Inhomogeneous Microstructure Research Articles

Influence of Microstructure on the Material Properties of LLZO Ceramics Derived by Impedance Spectroscopy and Brick Layer Model Analysis.

Variants of garnet-type Li7La3Zr2O12 are being intensively studied as separator materials in solid-state battery research. The material-specific transport properties, such as bulk and grain boundary conductivity, are of prime interest and are mostly investigated by impedance spectroscopy. Data evaluation is usually based on the one-dimensional (1D) brick layer model, which assumes a homogeneous microstructure of identical grains. Real samples show microstructural inhomogeneities in grain size and porosity due to the complex behavior of grain growth in garnets that is very sensitive to the sintering protocol. However, the true microstructure is often omitted in impedance data analysis, hindering the interlaboratory reproducibility and comparability of results reported in the literature. Here, we use a combinatorial approach of structural analysis and three-dimensional (3D) transport modeling to explore the effects of microstructure on the derived material-specific properties of garnet-type ceramics. For this purpose, Al-doped Li7La3Zr2O12 pellets with different microstructures are fabricated and electrochemically characterized. A machine learning-assisted image segmentation approach is used for statistical analysis and quantification of the microstructural changes during sintering. A detailed analysis of transport through statistically modeled twin microstructures demonstrates that the transport parameters derived from a 1D brick layer model approach show uncertainties up to 150%, only due to variations in grain size. These uncertainties can be even larger in the presence of porosity. This study helps to better understand the role of the microstructure of polycrystalline electroceramics and its influence on experimental results.

The effect of high-temperature ECAP on dynamic recrystallization behavior and material strength of 42CrMo steel

With the development of mechanical equipment towards long life and high reliability, the traditional heat treatment method is challenging to meet the high strength requirements of 42CrMo steel for mechanical equipment. High-temperature equal channel angular pressing (ECAP) of 42CrMo steel was carried out above austenite transformation temperature, and the microstructure and material strength of the specimen at different radial positions were characterized and tested. The dynamic recrystallization (DRX) kinetic model of 42CrMo steel was established. The distribution of strain, strain rate, temperature and DRX volume fraction in high-temperature ECAP was studied by coupling finite element software and DRX kinetic model, and the formation mechanism of microstructure and material strength difference was revealed. The results show that the different degrees of DRX caused by the uneven distribution of strain and strain rate on the specimen is the main reason for the gradient distribution of microstructure and material strength in the radial direction of the specimen. From the top to the bottom of the specimen, with the decrease of strain and strain rate, the degree of DRX increases continuously, the microstructure inhomogeneity decreases, the size of the martensite packets and blocks increases gradually, and the strength decreases gradually. The grain boundary strengthening caused by the decreased size of martensite packets and blocks is the main reason for the radial difference in material yield strength. Grain boundary strengthening and the coordinated deformation of geometrically necessary dislocations at the soft-hard grain interface of heterogeneous materials are the main mechanisms leading to the tensile strength gradient distribution. This study reveals the dynamic recrystallization behavior and material strength variation of high-temperature ECAP, which contributes to the qualification of the ECAP process and promotes the development of severe plastic deformation on the further strength enhancement of high-strength steel.

Effect of strain inhomogeneity on flow behavior, dynamic recrystallization and substructure evolution of a 904 L super austenitic stainless steel

In this work, two typical deformation processes were conducted on a 904 L super austenitic stainless steel to investigate its hot deformation behavior. Two regions with distinct effective strains were selected on identical cross section to examine the microstructural features, including substructures in the deformed matrix and dynamically recrystallized grains. The flow stress curves generally reflect the physical events that occurred at low strain range. When inhomogeneous microstructure and external factors such as friction become more remarkable at high strain level, some metallurgical events (e.g., partial dynamic recrystallization) are hard to be deduced from the flow curves. Meanwhile, strain dependent friction correction on the flow curves does not completely eliminate an obvious abnormal increase in flow stress, especially for the process with the deformation condition of 900 ℃ - 1 s−1. Based on the crystallographic analysis on the barreling zone of the deformed samples, it can be concluded that the strain-induced sub-boundaries that formed in the deformed matrix play a vital role in restricting the material flow behavior because these dense boundaries have a Hall-Petch strengthening effect. This is an intrinsic microstructure mechanism to explain the reason that the flow stress increases abnormally at large strains in the absence of dynamic recrystallization. In addition, the formation of twin boundaries at different deformation conditions are also discussed.

