Straight Grain Research Articles

Inhibition of interfacial cracks in 304L-Inconel718 bimetal fabricated via laser powder bed fusion

Multi-material additive manufacturing is crucial for intricate component fabrication, yet challenges, such as interfacial cracks and weak bonding, persist. This work investigated the laser powder bed fusion (L-PBF) of bimetallic components (stainless steel 304L-nickel-based alloy Inconel718) crucial in aerospace and nuclear applications. It is found that the interfacial cracks predominantly occur within the compositional transition zone where the proportion of 304L is between 45wt.% and 75wt.%, characterized by brittle Laves phases along grain boundaries. Experimental and finite element simulations of melt pool reveal that a higher ratio of temperature gradient (G̅) to the grain growth rate (R̅) (G̅/R̅) results in straight grain boundaries with underdeveloped secondary dendrites. This leads to the formation of continuous liquid film and strip-like Laves phase at grain boundaries, causing interfacial cracks during L-PBF. To suppress these cracks, this work proposes manipulating grain boundaries into a tortuous morphology through promoting the growth of secondary dendrites. By controlling the G̅/R̅ ratios below the critical value (<147.9×106K∙s/m2) and combining with a high cooling rate (G̅×R̅) during L-PBF, a well-developed secondary dendritic structure and grain refinement are achieved, significantly enhancing grain boundary tortuosity and forming discretely distributed Laves phases. As a result, interfacial cracks are completely suppressed, enabling the successful manufacturing of crack-free 304L-Inconel718 bimetallic components. The approach of tailoring the distribution of brittle precipitates through manipulating grain boundary morphology proposed in this work provides a novel and practical pathway for inhibiting cracks in multi-material additive manufacturing.

Combustion Characteristics of Vat-Photopolymerized Fuels with a Spiral Star Port for Hybrid Propulsion.

Despite hybrid rocket motors offering distinct advantages over solid or liquid rocket motors, their low regression rate and insufficient combustion efficiency remain significantly unimproved. This study focuses on the effects of the helix lead on the regression rate distribution and combustion efficiency of vat-polymerized fuel grains with a spiral star port for a hybrid rocket. Both experimental and numerical investigations were conducted to study the combustion characteristics and regression rate distribution of three-dimensional (3D) print grains. Spiral star grains with varying helix leads of 60, 90, and 120 mm were fabricated using light-curing 3D printing technology. A 3D simulation model was developed to obtain the temperature distribution, species mass distribution, and combustion efficiency. Furthermore, firing tests were performed on a two-dimensional radial hybrid combustion test stand to measure the regression rate. Digital image processing of computed tomography images was used to determine the regression rate. Simulation results indicated that the spiral star grain port helps to improve the combustion efficiency compared with those seen with round tube and straight star port grains. With an increase in the axial distance, the flame zone gradually shrinks, and the smaller the helix lead, the faster the shrinkage. At a mass flow rate of 1.50 g/s for oxygen, the regression rate of the spiral star grains is significantly higher than that of the straight star grain and the conventional round tubular grains, and the regression rate gradually increases with a decrease in the helix lead. This finding is expected to solve the problem of the low regression rate of solid fuels with spiral star pore-shaped grains prepared by the light-curing 3D printing method.

The comparison of densification mechanisms and microstructure evolution among TiB2 ceramics sintered under different levels of high pressure

TiB2 shows significant advantages in high-tech industry but it is hard to fulfill densification. High pressure sintering is an effective method to obtain TiB2 ceramics. This study reports the sintering condition of TiB2 ceramics under different levels of high pressure from 150 MPa to 15 GPa. The results show that the densification temperature reduced with the applied pressure. The densification mechanism of TiB2 ceramics sintered under 150 MPa during the final holding time at 1900℃ was dominated by the mixture of dislocation motion and grain boundary sliding. When the applied pressure achieves at 1.5 GPa, the densification mechanism was transformed into the plastic deformation of grains and grain boundary diffusion, a number of small angle grain boundaries, dislocation cells, substructures and straight grain boundaries were observed. Finally, the densification mechanism was transformed into plastic deformation of grains and grain boundary while the driving force increased to 15 GPa. The dislocation and substructures density increased with pressure, leading to the grain refinement. Our study compared the densification and microstructure evolution of TiB2 ceramics sintered under high pressure, proving the high pressure sintering could regulate the microstructure and refine grains.

