Showing papers on "Texture (crystalline) published in 2018"

Simultaneously enhanced strength and ductility for 3D-printed stainless steel 316L by selective laser melting

Abstract: Laser-based powder-bed fusion additive manufacturing or three-dimensional printing technology has gained tremendous attention due to its controllable, digital, and automated manufacturing process, which can afford a refined microstructure and superior strength. However, it is a major challenge to additively manufacture metal parts with satisfactory ductility and toughness. Here we report a novel selective laser melting process to simultaneously enhance the strength and ductility of stainless steel 316L by in-process engineering its microstructure into a crystallographic texture. We find that the tensile strength and ductility of SLM-built stainless steel 316L samples could be enhanced by ~16% and ~40% respectively, with the engineered textured microstructure compared to the common textured microstructure. This is because the favorable nano-twinning mechanism was significantly more activated in the textured stainless steel 316L samples during plastic deformation. In addition, kinetic simulations were performed to unveil the relationship between the melt pool geometry and crystallographic texture. The new additive manufacturing strategy of engineering the crystallographic texture can be applied to other metals and alloys with twinning-induced plasticity. This work paves the way to additively manufacture metal parts with high strength and high ductility. A steel alloy with both high tensile strength and ductility has been three-dimensional (3D) printed by researchers in Singapore. Additive manufacturing builds 3D objects by adding materials layer by layer, a relatively simple process for plastics. However, this manufacturing process is much more difficult for metals, which are susceptible to defects and internal pores. This is particularly problematic when the final product needs excellent mechanical properties, such as hardness or strength. Xipeng Tan and co-workers from Nanyang Technological University used a specific laser scanning strategy to melt metallic powders and form a stainless steel alloy with a zig-zag crystallographic microstructure. The tensile strength and ductility of their stainless steel samples were increased by approximately 16% and 40%, respectively, compared to an alloy with the typical microstructure. A creative approach to substantially enhance both the strength and ductility of SLM-printed metal parts was successfully demonstrated on the ubiquitous marine-grade stainless steel 316L. The new discovery improves the strength and ductility of stainless steel parts by ~16% and 40% compared with the typical 3D printing process and conventional manufacturing methods. Control of the crystallographic texture is key for this breakthrough, which was achieved by tailoring the geometrical features of the melt pool involved in the laser-based 3D printing process. The desired crystallographic texture favors the activation of the nano-twinning mechanism, which simultaneously enhances the strength and ductility.

Correlation between process parameters, microstructure and properties of 316 L stainless steel processed by selective laser melting

Abstract: High-density 316 L specimens were fabricated by selective laser melting (SLM). Different processing parameters, including laser power (100, 200 W) and scanning strategies (alternating stripes without and with re-melting after every layer) were employed to evaluate their impact on microstructure and texture of the specimens. Microstructures of the specimens in as-built condition were characterised by columnar grains of austenite with intercellular segregation of Mo, Cr and Si, resulting in creation of non-equilibrium eutectic ferrite. It was found that laser energy density and scanning strategy strongly affect cellular substructure of austenite and amount of ferrite, as well as kind and degree of texture. Specific microstructure of austenite in as-built condition is the cause of almost double increase of yield strength accompanied by much smaller improvement of ultimate tensile strength and 1.4 times reduction of elongation at fracture in comparison of properties of hot-rolled SS316L sheet. Moreover, features of this substructure determine kind of the changes occurring during stress relieving at 800 °C for 5 h (among others, precipitation of sigma-phase strongly activated by presence of ferrite and residual stresses), demonstrated by decreased yield strength value with no significant changes of ultimate tensile strength and elongation. At the same time, an attempt was made to explain some unclearly interpreted observations in the literature related to a correlation between process parameters, microstructure and properties of SLM-processed steel 316 L.

Thermoelectric Properties of Poly(3-hexylthiophene) (P3HT) Doped with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) by Vapor-Phase Infiltration

Abstract: Doping of thin films of semiconducting polymers provides control of their electrical conductivity and thermopower. The electrical conductivity of semiconducting polymers rises nonlinearly with the carrier concentration, and there is a lack of understanding of the detailed factors that lead to this behavior. We report a study of the morphological effects of doping on the electrical conductivity of poly(3-hexylthiophene) (P3HT) thin films doped with small molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Resonant soft X-ray scattering shows that the morphology of films of P3HT is not strongly changed by infiltration of F4TCNQ from the vapor phase. We show that the local ordering of P3HT, the texture and form factor of crystallites, and the long-range connectivity of crystalline domains contribute to the electrical conductivity in thin films. The thermopower of films of P3HT doped with F4TCNQ from the vapor phase is not strongly enhanced relative to films doped from solution, but the el...

