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In general, the Ti–6.5Al–2Zr–1Mo–1V (TA15) components fabricated via laser powder bed fusion (L-PBF) exhibit high strength and low ductility. Herein, we report a novel approach to enhance the comprehensive mechanical properties of L-PBF TA15 alloy by adjusting the laser power. Samples processed using the optimal laser power exhibit a grid structure of alternating wide and narrow prior-β grains (PBGs), the inside of which is composed of a fully martensitic microstructure. The paper discusses in detail the alterations in the microstructure of samples processed at both low and high laser powers, clarifying the relationship between microstructure and mechanical properties. Thinner martensite contributes to higher strength, while a homogenous microstructure improves ductility. These findings provide valuable insights for controlling microstructure and achieving strength-ductility synergy in L-PBF additive manufacturing of titanium alloys.

Compressive strength and compressive strain, which are important mechanical properties of directionally solidified TiAl alloy, were predicted using machine learning algorithms, specifically Multiple Linear Regression (MLR), and Random Forest Regression (RFR). The input variables for the machine learning model were designated as the composition of the directionally solidified TiAl alloy, experimental parameters (pulling velocity, input power), and compression test parameters (strain rate, compression temperature). Compressive strength and compressive strain were designated as the output variables. Although the typical ratio of training and test data is 8:2, this study used different ratios of 9:1, 7:3, and 6:4 for machine learning, and excellent R2 values were obtained for all ratios. The feature importance, which can identify the factor that has the most influence on the output variables, was obtained through the RFR algorithm. According to the feature importance, temperature was found to have the greatest influence on compressive strength, while the Erbium (Er) element had the most significant influence on compressive strain. Through the results of feature importance, it was possible to quantitatively investigate the relationship between the Er element, a microalloying element that affects the microstructure of TiAl alloy, and the compressive properties. Furthermore, the study was conducted on which data ratio between training and test data is most suitable for predicting the compressive strength and strain of TiAl alloy.

To explore the mechanism of the refinement and toughening of Nb-Si-based composites with different Y contents, Nb-16Si-20Ti-2ZrC-(0, 0.05, 0.1, 0.5) Y composites were prepared by arc melting. The results show that Y has a positive effect on the shift of the eutectic point and promotes it moving to the low Si side. Adding 0.1 at.% Y refines the Nbss phase from 38.7 to 11.2 μm. Y2O3 forms due to the strong affinity of Y to O. The edge-to-edge model predicted that there was no phase relationship between Y2O3 and Nbss and primary γ-(Nb,X)5Si3, which was verified by TEM. Y refines the Nbss and the Nbss/γ-(Nb,X)5Si3 cluster with compositional supercooling. As Y content increased, the fracture toughness at room temperature increased and decreased. When the Y content was 0.1 %, the KQ value reached 17.2 MPa·m1/2. The internal oxygen consumption of Y and the refinement of Nbss lead to the toughness improvement of the Nbss phase, promoting the formation of a large number of dimples. Microcracks form in the fine γ-(Nb,X)5Si3 of eutectoid cluster, which hinder the primary crack growth and enhance the ambient fracture toughness. These findings are of great significance for the further development of high-performance Nb-Si-based matrix composites.

In this paper, Zr and Mo elements are introduced into the Nb–Si based alloy to improve the room-temperature fracture toughness and oxidation resistance at high-temperature environment. Nb–16Si–24Ti-xZr-yMo (x = 4, 8; y = 3, 6) alloys were prepared by arc melting and their room-temperature fracture toughness and oxidation resistance at 1250 °C is investigated. Results show that the addition of Zr and Mo element promotes the formation of lamellar Nbss-(Nb, X)5Si3 eutectic structure, which is beneficial to the fracture toughness. The room-temperature fracture toughness increases with the content of alloying element. The KQ value of Nb–16Si–24Ti–8Zr–6Mo alloy reaches 14.88 MPa ·m1/2, which is 72% higher than that of the Nb–16Si–24Ti original alloy. The oxidation resistance decreases due to the presence of more Nbss phase and the holes left by Mo oxide. However, the generation of ZrO2 and the volatilization of Mo oxide relieve the internal stress of oxide film and improve the integrity and adhesion of the oxide layer.

