Showing papers on "Texture (crystalline) published in 2019"

Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy

Abstract: We describe here a comprehensive study on the effect of cellular structure and melt pool boundary (MPB) condition on the mechanical properties, deformation and failure behavior of AlSi10Mg alloy processed by selective laser melting (SLM). The morphology of melt pool (MP) on the load bearing face of tensile samples was significantly different with build directions. It resulted in different mechanical properties of the samples with different build directions. Furthermore, the microstructure analysis revealed that the MP in the SLM AlSi10Mg alloy mainly consisted of columnar α-Al grains which were made of ultra-fine elongated cellular structure. Electron back-scatter diffraction (EBSD) analysis revealed that the long axis of cellular structure and columnar grains were parallel to , which resulted in fiber texture in SLM AlSi10Mg alloy. However, Schmid factor calculation demonstrated that the anisotropy of mechanical properties of the SLM AlSi10Mg alloy built with different direction was mainly dependent on the distribution of MPB on the load bearing face, and not texture. The defects including pores, residual stress and heat affected zone (HAZ) located at MPB made it the weakest part in the SLM AlSi10Mg. The sample built along horizontal direction exhibited good combination of strength and plasticity and is attributed to the lowest fraction of MPBs that withstand load during tensile. MPB had strong influence on the mechanical properties and failure behavior of SLM AlSi10Mg built with different directions.

3D Printing of highly textured bulk thermoelectric materials: mechanically robust BiSbTe alloys with superior performance

Abstract: While zone-melted (ZM) Bi2Te3 is a standard commercially available thermoelectric (TE) material, it suffers from not being sufficiently mechanically robust due to the presence of the van der Waals bonded Te–Te layers that reduce the product yield and compromise operational reliability. Polycrystalline materials prepared by powder metallurgy techniques exhibit improved mechanical properties but usually lose the desired texture exhibited by ZM ingots, and this affects their TE performance. It is highly desirable to be able to fabricate Bi2Te3-based bulk materials with anisotropies similar to a single crystal, yet being mechanically strong as the polycrystalline specimens. Herein, we combine for the first time the thermal explosion technique with selective laser melting (SLM) to synthesize the highly textured p-type Bi0.4Sb1.6Te3 bulk material. Structural analysis (FESEM and XRD) indicates that the slender columnar grains grew along the building direction (BD) of the structure and the orientation factor reached up to 0.9, close to that representing a single crystal. TEM images revealed a high density of dislocations inside the grains. Since the printed compound has a high degree of texture, the TE and mechanical properties exhibit a highly anisotropic behavior. The maximum ZT of annealed samples parallel to the BD was 1.1, similar to that of the single crystal. However, the compressive strength of the structure reached up to 91 MPa, some 2.5 times the strength of a typical single crystal (37 MPa), and even higher than that of Spark Plasma Sintered (SPS) polycrystalline samples (80 MPa). Meanwhile, the mechanical cutting performance was much superior compared to that of the ZM ingot, and TE legs could be cut to sizes as small as 0.2 mm. A micro-TE module assembled using SLM-printed high performance p-type BiSbTe and SPS-compacted n-type BiTeSe materials showed the maximum cooling temperature difference of 62 °C. The work provides a facile and effective solution for preparation of Bi2Te3-based materials with high texture, robust mechanical properties, and excellent TE performance. As such, it lays a solid foundation for rapid in situ 3D printing of Bi2Te3-based micro-TE devices.

Non-equilibrium microstructure, crystallographic texture and morphological texture synergistically result in unusual mechanical properties of 3D printed 316L stainless steel

Abstract: Mechanisms underlying the evolution of texture and microstructure during selective laser melting (SLM) and their combined effects on the mechanical response of 316L stainless steel are presented. Long columnar grains with a fiber texture || build direction (BD) evolved in the SLM printed material. Fiber texture was stronger in the horizontal build compared to the vertical build. Use of bidirectional scanning strategy enforced epitaxial growth of grains across melt pools present within a single printed layer. || BD texture evolved as a consequence of maintaining the balance between epitaxy and growth of [100] along maximum thermal gradient. High dislocation density and not grain size effect of the ultra-fine cellular structure, imparted high strength to 316L. Lower average Schmid factor and smaller effective grain size in the horizontal build by virtues of crystallographic and morphological textures, respectively, imparted higher yield strength than the vertical build. The horizontal build demonstrated higher strain hardening rate in the early stages of deformation compared to the vertical build due to higher crystallographic texture dependent twinning. However, the higher rate of dislocation annihilation led to a continuous decline in the strain hardening rate of the horizontal build. In contrast, a stable strain hardening rate was maintained in the vertical build, which led to higher ductility than the horizontal build. In summary, the roles of non-equilibrium microstructure and texture (crystallographic and morphological) in regulating mechanical properties elucidated here, can be utilized in designing additively manufactured structural components of 316L stainless steel.