A new grain orientation spread approach to analyze the dynamic recrystallization behavior of a cast-homogenized Mg-Zn-Zr alloy using electron backscattered diffraction

Abstract: A new grain orientation spread approach (GOS ≤ 5°) was proposed to study DRX of a Mg-Zn-Zr alloy during hot deformation. DRXed grains possessed random texture while the deformed grains contributed to basal texture. The alloy exhibited rapid DRX at low deformation strains followed by near-saturated behavior as strain increased.

Tailoring sample-wide pseudo-magnetic fields on a graphene-black phosphorus heterostructure.

Abstract: Spatially tailored pseudo-magnetic fields (PMFs) can give rise to pseudo-Landau levels and the valley Hall effect in graphene. At an experimental level, it is highly challenging to create the specific strain texture that can generate PMFs over large areas. Here, we report that superposing graphene on multilayer black phosphorus creates shear-strained superlattices that generate a PMF over an entire graphene-black phosphorus heterostructure with edge size of tens of micrometres. The PMF is intertwined with the spatial period of the moire pattern, and its spatial distribution and intensity can be modified by changing the relative orientation of the two materials. We show that the emerging pseudo-Landau levels influence the transport properties of graphene-black phosphorus field-effect transistor devices with Hall bar geometry. The application of an external magnetic field allows us to enhance or reduce the effective field depending on the valley polarization with the prospect of developing a valley filter.

Microstructure evolution of 316L produced by HP-SLM (high power selective laser melting)

Abstract: New generation of selective Laser Melting (SLM) machines are evolving towards higher power lasers as well as multi laser systems in order to increase the productivity. The increase in laser power and the modification of the laser power distribution leads to microstructural and mechanical property variations that are still not well understood. This work aims at better understanding the interaction of a 1 kW top-hat power distribution laser on a well know material, 316 L stainless steel. The influence of texture and microstructure on relative density and crack density, when varying scan rotation, was evaluated. The high power (HP) laser and low power (LP) laser were compared with respect to microstructure and mechanical properties. HP leads to an increase in morphological and crystallographic texture together with a coarsening of the cell structure in contrast to the more random and finer cells found in LP processed material. Hot isostatic pressing was applied as a post-process treatment in order to close remaining pores and cracks. This helped in achieving higher elongations for LP and HP processed materials, while competitive mechanical properties to the 316 L material specifications were obtained in both cases.

Columnar to equiaxed transition during direct metal laser sintering of AlSi10Mg alloy: Effect of building direction

Abstract: In the current study, cylindrical samples of AlSi10Mg alloy were fabricated using direct metal laser sintering (DMLS) technique in vertical and horizontal directions. The microstructure of the samples was analyzed using scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. It was observed that, by changing the building direction from vertical to horizontal, columnar to equiaxed transition (CET) occurred in the alloy. While 75% of the grains in the vertical sample were columnar, by changing the direction to horizontal, 49% of the grains evolved with columnar shape and 51% of them were equiaxed. Moreover, the texture of DMLS-AlSi10Mg alloy changed due to CET. While {001} fiber texture evolved in the vertical sample, the direction tilted away from the building direction in the horizontal one. Using the fundamentals of solidification and constitutional undercooling, the solidification behavior of AlSi10Mg alloy during DMLS process was modeled. It was observed that, the determinant parameter in CET during DMLS of AlSi10Mg alloy is the angle between the nominal growth rate and h k l > direction of the growing dendrite, which is controlled by the geometry and building direction of the sample. Further TEM studies confirmed that, CET alters the shape and coherency of Si precipitates and dislocation density inside the α-Al dendrites in DMLS-AlSi10Mg alloy.