The influence of temperature on mechanical properties and fracture behavior of directionally solidified (DS) Ti47Al2Cr2Nb alloy was investigated in detail. Tensile tests and single-edge notched bending tests were performed in the temperature range of room temperature (≈25 °C) to 900 °C. The results revealed that both the yield strength and tensile strength of DS alloy specimens were insensitive to temperature until 850 °C. The yield strength and tensile strength at room temperature were found to be 500 MPa and 610 MPa, respectively, which were close to those at 850 °C (i.e., 534 MPa and 575 MPa). The ductility increased with the increase in temperature and the ductile-brittle transition (elongation >5%) occurred in the temperature range of 700–800 °C. Furthermore, the fracture toughness initially increased with the increase in temperature and reached the maximum value of 45 MPa m1/2 at 850 °C, followed by a decrease. It is worth emphasizing that the DS Ti47Al2Cr2Nb alloy exhibited a brittle trans-lamellar fracture at room temperature, which was similar to cleavage fracture. On the other hand, the fracture at high temperatures was influenced by the propagation and coalescence of microcracks at the lamellar interface, similar to micro-void accumulation fracture.

Mg2Ni is considered as a promising material for both storage hydrogen and catalyze MgH2 decomposition. Using Mg2Ni–Mg as a matrix material may achieve the combination of both advantages. Herein, hypoeutectic and hypereutectic Mg100-xNix (x = 11, 15, 22) alloys are prepared in this work to investigate the relationship between Mg2Ni in different microstructures and hydrogen storage performances. The results indicate that Mg2Ni connected Mg–Mg2Ni eutectics are formed besides the lamellar eutectic in Mg85Ni15 and Mg78Ni22 alloys because Mg2Ni precipitates firstly in eutectics, which are different from the α-Mg connected Mg–Mg2Ni eutectics in Mg89Ni11 alloy. Mg100-xNix alloys show rapid hydrogen absorption rates at ranges of both 125–175 °C and 275–325 °C because the presents of abundant interfaces generated by nano-Mg2Ni particles. Mg89Ni11 alloy with hydrogenation capacities of 3.6 wt% at 125 °C due to the large hydrogenation driving force of α-Mg. The Mg78Ni22 hydride with the highest proportion nano-Mg2Ni phase presents the fastest dehydrogenation rates. Two independent decomposition peaks are observed in the DSC curves of Mg100-xNix hydrides and the desorption temperature of Mg2NiH4 becomes lower with increasing Mg2Ni content due to Mg2Ni nanoparticles providing channels for the rapid diffusion of H atoms, and the reduced H concentration is conducive to the decomposition of MgH2.

TiB and TiC reinforced Ti–6Al–4V matrix composites were prepared by melt hydrogenation, in which the total fraction of the reinforcements was 5 vol % and the ratio of TiB to TiC was 1:1. The effects of melt hydrogenation on the solidification process, phase composition, and microstructure of the titanium matrix were studied. The morphology and distribution of the reinforcement without and with hydrogen were observed. Moreover, the nano-hardness of the composites was measured. The results show that the solidification process of the matrix can be changed by melt hydrogenation, which resulting the increase of β phase and refining of α lamellar in titanium alloy. Hydrogen increased the size of the reinforcements, which aggregated at the grain boundary of the primary β phase and formed a network structure. Hydrogen reduced the nano-hardness both of the titanium alloy and the reinforcements slightly. In summary, melt hydrogenation can maintain or slightly reduce the strength of the composites at room temperature and decrease the nano-hardness.

Abstract Laser three-dimensional (3D) printing has become a significant technique to fabricate high-performance Al2O3-based eutectic ceramics based on melt growth. However, oxygen vacancies are inevitable crystal defects during this process, and their formation mechanism and roles in the as-deposited ceramics are still unclear. In this paper, Al2O3/GdAlO3/ZrO2 ternary eutectic ceramics were prepared by laser 3D printing, and the formation mechanism of the oxygen vacancies was revealed by conducting a well-designed annealing experiment. In addition, the effects of the oxygen vacancies on the structure and mechanical property of the as-solidified eutectic ceramic were investigated. The formation of oxygen vacancies is revealed to be a result of the transfer of oxygen atoms from the oxide ceramic to the oxygen-deficient atmosphere by means of vacancy migration mechanism. Besides, the presence of oxygen vacancies has no obvious effects on crystalline structure and microstructure of the additively manufactured eutectic ceramic. However, the chemical bond property changes to some extent due to the formation of these crystal defects, which may affect the mechanical property of the as-deposited eutectic ceramic. It is found that the hardness decreases by 3.9%, and the fracture toughness increases by 13.3% after removing the oxygen vacancies. The results may provide a potential strategy to regulate the mechanical property of the oxide ceramic materials.

