During the deformation of body-centered cubic (BCC) structured lightweight refractory high-entropy alloys (LRHEAs), strain localization caused by a low strain-hardening rate (SHR) induces premature alloy necking, resulting in poor uniform tensile ductility (UTD) and restricts their processability and applicability. In this study, we improved the SHR of the alloys from negative to 1.5 GPa by tailoring multi-scale heterostructures, including the microscopic bimodal grain distribution, submicron spherical C14 Laves phase, nanoscale local chemical fluctuations (LCFs), and atomic clusters less than 1nm. The strength of the alloy was raised by 13.8%, and the UTD increased by 710% compared with the initial homogenized sample, and overall performance was superior to most LRHEAs. Bimodal grain interfaces can effectively coordinate the strain distribution between the two during deformation, accelerating the generation and storage of geometrically necessary dislocations (GNDs), and the back stress accumulates and increases with strain, stabilizing the hardening ability. Meanwhile, the meticulously dispersed C14 Laves phase plays a role in precipitation strengthening without compromising plasticity. The matrix's LCFs and Al-Zr atomic clusters can further regulate the morphology and distribution of statistically stored dislocations (SSDs). On the one hand, they could effectively pin dislocations and cause them to bend, increasing the migration resistance of SSDs; on the other hand, dislocation tangles resulting from microbands blocking and the interaction of multi-slip systems activate new dislocation sources, which lead to the rapid expansion of secondary microbands in a reticular manner. Those significantly increase the synchronous dislocation multiplication rate and dynamic dislocation density during plastic deformation, maintaining high and sustained SHR of alloys. Therefore, the SHR of LRHEA can be effectively improved by introducing multi-scale heterogeneous structures to optimize the coordination of GND and SSD density and distribution, thus achieving an excellent match between strength and UTD.

The distribution of TiB precipitates in laser additive manufacturing of boron-modified titanium alloys significantly affects the mechanical properties of the alloys. In this study, various distribution circumstances were achieved by controlling the boron content and cooling rate. With the slow cooling rate, TiB clusters at the grain boundaries tend to share [010] direction near perpendicular to the building direction of the samples. With rapid cooling and minor boron, orthogonal TiB whiskers precipitated from the supersaturated beta grains observe < 010>TiB\parallel<001>beta, (001)TiB\parallel(010)beta with the matrix, and the alloy possesses much higher strength than those at the slow cooling rate and equal plasticity with orthogonal dimples in the fracture.

Laser-directed energy deposition is extensively used for the production of titanium alloy components. These additively manufactured titanium alloys tend to show anisotropy owing to the coarse columnar prior-β grains formed under the ultra-high thermal gradient. Boron addition has proved to be a powerful method for controlling solidification grains. However, it is still limited due to the lack of quantitative regulation on the grain morphology. To explore the relationship between boron content and the grain morphology, boron-modified α + β titanium alloys Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) were laser-directed energy deposited as multilayer walls. Columnar-to-equiaxed transition (CET) and obvious grain refinement occur when boron content reaches 0.35 wt

