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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.

Herein, we investigate the influence of heat treatment on microstructure and thermo-physical properties of Al-50wt%Si alloy fabricated by selective laser melting (SLM). Microstructure characterizations reveal that SLM and heat-treated Al-50wt%Si alloys are all characterized by the primary Si and eutectic structure consisting of dominant Al matrix and tiny eutectic Si. However, the size of primary Si monotonously increases with increasing the heat treatment holding time at 550 °C. In addition, the SLM Al-50wt%Si alloy displays a highest coefficient of thermal expansion (CTE) among all examined alloys. As the heat treatment holding time increases, the Si precipitates from the Al(Si)-supersaturated solid solution, leading to the formation of a skeleton-like Si phase, which effectively alleviates the thermal expansion. Furthermore, the SLM Al-50wt%Si alloy exhibits a lowest coefficient of thermal conductivity (CTC), since the significantly increased size of primary Si with increasing the heat treatment holding time leads to a substantial reduction in the number of grain boundaries, which severely restricts the scattering of heat transfer phonons. Moreover, theoretical models are employed to calculate the CTE and CTC. Results indicate that the Schapery lower model provides a better prediction of CTE, and the actual CTC values of Al-50wt%Si alloys after heat treatment are closer to theoretical values. These findings can provide new insights into designing Al matrix composites with low CTE and high CTC through microstructural optimization.

Scandium is very expensive and limits the application potential of Sc containing alloys. A novel high-strength Al–Mg–Mn-Sc-Zr alloy was designed for reducing Sc. Multiple strengthening mechanisms were utilized for designing this alloy, especially for the solid solution strengthening which benefits from ultrahigh cooling. With the addition of Mn and Si, the strength of our alloy reaches comparable level of similar alloys while the Sc content is 40–60 % less than them. The effect of Mn addition on microstructure and mechanical properties were carefully studied. Near full-dense samples with relative density >99.88 % were obtained with the help of overlapping prediction model. The continuous Mg2Si network was observed with Si introduction and broken up with increasing Mn content. According to the tensile experiment results and corresponding calculations, it was considered that the thermal stability and the effect of precipitate strengthening was slightly deteriorated. However, obvious improvement of the tensile strength was observed with increasing Mn addition, and the ductility maintains in a certain range. After aging at 280 °C for 4 h, the sample of 1.4 wt% Mn exhibits the highest ultimate strength of 497.08 ± 7.22 MPa, and possesses a considerable ductility of 8.01 ± 0.34 %. The Portevin-Le Chatelier (PLC) effect was observed in stress-strain curves, which reflects a dynamic strain aging of mobile dislocations and solute atoms. After aging, the stress amplitude and waiting time of serration flow decrease obviously, because of the introduction of bypassing precipitates and reduction of diffusion time. Compared with other similar alloys, our alloy exhibited comparable yield strength and elongation, but considerable advantages in material costs resulting from lower Sc content. It will offer not only an idea for alloy design utilizing the ultra-high solid solubility of elements, but also a feasible way to further expand application field of high-strength Al-based alloys through reducing the material costs.

In this paper, the microstructural evolution and failure behavior of GH4169 superalloy repaired using wire-fed electron beam directed energy deposition (EB-DED) are investigated in detail. Results show that in the deposited zone (DZ), the Laves phase is formed in the interdendritic zone, and the γ'' phase and γ′ phase precipitate around the Laves phase and are distributed gradiently along the building direction. In contrast, the substrate zone (SZ) is composed of fine equiaxed grains and uniform distributed γ'' and γ′ phases. Direct aging heat treatment cannot effectively dissolve the Laves phase, but can further precipitate the γ'' and γ′ phases, resulting in an increase in yield strength and ultimate tensile strength and a decrease in elongation. During the plastic deformation process, dislocation activity preferentially occurs in the γ matrix and forms accumulation at the Laves/γ interface, leading to the formation of cracks in the DZ, so that all tensile samples fail in the DZ. In addition, this study further elucidates the fracture mechanism of the Laves phase within grains and at grain boundaries.