The effect of Mn element on the microstructure and mechanical properties of Nb–Si based alloys is studied, and the investigated alloys compositions are Nb–16Si–22Ti–xMn (x = 0, 2, 4, 8 at.%). Solubility of Mn in Nbss phase is higher than that in silicide. This dissolution of Mn in Nbss phase and silicide causes solid solution strengthening. Tiss phase and Mn2Ti phase eutectic structure precipitates between dendrites of 8Mn alloy, which affect room-temperature fracture toughness and compressive strength adversely. After heat treatment, Tiss phase and Mn2Ti eutectic structure almost disappears in Mn-contained alloys (Nb, Ti)3Si phase decomposes and transforms to Nbss phase and α-(Nb, Ti)5Si3 phase. The fracture toughness of heat treated 8Mn alloy has been improved greatly due to Mn solution strengthening in Nbss phase.

Directional solidification of γ-TiAl alloys with abruptly varying cross-sections was performed in yttria molds using an electromagnetic cold crucible. The solid–liquid interface at different growth positions was revealed by quenching using a liquid metal coolant, and the temperature field was mapped with the assistance of a finite element simulation. On the basis of these findings, the process of grain growth was summarized. The columnar–equiaxed transition was found to occur during directional solidification, and a criterion for continuous growth was established, which demonstrates that increasing the withdrawing rate or decreasing the nucleation rate can promote continuous growth. The continuity of columnar grains is confirmed by analysis in orientation evolution which also reconstructs the orientation of α and β grains at elevated temperatures. The degree of directionality of microstructures at elevated temperatures is determined from the angle between the preferential growth direction and the actual growth direction of the β phase. It is found that the degree of directionality increases with increasing withdrawing rate and nucleation rate.

The mechanical properties of a directionally solidified (DS) TiAl alloy were predicted through a random forest regression (RFR) machine learning algorithm. The prediction results were evaluated using the R2 value. As a result, the R2 values of prediction for the tensile strength, elongation, nanoindentation hardness, and interlamellar space were 0.9336, 0.9902, 0.8104, and 0.9810, respectively. To observe the correlation between the microstructure and mechanical properties, RFR prediction was conducted with a double input variable. This yielded a R2 value of tensile strength of 0.9856, which was higher than the tensile strength derived with a single input variable. The R2 value of nanoindentation hardness increased to 0.9902, which was higher than the nanoindentation hardness value with a single input variable. Through the use of the double input variables, the relationships among tensile strength and elongation, nanoindentation hardness, and interlamellar space were observed. Using feature importance, which could not be obtained in a previous study using the MLR algorithm, it was possible to determine which input variable had the most efficiency with respect to the output variable. Based on these research results, the speed and accuracy of new alloy development specifically in the design and processing, can be increased. In addition, metallurgical research on the relationship between the interlamellar space and mechanical properties was conducted, and the relationship was verified through the results of machine learning training.

A near fully lamellar Ti-43Al-5Nb-3.5Cr-1Zr alloy with different lamellar orientations was investigated by electron backscattered diffraction and nanoindentation measurements to study the influence of lamellar orientation on the nanohardness of α2/γ lamellae in a β-solidifying TiAl alloy. The results indicate that nanohardness of the α2/γ lamellae exhibits apparent plastic anisotropy, which is simultaneously determined by the oriented angle θ1 between the indentation direction and the lamellar plane and the deformation characteristics of the α2 and γ phase. Furthermore, the nanohardness of the individual lamellar colony decreases with an increase in the oriented angle from hard (0°) to soft (45°) orientations. Based on the analysis of the Schmid factor, the nanohardness value of the hard oriented α2/γ lamellae with a small Schmid factor is greater than the soft orientation with a large Schmid factor. The pile-up morphology on the γ phase is determined to be higher than that on the B2 phase at the interface through atomic force microscopy characterizations, which is attributed to the stronger plastic deformability of the γ phase. Therefore, the results can be successfully applied to understand the microscopic deformation mechanism of near fully lamellar β-solidifying TiAl alloy from a new perspective.