Substantial toughening by thick nanoscale amorphous intergranular films in nanocrystalline materials

Amorphous intergranular films (AIFs) have been proven in experiments to improve the damage tolerance of nanocrystalline materials. However, a quantitative study is still lacking. Molecular dynamics simulations were performed here to investigate the effect of CuNb AIFs on the fracture toughness of nanocrystalline Nb. In order to clarify the role of AIFs, a bicrystal Nb model with one straight symmetrical tilt grain boundary and a mode-I crack in one of the grains was constructed, in which the AIF effect was introduced by replacing the normal grain boundary with a CuNb AIF. Then, AIF thickness-dependent tensile deformation of the bicrystal Nb samples was simulated. The work-of-fracture, which is defined as the released strain energy due to the newly generated unit area in the crack during stretching, was employed to quantify the fracture toughness of the bicrystal systems. The results show that the fracture toughness of the AIF sample can be tripled due to the blunted crack tip and the relieved stress concentration at the crack tip as compared to the AIF-free one that exhibits a brittle crack propagation behavior. Also, the thicker the AIFs, the more pronounced this reinforcing effect. More importantly, it is found that there exists a critical AIF width of 1.7 nm, below which the crack will eventually break through the AIF, and above which the crack failed to do this. It is revealed that the enhanced fracture toughness originated from the transformation of brittle crack propagation to abundant dislocation emission from AIFs.

The welding kinetics of interface evolution and mechanism in IN718 alloy solid-state welding

The welding kinetics of interface evolution and mechanism inIN718 alloy solid-state welding are investigated systematically by comparing the hot compression behavior of the separated samples and conventional samples in the temperature range of 1000 °C–1160 °C and strain rate range of 0.1/s to 5/s. The separated samples obtain higher peak stress than conventional samples, and the kinetics analysis results show that separated samples provide higher structural factors and deformation activation energy. The activation energy of the interfacial microstructure evolution of the IN718 alloy was calculated to be 29.7948 kJ/mol. The evolution mechanism of the interface microstructure is characterized by the elimination of the interface second phase and the migration of straight interfacial grain boundaries (IGBs), which is inseparable from the occurrence of interfacial dynamic recrystallization. Interfacial dynamical recrystallization helps to regulate the chemical potential around the second phase for a continuous dissociation. The migration mechanism of the IGBs is influenced by the strain rates, which is continuous dynamic recrystallization (CDRX) for high strain rates and discontinuous dynamic recrystallization (DDRX) for low strain rates.

A novel strategy creating serrated grain boundaries to improve ductility in a Fe–Cr–Al alloy

Grain boundaries serration has long been regarded as a way to improve mechanical properties at high temperatures in alloys. However, the traditional method of creating serrated grain boundaries (SGBs), solution treatment followed by slow cooling, could induce a significant coarsening behavior of the second phase, thereby adversely affecting the properties. In this work, a new strategy is proposed to obtain numerous irregular SGBs without producing coarser Laves precipitates (<400 nm) in Fe–Cr–Al alloys, in which these SGBs are formed through recrystallization annealing of a warm-rolled microstructure featuring kink bands (KBs). These SGBs can be divided into two types: low amplitude SGBs (LA-SGBs, in the range of 1∼10 μm), and high amplitude SGBs (HA-SGBs, over 20 μm). Through SEM and quasi in-situ EBSD observation, and combined with the Taylor factor model, the formation mechanism of irregular SGBs is revealed in detail. The pinning effect of Laves phase precipitated during annealing on the grain boundaries migration induces the formation of LA-SGBs. On the other hand, the discrepancy in deformation stored energy between the matrix and the KBs promotes the special nucleation and growth of new grains, thus resulting in the formation of HA-SGBs. Moreover, tensile tests are conducted at 298 K and 873 K. SGBs can significantly enhance ductility while maintaining strength, exhibiting an increase of 52.1% and 41.1% respectively. Compared to straight grain boundaries, at 298 K, SGBs can reduce the accumulation of the GNDs in the vicinity of themselves through the unique strain-hardening behavior mediated by high-density dislocation walls (HDDWs), alleviate the stress concentration, make the plastic strain distributions more uniform, and thus retard the crack initiation. As the deformation temperature rises to 873 K, the improved ductility is mainly ascribed to the deflection and passivation effects of SGBs on crack propagation during straining from necking to fracture while their effects on the strain-hardening stage are weakened.