Assessment of different processing strategies to fabricate bulk Mg-Fe nanocomposites

Recent observations indicate that very high deformation temperatures (673 K corresponding to 0.7 of Mg's melting point) by high pressure torsion (HPT) are mandatory to successfully synthesize bulk Mg-Fe nanocomposites with homogenous microstructure and exceptionally high strength (2.2 GPa Hardness). This spurs the development of new fabrication strategies to further strengthen the material while maintaining microstructural homogeneity for lower processing temperatures (T < 673 K). In this regard the present work deals with various strategies of severe plastic deformation at temperatures below 673 K including ball milling the Mg50Fe50 powder followed by HPT, HPT at 673 K followed by HPT at room temperature, inter-stage annealing during deformation, and changing the geometry of HPT anvils. Microstructural analysis using a scanning electron microscope and results of microhardness tests revealed that although abovementioned strategies accelerated microstructural refinement in Mg-Fe composites, strain localization cannot be suppressed. The strain localization and consequently inhomogeneous microstructure can be attributed to the immediate strengthening of basal texture intensity in Mg phase at low temperature driven by shear strain. Hence, the lack of structural reproducibility and hardness values less than 2.2 GPa show that all strategies are not sufficiently effective to synthesize bulk Mg50Fe50 nanocomposites. Only a high deformation temperature is the main key to fully transform a coarse structure into a homogeneous nano-structure with exceptional strength and good reproducibility.

Inhomogeneity of microstructure and mechanical properties in the interlayer regions for narrow gap laser wire filling welding of 316L stainless steel

A systematic and accurate analysis of microstructural evolution in the interlayer regions of multilayer welded joint is of great importance for in-depth understanding of influence mechanism of uneven microstructure on mechanical properties. Herein, the interlayer microstructure acted on different thermal cycle curves in the upper layer (UL), middle layer (ML) and bottom layer (BL) of narrow gap laser wire filling welding (NGLWFW) of 316L stainless steel with a thickness of 40 mm were compared and investigated through experiment and numerical simulation. Results indicated that the microstructures of components created by NGLWFW exhibit distinct inhomogeneity with multiple rapid thermal cycles caused by repeated weld filling process, which provides a great deal of effect when considering mechanical properties of welded components. The molten pool size and the peak temperature in the UL acquired by simulation are higher than that in other layers ascribed to the high heat accumulation effect. The remelting zone forms between layers owing to the repeatedly heating, cooling and solidification process, which leads to the variation of microstructure from the formed layer to the latter layer. The maximum orientation distribution density and average size of grain in different layers are characterized by distinction presenting a monotonically upward trend from BL to UL. The δ-ferrite phase content in the BL is higher than that in other layers due to higher cooling speed, which results in higher hardness of the BL.

Mechanisms of macrozone elimination and grain refinement of near α Ti alloy via the spheroidization of the Widmannstätten structure

Regions with sharp local textures, i.e., macrozones, are common microstructural inhomogeneities in titanium (Ti) alloys that significantly reduce the fatigue resistance of Ti alloy parts. In this study, a two-stage thermomechanical operation was developed to eliminate macrozones and optimize microstructure in a near α Ti alloy. Multiscale microstructure characterization and full-field crystal plasticity (CP) modeling were conducted to uncover the underlying mechanisms. Spheroidizing through dynamic recrystallization (DRX) and superplasticity are indispensable ingredients. The alloy exhibited multiple DRX phenomena, which are strongly associated with grain morphologies. The discontinuous DRX and the main spheroidizing mechanism of the equiaxed primary α phase failed to eliminate macrozones but exacerbated microstructure heterogeneities due to the strong anisotropy of the hexagonal close-packed α phase. The Widmannstätten structure (WS), fabricated through the β heat treatment that generated abundant nano-defects and serrated grain boundaries, significantly promoted the occurrence of geometric dynamic recrystallization (GDRX). A complete GDRX of lamellar α grains can generate a sufficiently fine-grain structure to achieve superplastic deformation in the subsequent hot compression at a moderate deformation. The full-field CP simulations considering slip only versus slip + superplasticity confirmed that the dislocation slip dominated deformation aggravated deformation heterogeneity and the formation of macrozones. In contrast, the superplastic deformation suppressed the heterogeneous deformation and randomized the grain rotation, which in turn homogenized the microstructure and eliminated macrozones. Based on these observations, the β heat treatment followed by α hot compression is an effective method to achieve grain refinement and macrozone elimination of Ti alloys.