Investigation about how to enhance the corrosion properties of Ti alloys is essential to their actual applications in corrosive environments. Herein, the specific influences of Zr content on the corrosion behavior of Ti-6Al-3Nb-xZr-1Mo alloys (x = 2, 6, 10, 20 and 40 wt%) are systematically investigated using microstructure characterization, electrochemical and weight loss measurements. The results signify that all investigated alloys are composed of α and β phases; however, the increase of Zr content endows alloys with the refined thickness of lamellar α phase and enhanced volume fraction of intergranular retained β phase. Meanwhile, the corrosion resistance monotonously improves along with increasing the Zr content, directly characterized by the decreased corrosion current density, improved polarization resistance as well as reduced corrosion rate. The improvement in corrosion performance is a result of the alleviated micro-galvanic corrosion formed between α and β phases, increased volume fraction of corrosion-resistant β phase and modified composition of oxide film. One should note that the corrosion mode changes from the uniform corrosion to the mixed pitting and uniform corrosion with the Zr addition up to 40 wt%. Besides, surface analysis of samples after immersion tests reveals that the lamellar α phase is more susceptible to corrosion compared to the adjacent intergranular β phase, attributed to its higher concentration of Ti and Al elements. The current findings provide data that can be referenced for designing high-corrosion resistance materials for marine, biomedical applications and hydrochloric acid industries through composition optimization.

A multiphase TiAl alloy with nominal a composition of Ti–43Al–5Nb-3.5Cr–1Zr (in at. %) was fabricated using cold crucible directional solidification equipment at multiple growth rates. Effect of growth rate on the microstructure evolution, element distribution and mechanical property of phases in TiAl alloy were experimentally and statistically studied, especially the mechanism that affects the phase nanohardness. Multiphase microstructures consisting of α2/γ lamellar colonies and blocky γ phases as well as strip-like B2 phases were investigated through SEM and EDS identifications. With the increase of growth rates, the volume fractions of B2 and blocky γ phase increased accordingly from 7.3% to 13.3% and 4.6% to 12.6%, respectively. Furthermore, refinement of colonies can also be observed, which is attributed to phase interface migration driven by the change in growth rate. The content of Cr and Zr in the B2 phase is positively correlated with the growth rate, because the increase of growth rate will increase the effective distribution coefficient and make the solidification process deviate from the equilibrium state. Nanoindentation test results showed that nanohardness of B2 and blocky γ phases changed along with the growth rates owing to the enrichment or barrenness of alloying elements. The increase of Zr in blocky γ phase will increase its nanohardness, while the increase of Cr in B2 phase will lead to a decrease in its nanohardness, which is ascribed to lattice distortion caused by atomic substitution.

Perovskite solar cells (PSCs) have been developed rapidly in recent years because of their excellent photoelectric performance. However, interfacial non-radiative recombination hinders the improvement of device performance. The buried interface modification strategy can minimize the non-radiation recombination in the interface and can obtain the high efficiency and stability of PSCs. In this review, we introduce the device structure and the charge carrier dynamics (charge transfer, extraction, and collection) at the interface. We further summarize the main sources of non-radiative recombination at the interface, such as energy alignment mismatch and interface defects, and methods to characterize them. In contrast to the previous review of perovskite solar cells, the important roles of buried interfaces in regulating energy level alignment, passivating surface defects, modulating morphology, and so on are reviewed in detail based on the latest research, and strategies for reducing interfacial nonradiative recombination are provided. In the end, the potential development and challenges of buried interfaces for high-performance and stable PSCs are presented.

In this work, different interlayer remelting strategies are applied to regulate the microstructure evolution, martensitic transformation, and superelastic features of NiTi shape memory alloys prepared using the EBF3 additive manufacturing technique. The NiTi deposits prepared under different remelting beam currents are all composed of the B2 austenite, residual B19′ martensite, and submicron-scale Ti4Ni2Ox precipitates, and exhibit a one-step phase transformation (B2 ↔ B19′). Meanwhile, the crystallographic orientation, grain boundaries, and residual strain of these alloys present a distinct variation with the application of different remelting beam currents. During mechanical testing, the critical stress (σMs) of the EBF3-fabricated NiTi alloys was seen to possess a significant dependence on the martensitic transformation behavior, namely with the amount of B19′ martensite and the corresponding Ms. However, the broadening and stabilization of lamellar martensite created by dislocation pile-ups and plastic deformation in the cyclic loading–unloading procedure is the key reason for the deterioration of the superelastic response of the NiTi deposits. This work confirms that the mechanical and functional performances of NiTi alloys produced via EBF3-technique can be modified upon the application of proper interlayer remelting strategies, which can be extrapolated to the directed energy deposition of shape memory alloys or other metal components.

