Microstructural Defects Research Articles

Scan strategy induced microstructure and consolidation variation in the laser-powder bed fusion (L-PBF) additive manufacturing of low alloy 20MnCr5 steel

The paper focuses on the effect of the scanning strategies on the microstructural evolution, defect formation, and macro-hardness performance of laser-powder bed fusion (L-PBF) produced samples of low alloy 20MnCr5 steel. Respect to the scanning strategies, advanced characterization techniques were employed to study (i) as-built microstructure, (ii) inclusion size and distribution, and (iii) details of compositional variation around porosity and within the build. Microstructural characterization shows that the chessboard scanning strategy can provide a favorable microstructure for the improvement of mechanical performance. However, macro-hardness results show a lower mechanical performance compared to the linear scanning strategy samples, which is contradicted by the improved microstructure. Experimental results reveal that the chessboard scanning strategy promotes the oxidation reaction and in-situ oxide (SiO2) formation in L-PBF, which leads to significant defect formation due to the excessive thermal profile from the overlap of the laser. This has been validated through finite element analysis and thermodynamic computation. The advantages of microstructural improvement using the chessboard strategy can only be realized with strict control of the metallurgical quality during the L-PBF process. Thermal profile optimization and oxygen elimination during the L-PBF process could be critical for the improved metallurgical quality and superior mechanical performance of the as-built components.

Microstructure and mechanical properties of WRe/GH3128 alloy electron beam welded joint

This study investigates electron beam welding of WRe/GH3128 alloy, focusing on the microstructure, defects, mechanical properties, and fracture characteristics of the welded joint. Competitive grain growth within the weld, attributed to solidification rate and grain orientation, as well as the influence of electron beam keyhole oscillation on grain morphology, is analyzed. The study discusses the uneven distribution of solute concentration within the solid phase during the non-equilibrium solidification stage of the weld, along with dendritic segregation and phase transformation processes. Segregation of Cr, Re, and C elements in the interdendritic region results in abundant precipitation of σ phase and M6C. Non-equilibrium solidification phase fractions and phase transformation processes within the weld are calculated using JMartPro software. Furthermore, the potential relationship between the uneven diffusion of elements within the reaction layer and the formation of intermetallic compounds is discussed. XRD testing confirms the presence of Cr7Re3, Ni4W, Ni4Mo, and M6C within the reaction layer. The microhardness and tensile properties of the joints are evaluated, with a tensile strength of 600 MPa, representing 77% of GH3128's strength, while the elongation is only 2.5%. The increase in weld microhardness is attributed to changes in grain morphology and precipitation of M6C and σ phases, with M6C reducing joint plasticity. Cracks and defects are observed in the WRe heat-affected zone, and the role of metallurgical bonding and solid-state healing in crack repair is elucidated. Finally, an analysis is conducted on factors such as crystal microstructure defects that can lead to crack nucleation and propagation, providing separate explanations for intergranular fracture in the WRe base material and brittle fracture in the reaction layer.

Effect of Current Density on the Corrosion Resistance and Photocatalytic Properties of Cu-Ni-Zn0.96Ni0.02Cu0.02O Nanocomposite Coatings.

2 at.% Cu + 2 at.% Ni were co-doped in ZnO nanoparticles by a simple hydrothermal method, and then the modified nanoparticles were compounded into Cu-Ni alloy coatings using an electroplating technique. The effects of the current density (15-45 mA/cm2) on the phase structure, surface morphology, thickness, microhardness, corrosion resistance, and photocatalytic properties of the coatings were investigated. The results show that the Cu-Ni-Zn0.96Ni0.02Cu0.02O nanocomposite coatings had the highest compactness and the best overall performance at a current density of 35 mA/cm2. At this point, the co-deposition rate reached its maximum, resulting in the deposition of more Zn0.96Ni0.02Cu0.02O nanoparticles in the coating. More nanoparticles were dispersed in the coating with a better particle strengthening effect, which resulted in a minimum crystallite size of 15.21 nm and a maximum microhardness of 558 HV. Moreover, the surface structure of the coatings became finer and denser. Therefore, the corrosion resistance was significantly improved with a corrosion current density of 2.21 × 10-3 mA/cm2, and the charge transfer resistance was up to 20.98 kΩ·cm2. The maximum decolorization rate of the rhodamine B solution was 24.08% under ultraviolet light irradiation for 5 h. The improvement in the comprehensive performance was mainly attributed to the greater concentration of Zn0.96Ni0.02Cu0.02O nanoparticles in the coating, which played the role of the particle-reinforced phase and reduced the microstructure defects.