Innovative powder-based wettability evaluation of HfB2-ZrB2-SiC-B4C-CNT composite: Effect of surface roughness and ambient conditions

Active metal brazing, the most common method for joining UHTCs, necessitates the wettability of diborides in contact with filler layer, which is evaluated in present work using a powder-based approach. Ni powder is placed on pre-spark-plasma-sintered HfB2-ZrB2-SiC-B4C-CNT (HZSBC) composites with varying surface roughness (0.1 and 0.7 μm) and heated to 1450°C in Ar and Ar+H2 atmosphere. Formation of gas-liquid-Ni interface for rough surface (Ra:0.7 µm) restricted the spreading of molten Ni on HZSBC surface and increased contact angle (by ∼30°) compared to smooth surface (Ra:0.1 µm). Addition of 5% H2 into Ar atmosphere suppressed the oxide formation and improved molten Ni spreading at a contact angle of ∼10°. Cross-sectional analysis of droplets supports dissolutive wetting where enhanced surface roughness has reduced the dissolution depth (by ∼50% for Ar and ∼12% for Ar+H2 atmospheres). Furthermore, brazing of HZSBC composites with Ni interlayer is performed at 1450°C, in Ar and Ar+H2 atmosphere. Lower wettability of Ni with HZSBC in Ar resulted in detachment of composites during polishing, thus, indicating poor interfacial bonding. Successful brazing of these composites in Ar+H2 atmosphere resulted in microstructural inhomogeneity with decreased hardness and Young's modulus by ∼68% and ∼58%, respectively, from HZSBC region to Ni-Si rich filler layer. Lower interfacial residual stresses (∼0.4 GPa) have provided the structural integrity between ceramic and metal layer, thus making them suitable for applications involving complex shapes.

Microstructure and strengthening of Al-6Ce-3Ni-0.7Fe (wt%) alloy manufactured by laser powder-bed fusion

An Al-6Ce-3Ni-0.7Fe (wt%) alloy was fabricated via laser powder-bed fusion and its microstructure, thermal stability, tensile properties, and creep properties are investigated for two rapid-solidification rates with different eutectic spacings. For both faster- and slower-cooled states, the as-fabricated alloy mostly shows elongated grains with very fine eutectic networks (∼40 nm lamellar width) comprising a high-volume fraction (∼18 %) of intermetallic phases. Faster-cooled samples, however, display a less continuous eutectic network with finer spacing (∼100 nm), leading to higher Orowan strengthening and therefore superior microhardness and yield stress up to ∼350 °C. Upon aging (300 – 450 °C), micron-size Al9(Ni,Fe)2 needle-like precipitates form within grains, and the Al11Ce3 eutectic network spheroidizes, while retaining a submicron width and spacing, resulting in 33–37 % drop in microhardness after 144 h aging at 400 °C. Limited tensile ductility (∼6 %) and creep ductility (∼1–2 %) are measured for both faster- and slower-cooled alloys due to an inhomogeneous microstructure, where cavitation preferentially initiates at melt-pool boundaries, precipitate-free zones, and/or denuded zones, eventually leading to local fracture. Denuded zones, induced by stress, form due to diffusional flow with stress-dependent orientations, and are identified as microstructurally weak regions leading to strain localization. The present alloy does not show a significant difference in 300 °C creep resistance between: (i) tensile and compressive loading, (ii) faster- and slower-cooled samples, and (iii) continuous and spheroidized eutectics. Creep resistance at 300 °C is comparable to that of a Ce-richer Al-10.5Ce-3.1Ni-1.2Mn (wt%) alloy, despite the formation of denuded zones and needle-like Al9(Ni,Fe)2 precipitates.