In order to study the factors of columnar to equiaxed transition (CET) of high-Nb TiAl alloys, Ti46Al7Nb0.4W0.6Cr0.1B alloy has been fabricated by cold crucible directional solidification (CCDS) technique under different pulling rate from 3.3 μm/s to 16.7 μm/s. The marco/micro-structure and phase composition near solid–liquid interface have been characterized. Results show that the CET of the high-Nb TiAl alloy occurs with the increase of the pulling rate at the constant temperature gradient. The microstructure of the columnar grain is composed of α2/γ lamellar matrix and a coupling structure of striped-like B2+γ phases. The lamellar colonies in a columnar grain possess the same orientation, while the arrangement direction between the striped-like B2 phase and growth direction is 0° or 45°. A solidification map for CCDS is established which predicts columnar or equiaxed morphology according to the growth rate (R) and temperature gradient (G). The dendrite morphology at the solid–liquid interface after quenching and the CET is controlled by the actual temperature gradient at the tip of the dendrite. Meanwhile, the increase of growth rate and the satisfaction of heterogeneous nucleation conditions are the main factors for CET. The decrease of actual temperature gradient caused by quenching or the increase of liquidus gradient caused by increasing growth rate can increase the maximum supercooling degree ΔTC. When it reached the supercooling degree ΔTN required to form a new nucleus, equiaxed grains will be produced. In addition, the boride in this alloy can act as a heterogeneous nucleation core to promote CET.

In order to fabricate in-situ synthetic (TiB + TiC)/Ti-6Al-4V composites, we conducted arc-smelting in the atmosphere of hydrogen and argon gaseous mixture using a melting casting method of melt hydrogenation technology. Some experimental parameters such as arc current, voltage, gas partial pressure and smelting time were precisely controlled. The molar ratio of the ceramic phase is 1:1 at a total integral of 5%. The microstructure evolution, tensile properties and fracture mechanism of the composite at room temperature are revealed and discussed.The results of the tensile test demonstrate that melt hydrogenation technology remarkably improves the ultimate strength of (TiB + TiC)/Ti-6Al-4V composites accompanied by slight increase of elongation, in which the 5 vol% (TiB + TiC)/Ti-6Al-4V increased by 15.19% from 901.42 MPa to 1038.42 MPa with elongation increasing from 0.47% to 0.78%, compared with that of the unhydrogenated composites. The improvement in the strength of the composite can be attributed to the refinement in matrix α layer and the increasement of the aspect ratio of TiB whiskers. The use of melt hydrogenation technology is found to change the trend of segregation of ceramic phase to primary β phase at grain boundaries into random dispersion distribution.

Herein, we systematically investigate the corrosion behaviour of a wrought Ti-6Al-3Nb-2Zr-1Mo (Ti80) alloy in artificial seawater with various fluoride ion (F−) concentrations and pH values using electrochemical, static immersion measurements and surface characterizations. Results of electrochemical tests manifest that increasing the F− concentration and/or reducing pH value can deteriorate the corrosion performance of Ti80 alloy due to the dissolution of the compact passive film into the porous passive film. Besides, the electrochemical parameters obtained from polarization curves indicate that the F− concentration has few effects on the cathodic reaction but noticeable promoting effects on the anodic process, whereas the increase of pH plays a significant role in inhibiting both cathodic and anodic processes. In addition, surface characterizations reveal the preferential dissolution of the equiaxed α phase compared to the intergranular β phase, resulting from its relatively low Volta potential. We also interpret the F− concentrations and pH values dependent corrosion behaviour based on the mixed potential theory and illustrate the corrosion mechanisms associated with F− in artificial seawater. We believe that the insights into the F− concentration and pH value dependent corrosion behaviour can provide practical guidelines for the design and development of Ti alloys with high-corrosion resistance.

Abstract The effect of substitution of Nb by Mo in Nb40Ti30Ni30 was investigated with respect to microstructural features and hydrogen dissolution, diffusion and permeation. As-cast Nb40−xMoxTi30Ni30 (x = 0, 5, 10) alloys consist of primary bcc-Nb phase and binary eutectic (bcc-Nb + B2-TiNi). The substitution of Nb by Mo reduces the hydrogen solubility in alloys, but may increase (x = 5) or decrease (x = 10) the apparent hydrogen diffusivity and permeability. As-cast Nb35Mo5Ti30Ni30 exhibits a combined enhancement of hydrogen permeability and embrittlement resistance as compared to Nb40Ti30Ni30. This work confirms that Mo is a desirable alloying element in Nb that can contribute to a reduction in hydrogen absorption and an increase in intrinsic hydrogen diffusion, thus improving embrittlement resistance with minimal permeability penalty.