Comparative Transcriptomic and Metabolomic Analyses of Differences in Trunk Spiral Grain in Pinus yunnanensis.

Having a spiral grain is considered to be one of the most important wood properties influencing wood quality. Here, transcriptome profiles and metabolome data were analyzed in the straight grain and twist grain of Pinus yunnanensis. A total of 6644 differential expression genes were found between the straight type and the twist type. A total of 126 differentially accumulated metabolites were detected. There were 24 common differential pathways identified from the transcriptome and metabolome, and these pathways were mainly annotated in ABC transporters, arginine and proline metabolism, flavonoid biosynthesis, isoquinoline alkaloid biosynthesis, linoleic acid metabolism, phenylpropanoid, tryptophan metabolism, etc. A weighted gene coexpression network analysis showed that the lightblue4 module was significantly correlated with 2'-deoxyuridine and that transcription factors (basic leucine zipper (bZIP), homeodomain leucine zipper (HD-ZIP), basic helix-loop-helix (bHLH), p-coumarate 3-hydroxylase (C3H), and N-acetylcysteine (NAC)) play important roles in regulating 2'-deoxyuridine, which may be involved in the formation of spiral grains. Meanwhile, the signal transduction of hormones may be related to spiral grain, as previously reported. ARF7 and MKK4_5, as indoleacetic acid (IAA)- and ethylene (ET)-related receptors, may explain the contribution of plant hormones in spiral grain. This study provided useful information on spiral grain in P. yunnanensis by transcriptome and metabolome analyses and could lay the foundation for future molecular breeding.

Effect of Grain Orientation on Microstructure and Mechanical Properties of FeCoCrNi High-Entropy Alloy Produced via Laser Melting Deposition.

The long, straight grain boundary of the high-entropy alloy (HEA) produced via laser melting deposition (LMD) is prone to cracking due to unidirectional scanning (single wall). To enhance the competitive growth of columnar grains and improve the overall performance of the alloy, a vertical cross scanning method was employed to fabricate FeCoCrNi HEA (bulk). The influence of grain orientation on the microstructure and mechanical properties of FeCoCrNi-LMD was systematically investigated. Microhardness tests and tensile tests were conducted to assess the mechanical property differences between the single-wall and bulk samples. This study shows that using a single scanning strategy results in monolayer wall grains sized at 129.40 μm, with a max texture strength of 21.29. Employing orthogonal scanning yields 61.15 μm block-like grains with a max texture strength of 11.12. Dislocation densities are 1.084 × 1012 m-2 and 1.156 × 1012 m-2, with average Schmid factors of 0.471 and 0.416. In comparison to the FeCoCrNi-LMD single wall, the bulk material produced through cross-layer orthogonal scanning exhibited reduced residual stress, weakened anisotropy, and improved mechanical properties. These findings are expected to enhance the potential applications of FeCoCrNi-LMD in various industries.

Advancements in processing of Ni-based superalloys by microstructure engineering via discontinuous γ′ break-down

