Showing papers on "Electron backscatter diffraction published in 2017"

Microstructure and mechanical properties of Inconel 718 produced by selective laser melting: Sample orientation dependence and effects of post heat treatments

Abstract: Inconel 718 produced by selective laser melting (SLM) has been characterized with focus on the microstructure, the dependence of sample orientation on the mechanical properties and the effects of post heat treatments. The as-manufactured IN718 has a very fine cellular-dendritic structure with fine Laves phases precipitating in the interdendritic region, and electron backscatter diffraction (EBSD) analysis shows that both the vertically and horizontally built samples have relatively weak texture. The vertically built samples show lower tensile strength but higher ductility than the horizontally built samples, and the mechanism is shown to be partly due to the crystallographic feature but more importantly due to the different amount of residual stress and dislocations accumulated in these two kinds of samples. Applying heat treatments can significantly increase the strength while decrease the ductility correspondingly, and difference in yield strength between the vertically and horizontally built samples decreases with increasing the heat treatment temperatures, mainly due to the removal of residual stress and dislocations.

Controlling and correlating the effect of grain size with the mechanical and electrochemical properties of Li7La3Zr2O12 solid-state electrolyte

Abstract: Li7La3Zr2O12 (LLZO) solid-state electrolyte is garnering interest due to its potential to enable solid-state batteries (SSBs) using metallic Li anodes. However, Li metal propagates along LLZO grain boundaries at high Li plating current densities (above the critical current density, CCD). In the present study, we examined whether microstructural aspects, such as grain size, could influence mechanical and electrochemical properties thereby affecting the CCD. A unique densification technique (heating between 1100 and 1300 °C) was used to control grain size. Electron backscatter diffraction determined that the grain size and the misorientation angle varied from 5 to 600 μm and 20 to 40°, respectively. Vickers indentation was used to characterize the mechanical properties and revealed that hardness decreased (9.9–6.8 GPa) with increasing grain size, but the fracture toughness was invariant (0.6 MPa m−1/2) at grain sizes ≥40 μm. DC and AC techniques were used to measure and correlate the CCD with grain size and showed that the CCD increased with increasing grain size achieving a maximum of 0.6 mA cm−2. We believe the implications of this work could be far-reaching in that they represent a significant step towards understanding the mechanism(s) that control the stability of the Li–LLZO interface and a rational approach to increase the CCD in SSBs.

Tensile deformation behavior and deformation twinning of an equimolar CoCrFeMnNi high-entropy alloy

Abstract: The tensile deformation and strain hardening behaviors of an equimolar CoCrFeMnNi high-entropy alloy (HEA) were investigated and compared with low and medium entropy equiatomic alloys (LEA and MEA). The HEA had a lower yield strength than the MEA because the addition of Mn weakens solid solution hardening in the HEA. However, deformation twinning induced the multiple stage strain hardening behavior of the HEA and enhanced strength and elongation. Using tensile-interrupted electron backscatter diffraction analysis, geometrically necessary dislocations were observed as plume-shaped features in grain interior, and a considerable texture was characterized, which is typical of face centered cubic metals. Moreover, the relationship between favorably oriented grains and twinning in the HEA bore a clear resemblance to the same tendency in TWIP steels. The thickness of the twin bundles was less than 100 nm. A high density of stacking defects was found in the nanotwins. Nano twinning and stacking faults were found to contribute to the remarkable mechanical properties. Deformation induced twinning not only demonstrated the dynamic Hall-Petch effect but also changed dislocation cell substructures into microband structures.

Microstructure evolution characteristics of Inconel 625 alloy from selective laser melting to heat treatment

Abstract: Superalloy Inconel 625 has been widely used in selective laser melting (SLM). Since SLM-induced microstructure with columnar grains, strong texture, porosity, and undesired properties, heat treatment is often used to tune the microstructure and mechanical properties. However, the microstructure evolution of IN 625 from SLM to heat treatment is poorly understood. In this study, IN 625 samples were SLMed and then heat treated at elevated temperatures. Microstructure evolution characteristics of the processed IN 625 alloy have been characterized using optical metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction, and micro indentation. Fine dendrite microstructures with strong texture parallel to layer build-up direction was observed in the as-SLM samples due to rapid cooling and epitaxial growth. High dislocation density and high microhardness were found in the γ matrix which also contains high Z-contrast precipitates. After annealing at high temperatures, random grain growth accompanied by dislocation annihilation and twinning occurs. The decrease in lattice parameter and the prevalence of large grain boundary misorientation in the γ matrix suggests that SLM-induced residual stress be significantly reduced. In addition, the uncertainty of mechanical properties due to the process variations was quantified.