Objective Additive manufacturing of Cu/Ni dissimilar metals, integrating high- thermal conductivity, high strength, high reliability, and low cost, is one of the best choices for the generation of liquid rocket engines, capable of achieving material- structure- function engine integration. Currently, the main challenges faced in the preparation of GH4169 using laser- directed energy deposition (LDED) technology on CuCrZr substrates are the high- laser reflectivity and thermal conductivity of copper alloys, as well as the considerable differences in the physical properties of the two alloys, making it difficult to achieve defect- free metallurgical bonding. Stable process manufacturing and control of microstructural properties are also challenges. Herein, we use surface pretreatment processes followed by additive manufacturing to solve the problems of high reflectivity of Cu alloys and interface metallurgical defects and prepare CuCrZr/GH4169 dissimilar metals. Methods This study utilizes five processes, namely thermal spraying, cold spraying, electroplating, physical vapor deposition (PVD), and powder spreading, for surface pretreatment of the CuCrZr alloy. On the pretreated surfaces of the CuCrZr substrates, three processes are employed for single-track cladding experiments; Process 1: laser power set at 4.5 kW, and scanning speed set to 1000 mm/min, without powder feeding; Process 2: laser power set at 4.0 kW, scanning speed set at 1000 mm/min, and a powder feeding rate of 35 g/min, and Process 3: laser power set at 4.5 kW, scanning speed set at 1000 mm/min, and powder feeding rate of 40 g/min. The optimal process is selected for overall additive manufacturing of CuCrZr/GH4169 alloy. Microstructural observations are conducted on the interface of single-track cladding specimens (perpendicular to the laser scanning direction) and the interface of overall dissimilar metals specimens. Ultrasonic nondestructive testing and tensile performance testing are performed on the dissimilar metals specimens. Results and Discussions 1) Single-track cladding deposition GH4169 experiments are conducted on CuCrZr substrates using different surface pretreatments. The results for thermal spraying (Fig. 6) and cold spraying (Fig. 7) samples are excellent . The thermal spraying samples formed a stable melt pool without microcracks, pores, or local unmelted areas at the interface. In the cold spraying samples, local unmelted areas appeared at the edges of the melt pool when the coating thickness increased to 150 mu m. However, the electroplating (Fig. 8) and PVD (Fig. 8) samples did not produce stable melt pools, and there were obvious microcracks and local unmelted areas between the coating and the weld track. The coating prepared by powder spreading process (Fig. 8) showed many unmelted adhesive powders around the weld track, poor continuity of the weld track, unstable melt pool fusion line, and small penetration depth. The thermal spraying experiment yielded the best results. By comparing the main parameters of single-track cladding, the optimal process was selected: thermal spraying was used to deposite the GH4169 alloy coating with 100 mu m thickness on CuCrZr substrate, Process 1 was used to remelt the first layer, followed by Process 2 for deposition. 2) The prepared samples exhibited good bonding characteristics between the two materials. Nondestructive testing results did not reveal major defects (Fig. 10). The interface area of the samples along the deposition direction from the bottom to the top parts can be divided into five regions: CuCrZr substrate, columnar crystal, diffusion, Cu element diffusion, and GH4169 regions (Fig. 11). The tensile strength of the laser- directed deposited CuCrZr/GH4169 dissimilar metals along the deposition direction was 280 MPa +/- 4.24 MPa, with the fracture occurring at the interface, slightly toward the copper alloy side, indicating a ductile fracture. Conclusions This study primarily investigates the impact of surface modification on the laser reflectivity of copper alloys. Various surface modification processes, including thermal spraying, cold spraying, electroplating, PVD, and powder spreading, were used to prepare nickel- based alloy coatings with different compositions and thicknesses on copper alloy surfaces. After determining the optimal preparation process, coating composition, and thickness, the overall material deposition process was explored. The main conclusions are as follows: 1) The results of the single-track cladding experiments showed that the thermal spraying is the preferred pre-treatment process to achieve defect- free bonding between the CuCrZr alloy and the GH4169 nickel- based high- temperature alloy with a stable melt pool. The GH4169 alloy coating prepared by cold spraying was less effective than that prepared by thermal spraying. The interfaces of single-track deposition after surface modification using PVD, electroplating, and powder spreading processes exhibited defects such as local unmelted areas, microcracks, and poor metallurgical quality. 2) The thermal spraying surface modification process on copper substrates can effectively avoid interface defects and forming difficulties under various process conditions, when the surface GH4169 coating thickness reaches 50-150 mu m, resulting in metallurgically bonded interfaces. After comparing the main process parameters using a radar chart, a CuCrZr substrate pretreated with a thermal spray coating (thickness of 100 mu m) was selected. Using Process 1 to remelt the first layer and Process 2 for deposition, samples with dense metallurgical bonding and good appearance were obtained. Nondestructive testing confirmed that internal defects met the GJB 1580A-2004 (Class AA) requirements. 3) The interface region of the prepared CuCrZr/GH4169 dissimilar metals is mainly divided into the CuCrZr alloy substrate, columnar crystal, unmelted powder, Cu element diffusion, and GH4169 alloy zones. The columnar crystal zone is formed by remelting the copper alloy. The unmelted powder zone consists of flaky and spherical particles, mainly from insufficiently melted GH4169 coating and powder. The tensile strength of the CuCrZr/GH4169 dissimilar metals along the deposition direction was 280 MPa +/- 4.24 MPa. The fracture of the tensile samples occurred at the interface, slightly toward the copper alloy side, indicating good bonding at the interface, with the fracture surface confirming ductile fracture.