Microstructure and mechanical properties of stainless steel 316L vertical struts manufactured by laser powder bed fusion process

Abstract: Stainless steel 316L (SS316L) vertical struts with various diameters ranging from 0.25 mm to 5 mm were manufactured by laser powder bed fusion (LPBF) process. A systematic investigation was conducted on the microstructure and mechanical properties of the produced struts. The struts possessed hierarchical microstructures consisting of cellular sub-grain structures inside columnar grains. The primary dendrite arm spacing (PDAS) of the cellular sub-grains decreased monotonically with increasing strut diameter until reaching a plateau after 1 mm. In contrast, the columnar grain width did not show a clear relationship with respect to the variation in the strut diameter. A to texture transition along the building direction (BD) of the struts was observed as the strut diameter decreased from 5 mm to 0.25 mm, which was attributed to the change of the heat extraction direction. Microstructure-property relations were established via Hall-Petch type correlations between the PDAS and the microhardness as well as the PDAS and the strengths of the struts, suggesting the importance of the role played by the cellular sub-grain structures in the strengthening of LPBF manufactured SS316L. Electron backscatter diffraction (EBSD) analysis confirmed that the strong texture within the thicker struts promoted the twinning-induced plasticity, and thus resulted in a better strength-ductility combination compared with that of the thinner struts with texture or weak texture.

Microstructure, solidification texture, and thermal stability of 316 L stainless steel manufactured by laser powder bed fusion

Abstract: This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 °C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 °C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.

Anisotropic tensile and actuation properties of NiTi fabricated with selective laser melting

Abstract: This study evaluates the anisotropic tensile properties of Ni50.1Ti49.9 (in at%) components fabricated using an additive manufacturing (AM) process of selective laser melting (SLM). Dog-bone shaped tensile specimens were fabricated in three orthogonal building orientations (i.e., horizontal, edge, and vertical) with two different scanning strategies (i.e., alternating x/y and alternating in ± 45° to the x-axis). Next, the samples were subjected to tensile testing until failure, shape memory effect tests and thermal cycling under constant tensile stresses up to 500 MPa. Their failure surfaces were analyzed for possible microstructural defects. It was revealed that the build orientation and scanning strategy affect the texture/microstructure, and hence the failure stress, ductility, shape memory effect, and functional stability. Samples fabricated in the horizontal orientation with alternating x/y scanning strategy had the highest ultimate tensile strength (606 MPa) and elongation (6.8%) with the strain recovery of 3.54% after 4 shape memory effect cycles. At stress levels less than or equal to 200 MPa, these samples had the actuation strain greater than 3.8% without accumulation of noticeable residual strain. It was observed that the scanning strategy of alternating in ± 45° result in degraded mechanical and shape memory response, particularly in horizontal and edge samples.

High Thermoelectric Performance in Crystallographically Textured n-Type Bi2Te3–xSex Produced from Asymmetric Colloidal Nanocrystals

Abstract: In the present work, we demonstrate crystallographically textured n-type Bi2Te3–xSex nanomaterials with exceptional thermoelectric figures of merit produced by consolidating disk-shaped Bi2Te3–xSex colloidal nanocrystals (NCs). Crystallographic texture was achieved by hot pressing the asymmetric NCs in the presence of an excess of tellurium. During the hot press, tellurium acted both as lubricant to facilitate the rotation of NCs lying close to normal to the pressure axis and as solvent to dissolve the NCs approximately aligned with the pressing direction, which afterward recrystallize with a preferential orientation. NC-based Bi2Te3–xSex nanomaterials showed very high electrical conductivities associated with large charge carrier concentrations, n. We hypothesize that such large n resulted from the presence of an excess of tellurium during processing, which introduced a high density of donor TeBi antisites. Additionally, the presence in between grains of traces of elemental Te, a narrow band gap semicond...

Effects of graphene nano-platelets (GNPs) on the microstructural characteristics and textural development of an Al-Mg alloy during friction-stir processing