Wrought γ′-strengthened Ni-based superalloys are critical materials for gas turbine disks in aircraft and power-generation applications. However, their high inherent strength makes their hot formability challenging, and this is often related to intergranular cracking. We propose that this type of crack formation may be mitigated by grain boundary serrations which has not yet been demonstrated in superalloys. Here, we introduce engineered grain boundary serrations in a Ni-based superalloy via discontinuous γ′ precipitation during slow cooling after super-solvus solutionising. We compare the formability of this microstructure to a counterpart with globular γ′ and straight grain boundaries, during sub-solvus compression at 950–1130 °C. We demonstrate that the microstructure with discontinuous γ′ shows excellent cracking resistance, while the one with globular γ′ develops severe intergranular cracks during compression ≤1100 °C. The discontinuous γ′ microstructure exhibits up to 26% lower peak flow stresses and recrystallises at temperatures ≤1100 °C, compared to the globular γ′ microstructure where recrystallisation does not start until 1130 °C. Compression of the discontinuous γ′ microstructure at 1000–1050 °C yields recrystallised fractions of >75 vol.% and results in a duplex γ-γ′ microstructure with grain diameters <4 μm. We argue that its significantly improved formability results from these engineered serrated grain boundaries, the coarsened γ′ morphology, energy absorption by the break-down of discontinuous γ′, and the resulting development of a fine-grained duplex γ-γ′ microstructure. Integrating such discontinuous γ′ precipitation into advanced manufacturing routes will facilitate or, in some cases, even enable manufacturing of parts for aircraft turbines and gas engines at lower temperatures and forces.

Wood color modification with iron salts aqueous solutions: effect on wood grain contrast and surface roughness.

Wood is a biosourced material with unique aesthetic features due to its anatomy and chemical composition. White oak wood surface color can be modified with the use of iron salts, which react with wood phenolic extractives, present as free molecules in wood porous structure. The impact of modifying wood surface color with iron salts on the final appearance of wood, including its color, grain contrast and surface roughness, was evaluated in this study. Results showed that following the application of iron (III) sulphate aqueous solutions on white oak wood surface, its roughness increased, which is due to grain raising after wetting of wood surface. The color modification of wood surface with iron (III) sulphate aqueous solutions was compared with a non-reactive water based blue stain. The contrast associated to wood grain that was expressed by the standard deviation of luminance values in wood images, also increased after application of the iron (III) sulphate aqueous solution on white oak wood surface. The comparison of contrast changes showed that wood samples stained with iron (III) sulphate on their curved surface had the highest increase in grain contrast compared to iron-stained wood showing the straight grain and to wood surfaces colored by a non-reactive water-based stain for both curved and straight grains.

Microstructure, properties and deformation mechanism of HCCM horizontal continuous casting Cu–Ni–Co–Si alloy strip during cold rolling

Cu-1.3 wt%Ni-1.0 wt%Co-0.9 wt%Si-0.1 wt%Mg alloy strip with 10 mm in thickness produced by heating-cooling combined mold (HCCM) horizontal continuous casting was cold rolled. Microstructure evolution and mechanical properties of the cold-rolled alloy strips with different reduction were studied, and the deformation mechanism was clarified. The results showed that the continuous casting strips had axial columnar grain with main <001> orientations and straight small angle grain boundaries, the elongation after fracture was up to 25.0%, which can be directly processed by large deformation cold rolling, and the maximum cumulative cold deformation was up to 99%.When the reduction was 40% and 60%, the dislocation bands crossed each other and shear bands formed, and the alternating distribution of soft/hard oriented grains were conducive to improve the deformation coordination ability of micro-region under high strain. When the deformation exceeded 80%, deformation twins formed and the twins had strong interaction with the shear band and grain boundaries, resulting in strong shear and rotation of the local grains. The tensile strength and hardness increased from 464 MPa to 137HV of the as-cast strip to 629 MPa and 208HV with reduction 90%, respectively, while the elongation decreased from 25.0% to 4.1%. These results can lay a foundation for the development of compact method for producing high performance Cu–Ni–Co–Si alloy strips.

Influence of microstructure on the micro-region fracture toughness of the 30Cr2Ni4MoV turbine rotor welded joint

