Difference In Thermal Expansion Coefficients Research Articles

Effect of the B4C particles on the brazed joints of SiC ceramics and 2219 aluminum alloy

AbstractA two‐step process, including the premetallization of the SiC ceramic by Ag‐26.7Cu‐4Ti+B4C and the followed brazing with the Al‐based filler alloy, was developed to join 2219 aluminum alloy and the metallized SiC. The influence of metallization temperature and the B4C addition on the microstructure and mechanical properties of the joint was analyzed. The reaction products of Ti and B4C were detected in the brazing seam, such as Ti2B, TiC and graphite. With increasing of metallization temperature and B4C addition content, the shear strength of joint first increased and then decreased. The joint shear strength reached 14.0 MPa when SiC metallized with Ag‐26.7Cu‐4Ti+1%B4C at 930°C for 10 min, which showed that appropriate addition of B4C in Ag‐26.7Cu‐4Ti metallization layer could regulate the coefficient of thermal expansion difference between 2219 aluminum alloy and SiC to relieve the residual stress and improve the shear strength of their brazed joint.

Revealing the mechanism of laser welded aluminum alloy joints in-situ reinforced by carbon nanotubes based on microstructural evolution

In-situ addition of carbon nanotubes (CNTs) was innovatively applied to laser welding Al-Cu-Mg alloys to form composite joints, and the mechanism of CNTs strengthening joints was revealed from the viewpoint of microstructure evolution. Tensile test showed that the average tensile strength of the CNTs-reinforced joint increased by 37.3% to 306 MPa. Initially, microscopic observation and elemental analysis found that part of the CNTs were combined with the precipitate, and part of the CNTs were dispersed in the matrix, which provided conditions for the transfer of the load from the matrix to the CNTs. In addition, the fusion zone (FZ) grains of the joints in-situ strengthened by CNTs are finer and isotropic compared to the joints without CNTs. Since CNTs and aluminum alloys have remarkable differences in thermal expansion coefficients to form thermal mismatches, and part of CNTs may act as grain boundaries, resulting in dislocations cannot cut through them to form Orowan strengthening. As a result, CNTs-reinforced joints achieved significant improvements thanks to these primary strengthening mechanisms. The mechanism of CNTs-strengthened aluminum alloy joints is comprehensively and deeply revealed from the microscopic level, which provides a reference for the design of Al/CNTs composite joints and performance regulation.

Adhesion of nanodiamond composite films on Ti substrates at room temperature via hybrid ion etching gun and coaxial arc plasma deposition

It has been extremely difficult for nanodiamond composite (NDC) films to be deposited on Ti due to a large thermal expansion coefficient difference. The native oxide layer on Ti is another problem preventing the appropriate adhesion of NDC films and subsequent delamination. In this work, innovative room temperature adhesion of 3 μm NDC films with 54 GPa hardness on Ti substrates was accomplished via a hybrid system of ion etching gun and coaxial arc plasma deposition (CAPD). Ar+ plasma etching is capable to terminate the superficial TiO2 layer and manipulates substrate morphology during CAPD provides instantaneous deposition of NDC films at room temperature.

Design, preparation, and performance characterization of low temperature environment-resistant composite resin matrix in rocket fuel

Cryogenic technology is a vital part of our society, not only in our lives, but also in the field of cutting-edge technology. The research and application of cryogenic technology is involved in many important projects in many countries and even in the world, such as aviation, aerospace, energy, transportation, and medicine. With its high specific strength and high specific modulus, carbon fiber reinforced resin matrix composite materials have gradually become the key material for aerospace vehicles, with significant advantages in reducing structural weight and improving structural efficiency. However, in the ultra-low temperature environment such as liquid hydrogen and liquid oxygen, the overall structure of carbon fiber reinforced resin matrix composites is severely tested by the environment, and it is extremely important to evaluate the mechanical properties of the composites under ultra-low temperature due to the difference of their material structure and properties from those of traditional materials, and the difference of thermal expansion coefficients between the reinforcing material carbon fiber and the resin matrix in rocket fuel. In this paper, the epoxy resin-based composite system was prepared by modifying TDE-85 epoxy resin with low-viscosity cyanate resin through molecular structure design, which is suitable for ultra-low temperature environment. The surface tension and dynamic contact angle of the modified epoxy resin are better than those of the pure epoxy resin, and it can form a good infiltration with carbon fiber and the interfacial properties of the composite are excellent. Secondly, the modified epoxy resin-based composite unidirectional plate was prepared by wet winding molding process, and the low-temperature mechanical property test specimens were prepared according to the relevant test standards. The mechanical properties were tested at −196℃, −150℃, −90℃ and room temperature 25℃ to obtain the low-temperature mechanical properties of the composite system, which provided the basis for the design of the composite system in the ultra-low temperature environment. Finally, the microstructure of the epoxy matrix composites was characterized by SEM method after the different temperature tests. The material structure, morphology, and composition were characterized by field emission scanning electron microscopy (FESEM, M400 FEI) with energy dispersive X-ray spectroscopy (EDS). In this paper, TDE-85 epoxy resin was modified with low viscosity cyanate resin to produce a modified epoxy resin suitable for ultra-low temperature environment, and the process properties of the epoxy resin were characterized. The surface tension of the modified epoxy resin was 43.405 mN/m, which was significantly lower than that of the pure epoxy resin at room temperature of 48.814 mN/m. Therefore, the modified epoxy resin had better flowability and required less time to wet the fibers during the molding process, resulting in higher molding efficiency.