Microstructure and Early-Stage Oxidation Behavior of Co-Cr-Cu-Fe-Mn-Ni High-Entropy Alloys

The microstructure and early-stage oxidation behavior of the equiatomic CoCrCuFeMnNi high-entropy alloy (HEA) and its six sub-alloys, obtained by omitting one element each, were investigated. Alloys were prepared using induction levitation melting, cold rolled, and oxidized for 1 h at 800°C in air. The Ni-free and Co-free HEAs showed an inhomogeneous microstructure associated with liquid phase separation. The other alloys were either single-phase (Cu-free HEA) or contained two face-centered cubic phases, one Cu-rich and one Cu-poor. The Cu and Mn-containing two-phase alloys showed preferential oxidation of the Cu/Mn-rich phase, leading to Mn-rich oxides that are prone to spallation. The Mn-free alloy exhibited a thicker oxide (~ 5 µm) on the Cu-rich phase, whereas the Cu-poor phase was covered by a thin base oxide (< 1 µm). The single-phase Cu-free (‘Cantor’) alloy formed an approximately 1-µm-thick oxide of the crystal structure types of Mn3O4, Mn2O3, MnCr2O4, and Cr2O3. For prospective high-temperature applications, reducing the Cu and Mn content and thus avoiding formation of a second Cu-rich phase is a promising route to facilitate formation of a protective oxide.

Enhanced microstructural homogeneity and strength-ductility synergy of Ti-6Cr-5Mo-5V-4Al β-titanium alloy wires through boron addition

One of the long-term concerns regarding titanium alloy wires involves microstructural inhomogeneity, mechanical instability and product unreliability in service. In this study, a strategy of micro-alloying was utilized to address the issue in Ti-6Cr-5Mo-5V-4Al (Ti6554) β-titanium alloy wires. The microstructures and mechanical properties of Ti6554 hot-rolled wires were optimized through boron (B) additions. The results manifest that B-addition promotes microstructural homogenization of wires, wherein the microstructure nearly completely consists of normal grain region (NGR) without abnormal grain region (AGR) like that in the B-free alloys. This microstructural homogenization relieves mechanical fluctuation amongst different samples and renders plastic flow highly repeatable upon stretching. Meanwhile, B-addition strengthens wires whilst maintains decent ductility and thereby achieves the better combination of strength and ductility in comparison with that in the B-free wires. The analyses indicate that the addition of B facilitates the universal occurrence of dynamic recrystallization (DRX) to form NGRs at the expense of AGRs, which is beneficial to microstructural homogenization of hot-rolled wires. A good plastic accommodation resulting from NGRs and strengthening caused by TiB laths endow Ti6554 wires superior strength-ductility synergy. These findings provide helpful guidance for microstructural and performance optimization of titanium alloy wires.

Application of Static Magnetic Field to Modify Heat and Mass Transfer during Welding of Shipbuilding Steel

Welding process is very important in numerous industries ranging from automotive and aviation to shipbuilding and pressure vessel production. Nowadays new methods to improve the productivity and quality of various welding techniques are searched. In many industrial applications even small process optimizations may lead to significant cost and energy savings and new applications. Large plate welding is particularly important in shipbuilding industry. It is common to weld multiple times to join thick plates, but this approach is not optimal from energy and time effectiveness and outcome quality is limited. Alternative is single high heat input welding, which causes various problems related to rapid local overheating and the formation of inhomogeneous post-weld microstructure. There are several heat affected zones near the weld pool, which has different properties and microstructure due to different cooling rates and heat flux orientation during solidification. Since welding is a complex multiphysical process there are various parameters such as electric current, oxygen presence, heat flow and weld pool flow which influence the quality of welding joint and efficiency of the process. In this paper we aim to experimentally and theoretically investigate how to modify heat and mass transfer in the weld pool and heat affected zone by static magnetic fields. Electromagnetic force is one of the ways how to affect the weld pool flow and to influence the heat and mass transfer from the weld pool to the base metal. Our research demonstrates that moderate DC magnetic field can cause various effects on the post-weld morphology, depending on the magnetic field direction. Analytical estimates and similarity analysis for high heat input welding on EH36 shipbuilding steel shows that electromagnetic methods, like application of DC magnetic field can be promising approach for improved welding outcome in some cases.