The fracture toughness of each micro-region in 30Cr2Ni4MoV steam turbine rotor welded joint was studied, including the central zone of the SAW bead (SAW-C), the overlapping zone of the SAW bead (SAW-Z), and the central zone of the TIG weld metal (TIG), coarse-grained heating affected zone (CGHAZ), fine-grained HAZ (FGHAZ) and base metal (BM). The microstructure, fracture morphologies and cracking paths were observed by OM and SEM microscope, and the influence of microstructure on fracture toughness was analyzed. The research results show that the fracture toughness of the weld metal is lower than that of BM and HAZ, and the fracture toughness of the weld metal is non-uniform. TIG and SAW-C are composed of columnar tempered bainite, intergranular tempered lath martensite and carbides, and the fracture toughness is the worst. SAW-Z is zigzag columnar tempered bainite and equiaxed granular tempered bainite between weld passes, which has good fracture toughness. CGHAZ and FGHAZ are equiaxed tempered martensite with the best fracture toughness. By observing the fracture morphology and cracking path, SAW-C and TIG are dominated by brittle cleavage fracture mode, and SAW-Z, FGHAZ, CGHAZ and BM are in the ductile fracture mode. The straight columnar grains of SAW-C and TIG are prone to crack propagation, but the tempered bainite acts as a bridge. SAW-Z has a zigzag tempered bainite grains, and the crack propagation path is tortuous. For FGHAZ, CGHAZ and BM with equiaxed grains, the fracture toughness is related to the grain size and the dimple size around the crack tip. The crack propagation process is that the dimples are first formed in a certain range around the crack tip, but only the dimples in the lower strength micro-regions grow and merge preferentially, and rapidly form new cracks.

Influence of Grain Boundary Morphology on DDC Sensitivity of Nickel Base Alloy Deposited Metal

Effect of grain boundary morphology on ductility dip cracking (DDC) sensitivity of nickel base alloy inconel52 deposited metal was researched by welding thermal simulation method and high temperature tensile test. The sample was hold at 1300 °C for 2S ~ 10s and then stretched at its DDC sensitive temperature 1050 °C at different tensile rates. The DDC sensitivity was compared by reduction of area (VoA) of tensile test sample. The results show that straight grain boundary reduces VoA, precipitates in grain boundaries increases VoA, and VoA increases with the increase of tensile rate. Straight grain boundary causes stress concentration and strain localization at the trigeminal grain boundary, curved grain boundary decreases the maximum Mises stress which make more uniform stress distribution. Precipitates on the grain boundary can play a role of locking the grain boundary migration and disperse the strain concentration at the trigeminal grain boundary. The lower the strain rate, the longer the deformation time, which will lead to decrease of dislocation movement rate. The smaller the critical shear stress of grain boundary sliding, the smaller the deformation resistance, and the full progress of dislocation movement and climbing. Effect of strain rate on DDC needs more research.

Influence of Grain Orientation and Grain Boundary Features on Local Stress State of Cu-8Al-11Mn Alloy Investigated Using Crystal Plasticity Finite Element Method.

Reducing the local stress in the vicinity of the grain boundaries is a favorable way to improve the super-elastic properties of super-elastic alloys. The crystal plasticity finite element method (CPFEM) was applied in this study to simulate the deformation behavior and local stress of a super-elastic Cu-8Al-11Mn (wt.%) alloy containing single grains with various orientations, columnar grains with different misorientation angles, and tri-crystals with distinct grain boundary morphologies. The results indicated that the stress distribution of single grains presented obvious orientation dependence during deformation. Uniformly distributed stress was observed in grains with orientations of 0° and 90° when more slip systems were activated during deformation. With the increase in the misorientation angles of columnar grains, the stresses in the vicinity of the grain boundaries increased, which was related to the difference in the shear stress of the slip systems in adjacent grains. When the difference in the shear stress of the slip systems in two adjacent grains was large, a local stress concentration formed in the vicinity of the grain boundary. Compared with the triple-junction grain boundaries, the local stresses of the straight and vertical grain boundaries were smaller, which was closely related to the number of activated slip systems on both sides of the grain boundary. The above results were obtained experimentally and could be used to design super-elastic alloys with high performance.