Titanium/steel multi-material, given full play to their superiorities, has tremendous value for parts exposed to complexity service environments, yet invariably, its common joining techniques are lack of design freedom. To this end, we employ electron beam freeform fabrication (EBF3) to successfully prepare this multi-material system for the first time, whose interfacial microstructure and property are systematically investigated. Applying Cu as an interlayer, instead of causing extensive cracking and delamination at the interface of Ti64/316L, can effectively suppress the formation of continuous Fe–Ti intermetallic compounds (IMCs), and thus improve the strength of Ti64/Cu/316L interfaces. For the Cu/316L interface, besides the characteristic spherical Cu-rich and Fe-rich solid solutions, a few Fe–Ti IMCs appear within the Fe-rich solid solutions owing to the long-range diffusion of minor Ti atoms. Correspondingly, the dendritic Cu-rich solid solutions can also be found at the Ti64/Cu interface, which is regarded as a critical interface due to the concomitant of complex Cu–Ti IMCs. These Cu–Ti IMCs have less negative effect on the Ti64/Cu interface property relative to Fe–Ti IMCs, and the local strain produced by the deformed α-Ti near the interface and Cu-rich solid solutions can effectively relieve the concentration of residual stress. Consequently, the Ti64/Cu interface exhibits maximum micro-hardness to 490 HV and superior shear strength to 196.5 ± 2.2 MPa, which was attributed to the reinforcement for the tip of keyhole molten pool, as well as the synergy between Cu-rich solid solution and interdendritic Cu–Ti IMCs.

Dual-wire-fed electron beam directed energy deposition (EB-DED) is a highly efficient technique for additive manufacturing (AM) of TiAl alloys, but research into the evolution of microstructure and mechanical properties within the melt pool is limited. Experimental and numerical simulations are used together to study the evolution along the vertical direction within the TiAl alloy melt pool. The chemical composition, microstructure and microhardness in the top region (TR), middle region (MR) and bottom region (BR) within the melt pool were characterized and comparatively analysed. The results showed that the Al content decreased from 52 at. % to 49 at. % from the top to the bottom, the grain size increased and then decreased, and the grain aspect ratio increased from 1 to over 3. The decreasing size factor and increasing shape factor result in variations in grain size and shape. The improvement in microhardness from 226 HV0.5 to 280 HV0.5 was quantitatively analysed as being mainly due to the reduction in Al content. The results of this study are of paramount importance in facilitating the use of the dual-wire-fed EB-DED technique for the production of high performance TiAl alloy components.

In this paper, the effect of TB2 additions on a noval Al–Mg–Sc alloy was studied, and the microstructure and mechanical properties of SLM-fabricated TiB2/Al–Mg–Sc alloys were investigated. The results showed that molten pool width increased and molten pool depth slightly decreased with TiB2 increase, due to the laser absorptivity of the TiB2/Al–Mg–Sc alloys higher than that of Al–Mg–Sc alloy under the same process parameters. The microstructure of SLM-fabricated TiB2Al–Mg–Sc alloys were composed of fine-grained, coarse-grained and columnar-grained regions, and the fine-grained region increased with the increase of TiB2 content. The recrystallization regions in SLM-fabricated TiB2/Al–Mg–Sc alloy decreased with the increase of TiB2 content, and TiB2 additions used as heterogeneous nucleation to refine grains, which can pin grain boundaries and prevent recrystallization during SLM process. TiB2/Al–Mg–Sc alloy with 1% TiB2 additions showed a relatively higher comprehensive mechanical properties, and the tensile strength is 483 MPa, the elongation is 15.8%. Tensile strength of 1%TiB2/Al–Mg–Sc alloy was increased by 37 MPa and the elongation is also increased by 2.1% compared with 0%TiB2/Al–Mg–Sc alloy, and 2%TiB2/Al–Mg–Sc alloy shown a relatively lower comprehensive mechanical properties due to its lower density and TB2 agglomerated.

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.

In this paper,a high strength Al-4.82Mg-0.75Sc-0.49Mn-0.28Zr (Al–Mg-Sc-Mn) alloy was prepared by selective laser melting (SLM). The effects of laser power on microstructures, relative density, and mechanical properties of SLM-fabricated Al–Mg-Sc-Mn alloy were studied. The results shown that the bimodal structure with an ultra-equiaxed grain and columnar crystal is formed in Slammed-fabricated Al–Mg-Sc-Mn alloy. The columnar crystal is surrounded by ultra-fine equiaxed crystal, and the columnar crystal shows (001) preferred orientation, the ultra-fine equiaxed structure shows no anisotropy in SLM-fabricated alloy. With increase of laser power, the molten pool size increases during SLM process, and the samples relative density and hardness decrease as laser power increase. The relative density of SLM-fabricated Al–Mg-Sc-Mn alloy is 99.74% under the process parameters of laser power 310W and scanning speed 1200 mm/s, and the hardness of alloy is as high as 107.73 ± 1.27 HV. The tensile strength of SLM-fabricated Al–Mg-Sc-Mn alloy samples are all higher than 400 MPa, and elongations are more than 19%. The fracture morphology of SLM-fabricated Al–Mg-Sc-Mn alloy is a typical ductile fracture feature, which shows a high comprehensive mechanical property. Supersaturated solid solution is formed in Al–Mg-Sc-Mn alloy because of rapid cooling during SLM process, resulting in grain refinement and improvement of mechanical properties. Fine-grain strengthening and second phase strengthening are the main reasons for the improvement of mechanical properties of SLM-fabricated Al–Mg-Sc-Mn alloy.