Evaluation of the ATI 718Plus repaired by laser metal deposition under heat treatment regimes: Microstructures, mechanical properties, and their heterogeneity

Laser metal deposition (LMD) is an attractive repaired technology for integral impellers and blades. After maintenance, the LMD usually generates an inhomogeneous microstructure joint consisting of two zones: repaired zone (RZ) is mainly composed of highly textured columnar dendrite and long striped Laves phases in the inter-dendrite, while the substrate zone (SZ) displayed equiaxed grains with rod-like η phases on grain boundaries. RZ is the limiting factor due to the absence of γ′ and the presence of Laves phase. Three specific heat treatment regimes, namely direct aging (DA), solution and aging (SA), and homogenization and aging (HA) were chosen to investigate the microstructure response of heat treatments including the grain size, Laves phase, η phase, and γ′ phase of RZ and SZ. After heat treatment, the mechanical compatibility was highly improved, and the global tensile properties were analyzed based on the microstructure evolution and respective tensile properties. The tensile properties of the DA and SA joint reached the same level of wrought ATI 718plus alloys, whereas the ductility is lower due to the inhomogeneous deformation. Comparatively, the HA sample showed higher ductility and lower strength due to abnormal grain growth.

Strong yet ductile self-passivating W-10Cr alloy by laser powder bed fusion

Compared to pure tungsten, self-passivating tungsten alloys (SPTAs) for the future nuclear fusion armour are expected to offer a major safety guarantee in a loss-of-coolant accident (LOCA) event. Here we used laser powder bed fusion (LPBF) to print a self-passivating W-10Cr system. The main challenges for processing W-10Cr alloys are balling phenomena and chromium loss due to their melting point differences. After laser parameters optimizations, W-10Cr blocks with relatively dense and smooth surface were fabricated. As-built specimens exhibited an inhomogeneous microstructure with W-rich regions and Cr-rich ones. In the Cr-rich region, ultrafine equiaxed grains were obtained in view of Spinodal decomposition. Owing to the co-contribution of Spinodal decomposition and fine-grain structure, the compressive strength, fracture strain and Vickers hardness (HV0.005) of W-10Cr alloy reach 1451 MPa, 21.5% and 9.4 GPa, which are obviously superior to pure tungsten counterparts, with the improvement of 41%, 115% and 43%, respectively. Based on the indentation responses, as-measured indentation hardness followed typical indentation size effect (ISE). A higher contribution from elastically surface work in the early part of indentation is responsible for the hardening mechanism of W-10Cr alloy. Additionally, the strengthening and toughening mechanisms associated with improved properties are also discussed. The mechanistic insights into the deformation behavior have extensive implications for the development of additively manufactured SPTAs with exceptional mechanical properties.

A macro prediction model for butt-welded (U)HSS connections with softened HAZ

This paper reports an investigation on the tensile resistance of fully-penetrated butt-welded connections of (ultra-)high-strength steels considering the strength softening of the heat-affected zone (HAZ) and the strengthening due to the transverse constraint. Firstly, a parametric study is carried out to analyse the tensile behaviour of fully-penetrated butt-welded S700 connections using validated finite element (FE) models. Based on the FE results, the effect of the investigated parameters on the ultimate resistance of the connections is quantified. Consequently, a Macro prediction model is proposed, neglecting the detail of HAZ inhomogeneous microstructures but considering five design parameters, which are the width and thickness of the specimen, the width of HAZ, the softening ratio, and the matching ratio. Finally, the proposed Macro model is validated against the available experimental data of fully-penetrated butt-welded (ultra-)high-strength steel connections in literature. The results show that the Macro model is proven successful in accurately quantifying the effect of HAZ, concerning the strength softening and the transverse constraint strengthening, on the tensile resistance of fully-penetrated butt-welded (ultra-)high-strength steel connections. It is also found that 157 and 96 out of 185 experiments are predicted within a 5% deviation range using the Macro model and prEN 1993-1-8:2021, respectively, proving that the Macro model is more consistent with the experimental data compared to the prediction of prEN 1993-1-8:2021.