Preparation and anisotropic properties of textured structural ceramics: A review

Abstract: Ceramics are usually composed of randomly oriented grains and intergranular phases, so their properties are the statistical average along each direction and show isotropy corresponding to the uniform microstructures. Some methods have been developed to achieve directional grain arrangement and preferred orientation growth during ceramic preparation, and then textured ceramics with anisotropic properties are obtained. Texture microstructures give particular properties to ceramics along specific directions, which can effectively expand their application fields. In this review, typical texturing techniques suitable for ceramic materials, such as hot working, magnetic alignment, and templated grain growth (TGG), are discussed. Several typical textured structural ceramics including α-Al2O3 and related nacre bioinspired ceramics, Si3N4 and SiAlON, h-BN, MB2 matrix ultra-high temperature ceramics, MAX phases and their anisotropic properties are presented.

Selective electron beam melting of NiTi: Microstructure, phase transformation and mechanical properties

Abstract: Recent years have witnessed extensive investigations of additively manufactured NiTi alloys using laser-source techniques. This study reports a selective electron beam melting process to make Ni-rich NiTi parts using the plasma rotating electrode processed pre-alloyed NiTi powder. Microstructures of the as-printed NiTi samples were investigated, including micro-morphology, phase constituent and texture. Chemical compositions (primarily oxygen and nickel) and martensitic transformation temperatures were compared between the in house made NiTi powders and as-printed NiTi samples. X-ray diffraction results show that the as-printed samples exhibit a dominant austenitic B2 phase and a minor in situ formed Ni4Ti3 phase. The as-printed samples demonstrate a strong B2 (001) orientation texture along the building direction. Differential scanning calorimetry results show asymmetric phase transformations: a two-step of B2 → R → B19′ phase transformation during cooling but a one-step B19′ → B2 transformation during heating. Excellent superelasticity and large reversible strain have been observed. The as-printed NiTi samples achieve a fracture tensile stress of 1411.0 ± 59.3 MPa and an elongation of 11.8 ± 0.9%. The observed tension-compression asymmetry has been also discussed.

Controlling mechanical properties of additively manufactured hastelloy X by altering solidification pattern during laser powder-bed fusion

Abstract: Like other manufacturing processes, controlling the microstructure of additively manufactured parts is essential to reach the desirable mechanical properties. However, available reports on the control of as-build microstructure and mechanical properties of Ni-base superalloys during laser powder-bed fusion (LPBF) process are not comprehensive. This article aims at a systematic approach to study the effect of scanning strategies and build orientations on solidification patterns in the printed LPBF Hastelloy X parts. The as-built microstructure (grain size, texture) and mechanical responses (yield strength, ultimate tensile strength (UTS), and elongation) are also presented. Results reveal that the stripe unidirectional scan pattern leads to the largest grain size (>850 μm) with the lowest mechanical strength. These samples also exhibit the strongest crystallographic texture, resulting in a planar anisotropic mechanical response (~22 MPa difference in UTS). On the other hand, the stripe rotation scan strategy (67° rotation) leads to a randomly oriented and finer grain structure (~110 μm) with a higher UTS (~800 MPa) due to grain refinement observed in these samples. In addition, the aspect ratio of the columnar grain structure was observed to influence the mechanical response of these parts. UTS of horizontally printed parts were ~26% more than the vertical parts for the stripe scan strategy (67° rotation). However, changing the solidification pattern (stripe XY with 90° rotation) was observed to reduce this difference to ~18%. These findings can be used to tune the microstructure of as-built LPBF parts to obtain an optimal mechanical behaviour.

