Columnar Substructure Research Articles

Effect of Printing Parameters and Heat Treatments on Microstructure Development of LPBF Manufactured Inconel 718 Alloy

Abstract Processing parameters of the laser powder bed fusion (LPBF) technique strongly govern achieved performances and manufacturing defects of printed alloys. In this work, it was aimed to study the effects of LPBF printing parameters and subsequent heat treatments on resulted microstructure characteristics and tensile properties of Inconel 718 alloy. Inconel samples were fabricated using three different energy densities. Then, microstructure features such as Lave phase, primary dendrite arm spacing, and internal residual stresses as microstrains of both as-built and heat-treated specimens were determined. It was found that in the range of used energy densities, alterations of phase fractions and average sizes of the Laves phase were insignificant. Decreased energy density led to microstructures with smaller primary dendrite arm spacing and thus principally contributed to enhanced yield and tensile strengths of as-printed samples, whereas increased porosity greatly deteriorated elongation. Moreover, their flow stress curves could be significantly increased by direct aging; however, typical cellular and columnar substructures occurring during the LPBF printing remained. Homogenization treatment could entirely eliminate such substructures and otherwise caused different formations of delta phase when it was performed prior to a delta process.

Strength-ductility synergy in a non-equiatomic Co–Cr–Ni medium entropy alloy wall structure deposited using twin-wire arc additive manufacturing

An innovative twin-wire arc additive manufacturing (T-WAAM) has been deployed to fabricate a Co-rich non-equiatomic Co–Cr–Ni medium entropy alloy. The TWAAM-built single-wall structures of Co50Cr10Ni40 alloy were fabricated under argon gas shielding using dual gas metal arc welding sources. The microstructure studies revealed a single-phase FCC crystal structure and defect-free deposition. The slender columnar substructure and cellular substructure are formed along the build direction (BD) and scan direction (SD) respectively, which are commonly observed in the wall structures of other conventional FCC alloys. The tensile coupon SD and BD displayed a superior high strength and ductility combination. The T-WAAM-fabricated Co50Cr10Ni40 alloy showed both strength and elongation 1.4 times greater than that of the laser additive manufactured equiatomic CoCrNi wall structure. Thus, the study opens up a new avenue to tailor the mechanical properties of CoCrNi MEA by composition variation, with an added advantage of higher deposition rates of TWAAM.

Investigation of 316L microstructure evolution mechanism and mechanical properties in dual-laser powder bed fusion with controllable remelting time interval

Remelting has a significant effect on controlling microstructure and enhancing properties of components prepared by laser powder bed fusion. In order to reveal the mechanism of the effect of remelting time intervals, this study employed a dual-laser powder bed fusion equipment to ensure precise and controllable time intervals of remelting. The effects of intervals (2 ms, 5 ms, and 50 ms) on substructure morphology, microstructure and mechanical properties of 316L were analyzed. The results indicate that the grain size ranged from 13.03 μm to 26.55 μm due to the influence of initial temperature at different remelting intervals on grain size. Additionally, the composition ratio between columnar and cellular substructure varies with the time interval, which greatly influences mechanical properties. Longer intervals favor columnar substructures in high-temperature gradients, and dislocation motion is impeded, leading to an increased strength of up to 689.6 MPa. Shorter intervals promote cellular substructures in low-temperature gradients, and dislocations move smoothly along the boundaries of substructure, leading to an elongation of up to 49.3 %. Therefore, a novel method for controlling microstructure and properties is provided in this study utilizing the precise controllable remelting time intervals of dual-laser powder bed fusion.

Nano to macro-mechanical properties of laser directed energy deposited CoCrNi medium entropy alloy

The CoCrNi Medium Entropy Alloy is a popular subset of the Cantor alloy due to its superior cryogenic and elevated temperature properties. This study reports the successful deployment of a laser-based near-net shaping technique known as laser directed energy deposition (LDED) to fabricate the CoCrNi MEA. The effect of laser power and scan speed on track geometry is studied in detail. The bulk deposition is performed using a parametric combination yielding regular defect-free tracks with an aspect ratio greater than five and a maximum deposition rate. The EBSD analysis revealed a strong < 001 > cubic texture along the build direction, due to the thermal gradient in that direction. Across both the scan and build directions, the cellular and columnar substructure is observed. Solid solution strengthening and dislocation strengthening contribute 39 % and 42 % to yield strength. The nanoindentation analysis revealed hardness, reduced modulus, and elastic modulus of 6.32 ± 0.32 GPa, 222.08 ± 5.72 GPa, and 250.62 ± 8.25 GPa, respectively. The contact stiffness, reduced modulus, and hardness showed a positive linear relationship with strain rate. The strain rate sensitivity of MEA is in the range of 0.030–0.042, which is higher than that of conventional fcc metals. It is attributed to the higher lattice frictional stresses and different atomic-level structures of MEA than conventional fcc metals. Furthermore, chemical short-range ordering and a stronger peierls barrier resulted in a lower activation volume of the order of ∼100b3. The present study provides a detailed understanding of the effect of process parameters on melt-pool geometry, microstructure evolution, as well as the influence of strain rate on nanomechanical properties.