Recent developments in utilization of nano silica in self‐compacting concrete: A review

AbstractSelf‐compacting concrete (SCC) is a type of flowable, high‐strength and high‐performance concrete in the construction field, which can flow and compact under its own weight. The damage of concrete is often caused by internal microstructural defects. Nano silica (NS) has excellent pozzolanic properties, and with an appropriate content of NS as auxiliary cementing material, it can promote the generation of C‐S‐H gels in addition to serve as internal filler, thus improves the micro‐scale defects in concrete. This paper extracts data from studies published over the last decade which focus on the application of NS in SCC and discusses the similarities and differences between the studies. Ultimately, a systematic review of the physical and chemical characteristics of NS, and the effect of NS on the workability, mechanical and durability properties of SCC is presented. Based on the review, the most common NS content is 0%–6%, where the negative effect of 0%–6% NS on the workability of SCC can be controlled by increasing the dosage of superplasticizer. The NS content of 2%–3% is optimum for improving the mechanical strength and the ability of SCC to resist water permeability, corrosion, acid and sulfate attacks, elevated temperature as well as shrinkage.

Metallurgical Characterization of SS 316L Repurposed by Wire Plus Arc Additive Manufacturing

This research aims to analyze the microstructures and mechanical characteristics of stainless steel (SS) 316L repurposed by wire plus arc additive manufacturing (WAAM). The SS 316L wire is deposited on a SS 316 substrate, which can be repurposed. This deposited material underwent optical microscopy, X-ray diffraction, and tensile test, and the results indicate that it features cellular and columnar dendrites at the bottom and equiaxial grains at the top. The tensile strength of the interface region between the deposited material (DM) and the base material (BM) is the highest (559 ± 4.16 MPa vs. 510 ± 4.93 MPa in DM and 540 ± 2.65 in BM), indicating that the BM and the deposited layers are strongly bonded. All the results from the defect observation, microstructures, and mechanical characteristics confirm the potential of the WAAM process for repurposing.

Early-stage structure and evolution mechanism of hydrogen supersaturated surface layers on tungsten under low-energy plasma exposure

Trapping at irradiation-induced defects significantly enhances hydrogen-isotope retention in metals with low hydrogen solubility. Here, we present the first microscopic observation of nanocavity formation on tungsten surfaces exposed to low-dose hydrogen plasma at room temperature. The nanocavities exhibit very high aspect ratios between their lateral size L and thickness ω (L/ω: 60-90) and represent the predominant microstructural defects in the early development stage of the hydrogen supersaturated surface layer on hydrogen plasma-irradiated tungsten (see Nucl Fusion57 (2017) 016026, Acta Mater201 (2020) 55). We also quantify the vacancy yield in tungsten under hydrogen irradiation with ion energies below the threshold to create stable Frenkel pairs. Such defect production at sub-threshold energy is conventionally deemed impossible, but we consider that the exposed surface acts as a defect sink with strong absorption bias for mobile self-interstitial atoms from temporarily created Frenkel pairs. This results in a high excess of immobile vacancies in the nanometer-thin surface layer and their agglomeration causes the evolution of large, disc-like nanocavities.

Digital light processing of high-strength hydroxyapatite ceramics: Role of particle size and printing parameters on microstructural defects and mechanical properties

Dense hydroxyapatite (HA) bars were fabricated using digital light processing. The roles of HA median particle size (MPS), curing depth-to-layer thickness ratio (CD/LT), and debinding process on the printing/debinding flaws and flexural strength of the sintered parts were investigated. Commercial HA was milled for different times to provide powders with an MPS ranging from 0.3 to 2.7 µm. Thermal debinding led to delamination and vertical cracks, which decreased with increasing MPS; the minimum value required to fabricate specimens with appreciable flexural strength was 0.9 µm. At a given MPS (2.7 µm), the CD/LT varied between 1.4 and 3.3, indicating a progressive disappearance of the above major flaws. Finally, the positive effect of water debinding prior to thermal debinding on reducing crack formation was demonstrated. After optimisation, the bars achieved a flexural strength greater than 100 MPa, which is the highest value among dense HA fabricated using lithography-based techniques.

Effect of macroscopic defects on optical properties of fluorophosphate glasses

Fluorophosphate glass is the core of laser materials and also has an important reference value for an optical lens. According to Kirchhoff’s law, this paper focuses on the optical loss of incident light entering fluorophosphate glass. The transmittance and refractive index of optical glass were measured by an ultraviolet-visible spectrophotometer (Uv) and V-prism tester. With the sample thickness increasing from 10 mm to 50 mm, the transmittance of grade 4 fluorophosphate glass decreases from 85.32 % to 76.91 %, and the scattering rate increases from 8.4704 % to 17.0591 %. The microstructure and internal images of macroscopic defects were obtained by projective optical microscopy. In addition, an electron probe (EPMA) was used to analyze the defective components and identify the main components of the defects. The results show that the high content of P/Al/O element is the main cause of optical loss.

