluctuations in the longitudinal direction. The wear resistance of the cladding layer is excellent, the wear rate is reduced from 1.75 to 1.44 &times, 10&minus, 6 mm3 N&minus, 1 m&minus, 1 or less, and the wear mechanism is a combination of abrasive wear and adhesive wear.

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-TiHx and e-TiH2, 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 (HnVm), 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 HnVm (n

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 (αp) and secondary α (αs) phases. At low strain levels, lasting hardening of BM was due to the interaction of multiple prismatic and basal slips, and pyramidal slip in αp particles, while slight softening of LM was owing to the activation of a few twins in lamellar αs. However, at high strain levels, dislocation annihilation and numerous twins were detected in both αp and αs 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.

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 Ti2Cu 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+Ti2Cu phases are uniformly distributed in the alloy after different cooling processes, and the morphology of Ti2Cu phase is controlled by the peritectic solidification and eutectoid transformation. Specifically, the thickness and the volume fraction of primary Ti2Cu phase are larger than the sample cooling from β phase field when the cooling rate is the same. The morphology of Ti2Cu 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 Ti2Cu phase provides a favorable nucleation site for the precipitation of eutectoid Ti2Cu. 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 Ti2Cu 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.

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 (dβ) grows significantly while α colony (dc) increases slowly with the solution temperature increases. Meanwhile, α plate (dα) 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. Keywords: TC21 alloy, Multi-level microstructure, Correlated microstructure unit, Strength, Toughness

The α + β → β phase transformation kinetics of Ti-1300 alloy during continuous heating at different heating rates were investigated using dilatometric method. Results show that the curves of the α + β → β phase transformation exhibit a typical S-shaped pattern, which indicates that the α + β → β phase transformation is a nucleation-growth-controlled process. The overall activation energy of the α + β → β transformation of the alloy is 797 kJ·mol−1. The nucleation and growth mechanism of the α + β → β transformation was also investigated using the non-isothermal Avrami exponent. The Avrami exponent during α + β → β transformation process significantly changes with transformed volume fraction increasing, which indicates that the α + β → β transformation mechanism in the Ti-1300 alloy varies at different sections.

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.