Void Formation Research Articles

Effect of long-term aging on the microstructure evolution and fracture mechanism of 9% Cr steels

The fracture toughness of pressure pipe steels is vital for their design and safe operation. Understanding the deterioration of the fracture toughness and the corresponding fracture mechanisms is instructive and meaningful for material selection and development. The elastic–plastic fracture toughness of 9% Cr steels aged at 873 K and 923 K from the initial state to 20000 h was investigated at ambient temperature and 873 K. The deterioration of the fracture toughness and the corresponding fracture mechanisms are discussed in detail. For 9% Cr steels, voids mainly initiated from primary inclusions such as silicides, sulfides, alumina and manganides in as-received state, while voids initiated from both primary inclusions and secondary particles such as the Laves phase after long-term aging. These voids grew and coalesced with neighboring voids and crack tips subsequently until ultimately fracture. After aging at 873 K, the fracture toughness values tested at 873 K decreased from the initial state to 3000 h and then increased to 10000 h. The peculiar deterioration of the fracture properties after 3000 h of aging at 873 K can be attributed to the high number density of Laves phase particles in the steels. It was found that the densely distributed hard Laves phase particles induced strain concentration at the interface between the Laves phase and the matrix under plane-strain tension condition and promoted nucleation of voids. It was suggested that the spatial distribution of the Laves phase played a significant role on the void formation. 9% Cr steels with a denser distribution of Laves phases brings about voids with shorter interspaces, which are more susceptible to coalesce into crack tip, resulting in lower fracture resistance. After aging at 923 K from 10000 h to 20000 h, the number density, size and volume fraction of precipitates, block size, misorientation angle and dislocation density remained almost constant. The slight increase in the fracture toughness from 10000 h to 20000 h can be mainly ascribed to the coarsening of the martensite lath in this stage.

Rim nucleation and step-train orientation effects in SOI(111) dewetting

We demonstrate that the step-train orientation of a vicinal surface plays an important role in solid-state dewetting. Focusing on SOI(111), we observe the formation of hexagonal voids in the silicon film due to dewetting. These voids are surrounded by an asymmetric rim, which is wider on the higher-terrace side compared to the lower-terrace side. Our findings also reveal a layer-by-layer growth of the rim around the dewetting voids during the early stages, before the formation of branched structures. Dewetting fingers develop faster and are thinner in the step-down direction compared to the step-up direction. Kinetic Monte Carlo (KMC) simulations confirm our experimental observations and provide insights into the atomic details underlying rim and finger formation. During the initial stages of dewetting, the nucleation of a new rim layer is not required: Atoms leaving the dewetting void are captured by the adjacent upper step, leading to the growth of the upper terrace that thus surrounds the dewetting void. The nucleation of a new rim layer only becomes necessary when the distance between the edge of the void and the upper terrace increases. The energetics involved in the dewetting kinetics of SOI(111) are consistent with those of SOI(100).

Characterization of the microstructure, texture, and tensile behavior of additively manufactured Hastelloy X superalloy: Insights into heat treatment at 900 °C

This research aimed at the microstructural characterization and elucidation of room- and high-temperature tensile properties of additively manufactured Hastelloy X Ni-based superalloy heat-treated at 900 °C. The samples were fabricated via the laser powder bed fusion method utilizing two scan strategies known as island and meander types. The results indicated that the cellular structure embedded in the columnar grains of the as-built microstructure disappeared after applying the heat treatment, linking to the reduction in dislocation density while keeping the former morphology of the grains unchanged. However, the intensities of the Brass, Goss, Goss-Brass, and rotated Goss texture components were reduced, leading to the generation of more homogenous fibers in the heat-treated microstructure. Additionally, it caused the formation of a Mo-rich phase on the grain boundaries, which improved the room temperature's ultimate tensile strength. However, it was elucidated that the Mo-rich phase-induced void formation and intergranular cracking failure caused a slight decrease in hot ductility. Overall, the obtained results revealed that the heat-treated island samples showed superior hot tensile behavior than the meander sample due to having a larger grain size.