Simulation of crack patterns in quasi-brittle materials under thermal shock using phase field and cohesive zone models

Complex crack patterns are usually formed on the surface of brittle and quasi-brittle solids under thermal shock. In this paper, the phase field coupled cohesive zone model is established to simulate the brittle and quasi-brittle fracture processes in three-dimensional solids, especially to reproduce the complex crack patterns under thermal shock. Several examples are given to illustrate the effectiveness of the model to complex fracture problems under thermal shock: from uniaxial tensile and thermal shrinkage fracture of a one-dimensional rod to nucleation, propagation, and crack pattern formation of thermal shock crack of a three-dimensional ceramic bottle. In addition, the formation mechanism of crack patterns in ceramic bottle is discussed, especially the dominant role of the difference in thermal expansion coefficients between the ceramic matrix layer and the glaze layer in the formation of crack patterns and the final crack density.

Lanthanum Strontium Cobalt Ferrite (LSCF) perovskites by Sol-Gel Method for Potential Application in Solid Oxide Fuel Cells (SOFC)

Fuel cells are one of the most efficient and effective solutions to environmental problems and high energy demand. They are devices which change chemical energy into electrochemical energy, allowing much higher efficiency than conventional thermomechanical conversion methods. Lanthanum Strontium Cobalt Ferrite (LSCF) perovskites have been widely studied for application as cathodes in solid oxide fuel cells (SOFC) due to their high electrical conductivity, high thermal and chemical stability, low difference in thermal expansion coefficient, and physico-chemical compatibility with the other cells components. The aim of this work was to synthesize perovskitas type La0.7Sr0.3Co0.5Fe0.5O3 by sol-gel method and evaluate the potential for application as a cathode for fuel cell. The results obtained by X-ray powder diffraction (XRD) indicate that the sol-gel method calcined at 900ºC obtained an amount of the perovskite phase above 95%. The Field Emission Gun-Scanning Electron Microscope (FEG-SEM) images of LSCF film produced with 4 layers showed a better quality. Thus, the results obtained by XRD and FEG-SEM, indicate that the sol-gel method calcined at 900ºC has a potential application as cathode in solid oxide fuel cells.

GaN/Al2O3 heterostructure and intrinsic lattice constant of GaN

The relaxation process of the thermal strain in a GaN film due to the thermal expansion coefficient difference in the GaN(000l)/ a-Al2O3(0001) heterostructure is studied by varying the film thickness of GaN in a wide range from 1 to 1200 µm. The lattice constant c has a large value of 5.191 A at a film thickness less than a few microns, while it decreases to about 150, m, and becomes constant above 150, m, indicating that the strain is almost completely relaxed. The intrinsic lattice constants of wurtzite GaN free from the strain, a0 and c0 are determined to be 3.1892±0.0009 and 5.1850 ± 0.0005 A, respectively.

Stress Engineering in Germanium-Silicon Heterostructure Using Surface Activated Hot Bonding