Visualizing Exotic Orbital Texture in the Single-Layer Mott Insulator 1T-TaSe2

Abstract: Mott insulating behavior is induced by strong electron correlation and can lead to exotic states of matter such as unconventional superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of novel Mott systems in the two-dimensional limit. Here we report characterization of the local electronic properties of single- and few-layer 1T-TaSe2 via spatial- and momentum-resolved spectroscopy involving scanning tunneling microscopy and angle-resolved photoemission. Our combined experimental and theoretical study indicates that electron correlation induces a robust Mott insulator state in single-layer 1T-TaSe2 that is accompanied by novel orbital texture. Inclusion of interlayer coupling weakens the insulating phase in 1T-TaSe2, as seen by strong reduction of its energy gap and quenching of its correlation-driven orbital texture in bilayer and trilayer 1T-TaSe2. Our results establish single-layer 1T-TaSe2 as a useful new platform for investigating strong correlation physics in two dimensions.

The evolution of bainite and mechanical properties of direct laser deposition 12CrNi2 alloy steel at different laser power

Abstract: In this paper, 12CrNi2 alloy steel samples were successfully fabricated by direct laser deposition (DLD) technology. The evolution of bainite in the DLD 12CrNi2 alloy steel process at different laser power of 1800 W, 2000 W and 2200 W and mechanical properties of as-deposited samples were investigated. The results showed that the middle-upper microstructure of the samples fabricated at different laser power transformed from lath bainite (LB) to granular bainite (GB) with increasing laser power. In addition, granular bainite was divided into blocky granular bainite (GB1) and lath-like granular bainite (GB2) according to different formation mechanisms, and the amount of GB1 increased and GB2 decreased in the range of laser power from 2000 W to 2200 W. No preferred texture was observed in the EBSD maps due to the complex heat flux direction which was resulted in the reciprocating scanning strategy. With the increase of laser power, the proportion of high-angle grain boundaries increased from 33.1% to 46.4% and that of low-angle grain boundaries decreased from 66.9% to 53.6%. The sample fabricated at 2000 W had the highest mean microhardness (331 HV0.2) and the best combination of ultimate tensile strength (757 MPa) and elongation (9.1%). However, the sample fabricated at 2200 W had the best impact toughness (aku = 100.0 J/cm2) because it contained a large amount of GB1 with dispersive and spherical-like island structures. This study provides the theoretical and experimental basis for the design of laser power and controllability of microstructure and properties in the DLD alloy steel process.

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

Abstract: 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 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.

Effect of processing parameters on the densification, microstructure and crystallographic texture during the laser powder bed fusion of pure tungsten

Abstract: Laser Powder Bed Fusion is a leading additive manufacturing technology, which has been used successfully with a range of lower melting point materials (titanium alloys, nickel alloys, steels). This work looks to extend its use to refractory metals, such as those considered in this paper where the behaviour of pure tungsten powder is investigated. A strategy for fabricating high density parts was developed by creating a process map in which the effect of laser energy density was studied. The process quality was assessed using different techniques including light optical microscopy, XCT, SEM and EBSD. The results showed that the laser energy density was adequate to process tungsten to produce functional parts. The bulk density and optically determined densities, under different process conditions, ranged from 94 to 98%, but there was evidence of micro cracks and defects in specimens due to micro- and macro-scale residual stress. Analysis of the microstructure and local crystallographic texture showed that the melt pool formed under the laser beam favoured solidification in a preferred orientation by an epitaxial growth mechanism. The EBSD local texture analysis of the tungsten specimens showed a //Z preferential fibre texture, parallel to the build direction.

Build Orientation Effects on Texture and Mechanical Properties of Selective Laser Melting Inconel 718

Abstract: The production of components via selective laser melting (SLM) metal additive manufacturing results in microstructures unique to the process that are highly dependent on laser processing parameters and orientation of part geometry relative to the build direction. This study investigates the variation in tensile properties of SLM Inconel 718 at varying angles with respect to build direction. ASTM E8 tensile specimens were built in XY, Z, and B+45 from Z orientations to near-net shape, HIP and heat treated, and tensile tested at room temperature. Orientation dependence of mechanical properties was observed; Z samples had the lowest strength and highest elongation, while XY samples had the highest strength and lowest elongation. Optical, SEM, and EBSD analysis were conducted to probe for microstructure variation that could be the cause of the difference in mechanical properties. Analysis of pole figures and grain maps suggests a preferred grain texture and twinning orientation that aligns with the build direction, which may contribute to the anisotropic enhancement in ductility.