Reliability of FET-type gas sensor with asymmetric air-gap micro-heater structure considering thermoelectric effect

Poly-silicon has a wide range of applications because of its interesting properties in the thermoelectric effect. We propose an optimal field-effect transistor-type (FET-type) gas sensor platform with an embedded poly-silicon micro-heater designed considering both power consumption and reliability. Since the poly-silicon micro-heater has a thermoelectric effect, the reliability of the micro-heater can be improved without a significant increase in power consumption by adopting asymmetric etch-holes (finally asymmetric air-gap) along the heater electrode. The sensor with the shorter etch-hole (10 µm) around one electrode of the heater electrode increases power consumption by only 1.22 times but can increase lifetime by 14.2 times. The heat distribution and contact temperature of symmetric and asymmetric air-gap heaters are compared using COMSOL Multiphysics. Sensors with different lengths of etch-holes are fabricated, and the thermal response and reliability of the fabricated sensors are measured and analyzed. In addition, in a sensor in which a thin film of indium oxide is deposited as a sensing material on the gas sensor platform, the effect of air-gap design change on the gas sensing characteristics is investigated. Depending on the polarity of the voltage applied between the two terminals of the heater, the difference in void formation and damage at the heater contacts is confirmed by TEM and EDS analysis.

The effects of twist angle and loading mode on the deformation behavior of bicrystal copper

The effects of twist angle and loading mode on deformation behavior of bicrystal Cu containing symmetric twist grain boundary are investigated by molecular dynamics. Grain boundary dislocation network changes from square-like to ring-like with increasing twist angle. Tensile strength under loading parallel to grain boundary is higher than that under perpendicular loading to grain boundary for the same model, and the larger twist angle is, the larger difference under two loading modes is. Under perpendicular and parallel loadings, initial plastic yield of bicrystal Cu with low angle is triggered by slipping of grain boundary partial dislocations, which results in almost complete dissociation of grain boundary and formation of pillar-like voids inside grains. However, initial plastic yields of bicrystal Cu with higher angle are triggered by explosive collective slipping of short grain boundary partial dislocation and nucleating of new lattice dislocations on grain boundary, respectively, which result in direct splitting of grain boundary and formation of sphere-like voids, respectively. The calculated increase rates of partial dislocation number increase with increasing twist angle, but the increase amplitude is significantly larger under parallel loading. In later stage, void volume of each model is almost the same, but void surface area is significantly different.

Toward High‐Performance All‐Solid‐State Thin Film FeOxSy/LiPON/Li Microbatteries via Dual‐Interface Modification

AbstractFeaturing high theoretical capacity, low cost, and low preparation temperature, Li‐free cathodes are considered promising for all‐solid‐state thin‐film lithium microbatteries (TFBs). In this work, a Li‐free cathode of amorphous FeOxSy film is prepared, followed by the fabrication and investigation of FeOxSy/LiPON/Li TFBs. It is found that the structural degradation at both the LiPON/Li and FeOxSy/LiPON interfaces results in a rapid capacity loss for the TFB during electrochemical cycling. To achieve both high rate capability and long cycle life for the TFB, a dual‐interface modification approach using amorphous Al2O3 as an interlayer is proposed. During the TFB cycling, the Al2O3 interlayer not only enables robust LiPON/Li contact by preventing the formation of Li voids but also blocks the diffusion of Fe element from FeOxSy to LiPON electrolyte, effectively suppressing the interfacial resistance growth. Consequently, the dual‐interface modified TFB achieves greatly improved rate capability (194.3 mAh g−1 at 5 A g−1) and cycle performance (71% capacity retention after 800 cycles), which are superior to the pristine TFB and single‐interface modified TFB. This work advances the fundamental understanding of the failure mechanisms at the solid‐solid electrode/electrolyte interfaces, offering a feasible interfacial modulation strategy to develop advanced all‐solid‐state lithium batteries.

Elimination of buried interfacial voids for efficient perovskite solar cells

Establishing optimal interfacial contact is crucial for enhancing the efficiency of perovskite solar cells (PSCs). However, formation of voids at buried interface of perovskite films often hinders this critical objective. Our investigation reveals that these buried interfacial voids predominantly manifest at sites characterized by large-angle pits on rough substrate, due to their elevated nucleation barriers compared to their small-angle counterparts. To address this challenge and promote uniform nucleation and growth across all sites, regardless of their angles, we develop an innovative precursor regulation strategy to reduce the nucleation barrier discrepancy between different sites by introducing nonstoichiometric homologous ions or by facilitating the formation of intermediate phases. Notably, the incorporation of FACl additive into the FAPbI3 precursor combines the dual benefits of this approach, yielding high-quality perovskite film with intimate contact and reduced defect density. Consequently, this leads to a significant enhancement in the photovoltaic performance of PSCs.