Strength-ductility synergy of CoCrNi medium-entropy alloy processed with laser powder bed fusion

In this work, CoCrNi medium-entropy alloy (MEA) parts were fabricated by laser powder bed fusion (LPBF) with different laser powers. The microstructural evolution before and after tensile test was characterized to explore the strengthening and ductility improvement mechanisms. Similar microstructures and tensile performances were achieved among the samples that were fabricated with different processing parameters, which indicates a credible parameter window for fabricating high-quality CoCrNi samples. An excellent combination of tensile strength and ductility was acquired (yielding strength: 689–700 MPa, ultimate tensile strength: 963–973 MPa, and elongation after fracture: 31.9–37.7%), which were primarily attributed to the grain boundary strengthening mechanism and adequate working hardening capacity, respectively. After tensile test, most grains were rotated by aligning 〈110〉 directions, along with high-density low angle grain boundaries. Besides, both deformation twinning and dislocation multiplication were activated during tensile process, which acted as dominant deformation mechanisms. The findings above reveal that LPBF can fabricate high-quality CoCrNi MEAs with cellular and columnar sub-structures, and an excellent tensile strength without sacrificing ductility.

Growth-sector dependence of morphological, structural and optical features in boron-doped HPHT diamond crystals

Semiconducting boron-doped diamond single crystals of cubo-octahedral habit with prevalent development of octahedron {111} faces and insignificant area of cube {001}, rhombo-dodecahedron {110} and tetragon-trioctahedron {311} faces were obtained using solution-melt crystallization at high pressure 6.5 GPa and temperatures 1380…1420 °C. Using the Fe-Al solvent, which allows controlled incorporation of boron dopant between 2·10–4…10–2 at.% made it possible to vary the electro-physical properties of the crystals. Methods of micro-photogrammetry, atomic force microscopy, and micro-Raman spectroscopy were applied to reveal sectorial inhomogeneity of impurity composition and morphology of different crystal faces. The obtained crystals were shown to have high structural perfection and boron concentration ranging approximately from 1·1017 up to 7·1018 cm–3. An increase in boron concentration increases the area of {111} faces relatively to the total crystal area. Nanoscale morphological features like growth terraces, step-bunching, dendrite-like nanostructures, columnar substructures, negative growth pyramids on different crystal faces are shown to reflect peculiarities of carbon dissolution at high pressures and temperatures. The changes in the crystals’ habit and surface morphology are discussed in relation to inhomogeneous variation of thermodynamic conditions of crystal growth and dissolution at different boron concentrations.

Paramagnetic Molecular Semiconductors Combining Anisotropic Magnetic Ions with TCNQ Radical Anions.

We report the synthesis, magnetic properties, and transport properties of paramagnetic metal complexes, [Co(DMF)4(TCNQ)2](TCNQ)2 (1), [La(DMF)8(TCNQ)](TCNQ)5 (2), and [Nd(DMF)7(TCNQ)](TCNQ)5 (3) (DMF = N,N-dimethylformamide, TCNQ = 7,7,8,8-tetracyanoquinodimethane). All three compounds contain fractionally charged TCNQδ- anions (0 < δ < 1) and mononuclear complex cations in which the coordination environment of a metal center includes several DMF molecules and one or two terminally coordinated TCNQδ- anions. The coordinated TCNQδ- anions participate in π-π stacking interactions with noncoordinated TCNQδ- anions, forming columnar substructures that provide efficient charge-transporting pathways. As a result, temperature-dependent conductivity measurements demonstrate that all three compounds exhibit semiconducting behavior.

Fabrication of gradient-structure CuNiBe alloy bars by laser remelting and water-cooling

ABSTRACTLaser remelting (LR) is a typical laser manufacturing technique. In this study, LR was used to fabricate gradient-structured CuNiBe alloy bars. A series of LR experiments with different parameters (such as the output power of the laser transmitter, scanning speed, and cooling condition) and subsequent aging treatments were designed. The remelted zone depth (RZD), the hardness with depth on the plane perpendicular to the LR track, the microstructure, and the precipitation characteristics of different zones of the remelted samples were analyzed. The results showed that the RZD increased with a higher output power of the laser transmitter and lower scanning speed. The hardness of the CuNiBe was significantly improved by the LR and subsequent aging treatment. A remelted zone (RZ) and heat affected zone (HAZ) were found in the remelted surface layer, and equiaxed substructures and columnar substructures were present in the RZ. Well-dispersed NiBe precipitates on the substructure boundaries in the transitional area between the RZ and HAZ were the main cause of strengthening of the sample that underwent LR without water-cooling. The strengthening effect was significantly enhanced by the refined grains for the sample that underwent LR with water-cooling.