The manufacturing of heterostructures is interesting in fields such as photonics(1), solar energy production(2) and quantum technologies(3). This paper, dedicated to germanium on silicon heterostructure manufacturing and stress engineering, builds up on Ref. (4) findings.We are using the differences in terms of coefficients of thermal expansion (CTE) between germanium and silicon, to induce a tensile stress in a thin germanium layer transferred by the Smart CutTM [1] technique onto a silicon substrate. In this approach, a bulk germanium wafer is directly bonded on a bulk silicon wafer, using surface activated bonding (SAB)(5,6). Process integration advantages are the low defect density of bulk germanium and the tensile stress that can be tuned using the bonding temperature.The adherence of SAB was high during bonding at relatively high temperatures (~250°C). Without any further heat treatment, the adherence was found to be 3500mJ/m2 by the double cantilever beam method (under prescribed displacement control and in an anhydrous atmosphere). This remarkable adherence prevented the Ge/Si heterostructure from sliding, debonding and cracking (7). The CTE difference could thus generate a large amount of tensile stress during the heterostructure cooling and splitting steps.First of all, using finite elements simulation, a model was designed to predict the bow and the in-plane internal stress after bonding and cooling of the heterostructure. Thanks to it, we could also evaluate those two values after layer transfer (splitting step). This numerical approach was more accurate than the former analytic method (Timoshenko), which did not include the non-linearity of CTE with temperature and for large displacements. Figure 1 shows the bow and the in-plane stress expected for a bonding at 250°C of, respectively, a Si/Si homostructure (Figure1.a) and a strained Ge/Si heterostructure (Figure1.b), when cooled down to room temperature. The former is flat while the bow of the latter is large indeed.Then, based on indications collected from simulations, we performed the bonding of a Si/Si homostructure in an EVG®ComBond® at 250°C. The same bonding conditions were used for an implanted germanium on silicon heterostructure. As shown in Figure 2, both bond pairs exhibited a bow close to that from FE simulations.After that, a splitting step was carried out, resulting in a thin germanium film on a silicon wafer (200mm), as shown in Figure 3. The film was transferred almost to the whole surface. The bow after splitting was of 10µm, a value not so far from the simulated value of 28.6 µm.As far as tensile stress was concerned, we used X-Ray Diffraction to gain access to the lattice parameter deformation and thus the tensile stress, 0.05% and 72 MPa, respectively (Figure 4). In comparison, FE simulation gives us 107 MPa of tensile stress after splitting. The main advantage of our approach is the lack of misfit dislocations in the Ge film, at variance with Ge epitaxy on Si (with a threading dislocations density typically around 107 cm-2, then).The presentation will focus, first, on the simulation tool and the manufacturing process used to fabricate such stressed germanium films. Then, stress calculus will be detailed. Finally, TEM imaging of cross-sections, XRD and bow measurements will help us better understand the specificities of such thin Ge films transferred on Si bulk wafers. Higurashi E. Heterogeneous integration based on low-temperature bonding for advanced optoelectronic devices. Jpn J Appl Phys. 1 avr 2018;57(4S):04FA02.King RR, Law DC, Edmondson KM, Fetzer CM, Kinsey GS, Yoon H, et al. 40% efficient metamorphic GaInP∕GaInAs∕Ge multijunction solar cells. Appl Phys Lett. 30 avr 2007;90(18):183516.Scappucci G, Kloeffel C, Zwanenburg FA, Loss D, Myronov M, Zhang JJ, et al. The germanium quantum information route. Nat Rev Mater. oct 2021;6(10):926-43.Abadie K, Fournel F, Morales C, Vignoud L, Colona JP, Widiez J. Manufacturing of Optimized Ge Substrates Using a Covalent Bonding Process. ECS Meet Abstr. 23 nov 2020;MA2020-02(22):1623.Takagi H, Kikuchi K, Maeda R, Chung TR, Suga T. Surface activated bonding of silicon wafers at room temperature. Appl Phys Lett. 15 avr 1996;68(16):2222-4.Rebhan B, Vanpaemel J, Dragoi V. 200 mm Ge Wafer Production for Oxide-Free Si-Ge Direct Wafer Bonding. ECS Trans. 8 sept 2020;98(4):87-100.Chao YL, Tong QY, Gösele UM. Thermomechanical stress in silicon on quartz wafer bonding and Smart Cut® process. MRS Proc. 2001;681:I5.10. [1] Trademark from S.O.I.T.E.C. .S.A. Figure 1

Fabrication of AAO-Based Four-Stack Thin Film Solid Oxide Fuel Cells for the Low-Temperature Operation Using Sputtering Method