Advancements in emerging MXene-integrated nanocomposite coatings: Unraveling defect-free microstructure for superior tribological, mechanical, and anti-aging features

This study explored the enhancement potential of MXene, a novel two-dimensional material, in epoxy-based nanocomposites; which comprehensively examined the influence of MXene on epoxy's viscosity, void formation, aging resistance, mechanical properties, and anti-wear properties. MXene nanofillers, labeled as 25C and 80C, fabricated via different acid-etching methods, were incorporated into epoxy resin at varying weight percentages (0.1–2.0 wt%). Observations revealed that for both varieties of MXene, inclusion of 1.0 wt% MXene led to the mitigation of void content, whereas the incorporation of 2.0 wt% MXene yielded maximal enhancements in both tensile strength and abrasion resistance. Additionally, the integration of 1.0 and 2.0 wt% MXene displayed superior aging resistance, with around 80 % reduction in free radical formation compared to the unmodified epoxy, while maintaining its excellent mechanical properties after QUV exposure. Therefore, both MXene types significantly enhanced the performance of epoxy composites, with the 80C-MXene displaying marginally superior enhancement due to its smaller particle size and higher purity, as identified by SEM and TEM images.

Assessment of Critical Stack Pressure and Temperature in Li‐Garnet Batteries

AbstractStack pressure and temperature serve as effective means to induce deformation of the lithium metal anode toward the Li/solid‐state‐electrolyte interface, thereby mitigating the well‐known issue of void formation during high‐current‐density stripping. In this study, a compelling methodology is systematically assessed for determining the critical stack pressure and temperature of Li metal anode in conjunction with Li7La3Zr2O12 (LLZO) solid‐state electrolyte, which is the minimum set of values required to maintain conformal contact between Li and LLZO at a given current density. The methodology is based on the analysis of the second derivatives of the voltage profiles of identical Li/LLZO/Li symmetric cells measured during one half‐cycle (3 mAh cm‐2) at the same current density but different stack pressures. The effectiveness of the presented approach in assessing conditions for mitigating void formation during Li stripping is evaluated through cycle stability tests performed on Li/LLZO/Li symmetric cells.

Understanding a novel form of intergranular corrosion of stainless steel 316L exposed to molten LiCl-Li2O-Li

Stainless steel 316 L exposed to LiCl-Li2O molten salt containing Li metal for 500 and 1000 h experiences deep intergranular attack and bulk void formation. This degradation is due to a synergistic corrosion mechanism, different than other forms of molten salt corrosion, where Cr and Mn oxides and carbides form along grain boundaries and are dissolved by Li metal. Synchrotron transmission X-ray microscopy 3D imaging is used to show corrosion morphology and Cr enrichment and depletion in a novel format that has not previously been used for bulk corrosion samples.

Quasi-direct Cu–Si3N4 bonding using multi-layered active metal deposition for power-module substrate

The advancement of power modules demands more reliable insulating circuit substrates. Traditional substrates, comprising Cu and Si3N4, are produced using active metal brazing (AMB). However, AMB substrates have reliability concerns owing to electrochemical migration and void formation from brazing filler metals. This study introduces a quasi-direct Cu–Si3N4 bonding technique using a Ti/Al bilayer active metal deposition at the bonding interface. A sputtered Ti/Al bilayer was formed on the Si3N4 surface, then heated and pressurized the sputtered Si3N4 substrate with Cu sheets in vacuum to bond each other without voids or delamination. The Ti/Al layers reacted with Si3N4 and Cu, forming a 300 nm intermediate layer. TEM observations show this layer contains segregated Ti–N and Cu–Al phases, with a good lattice match to Si3N4 and Cu–Al. Temperature-cycling tests on the Cu/Si3N4/Cu substrate revealed delamination caused by increased tensile stress at the periphery of the bonding area due to asymmetrical Cu patterns. This novel quasi-direct Cu–Si3N4 bonding technique addresses issues of electrochemical migration and void formation seen in AMB substrates, offering a reliable bonding interface for power electronic substrates.