Interfacial voids and microstructure evolution, bonding behavior and deformation mechanism of TC4 diffusion bonded joints

The interfacial voids closure and related microstructure evaluation of diffusion bonding process was studied by the method of combining simulation and experiment on TC4 joints. It was found that the interfacial microvoids of joints transformed narrow oval to rounded circle with the increase of bonding time. Meanwhile, the distance of two adjacent interfacial microvoids increased, indicating the improvement of the bonded ratio. At the tip of microscopic convex region, the initial boundary was replaced by recrystallized grains. At the voids neck region, the initial bonding interfaces were replaced by straight grain boundary for the inadequate deformation, resulting from the incomplete driven force of recrystallization and grain boundary migration. The bonded interface developed from straight grain boundary to grains across the original interface. The evolution of Mises equivalent stress fields and the local strain fields of the interfacial region were by Finite Element Analysis (FEA) method. The results indicated that the stress at the asperity tip region was always higher than that at the neck of the void. The distribution of dislocations on the bonding interface region demonstrated that the grain boundary migration was controlled by the multiple slip of dislocations, which controlled by Power-Law Creep mechanism.

Prolonged grain boundary sliding in naturally deformed calcite marble at the middle crustal level

It is generally accepted that grain boundary sliding (GBS) is the dominant deformation mechanism in fine-grained strain localization zones. However, the lifetime of GBS during deformation in monomineralic rocks is hotly debated, as grain growth may inhibit the operation of GBS. In this contribution, recrystallized matrix grains in calcite marbles from the Jinzhou detachment fault zone at the middle crustal level, Liaodong Peninsula, Northeast China, show clear evidence of GBS, e.g., 1) fine grains with sizes of 5–30 μm, nearly equant grain shapes and slightly curved grain boundaries, 2) rare evidence for intracrystalline deformation, 3) existence of four-grain junctions and aligned straight grain boundaries, and 4) weak crystallographic preferred orientations. The existence of high misorientation zones along the margins of the matrix grains, especially at their triple junctions, and differences in cathodoluminescence (CL) characteristics between rims and cores of the grains indicate simultaneous accommodation of GBS by dislocation motion and grain boundary diffusion. The different CL characteristics between rims and cores of calcite matrix grains and the presence of calcite-filled cracks imply fluid activity during deformation. Fluids could dramatically enhance GBS by promoting dislocation motion and grain boundary diffusion. Additionally, the nucleation of fine grains by dynamic recrystallization and cavitation-sealing processes during GBS guaranteed the maintenance of GBS. At the same time, the occurrence of micropores and the existence of second phases also inhibit grain growth. These features make GBS the dominant deformation mechanism in naturally deformed marbles in the middle-upper crust.

Grain refinement and enhanced diffusion of W[sbnd]Cu gradient material processed by high pressure torsion with floating cavity

High-pressure torsion (HPT)processing with floating cavity was carried out under the applied pressure of 1.5 GPa with the revolution from 5 to 20 turns at the temperature of 300 °C, and the WCu gradient material with noble bonding interface and significant grain refinement was successfully obtained. The microstructure evolution and atomic diffusion around the bonding interface were analyzed by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) in a field emission scanning electron microscope (SEM). The mechanism of large shear strain on the microstructure evolution and diffusion ability of tungsten and copper at the interface was investigated. The results show that HPT processing with floating cavity can effectively refine microstructure of tungsten and copper to submicron. After 5 turns of HPT deformation, the grain size of copper was significantly refined to 0.59 μm but experiences slight increases to 0.8 μm with the additional revolution number to 20 turns, and the ultrafine equiaxed grains with relatively straight grain boundaries and few dislocations within grains were formed under the continuous dynamic recrystallization mechanism. However, the microstructure of tungsten was continuously refined within 20 turns of HPT processing due to the accumulation shear deformation, and a lamellar microstructure was formed along the WCu bonding interface with a gradient distribution in the width of banded grains from 0.06 μm at the interface to 0.33 μm towards the tungsten matrix. In addition, with the increase of torsion strain, the diffusion lengths of tungsten and copper gradually increasing to 1.6 μm and 6.2 μm, the interdiffusion coefficient of tungsten in copper remained basically constant, while the value of copper in tungsten increases continuously. The high density of defects and ultrafine grains with high angle boundaries introduced by large shear deformation of 20 turns of HPT processing play roles in the rapid diffusion paths to accelerate the atom diffusion and the interdiffusion coefficient of tungsten and copper is enhanced by 0.48– 1.79× 108 times compared to the lattice diffusion.