Comparative study of the microstructures and mechanical properties of laser metal deposited and vacuum arc melted refractory NbMoTa medium-entropy alloy

NbMoTa refractory medium-entropy alloy (MEA) was fabricated by laser metal deposition (LMD) and vacuum arc melting (VAM) respectively. The crystal structures of NbMoTa MEA under two processes are all single-phase solid solution of BCC structure. Compared with the MEA formed by VAM, the NbMoTa MEA formed by LMD has smaller grain size and component microscopic segregation. Due to the difference in cooling rate, cellular and columnar substructures are demonstrated within the grain of LMDed MEA while the substructure within the grain of VAMed alloy is typical dendritic. However, the refinement of the grains in LMDed MEA does not lead to improvements in mechanical properties. In this study, the theoretical yield strength of NbMoTa MEA is calculated through solid solution strengthening (SSS) theory. The theoretical value is consistent with the experimental value of VAMed MEA, which is higher than the experimental value of LMDed MEA. The main reason for this result is that there are some metallurgical defects like porosities and intergranular cracks appeared in the LMDed MEA. The EDS test showed there is no elemental segregation seen at both sides of the crack. The reason for the intergranular cracks can be attributed to the high residual thermal stress caused by the rapid solidification characteristic of LMD process.

AlCoCuFeNi high-entropy alloy with tailored microstructure and outstanding compressive properties fabricated via selective laser melting with heat treatment

The present study focused on changes in the microstructures and mechanical properties of AlCoCuFeNi high-entropy alloy (HEA) fabricated via selective laser melting (SLM) and subsequent heat treatment at temperatures of 900 °C and 1000 °C. The as-fabricated sample exhibits a single ordered body-centred-cubic (BCC)(B2) solid solution phase and fine columnar substructure with a strong <001> texture along the layer build-up direction. The heat treatment caused precipitation of the Cu-rich face-centred cubic (FCC) phase from the metastable BCC(B2) matrix, thus forming a dual phase structure in the heat-treated alloys. Heat treatment decreased the microhardness and compressive yield strength, but increased the ductility significantly as compared to the as-fabricated sample. This strength-ductility trade-off is related to the precipitation of the FCC phase, which can toughen the brittle HEA and result in apparent strain hardening. In particular, the sample heat-treated at 1000 °C exhibited a better compressive fracture strength of 1600 MPa, a yield strength of 744 MPa, and a strain of 13.1%. The improvement in mechanical properties is mainly attributed to the effective combination of the BCC(B2) and FCC phases. This study provides novel insights into the fabrication of AlCoCuFeNi HEAs with tailorable microstructures and superior mechanical performance via a combined process of SLM and subsequent heat treatment.

About properties of ZrO2 thermal protective coatings obtained from spherical powder mixtures

It is developed the technology of high-energy plasma spraying of the zirconium dioxide (ZrO2) thermal protective coating on the basis of ZrO2 tetragonal and cubic phases with the spheroidal grain shape and the columnar substructure, with the total porosity P = 4 %, the hardness HV = 12 GPa, the roughness parameter Ra ∼ 6 μm, the thickness 0.3–3 mm. As a sublayer it is used the heat-resistant coating of “Ni-Co-Cr-Al-Y” system with an intermetallic phase composition and the layered microstructure of the grains.

Growth stages and hexagonal-rhombohedral structural arrangements in spheroidal graphite observed in ductile iron

Crystallographic structures of spheroidal graphite particles (graphite nodules) were examined using transmission electron microscopy (TEM). Structures of graphite nodules were investigated relative to different stages of nodule growth in ductile iron. Curved graphene layers were observed during the early growth of the graphite nodules. Thin layered stacking faults give rise to streaking in the basal reflections, which give rise to curvature of the nodule and growth steps on the surface. Columnar substructures consisting of parallel peripheral subgrains were found in the outside region of graphite nodules that were formed during the intermediate growth stage. Straight planar graphene layers were observed in the columnar substructures. Crystallographic orientation of graphite showed little change through the subgrain and the c-axes of multiple subgrains in a single columnar substructure were parallel. A method for characterizing the crystal structures of graphite based on the selected area diffraction pattern was introduced. Both hexagonal structure and rhombohedral structure were found in the spheroidal graphite particles. Possible crystallographic defects associated with hexagonal-rhombohedral structure transition were discussed.

Crystal structure of 2,2'-({[2-(trityl-sulfan-yl)benz-yl]azane-diyl}bis-(ethane-2,1-di-yl))bis-(isoindoline-1,3-dione).