Influence of erosion voids and traffic loads on buried large-diameter reinforced concrete pipes

Geotechnical centrifuge tests were conducted to examine the influence of invert voids and surface traffic loads on 1400 mm diameter reinforced concrete pipes buried with a shallow soil cover depth of 700 mm. Void formation beneath the pipe was simulated during centrifuge testing. The test results revealed that before void formation, the surface load directly above the middle of the pipe caused a significant increase in not only the circumferential bending moments but also the longitudinal bending moments, the latter of which was considerable and could not be ignored. Void formation beneath the middle of the pipe led to a reduction in both the circumferential bending moments and longitudinal bending moments at all measuring positions, i.e., crown, springline, and invert. The most significant reduction occurred at the invert, and there was even a reversal in the sign of the invert longitudinal bending moment. A comparison was made between centrifuge tests with erosion voids and surface loads at different horizontal positions, which had a marked influence even when the positions differed by half a pipe length. Joint rotation played an important role in relieving large bending moments of pipe barrels in a jointed pipeline when the void and surface load were located at the joint.

Epoxy filled with nylon powder—An approach to reduce void formation via fused particle method

AbstractIn the quest for superior materials, especially in industrial applications demanding strength, stiffness, low density, and cost‐efficiency, composite materials have emerged as game changers. Combining a polymer matrix with reinforcement materials, they hold great promise. However, the challenge of particle agglomeration looms large, especially at high loadings. Particle agglomeration disrupts filler distribution and gives rise to voids in polymer composites. This study investigates the fusion behavior of agglomerated Nylon particles and their influence on Nylon/epoxy composites. Using epoxy resin and Nylon SP301, a micron‐sized Nylon 12 powder with a 185°C melting point, composites were prepared at 3%, 9%, and 15% Nylon loading. After curing, these composites underwent controlled heating at 185, 195, and 205°C, with a fusion of 20–100 min. At 185°C, particles initially remain separate, forming slight clumps after 20 min and increasingly sticking together at 60 and 100 min. Shifting to 195°C, particles begin consolidating into a solid mass even after 20 min. The introduction of Nylon decreases composite density compared to pure epoxy, and density changes vary with fusion time, exhibiting complete fusion, partial fusion, and shrinkage‐induced gap formation. Differential scanning calorimetry analysis reveals evolving glass transition temperatures (Tg) influenced by the fusion process, with longer fusion times yielding higher Tg and greater heat capacity.

Simulation of fiber-induced melt pressure fluctuations within large scale polymer composite deposition beads

The process-structure-property relationship in Large Area Additive Manufacturing (LAAM) technology is an ongoing area of research as the inherent microstructural properties (chiefly fibers and voids) affect the performance of printed parts. Unfortunately, we currently lack adequate understanding of micro void nucleation and evolution during the LAAM and fused deposition modelling (FDM) processes. Modeling of the polymer melt flow during the extrusion process is important in understanding the underlying microstructural formation and associated properties of the print, that determines the part performance in service conditions. In this paper we compute fiber-induced local pressure fluctuations which may promote void formation in the bead’s microstructure. On a macro-scale, we determine flow fields of a purely viscous, Newtonian planar polymer deposition flow through a LAAM nozzle which are utilized on a micro-scale model where we simulate the evolution of a single ellipsoidal fiber along streamlines obtained from the macro-model. On the micro-scale, we determine instantaneous values of the translational and rotational velocities of the rigid ellipsoidal fiber that satisfies a balance of hydrodynamic forces and couples on the fiber’s surface based on a Newton Raphson algorithm and we track the fiber’s motion along the flow path via an explicit numerical iterative algorithm. Model verification is achieved by benchmarking results with solutions from well-known Jeffery’s equation of motion of a particle in homogeneous simple shear flow. We account for rotary diffusivity due to short-range fiber-fiber interaction in the FEA simulation by determining an effective fluid domain size representative of the interaction coefficient of the melt flow through a correlation analysis that yields an equivalent steady state orientation based on the Advani-Tucker equation. We also consider different possible motions of the fiber along individual LAAM flow paths from a given set of random initial fiber conditions to determine pressure bounds on the fiber surface along each streamline. For improved computational efficiency, calculations are carried out with respect to the fiber’s local coordinate axes to overcome the rigor of adaptive remeshing during the quasi-transient analysis. Results show low pressure extremes near the fiber’s surface which varies across the printed bead as well as through its thickness. Discussion is provided to gain insight into the effect of low-pressure extremes on micro void formation, particularly at the nozzle exit and during die swell/expansion.