Tensile fracture behaviors of a laser powder deposited Fe–30Mn–10Cr–10Co–3Ni high-entropy alloy: In situ x-ray computed microtomography study

The defect evolution and fracture mechanism of a laser powder deposited Fe–30Mn–10Cr–10Co–3Ni alloy in the course of uniaxial tensile tests was investigated by in-situ high-resolution computed microtomography (CT) and electron back-scattered diffraction (EBSD). The influences of surface wrinkling and the long and straight grain boundary on the defect evolution and fracture mechanism were discussed emphatically. The results showed that pores were not the source of crack initiation. During plastic deformation, the pore volume did not change significantly, but the pore shape varied. The development of voids was observed only in the zones with strain localization. However, the plastic damage voids were not the source of fracture cracks, neither growing nor aggregating into macroscopic cracks. The cracks in the specimens were initiated at the long and straight grain boundary with an angle of about 45° relative to the applied load and the Taylor factors on both sides of the grain boundary differed greatly. Severe plastic damage occurred at the surface and near-surface of the tensile specimens as a result of surface wrinkling. The void volume fraction of a superficial zone was several times higher than that in the bulk of the specimens, and there were many cracks with an angle of about 45°relative to the applied load. The tensile fracture behaviors of the laser powder deposited Fe–30Mn–10Cr–10Co–3Ni alloy were different from the microvoid coalescence fractures of traditional polycrystalline ductile alloys, and the fracture in the former ones was caused by the propagation of surface and near-surface cracks. Fractures had neither shear lips nor radial zones. The fundamental understanding will provide a theoretical basis for future fabrication and post treatment to enhance the mechanical properties of similar alloys.

Effects of surface roughness on interfacial dynamic recrystallization and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo alloy joints produced by hot-compression bonding

The influence of surface roughness on the interfacial dynamic recrystallization kinetics and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo hot-compression bonding joints was systematically investigated. It is found that for the bonding interface of rough surfaces, elongated fine grains are formed at the bonding interface due to shear deformation of the interfacial area. As the surface roughness increases, the proportion of elongated grains drastically decreases as they further reorient to form equiaxed grains along the bonding interface of rougher surfaces resulting from severe incompatible deformation of the interface area. Meanwhile, high-density geometrically necessary dislocations accumulate around the interfacial recrystallization area to accommodate the incompatible strain and lattice rotation. A rotational dynamic recrystallization mechanism is thereby proposed to rationalize the formation of fine interfacial recrystallization grains during bonding of rough surfaces. In contrast to that of rough surfaces, bonding interface of polished surfaces exists in the form of straight interface grain boundaries without fine grains under the same deformation conditions. While with the increase of deformation strain, small grain nuclei form along the bonding interface, which is associated with discontinuous dynamic recrystallization assisted by strain-induced boundary migration of interface grain boundaries. Moreover, the bonding joints of rough surfaces show lower elongation compared with that of polished surfaces. This is because the formation of heterogeneous fine grains with low Schmid factor along the bonding interface of rough surfaces, leading to worse compatible deformation capability and thereby poor ductility of bonding joints.

High temperature mechanical properties and strain hardening mechanism of directionally solidified Mg-Gd-Y alloy

The Mg-6.0Gd-0.5Y alloy, with straight grain boundaries and preferred growth orientation [224(_)3], was prepared by directional solidification. The microstructure evolution during tensile tests at 150–350 °C was investigated using Electron Back-Scattered Diffraction (EBSD) and High-Resolution Transmission Electron Microscopy (HRTEM). Subsequently, the relationship between the deformation mechanism and mechanical properties was discussed. The results show that there are obvious uniform plastic deformation stage and work hardening stage after yield for the tensile stress-strain curves of the Mg-6.0Gd-0.5Y samples tested at 150°C–250 °C, while evident dynamic recrystallization characteristic can be observed for the curve of the sample tested at 350 °C. It is found that the stable high-temperature strength of the Mg-6.0Gd-0.5Y alloy can be attributed to the delayed dynamic recrystallization, the formation of compression twins and I1-type stacking faults. In addition, the superior plasticity of the alloy is related to the high concentration of crystal orientation, the activation of tensile twins, and non-basal slip which can provide more than 5 independent systems.