Additively manufactured high-strength titanium alloys generally possess equal strength and lower plasticity compared to wrought alloys owing to the different microstructures formed in the aging treatment. To examine the formation mechanism of these microstructures, an ultra-high-strength titanium alloy TB18(Ti-4.2Al-5V-5Cr5Mo-1Nb) was prepared by laser direct energy deposition (LDED) and forging respectively, and the aging behaviors and microstructures were characterized and compared in depth. It is found that during aging, the precipitation of the LDEDed alloy is 1-2 h earlier than that of the wrought alloy, and precipitates primarily form at the reticular sub-grain boundaries. Fine short-rod alpha laths then form inside the sub-grains due to the inhibition of the reticulations. The sub-grain boundaries in LDEDed alloy are generated due to the local deformation and recovery of the inter-dendritic zone rich of Cr and O atoms and show high thermal stability in the solution treatment, which differs from that of the wrought alloys. These boundaries possess a dislocation density several times higher than that of the inner-grain zones and promote the prior precipitation of alpha laths with Type 2 orientations at the early stage of aging. In the tensile test of the aged alloys, the dislocations in the LDEDed alloy pile up at the alpha/(3 interface, which can cause stress concentration and damage the plasticity.

Lightweight refractory high-entropy alloy (LRHEA) has lower density and higher specific strength, which makes it very promising for future applications in rocket engine components. Their high-temperature resistance to softening and high microstructure stability in a wide temperature range are also essential to sustaining their performance due to being used in harsh, high-temperature situations. In this work, the high-temperature tensile behavior of Al0.3NbTi3VZr1.5 LRHEA fabricated by laser additive manufacturing has been systematically investigated from 400 degrees C to 800 degrees C, and the relationship between the deformed microstructure and the mechanical behavior was revealed, and analyzing the reasons for the thermodynamic instability of the solid solution in LRHEA are discussed in detail. It is found that the weakening of the high entropy effect at intermediate temperatures reduces the stability of the solid solution, and the accelerated diffusion between elements further promotes the solid solution destabilization, mainly manifested as the Laves phase and the local chemical fluctuation (LCFs). The high dislocation density introduced by the additive manufacturing technology leads to the irregular connection of the Laves phases within the grains, and it can be improved by grain growth. The high negative mixing enthalpy competes with the high-entropy effect, and temperature is the decisive factor. In this situation, the atomic clusters were promoted and induced spinodal decomposition for generating LCFs. The formation of LCFs exhibits solute competition with the emergence of the C14 Laves phase, and the phase proportions fluctuate at different temperatures. This work provides new thinking and attention for developing lightweight, heat-resistant materials.

Heat treatment is critical for enhancing the mechanical properties of high strength titanium alloys, especially for exploiting the potential of laser additive manufactured titanium alloys. In this work, the influence of the cooling rate of continuous cooling transition on microstructure evolution and mechanical behavior was investigated in TC17 titanium alloy fabricated by laser directed energy deposition (LDED) technology. It was found that the number density and orientation characteristics of grain boundary alpha phases (alpha(GB)) are jointly influenced by the cooling rate and the structure of beta/beta grain boundaries (GBs). The average number density (lambda(avg)) of alpha(GB) has a consistent trend with the degree of variant selection (DVS) for precipitated alpha clusters subsequently, which is attributed to the autocatalytic effect of the pre-existing alpha on the post-precipitated alpha phases. The largest lambda(avg) of alpha(GB) and the highest DVS of alpha clusters could be simultaneously obtained at a suitable cooling rate (4 degrees C/min). In that case, plenty of alpha/beta phase interfaces and dominant variant type ensure high strength, meanwhile, the combinations of activated multi-slip systems and varied crack propagation paths extend work-hardening to maintain greater plastic deformation. This paper provides a novel thought for designing customized heat treatments of LDEDed high strength titanium alloys, and more importantly, promotes the engineering applications of large and complex components prepared by additive manufacturing technology.