Performance evaluation of Charpy impact tests to investigate and detect process defects of a Ni-based superalloy elaborated by laser powder bed fusion

Additive manufacturing is a robust process for building complex parts with improved mechanical properties. However, if a problem occurs during manufacturing, the production batch will present microstructural defects that reduce the mechanical properties and reliability of the part. Part post-processing represents a significant production time and production cost. Therefore, it is necessary to find a fast, simple, cheap, and efficient test to check production batch quality. High deformation rate tests are promising. We performed the Charpy impact test with reduced-size samples to investigate a specific height where defects may be suspected or critical. Charpy impact test's ability to detect an intentionally introduced defect in a tested sample is studied, involving exploitation of force-displacement data recorded during the test on reduced-size samples. For a defect localized within the notch plane, the results show that reduced-size samples allow for detecting a localized defect. Resilience values are reduced by 10%, and the force-displacement curves show a divergence in the propagation regime. Defects shifted from the notch plane are also detected, but not more than 500 μm, allowing the detection of production batch break.

Micromechanical properties of Yttria-doped zirconia ceramics manufactured by direct ink writing

Yttria-doped zirconia ceramics have many applications in a wide range of industries mainly due to their excellent mechanical properties, corrosion resistance and biocompatibility. In this study, micromechanical properties of yttria-doped zirconia produced by Direct-Ink Writing (DIW) were investigated and compared to the ones produced by Cold Isostatic Pressing (CIP). In doing so, mechanical response was assessed at different length scales, from macro- up to submicrometric-, by means of Vickers hardness, nanoindentation, and nanoscratch tests. Microstructure was also characterized by determining grain size, crystal structure and phase tetragonal to monoclinic phase transformation. Results revealed that printed samples displayed 20–25% lower hardness values compared to those exhibited by the respective CIP pairs. Differences in hardness between 3 and 8 mol% yttria content evaluated for CIP samples were slight for printed samples, due to the effect of microstructural defects like porosity, resulting from the processing parameters used. At the local level, such an effect was found to be lower. In this sense, hardness and elastic modulus achieved by nanoindentation were closer, when comparing printed and CIP samples. Scratch tests carried out from 0 to 250 mN revealed that 3 mol% Y2O3 samples developed micro-fracture events in the track length, being the printed samples the ones heavily deformed.

Silicon nitride assisted carbon nanotubes induced stacking faults and twins in aluminum alloy composite joint

The extensive heat input during welding can generally disrupt the meticulously crafted microstructure of the Al alloy substrate, leading to a marked decrease in the mechanical performance of the traditional joint. Here, we conducted a study of adding silicon nitride and CNTs to form Al composite joints, which not only compensated for the microstructural defects caused by welding, but also improved the tensile strength of the joints by 31%. Importantly, microstructural analysis reveals that there are twinning and SFs in both Al and Al4C3 phase. It was contributed to Al4C3 growth and the reduction in Al alloy SFs energy by CNTs. Moreover, the residual CNTs and Al4C3 phase in the composite joints refined the original precipitation phase network. This work is supposed to provide new insights into the strengthening of composite joints based on SFs and twins.

Geometry and Microstructure Control of Remanufactured Metallic Parts by Cold Spray Additive Manufacturing.

Cold Spray Additive Manufacturing (CSAM) is a thermal spray technique that is typically used for the repair of metallic components. One of the challenges of CSAM is to improve the geometrical accuracy of the sprayed parts, along with overcoming the inferiority of the mechanical properties of the deposits by tailoring their microstructure with different deposition strategies. For this, Cu, Al, Ti, and Ti6Al4V substrates were reconstructed by two Cold Spray (CS) methods: Traditional (T) and a novel strategy, Metal Knitting (MK). The final geometry, microstructure, and mechanical properties of the reconstructed parts by these two methods were compared. Additionally, we investigated the effects of annealing on the microstructure of sprayed components and its influence on adhesion, resistance to erosion, and abrasive wear. The results indicate that annealing effectively reduces the microstructure defects of the remanufactured parts (up to 30% porosity reduction) and improves the adhesive strength (i.e., below 30 MPa for as-sprayed deposits, and up to 160 MPa for heat-treated Ti4Al4V deposits). Notably, the abrasive and erosive resistance of the Cu and Al annealed deposits sprayed by MK gave very similar results compared to their bulk counterparts, suggesting that it is an efficient method for the reconstruction of damaged parts.