In the structure of the title compound, C46H37N3O4S, the planes of the two isoindoline units make a dihedral angle of 77.86 (3)°. The dihedral angles between the benzyl plane and the isoindoline units are 79.56 (4) and 3.74 (9)°. The geometry at the S atom shows a short [1.7748 (17) Å] S—Cbenz­yl and a long [1.8820 (15) Å] S—Ctrit­yl bond and the C—S—C angle is 108.40 (7)°. N—C bond lengths around the azane N atom are in the range 1.454 (2)–1.463 (2) Å. he crystal packing exhibts two rather ‘non-classical’ C—H⋯O hydrogen bonds that result in stacking of the molecules along the a as well as the b axis and give rise to columnar sub-structures.

Non-uniformity of extrinsic connections and columnar organization.

Afferent columns (>200 microm in diameter) have been intensively investigated in the context of thalamocortical and intrinsic connections. Many extrinsic cortical connections also form columnar terminations, but less is known about their fine organization. Results from intracellular injections of neighboring neurons (in rats: Johnson et al., 2000) suggest that even neurons within a common domain may have non-stereotyped projection patterns, with only partial overlap of terminal arbors. The issue of non-stereotyped projections at the columnar level is further considered by analysis of V1 axons terminating in primate area MT/V5 (an early visual area), and of an axon from temporal cortex terminating in area 7b (a higher cortical area). Both these axons have multiple non-uniform arbors. The implication is that each arbor recruits different numbers and possibly different combinations of postsynaptic elements. While more data are needed concerning convergence of connectional systems, and the actual identity and numbers of postsynaptic targets, the distributed spatial and laminar patterns do not evoke a repetitive uniformity, but rather a columnar substructure and the combinatoric possibilities of the 3-dimensional cortical organization.

Network self-assembly patterns in Main Group metal chalcogenide-based materials

Technological interest in the design of multifunctional microporous materials has stimulated recent research into the development of mild solventothermal techniques for the construction of lamellar and framework Main Group chalcogenidometalates. Reaction pathways from elemental or metal chalcogenide sources can be influenced by a variety of often interdependent factors of which counter cation size and charge, solvent polarity, pH and temperature are of paramount importance. As reviewed in this article, the presence of predominant solution species such as cyclic tripyramidal M3S63− (M = As or Sb) or edge-bridged ditetrahedral Sn2E64− anions (E = S or Se) as molecular building units and their participation in columnar substructures is characteristic for M2S3- and SnE2-based anionic networks. Hierarchical topological relationships between individual members of structural families of the type AxMyEz (A = alkali metal or alkylammonium cation) can be established that provide a detailed insight into probable multiple-step cation-directed self-assembly mechanisms. These findings enable the development of rational guidelines for the employment of suitable counter cations in controlling the condensation of small solution species into chains, sheets or frameworks, whose cavities reflect the spatial requirements of the structure-directing agent.

Metallurgical reaction and deposit fusion boundary microstructure under a stationary plasma arc

Alloy element loss, decarburization in melted region and microstructural change of a deposit fusion boundary under a stationary plasma arc have been investigated by a first-order kinetic and interfacial microstructure. The cylindrical specimens of iron-based alloys, Fe-C, Ni-C, and Co-C alloys were locally melted by a plasma arc using argon plasma gas and Ar+H2 shielding gas, and the rates of alloy element loss and decarburization in the melted region were measured. Moreover, the fusion boundaries experienced when nickel, iron, Ni-Fe filler metals were deposited on iron, Fe-C, nickel and cobalt base metals, were evaluated metallographically. The initial rate of alloy element loss decreases as follows: Fe-Al>Fe-Mn>Fe-Cr>SGI-Mg>Fe-Ni. The loss reaction mechanism is metallic evaporation and the rate seems to be limited by transport in the gas boundary layer. The magnitude of decarburization is as follows: Ni-C>Fe-C> Co-C. The decarburization rate in Fe-C alloy is assumed to be governed by a process involving mass transfer in the gas phase and the molten metal. However, at low carbon concentrations, the rate appears to be limited by transfer in liquid metal. Fusion boundary deposited with nickel filler metal on iron is regular, but with carbon added to iron, an infiltration of deposit metal into adjacent base metal occurs. The fusion boundary with iron deposited on nickel is irregular where thin Ni-Fe solid solution is formed. In a deposited fusion boundary of cobalt with iron, nickel, and Ni-Fe filler metals, FeCo compound formation occurs, with cobalt dissolving into the nickel deposit metal resulting in a tongue-like structure produced by nickel penetration and a fine columnar substructure formation produced by Ni-Fe diffusion.

Erection of columnar substructures in depressions obtained by tamping of sagging grounds

Erection of columnar substructures in depressions obtained by tamping of sagging grounds