Lightweight refractory high-entropy alloys (LRHEAs) exhibit lower density and better ductility than other refractory high-entropy alloys, which bestows upon them substantial prospects for application across diverse engineering domains. laser directed energy deposition additive manufacturing (LDED) technology emerges as an ideal process for preparing such refractory metals owing to its exceptional attributes, such as rapid cooling rates and design flexibility. In this study, a new LRHEA Al0.3NbTi3VZr1.5 was fabricated by LDED, and a comprehensive investigation was conducted to explore the microstructure evolution and the plastic deformation mechanisms. A graded microstructure with body-centered cubic (BCC) matrix, Laves phases near grain boundaries, and coarsened omega particles in the matrix is detected in as-deposited samples. In solid solution treated (SST) samples, the Laves phase disappears, and the omega particles decrease in size, forming a nanoscale two-phase mixture with the matrix. The as-deposited samples exhibit distinct brittle fractures, with a fracture strength of 902 MPa and 1% fractured strain. While the SST samples exhibit a considerable fracture strain of about 25 +/- 2% with 1032 +/- 12 MPa yield strength and a specific yield strength of 180 MPa.g(-1).cm(3). The high strength is mainly attributed to the solid-solution strengthening, while the excellent ductility is achieved by activating the unique deformation mechanism, including the formation of multi-stage microbands and kinking. The refinement of microband spacing accommodates more plastic deformation, and cross-slip further refines the microbands spacing in three dimensions. Microband-induced kinking effectively relieves local stress concentration and slows fracture. This work provides new insights into the design and preparation of LRHEAs using additive manufacturing technology and makes a remarkable contribution to the comprehensive and in-depth understanding of their plastic deformation mechanisms.

Controlling the grain structure while avoiding the stray grain (SG) formation is the key to fabricate the nickel-based single crystal (SX) superalloy components during laser additive manufacturing. In this study, a new approach is developed to inhibit SG formation by the selection of optimizing the substrate crystallographic orientation. Besides, a numerical model of combined substrate orientation with the tendency of equiaxed grain formation during epitaxial growth processing is developed to predict the SG formation in the molten pool. Combining the results from both experiments and simulations, results indicate that (001)/[110] and (013)/[100] substrate orientation could suppress the formation of SGs effectively.

•An ultra-high strength titanium alloy was prepared with LDED and forging;•Segregation elimination of alloying elements was quantitatively characterized;•Microstructure evolution during thermal cycle of LDEDed alloy was analysized;•Microstructure and tensile properties of LDEDed and wrought alloys were compared.

Deformation kinking acts as an important complementary deforming mechanism which is usually discovered in low-symmetrical metallic materials with hexagonal close-packed (HCP) crystallographic structure. Interestingly, for titanium alloys, rather than taking place in alpha-Ti with HCP structure, deformation kinking prefers to occur in beta-Ti with the body-centered cubic (BCC) structure of higher symmetry. in this paper, the duplex titanium alloy with the widmannstatten microstructure was prepared, and uniaxially compressed at a strain rate of 3000 s(-1). Kink bands are independently observed for the first time in duplex titanium alloy. The 30 degrees<10<(1)over bar>0> KBs contain an unfavorable orientation for plastic deformation, while the KBs of about 35 degrees misorientation angle possess a favorable orientation, which consist of two subordinate deformation bands, i.e. 20 degrees<2<(2)over bar>01> and 15 degrees<1<(1)over bar>01> respectively. The detailed analysis of KBs is of great significance for both the fundamental understanding of duplex titanium deformation and their potential engineering applications.