High-temperature tensile deformation behavior and microstructure evolution of Ti55 titanium alloy

Abstract: High temperature tensile and electron backscatter diffraction (EBSD) techniques were combined to perform a systematic investigation for the hot deformation behavior and microstructure evolution of the Ti55 alloy Under temperatures ranging from 885 to 935 °C and stain rates of 83×10−4 s−1−133×10−2 s−1, all the flow curves of the Ti55 alloy exhibit similar behaviors: after reaching the peak flow stress, the curves enter into a softening stage and then remain a steady level, where the maximum superplastic elongation of 987% indicates good superplasticity Detailed microstructure characterizations under different deformation stages show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries (>15°) increase rapidly at the softening stage These observations are attributed to the dynamic recrystallization, in which low angle grain boundaries (

Growth and characterization of all-inorganic lead halide perovskite semiconductor CsPbBr3 single crystals

Abstract: As a typical representative of all-inorganic lead halide perovskites, cesium lead bromide (CsPbBr3) has attracted significant attention in the context of photovoltaics and other optoelectronic applications in recent years. In this paper, CsPbBr3 single crystal growth was conducted by a creative electronic dynamic gradient (EDG) method. The crystal structure was systematically investigated using scientific instruments and equipment. X-ray diffraction techniques, including X-ray diffraction (XRD), temperature-dependent X-ray powder diffraction and the X-ray rocking curve, were used to identify the phase and to investigate phase transition rules. Electron diffraction techniques, including high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and electron backscatter diffraction (EBSD), were used to investigate the crystal micro-structure. The final results indicated that the grown CsPbBr3 crystal was a perfect single crystal preferentially orienting in the (110) direction and met the basic demand of its applications.

Microstructures and mechanical behavior of Inconel 625 fabricated by solid-state additive manufacturing

Abstract: Here we introduce a novel thermo-mechanical Solid State Additive Manufacturing (SSAM) process referred to as Additive Friction Stir (AFS) manufacturing that provides a new and alternative path to fusion-based additive manufacturing processes for developing fully-dense, near-net shape components with a refined-equiaxed grain morphology. This study is the first to investigate the beneficial grain refinement and densification produced by AFS in IN625 that results in advantageous mechanical properties (YS, UTS, ef) at both quasi-static and high strain rate. Electron Backscatter Diffraction (EBSD) observed grain refinement during the layer deposition in the AFS specimens, where the results identified fine equiaxed grain structures with even finer grain structures forming at the layer interfaces. The EBSD quantified grains as fine as 0.27 µm in these interface regions while the average grain size was approximately 1 µm. Additionally, this is the first study to report on the strain rate dependence of AFS IN625 through quasi-static (QS) (0.001/s) and high strain rate (HR) (1500/s) tensile experiments using a servo hydraulic frame and a direct tension-Kolsky bar, respectively, which captured both yield and ultimate tensile strengths increasing as strain rate increased. The HS results exhibited an approximately 200 MPa increase in engineering strength over the QS results, with the fracture surfaces at both strain rates aligned with the maximum shear plane and exhibiting localized microvoids.

Effects of thermal cycles on the microstructure evolution of Inconel 718 during selective laser melting process

Abstract: A staircase Inconel 718 block was fabricated to investigate the effects of the thermal cycles on the microstructure evolution in the selective laser melting (SLM) part using optical scope (OM), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD). The laser beam scanning strategy was clearly shown in the part under OM, including laser scanning pattern and hatch spacing. The Y-plane (side surface) was characterized by elongated colonies of cellular dendrites with an average cell spacing of 0.511 ∼ 0.845 μm. In addition, Laves phase was observed in the inter-layers and inter-cellular regions. Under the continuing effects of the thermal cycles, the fraction of the Laves-phase showed a significant drop with their morphology changing from coarse and interconnected particles to discrete Laves phase. This is attributed to the reheating process as Laves phase can be dissolved at a proper heat treatment. In terms of the width of the cellular dendrites, the longer the thermal cycle period is, the coarser the elongated grains are. Due to the preferred orientation of the crystal, the Y-plane and Z-plane had a strong texture of and , respectively. With the repeating thermal cycle period elongating, the maximum intensity of the texture, together with the fraction of larger grains and the high misorientation angles, increased. At the same height, there was no significant changes with the grains size, but the fraction of the high misorientation angles (>15°) increased with the subsequent building of the part. Moreover, the area fraction of the porosity was below 0.2%, with no remarkable effects found from the thermal cycles and the build height.

Controlling the stainless steel surface wettability by nanosecond direct laser texturing at high fluences

Abstract: This work investigates the influence of the direct laser texturing at high fluences (DLT-HF) on surface morphology, chemistry, and wettability. We use a Nd:YAG laser (λ = 1064 nm) with pulse duration of 95 ns to process stainless steel surface. The surface morphology and chemistry after the texturing is examined by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD), while the surface wettability is evaluated by measuring the static contact angle. Immediately after the texturing, the surface is superhydrophilic in a saturated Wenzel regime. However, this state is not stable and the superhydrophilic-to-superhydrophobic transition happens if the sample is kept in atmospheric air for 30 days. After this period, the laser-textured stainless steel surface expresses lotus-leaf-like behavior. By using a high-speed camera at 10,000 fps, we measured that the water droplet completely rebound from this superhydrophobic surface after the contact time of 12 ms.

Crystallographic texture in an additively manufactured nickel-base superalloy

Abstract: Laser-based directed energy deposition was used to additively manufacture a wall out of pre-alloyed powder of a nickel-base superalloy−Inconel 625. The crystallographic texture of the wall has been characterized using neutron diffraction and electron backscatter diffraction. The measured pole figures show a strong Goss texture component ({011} ) plus a comparatively much weaker cube component ({001} ), both indicating that the -direction of the majority of grains lies along the laser-scanning direction (or the length direction). The origin of the Goss texture is hypothesized to be a result of the preferential -oriented dendritic solidification driven by the laser-induced heat flow, which is affected by the combined effect of laser power, absorption of powder, and laser scanning speed. The texture-induced mechanical softening is also presented. This study aids in understanding the processing-structure-property relationship in additive manufacturing.