In-situ investigation on tensile deformation and fracture behaviors of inhomogeneous microstructure during laser repair of Ti-6Al-4 V titanium alloy

Laser additive manufacturing, with its small heat-affected zone, high forming accuracy, and adjustable filler material, has great potential for use in the repair of aircraft engine blades. Nevertheless, it is inevitable to introduce a non-uniform microstructure during laser processing, which will cause a decrease in the components’ mechanical properties. To gain further insight into the mechanics of inhomogeneous structures under tensile load, digital image correlation (DIC) technology and in-situ tensile test are applied to study the mechanical response and fracture mechanism of inhomogeneous structures. It is indicated that the position of stress concentration changes in the β columnar during the tensile process, whereas the same remains constant in equiaxed crystals. The coordination deformation ability and mode of the RZ exhibit notable differences compared to that of the HAZ. The slips between the α phases in the β columnar lead to micro-cracks at the phase boundary and the propagation of Z-shaped cracks, while crack propagation along the grain boundary in the equiaxed crystal due to coordinated deformation by the different grains.

Microstructure of TiAl Capsules Processed by Electron Beam Powder Bed Fusion Followed by Post-Hot Isostatic Pressing.

The microstructures of intermetallic γ-titanium aluminide (TiAl) alloys are subjected to a certain degree of Al evaporation when processed by electron beam powder bed fusion (EB-PBF). The magnitude of the Al-loss is mainly correlated with the process parameters, and highly energetic parameters produce significant Al evaporation. The Al-loss leads to different microstructures, including the formation of inhomogeneous banded structures, thus negatively affecting its mechanical performance. For this reason, the current work deals with creating EB-PBFed TiAl capsules with the inner part produced using only the pre-heating step and melting parameters with low energetic parameters applying high beam speed from 5000 to 3000 mm/s. This approach is investigated to reduce the Al-loss and microstructure inhomogeneity after hot isostatic pressing (HIP). The results showed that the HIP treatment effectively densified the capsules obtaining a relative density of around 100%. After HIP, the capsules produced with the inner part melted at 3000 mm/s presented a lower area shrinkage (around 6.6%) compared to the capsules produced using only the pre-heating step in the core part (around 20.7%). The different magnitudes of shrinkage derived from different levels of residual porosity consolidated during the HIP process. The HIPed capsules exhibited the presence of previous particle boundaries (PPBs), covered by α2 phases. Notably, applying low energetic parameters to melt the core partially eliminates the particles' surface, thus reducing the PPBs formation. In this case, the capsules melted with low energetic parameters (3000 mm/s) exhibited α2 concentration of 3.5% and an average size of 13 µm compared to the capsules produced with the pre-heating step in the inner part with an α2 around 5.7% and an average size around 23 µm. Moreover, the Al-loss of the capsules was drastically limited, as determined by X-ray fluorescence (XRF) analysis. More in detail, the capsules produced with the pre-heating step reported an atomic percentage of Al of 48.75, while using low energetic melting parameters led to 48.36. This result was interesting, considering that the massive samples produced with standard parameters (so more energetic ones) revealed atomic Al percentage from 48.04 to 47.70. Finally, the recycled small particles showed a higher fraction of α2 phases with respect to the coarse particles, as determined by X-ray diffraction (XRD).

Effects of deposition paramaters on the microstructure evolution of wire arc additive manufactured Al–Zn–Mg–Cu alloy

Wire arc additive manufacturing (WAAM) is a suitable method for fabricating large high-strength components made of the Al–Zn–Mg–Cu alloy. However, the solidification behavior of this alloy is complex due to its composition. The inhomogeneous solidification microstructure results in numerous defects during the deposition process, hindering the application of the deposited Al–Zn–Mg–Cu alloy. This study aims to analyze the microstructure evolution under different process parameters and reveal the relationship between the solidification behavior of the molten pool, pore defects, and grain morphology. Thin-walled components based on Al–Zn–Mg–Cu alloy were fabricated using WAAM under different parameters. The results showed that increasing deposition current initially decreased and subsequently increased pore defects, while the proportion of twinned dendrites exhibited the opposite trend. A lower deposition speed reduced pore defects by promoting gas escape, whereas a higher deposition speed led to the precipitation of Al3Zr, grain refinement, and the suppression of twin dendrite growth. A faster wire feeding speed had a better suppression effect on defects, while a slower wire feeding speed inhibited the growth of twinned dendrites, promoting the directional growth of regular dendrites.