A TiB2/AlMgScZr composite was fabricated by selective laser melting (SLM) with direct powder mixing. The microstructure and mechanical properties of SLM-fabricated TiB2/AlMgScZr composites were investigated. Nearly fully dense and crack-free SLM-fabricated TiB2/AlMgScZr parts were obtained. The results showed that TiB2 formed a coherent relationship with the matrix and was swallowed by the solid–liquid interface as a nucleation point to refine grains during the SLM process. In addition, some TiB2 particles did not form a coherent relationship with the matrix, moving the solid–liquid interface at the grain boundary, which reduced the recrystallization fraction and refined the grains significantly. TiB2 also promoted the precipitation of Al3Sc and Al3Zr phases and increased the volume fraction of the Al3Sc and Al3Zr phases during the SLM process. The SLM-fabricated TiB2/AlMgScZr composites showed a very high ultimate tensile strength of ∼483 MPa and excellent ductility of ∼15.8%, which are higher than those of wrought Al alloys. The solid solubility of the Mg element slightly increased in the SLM-fabricated TiB2/AlMgScZr composites because of the high heating and cooling rate during the SLM process. The high heating and cooling rate caused discontinuous strain and Portevin–Le Chatelier effect and weakened the strengthening effect of TiB2 in SLM-fabricated TiB2/AlMgSc composites.

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.

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 this paper, high-silicon Al–42Si alloy was prepared by selective laser melting (SLM) with different process parameters. Microstructures evolution and defects formation were studied and process parameters were optimized. The results shown that the density of SLM-fabricated Al–42Si alloy increases as input energy density increases. The highest and lowest density of SLM-fabricated Al–42Si alloy are obtained, when input energy density is 42.9J/mm3 and 33.8J/mm3 respectively. The microstructures of Al–42Si alloy fabricated by selective laser melting is mainly composed of primary silicon phase and eutectic silicon phase, which is distinct from casting alloy because of diffient grains size and shapes of the primary silicon. With higher energy density, larger size of the primary silicon observed during process due to higher heat released by powder. The size of primary silicon phase particles is in the range of 2.9–9.4μm, and the size of molten pool during SLM process is in the range of 125 ± 10μm–140 ± 10μm in this study. Also the hardness of SLM-fabricated Al–42Si alloy increases as input energy density increases between 40.0J/mm3 and 42.9J/mm3. After heat treatment, the residual stress is eliminated, microstructure stability and homogeneous of SLM-fabricated Al–42Si alloy are improved. The silicon distribution is more uniform and sizes increases about 1∼2μm, and the hardness decreases after heat treatment. The optimal SLM parameters for Al–42Si alloy are laser power of 320W, scanning speed of 1355 mm/s, layer thickness of 50μm and scanning space of 110μm.

In this study, Ti-6Al-4V matrix composites reinforced with TiB ceramic whiskers were in situ synthesized and hydrogenated using the melt hydrogenation technique (MHT). The effects of MHT on the microstructure evolution and hot compression behavior of the composites were investigated by optical microscopy (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Hot compression tests were performed at strain rates of 0.1/s, 0.01/s, and 0.001/s and temperatures of 800 °C, 850 °C, and 900 °C; the hot workability of composites significantly improved after hydrogenation, for example, the 900 °C peak flow stress of hydrogenated composites (43 MPa) decreased by 53.76% compared with that of unhydrogenated ones (93 MPa) at a strain rate of 0.01/s. Microstructural observations show that MHT can effectively facilitate the dispersion of TiB whiskers and induce the α/β lath refinement of the matrix in our as-cast hydrogenated composite. During hot compression, MHT effectively promoted the as-cast composite microstructure refinement, accelerated the dynamic recrystallization (DRX) generation, and reduced the stress concentration at the interface between the reinforcement and matrix; in turn, the hydrogenated composites presented low peak stress during hot compression.

Single tracks of Inconel 718 were manufactured by selective laser melting (SLM) under the same volumetric energy density (VED), but via various scanning speed and laser power, with the aim of discerning the influence of the melting mode of molten pools on their solidification behavior, microstructure and nano-hardness. For this purpose, the geometric appearance of each Inconel 718 single track was firstly investigated. The results indicate that the molten pool geometries are significantly different despite melting under the same VED condition; the typical melting modes of molten pools, including the keyhole and conduction modes, can be obtained. The solidified microstructures of both modes evolve from cellular at the bottom to cellular dendrites at the top; but the dendrites are more developed under the keyhole mode. In addition, the grains of both modes follow the epitaxial columnar crystal growth pattern along the boundary of molten pool. The preferential growth direction of grains under the conduction mode is closer to the direction, but the grains orientation of the keyhole mode tends to be and direction. Moreover, in comparison with the conduction mode, the average nano-hardness for different regions of molten pools corresponding to the keyhole mode is slightly higher.