Hybrid manufacturing technique by combining of the conventional manufacturing techniques with the additive manufacturing technique has an attractive potential to fabricate the large and complex components. In this study, hybrid manufactured TC11 titanium part was produced by depositing TC11 alloy on a rolled TC11 plate via directed energy deposition (DED) process. Microstructure, microhardness and tensile properties of hybrid manufactured TC11 samples were examined. The deformation behavior of the hybrid manufactured TC11 samples was also investigated using in-situ tensile test and digital image correlation (DIC). The results demonstrate that the hybrid manufactured TC11 sample can be divided into three zones: the laser deposition zone (LDZ), the heat affected zone (HAZ) and the rolled substrate zone (SZ). The gradient microstructure is formed in the HAZ along the deposition direction due to the different thermal conditions. A special bimodal microstructure consisting of coarse primary α (αp) phase platelet and ultrafine lamellar α phase has been generated in the upper HAZ. The texture of α phase shows the highest intensity in the easily-activated orientation in the LDZ with larger Schmid factor leading to fracture when loading along the deposition direction. The small grains size with super fine αs in the HAZ results in the highest strength and microhardness.

Titanium alloys are prevalently applied in aerospace, ship, and nuclear power fields due to their excellent properties, such as high special strength, good corrosion resistance, and fine properties at elevated temperatures. Additive manufacturing technique provides a revolutionary way to process the large-scale integrate key parts of titanium alloys with short period and low cost. However, the coarse columnar grains produced in the forming process lead to the anisotropy of the components, which restricts the full play of the properties of the alloys. The methods to control the size and morphology of the grains for the purpose of anisotropy reduction and mechanical property enhancement are the research hotspot in recent years. This study described the feature and formation mechanism of typical grain microstructures in general additive manufacturing methods and summarized the grain-control methods, including the parameter optimization, novel processing methods, micro-alloying/new alloy composition design, subsequent heat treatment, external field assistance, and the combination methods. The grain regulation mechanisms were summarized and the control effects were evaluated. This research progress provides direction and prospect for the further development in the control of solidified grains of additively manufactured titanium alloys.

Attributing to the fast solidification and thermal cycling during additive manufacturing process, massive phase transformation could take place thus leading to the formation of unpredictable microstructures with large variation in mechanical properties. In this paper, a continuous direct energy deposition technique (CDED) is adopted to prepare TiAl alloy seceding from the thermal cycling, and the evolution of micro-structure, microhardness and tensile properties of alloy are investigated. Results indicate that the micro-structure of alloy composes of columnar grains with very fine (alpha 2 +gamma) lamellae, and no heat-affect band is observed. A heat transfer model is established to assist in explaining the solid-phase transformation and microstructure formation mechanism of TiAl alloy. Furthermore, a special duplex gamma microstructure (DP gamma) forms which composes of massive gamma phase surrounded by feathery-like gamma phase. By dissecting crystallographic orientation from EBSD results, the formation mechanism of DP gamma is clarified as the result of sequential solid-state phase transition in (alpha + gamma) phase region, in which the feathery-like microstructure forms firstly followed by the massive gamma phases nucleating at the inter-phase boundaries of feathery-like gamma. Tensile property of as-deposited alloy reveals of 535 MPa with 1% elongation, and the highest hardness of around 320HV is detected at the top region of alloy with the finest interlamellar spacing. (C) 2022 Elsevier B.V. All rights reserved.

Boron addition is prevalently applied for additively manufactured titanium alloys. The aspect ratio of α laths in titanium alloys tends to decrease with boron addition, while the critical cause is not clear. To reveal the mechanism of this circumstance, Ti-6.5Al-3.5Mo-1.5Zr-0.3Si-xB (x = 0, 0.14, and 0.35) alloys were laser directed energy deposited as multi-layer walls, and the morphology, size, and variant selection of α laths before and after thermal cycle were characterized. It is found that the contribution of heterogeneously nucleated α on TiB to the decrease is limited. The α laths are initially refined due to grain refinement and TiB precipitates from boron addition. However, the layer-by-layer thermal cycle renders the selective coarsening of αp laths, and thus the α laths widen with boron addition. In addition, branching of α also assists to widen the laths. Therefore, spatial inhibition and selective coarsening in thermal cycle predominately account for the aspect ratio decrease of α laths. Boron addition weakens the variant selection in both unstable and stable zones, which assists to prove the mechanism of aspect ratio decrease for α laths.