Interaction of Stacking Faults with point/extended defects in Fe-He irradiated 6H-SiC

The study explored the microstructure evolution of 6H-SiC that underwent sequential iron and helium ion irradiation with energies of 2.5 MeV and 500 keV, respectively, at room temperature, followed by annealing at 1500°C for two hours. Following irradiation, the entire damaged layer underwent amorphization. However, during subsequent annealing, epitaxial recrystallization took place, resulting in the formation of defected polycrystalline 6H-SiC characterized by the presence of Fe-rich clusters, cavities, and stacking faults. Fe-rich cavities were found to predominantly form at the edges of the stacking faults, as revealed by XTEM. The interaction of microstructural defects is further investigated via first-principles calculations. The periphery of the stacking faults has been identified as the primary location for the emergence of vacancy clusters, serving as favorable sites for the accumulation of point defects, including Fe atoms. This behavior can be attributed to the combined effects of mechanical and electronic energy relaxation mechanisms. Mechanically, the presence of stacking faults allows for the release of elastic energy that had been stored at the boundary. Electronically, the energy relaxation arises from the saturation of C- and Si-dangling bonds. Both of these processes contribute to the observed behavior, highlighting the intricate interplay between mechanical and electronic factors in the system. The low point defect migration energy barriers in the vicinity of the stacking faults promise high recombination, which can limit cavity growth and enhance radiation resistance. The study not only offers valuable insights into the mechanism of cavity/stacking faults interaction, contributing to a better understanding of radiation damage in 6H-SiC but also demonstrates that 6H-SiC material containing stacking faults could serve as a viable alternative to 3C-SiC for nuclear application.

Sb Alloying for Engineering High‐Thermoelectric zT of CuGaTe2

Decades of studies on thermoelectric materials have enabled the design of high‐performance materials based on basic materials properties, such as bandgap engineering. In general, bandgap energies correspond to the temperature at which the peak thermoelectric performance occurs. For instance, CuGaTe2 with a relatively wide bandgap of 1.2 eV has its peak zT > 1 at > 900 K. On the other hand, the zT is usually very low (<0.1) for this material at room temperature. This severely limits its average zT and hence overall performance. In this study, a phase diagram‐guided Sb alloying strategy to improve the low‐temperature zT of CuGaTe2 is used, by leveraging on the solubility limits to control the formation of the microstructural defects. The addition of Sb simultaneously improves the electrical conductivity and decreases the lattice thermal conductivity. For a low‐temperature range of 300–623 K, this Sb‐alloying strategy enables the achievement of a record high average zT of 0.33. The strategy developed in this study targets the improvement of the low‐temperature range of CuGaTe2, which is rarely focused on for wide‐bandgap ABX2 compounds, opening up more opportunities for holistic performance improvements, potentially enabling ultrahigh‐performance thermoelectrics over a wide temperature range.

Structure formation of Np-30KhGSA alloy in wire and arc additive manufacturing

The use of metallic products 3D-printing is a modern, promising technology that improves production efficiency. However, using this technology is associated with a number of problems, for example, with increased microstructural heterogeneity and defects in metal. Therefore, it is necessary to carry out researches to identify 3D-printing modes ensuring the most homogeneous, stable and non-defect structure. In this work, a study was made of the process of structure formation of 30KhGSA steel in the process of Wire and Arc Additive Manufacturing (WAAM) under various printing modes. Microstructural analysis, microhardness measurement and fractal analysis were used for assessment of the obtained billets. In all surfacing modes, a significant structural inhomogeneity of the deposited billet was revealed, which is explained by the thermal effect of the deposited layer on the already crystallized metal. Nevertheless, we found the mode that gives the most favorable microstructure in terms of its uniformity and equiaxed grains. With an increase in WAAM heat input values, an increase in the productivity of the process is observed and a decrease in the number of pores in the material is recorded. However, when the heat input of the surfacing process exceeds 1000 J/mm, the structural inhomogeneity of the material increases and its microhardness significantly decreases. Based on the studies, as a WAAM 3D-printing mode for Np-30KhGSA alloy, a mode with a heat input of about 920 J/mm can be chosen, which provides the lowest structural inhomogeneity and a sufficiently high productivity of the growth process with the absence of defects in the form of pores and elements of not melted wire.