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.

The microstructures and morphologies of directionally solidified Ti–47Al alloys with different growth rates ranging from 1 to 200 μm/s were investigated using the Bridgman directionally solidified method. The results showed that numerous columnar grains were formed along the growth direction with the onset of directional solidification. With a variation in the growth rate, the solid/liquid interface changed from a flat to cellular and to dendritic interface. The flat-to-cellular interface transition rate of the Ti–47Al alloy varied from 1 to 3 μm/s. When the growth rate was higher than 10 μm/s, the solid/liquid interface showed typical dendritic growth. During the directional solidification process, the main phase of the directionally solidified Ti–47Al alloy was the α phase, which can be attributed to the solute segregation, supercooling of the components, and contamination of the alloy melt by the Y2O3 ceramic shell. After reaching the steady growth state during the directional solidification process, the solidification path of the alloy was: L→α→α+γ→(α2+γ) + γ. With an increase in the growth rate, the primary dendrite spacing (λ) and lamellar spacing (λs) of the alloy decreased gradually.

A novel Co-free non-equiatomic Ni2FeCr medium-entropy alloy (MEA) was designed, and the Hall–Petch strengthening by grain boundaries and annealing twin boundaries was investigated. For this purpose, the alloy was prepared by cold rolling and recrystallization at 873–1323 K for 40 min–6 h. Annealing at different temperatures revealed that Ni2CrFe alloy forms a stable face-centered cubic (FCC) solid solution. Mean grain sizes (excluding annealing twin boundaries) and mean crystallite sizes (including both grain and annealing twin boundaries) were determined using the linear intercept method and the equivalent circle diameter in electron back-scattered diffraction (EBSD) soft. Tensile tests at 293 K indicated that the Hall-Petch slopes of grain sizes and crystallite sizes are 673 and 544 MPa μm1/2, respectively, and this contribution was then subtracted from the overall strength to calculate the intrinsic uniaxial lattice strength (90 MPa). Additionally, tensile tests, performed between 293 K and 873 K, revealed that the Ni2CrFe MEA has a stronger resistance to softening at high temperatures. Transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at annealing twin boundaries, indicating that it is also an obstacle to dislocation slip. Furthermore, the thickening of the annealing twin boundary after deformation was observed and illustrated by the interaction between different dislocations and annealing twin boundaries.

Because of their superior mechanical performance at ultra-high temperatures, refractory niobium–silicon-based alloys are attractive high-temperature structural alloys, particularly as structural components in gas turbine engines. However, the development of niobium–silicon-based alloys for applications is limited because of the trade-off between room temperature fracture toughness and high-temperature strength. Here, we report on the fabrication of a Nb-18Si alloy with dispersion of hafnium carbide (HfC) particles through selective laser melting (SLM). XRD and SEM-BSE were used to examine the effects of scanning speed on the microstructure and the phase structure of the deposited Nb-18Si-5HfC alloy. The results show that when the scanning speed rises, the solid solubility of the solid solution improves, the interlamellar spacing of eutectics slowly decrease into nano-scale magnitude, and the corresponding hafnium carbide distribution becomes more uniform. We also discover the hafnium carbide particles dispersion in the inter-lamella structure, which contributes to its high fracture toughness property of 20.7 MPa∙m1/2 at room temperature. Hardness and fracture toughness are simultaneously improved because of the control of microstructure morphology and carbide distribution.

The addition of interstitial elements generally has a significant impact on the macroscopic performances of structural alloys. Here, we report a systematic study on the microstructure, corrosion behaviors and mechanical properties of a series of as-cast Ti-0.3Mo-0.8Ni- x C ( x = 0.09, 0.12 and 0.15 wt.%) alloys. A typical basket-weave microstructure appears in all specimens, characterized by the lamellar α -phase and intergranular retained β -phase without TiC precipitates within the prior β grain boundary. Based on the electrochemical and immersion tests, doping with C element can significantly improve the corrosion resistance of TA10 alloys; with increasing the carbon content, the passivation current density decreases from 5.750 to 4.938 μ A·cm ^−2 , the breakdown potential increases from −1.115 to −1.223 V versus SCE, the corrosion rate reduces from 15 mm/a to 7.5 mm/a. Moreover, the mechanical properties of TA10 alloys, such as the strength and hardness, can also be enhanced via C alloying. Our results indicate that the addition of interstitial elements should be a potential effective mean to accommodate the comprehensive performances of TA10 alloys.