Study the effects of TiB2 reinforcement on the AA-2014 matrix fabricated by friction stir processing

This study explores the impact of TiB2 nanoparticles on the microstructure, microhardness, and tensile properties of AA-2014 surface composite manufactured through Friction Stir Processing (FSP). A series of experiments were conducted with varying FSP parameters, and the findings were compared with the base metal. Microstructure studies revealed optimal bonding between TiB2 and the AA-2014 substrate at 1200 rpm, surpassing results from 900 and 1600 rpm. However, microstructural analysis unveiled agglomeration of particles and void formation on the composite surface. Microhardness values indicated a substantial increase from 88 HV (AA-2014) to 138 HV (AA-2014/3%TiB2 @1200rpm), attributed to the presence of TiB2 nanoparticles and FSP-induced modifications. Ultimate Tensile Strength (UTS) values exhibited a noteworthy improvement from 290 MPa (AA-2014) to 465 MPa (AA-2014/3%TiB2 @1200rpm), emphasizing the positive influence of uniform TiB2 dispersion achieved during FSP. The study underscores the effectiveness of TiB2 reinforcement and FSP in enhancing the mechanical properties of AA-2014 surface composite, despite observed microstructural features.

Cracking and Void Formation in Bulk W Components Manufactured for JET ITER-Like Wall

Cracking and Void Formation in Bulk W Components Manufactured for JET ITER-Like Wall

Effect of kenaf fiber loading on mechanical and morphological properties of polyurethane shape memory polymer composites

Shape memory polymers (SMPs) have captivated researchers and engineers with their unique ability to recover their original shape after deformation. Among SMPs, shape memory polyurethane (SMPU) is promising due to its easy processing, cost-effectiveness, and biocompatibility. Despite its potential, SMPU faces limitations in strength and modulus, hindering structural applications. This study investigates the sustainable improvement of shape memory polymer composites (SMPCs) by integrating kenaf fibers (KF) as a reinforcing material at different percentages, specifically 5%, 10%, 15%, 20%, 30%, and 40% of the composite’s weight. A combination of hand lay-up technique and compression moulding technique was employed for the fabrication process to achieve the desired shape. The results reveal a clear correlation between the KF content and the mechanical properties. Tensile testing reveals improved strength and strain up to 20% KF, peaking at this level but declining at 30% and 40% due to fiber agglomeration and void formation. Scanning electron microscope (SEM) analysis highlights favourable fiber-matrix adhesion at optimal level. By shedding light on the mechanical behaviour of SMPC with KF reinforcements, this study paves the way for developing high-performance composites tailored for shape memory applications.

Stable polydisperse free-standing porous films made by mechanical deformation.

Using molecular dynamics simulations, we show that the methodology of making thin stable nanoporous monodisperse films by biaxial mechanical expansion and subsequent cooling into the glassy state, also works for polydisperse films. To test this, a bidisperse polymer system of an equal number of very long (≈72 entanglements) and short (≤4 entanglements) chains with a polydispersity index of 1.80 is considered. The void formation and the development of the local morphology upon expansion, relaxation, and cooling are investigated. As for the monodisperse case, long chains in thin porous polydisperse films extend over several pores, stabilizing the whole morphology. The short chains do not fill up the pores but tend to aggregate inside the polymer matrix and to avoid surface areas and reduce conformational constraints imposed by the surrounding, a scenario very similar to strain-induced segregation between the strained long and relaxed short chains.

Exploring Lime Mortar Microstructure: Investigating the Impact of Mixed-in Additives and their Role in Salt Efflorescence Inhibition

This study presents the analysis of the crystallization impact in four lime-based mortar mixtures in order to determine the extent of decay during salt crystallization, affected by the use of additives and change in microstructure. For this purpose, non-destructive micro-computed tomography was used to monitor salt crystallization in lime mortar cores. This technique allowed tracking of salt precipitation after accelerated weathering steps and further enabled the localization of damage due to salt crystallization in both time and space. Results indicate that the internal 3D microstructure of the lime mortars is greatly influenced by the type and the amount of additive used. Polyacrylic acid (PAA) acted as a superplasticizer and air entrainer in the mixture, resulting in the formation of larger air voids, which led to a decrease in compressive strength. Mobilization of salts within the mortar matrix was observed, causing accumulation and cracking of the mortar wall towards the end of the weathering experiment.

Na vs. Li metal anodes for batteries: unraveling thermodynamic and electronic origins of voids and developing descriptors for artificial surface coatings

This work examines the thermodynamics, interfacial chemistry, and stiffness variations between Na and Li void and pit formation in metal batteries, with the goal of developing accurate descriptors and coatings for a stable battery.