High-resolution Bragg-edge neutron radiography detects grain morphology in PBF-LB/M IN718

One of the main advantages of metal additive manufacturing (MAM) techniques is their ability to produce components with site-specific microstructural features. Nevertheless, microstructural defects and lack of repeatability are still major concerns in MAM. In this study, a laser powder bed fusion (PBF-LB/M) IN718 material, produced using two different scan length vectors, is investigated using Bragg-edge neutron 2D imaging (BENI) combined with electron backscatter diffraction (EBSD) analysis. BENI is able to detect, on a macroscopic scale, process-induced changes in texture in a large field of view covering the entire sample (20×80 mm2). In addition, high-resolution BENI (HR-BENI), with a pixel size of 12.8 µm, provides a micro-scale examination of the local variations of texture and grain morphology, otherwise undistinguishable using the standard resolution. As such, HR-BENI offers a straightforward and detailed way of screening the integrity of MAM parts at cm-length scales.

Formation of Twin Boundaries in Rapidly Solidified Metals through Deformation Twinning.

The rapid solidification process is relevant to many emerging metallurgical technologies. Compared with conventional solidification processes, high-density microstructure defects and residual thermal stress are commonly seen in rapidly solidified metals. Among the various defects, potentially beneficial twin boundaries have been observed in the rapidly solidified nanocrystalline microstructures of many alloy systems. In this work, a pathway for forming twin boundaries in rapid solidification processes is proposed. A detailed derivation of strain inhomogeneities upon thermal shrinkage and the deformation twinning phase field method is given. By calculating cooling-induced thermal strain inhomogeneity in nanocrystalline metals and growth thresholds for deformation twinning using the phase field method, it is shown that residual thermal strain hotspots in the microstructure can reach the threshold for deformation twinning when the shear elastic property of grain boundaries is significantly different from the bulk.

Numerical studies of self-organization of shear bands in one and two dimensions

In the present manuscript we perform a numerical analysis of the self-organization processes of adiabatic shear bands formation in depleted uranium, aluminum alloy, and high-strength steel in one- and two-dimensional cases. Since the processes of shear bands formation are strongly nonlinear, three materials with significantly different parameters considered allow us to generalize obtained dependencies. For all materials, we investigate the evolution of stress, temperature and velocity fields from initial to final stage of localization. It is well known that localization processes occur during the high shear rate loads in the presence of initial microstructural defects in metallic materials. Starting with a random distribution of initial stress, which models microstructural defects, we obtained new dependencies for such significant parameters of the problem considered such as localization time and spacing between shear bands. The influence of initial plastic strain rate on these parameters was also studied. We obtain the linear dependence of localization time on initial strain rate and show that spatial dimension significantly influences on its value. Also, we introduce the method of shear bands selection during computations and present new statistical distributions for band spacing. We show that the interaction between shear bands significant influences on the spacing between them.

A comprehensive study on the influence of the scan pattern in two porosity levels and surface roughness on the fatigue behavior of laser powder bed fusion manufactured specimens made of steel H13

As a developing and immature technique, additive manufacturing (AM) shows some limitations: depending on material and process parameters so far, it usually results in parts with residual porosity, high residual stresses and a surface with a certain level of roughness. Due to its weaknesses and high production costs, AM is more preferably used when the fabricated parts have a high geometry complexity, the material used is very expensive, or the parts can offer additional performance. In order to allocate AM further in industry, a better understanding of the not well-investigated fatigue behavior is necessary. This work focuses on the influences of some general process parameters including laser power, scan speed, scan pattern and postmachining on the resulting fatigue properties of H13 tool steel specimens generated through powder bed fusion (PBF) technique. Results reveal that scan patterns influence fatigue properties by affecting the largest porous defect size and microstructure thus matrix strength. The degree of porosity or roughness resulting from the energy input and postmachining has a significant inferior impact on the fatigue strength. Neither porosity nor tensile properties show a single direct mathematic correlation with the fatigue properties.

Microstructural evolution of extended defects in 25 μ m thick GaN homo-epitaxial layers

Defect origins and their propagation behavior were investigated in 25 μm thick homo-epitaxial GaN layers grown on ammono-thermal and void-assisted separation (VAS) substrates using multi-vector x-ray topography in both transmission and reflection geometries. Complex inclusions were identified and their microstructure was analyzed. Additionally, generation of threading dislocation clusters during epitaxial growth is analyzed. Various defects are delineated from the substrate vs epitaxial layers. Growth on the ammono-thermal substrate led to less defective and flatter epitaxial layers compared to the growth on the VAS substrate. Determining the origins and microstructure of defects is crucial toward developing defect mitigation strategies for reliable GaN devices.