Abstract: The aim of this research is to characterize the unique microstructural features of Al-matrix nanocomposites reinforced by graphene nano-platelets (GNPs), fabricated by multi-pass friction-stir processing (FSP). During this process, secondary phase GNPs were dispersed within the stir zone (SZ) of an AA5052 alloy matrix, with a homogenous distribution achieved after five cumulative passes. The microstructural characteristics and crystallographic textures of different regions in the FSPed nanocomposite, i.e., base metal (BM), heat affected zone (HAZ), thermo-mechanical affected zone (TMAZ), and SZ, were evaluated using electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The annealed BM consisted of a nearly random crystal orientation distribution with an average grain size of 10.7 μm. The SZ exhibited equiaxed recrystallized grains with a mean size of 2 μm and a high fraction of high-angle grain boundaries (HAGBs) caused by a discontinuous dynamic recrystallization (DDRX) enhanced by pinning of grain boundaries by GNPs. The sub-grains and grain structure modification within the HAZ and TMAZ regions are governed by dislocation annihilation and reorganization in the grain interiors/within grains which convert low-angle to high-angle grain boundaries via dynamic recovery (DRV). The FSP process and incorporation of GNPs produced a pre-dominantly {100} cube texture component in the SZ induced by the stirring action of the rotating tool and hindering effect of nano-platelets. Although, a very strong {112} simple shear texture was found in the HAZ and TMAZ regions governed by additional heating and deformation imposed by the tool shoulder. These grain structure and texture features lead to a hardness and tensile strength increases of about 55% and 220%, respectively.

Effect of laser scanning strategies on texture, physical and mechanical properties of laser sintered maraging steel

Abstract: Direct Metal Laser Sintering (DMLS) is one of the most emerging metal Additive Manufacturing (AM) process due to its ability to quickly form complex designs with maximal surface finish. In this research, DMLS is used to additively manufacture 18% Ni maraging steel 300 samples by adopting a bidirectional and a cross-directional laser scanning strategy. The density, surface finish, texture, residual stress and mechanical properties of the DMLSed samples are investigated. Higher densification and surface finish are obtained using the cross-directional scan strategy. The formation of γ-austenite in the bi-directional scanning strategy is found to be nearly 60% in comparison to the cross-directional scan strategy. A preferential growth of columnar cells followed by epitaxial formation was found in both the directions for cross-directional scan strategy due to the rotation of heat flux and transformation of strong crystallographic texture into weaker ones. This resulted in a reduction of anisotropy and higher compressive residual stresses and mechanical properties. The outcomes of this research are likely to help in a better understanding of the DMLS AM process for fabrication of high surface finish, density and mechanical properties maraging steel parts by controlling their crystallographic texture.

Crystallographically Textured Nanomaterials Produced from the Liquid Phase Sintering of BixSb2–xTe3 Nanocrystal Building Blocks

Abstract: Bottom-up approaches for producing bulk nanomaterials have traditionally lacked control over the crystallographic alignment of nanograins. This limitation has prevented nanocrystal-based nanomaterials from achieving optimized performances in numerous applications. Here we demonstrate the production of nanostructured BixSb2–xTe3 alloys with controlled stoichiometry and crystallographic texture through proper selection of the starting building blocks and the adjustment of the nanocrystal-to-nanomaterial consolidation process. In particular, we hot pressed disk-shaped BixSb2–xTe3 nanocrystals and tellurium nanowires using multiple pressure and release steps at a temperature above the tellurium melting point. We explain the formation of the textured nanomaterials though a solution–reprecipitation mechanism under a uniaxial pressure. Additionally, we further demonstrate these alloys to reach unprecedented thermoelectric figures of merit, up to ZT = 1.96 at 420 K, with an average value of ZTave = 1.77 for the r...

Controlling the microstructure and mechanical properties of a metastable β titanium alloy by selective laser melting

Abstract: Customized titanium alloy devices have become the most attractive orthopaedic implants owing to their effects in mitigating the pain and suffering of patients. This work demonstrates the feasible control of the microstructure, i.e., defects, element diffusion, and phase transformation, and the mechanical properties of a biocompatible Ti–37Nb–6Sn alloy obtained by selective laser melting (SLM). Defects such as voids and unmelted Nb particles, as well grains with random orientation or columnar grains with {100} fiber texture in the as-fabricated Ti–37Nb–6Sn alloy can be modulated by varying the solidification rate and aging effects during the deposition process. A high energy density input promotes the diffusion of Sn from the grain boundaries to the β-matrix, resulting in an increase in the lattice constants of the β-matrix, with low elastic modulus of the as-fabricated Ti–37Nb–6Sn alloy. However, reheating effects promotes the formation of nanoscale α-phase precipitates both at the grain boundaries and in the matrix. The combined effects of rapid solidification and aging induced by reheating result in a metastable β-type Ti–37Nb–6Sn alloy with a Young's modulus of 66 GPa, ultimate strength of 891 MPa, and elongation of 27.5%. This method can aid the design of customized titanium devices with low elastic modulus for orthopaedic implants applications.