Study on the Adsorption Deformation of a Substrate via Spin Coating Based on the 3D-DIC Method and Its Effect on the Homogeneity of Perovskite Films.

The physical and chemical stability of perovskite films has always been a key issue for their industrialization, which has been extensively studied in terms of materials, environment, and encapsulation. Spin coating is one of the most commonly used methods for the preparation of perovskite films in research. However, little attention has been paid to the deformation state of the substrate when it is fixed by means of adsorption and its impact. In this work, the three-dimensional digital image correlation (3D-DIC) method and hyperspectral technology are used to acquire and analyze the adsorption deformation characteristics of the substrate during spin coating, as well as the resulting inhomogeneity. Plastic and four different thicknesses of float glass (0.2, 0.5, 0.7, 1.1 mm) were selected as substrates, and they were tested separately on two suction cups with different structures. The results show that the plastic and 0.2 mm specimens exhibit obvious strain localization behavior. The distribution and magnitude of the strain are affected by the size of the sucker structure, especially the width of the groove. For glass specimens, this effect shows a nonlinear decrease with increasing substrate thickness. Compared to the strain value, the irregularity of local deformation has a greater impact on the non-uniform distribution of materials. Finally, inhomogeneities in the perovskite films were observed through optical lens and hyperspectral data. Obviously, the deformation of the substrate caused by adsorption should attract the attention of researchers, especially for flexible or rigid substrates with low thickness. This may affect the centrifugal diffusion path of the precursor, causing microstructure inhomogeneity and residual stress, etc.

The effect of microstructure on the fatigue crack growth behavior of Ni-Cr-Mo-V steel weld metal

Fatigue crack growth (FCG) rate, which could significantly affect fatigue lifetime of thick wall components, has been studied extensively in forgings with homogeneous microstructure. However, there were few studies on the effect of inhomogeneous microstructure on FCG rate in weld metal. In this work, the effect of microstructure on FCG behavior of Ni-Cr-Mo-V weld metal was investigated. The FCG rate in the Paris regime was similar to that reported in the literature. However, in the near-threshold regime, the FCG rate decreased rapidly at first with the decrease of stress intensity factor range (ΔK) and then increased slightly. Finally, the FCG rate slowly decreased with the decrease of ΔK. It was found that when the monotonic plastic zone size around the crack tip was equivalent to the ferrite grain size in reheated grain zone at a certain ΔK, the crack propagated along the ferrite grain boundaries. The occurrence of intergranular fracture increased the fracture surface roughness, and enhanced the roughness-induced crack closure effect, which increased the FCG resistance and reduced the FCG rate drastically at the early stage. At a lower ΔK level, frequent deflection and crack branching were observed and the mechanisms were investigated by crystallographic analysis.

Microstructure-based interior cracking mechanisms and life prediction of additively manufactured Ni-based superalloy with temperature effect

The localized microstructure related fatigue cracking of additively manufactured (AM) Ni-based superalloy has been attracting increasing attention. Understanding the connection between the inhomogeneous microstructure and the fatigue characteristics with small deformation is crucial. Herein, the microstructure-related high cycle fatigue and very high cycle fatigue properties of an Ni-based AM superalloy were examined. The fatigue tests were conducted with stress ratio of 0.1 at room (25 ℃) and elevated (650 ℃) temperatures. To support the modeling of the internal fatigue failure mechanism, additional studies on microstructure features and mechanical properties were carried out in conjunction with electron backscatter diffraction, energy dispersive spectrometry, and monotonic tensile tests. Results of 2D fracture surface observation showed that the interior facet-induced fatigue crack nucleated under relatively low applied stress at both temperatures, which were assisted by the interior abnormal structure. Furthermore, the interior crack nucleation region consisting many facets was formed under shear stress and strain based on 3D restructure of fracture surface. Combined with evaluation of threshold stress intensity factor, the transition lengths from small crack to long cracks for surface failure at 25 ℃ and interior failure at 650 ℃ were evaluated. Based on the above analysis, the interior failure mechanisms were summarized related with microstructure characteristics. Finally, fatigue index parameters based on different failure modes were calculated to predict the fatigue life. The anticipated and experimental data are determined to be in satisfactory agreement.