Anodic Aluminium Oxide (AAO) based four thin-film solid oxide fuel cell (TF-SOFC) stack was fabricated. The fuel cell stack was tested and analyzed at the low-temperature window. Since the low-temperature operation has many advantages, such as lowering the system cost and widening the material selection, there has been much effort to reduce conductive resistance, mainly at the electrolyte, which is the most problematic issue in low-temperature operation. One of the solutions is TF-SOFC deposited on the various substrates or supporting materials. AAO, a non-conductive and nanoporous substrate, has been widely used in fabricating TF-SOFC as a substrate because of its uniformly arranged pores and highly developed manufacturing process.However, AAO is a poor template for stacking TF-SOFCs because it has lower mechanical strength than metal, has no electrical conductivity, and has a different coefficient of thermal expansion from metal. In this study, therefore, planarly stacked TF-SOFC were fabricated to expand its limits of use as a template for the stacked fuel cell. Nickel-based anode, yttria-stabilized zirconia (YSZ) electrolyte with functional layers, and platinum cathode and current collector were sequentially deposited using sputtering and atomic layer deposition methods. All thin-film components of four cells were deposited on the AAO at the same time, making good connections between each electrode. By effectively setting the deposition process, unnecessary pores of AAO were blocked, and current collecting layers were stably formed in between the structure of the cells with a step difference.Then, the stacked TF-SOFC were electrochemically analyzed, including the current density-voltage-power density curve (i-V-P curve) and electrochemical impedance spectroscopy (EIS) being compared to a single cell. The microstructure of the fuel cell and its electrically connected structures were observed by scanning electron microscopy (SEM) and focused ion beam (FIB) cross-sectional SEM analyses. The cell’s overall voltage was comparable to four times that of a single cell, and the power density was obtained. We revealed the correlation between the electrochemical performance and the microstructure of individual cells and the whole stack. The full stack achieved electrochemical and mechanical stability by forming a complete fuel cell structure and enhanced the applicability of the planar cell stack on the AAO substrate.

Study on removal mechanism of oxide film in direct current straight polarity tungsten arc welding of aluminum alloy

The Direct Current Straight Polarity (DCSP) TIG welding process of 2219 aluminum alloy was realized by removing oxide film on aluminum alloy surface with the activating flux. The effect of activating flux removing 2219 aluminum alloy surface anode oxide film and welding slag distribution morphology under the action of welding arc were analyzed. The results showed that the activating flux can effectively remove the anode oxide film, and welding slag with discontinuous floccules was distributed on the basic surface of aluminum, which was easy to remove. The difference in thermal expansion coefficient between Al and Al2O3 oxide films caused Al2O3 oxide films to crack under the action of high-temperature electric arc in TIG welding, but the oxide films cannot be removed. However, the weld surface coated with fluoride activating flux, fluoride can permeate between the oxide film and the aluminum matrix from the place where the oxide film cracked, causing defects at the film/matrix interface, which was conducive to peeling off the oxide film on the weld surface.

Employment of 3D-Printed Bilayer Structures with Embedded Continuous Fibers for Thermal Management Applications: An Axial Cooling 4D-Printed Fan Application Case Study.

Bi-material composite structures with continuous fibers embedded on polymer substrates exhibit self-morphing under thermal stimulus induced by the different coefficients of thermal expansion (CTE) between the two constituent materials. In this study, a series of such structures are investigated in terms of fiber patterns and materials to achieve programmable and reversible transformations that can be exploited for thermal management applications. Stemming from this investigation’s results, an axial cooling fan prototype is designed and fabricated with composite blades that passively alter their shape, specifically their curvature and twist angle, under different operating temperatures. A series of computational fluid dynamics (CFD) simulations are performed, subjecting the fan’s geometry to different flow temperatures to measure differences in airflow deriving from the induced shape transformations. Corresponding experimental trials are additionally performed, aiming to validate the simulation results. The results indicate the potential of utilizing bilayer self-morphing configurations for the fabrication of smart components for cooling purposes.

Non-thermal and thermal effects on mechanical strain in substrate-transferred wafer-scale hBN films

Wafer-scale thin films of hexagonal boron nitride have exceptional thermal and mechanical properties, which harness the potential use of these materials in two-dimensional electronic, device applications. Along with unavoidable defects, grains, and wrinkles, which develop during the growth process, underlying substrates influence the physical and mechanical properties of these films. Understanding the interactions of these large-scale films with different substrates is, thus, important for the implementation of this 2D system in device fabrication. MOVPE-grown 2 and 30 nm hBN/sapphire films of size 2 in. diameter are delaminated chemically and transferred on quartz, SiO2/Si, and sapphire substrates. The structural characteristics of these films are investigated by employing Raman spectroscopy. Our results suggest that not only the roughness but also the height modulation at the surface of the substrates play a pivotal role in determining substrate-mediated mechanical strain inhomogeneity in these films. The statistical analysis of the spectral parameters provides us with the overall characteristics of the films. Furthermore, a Stark difference in the thermal evolution of strain in these films depending on substrate materials is observed. It has been demonstrated that not only the differential thermal expansion coefficient of the substrates and the films, but also slippage of the latter during the thermal treatment determines the net strain in the films. The role of the slippage is significantly higher in 2 nm films than in 30 nm films. We believe that the observations provide crucial information on the structural characteristics of the substrate-coupled wafer-scale hBN films for their future use in technology.

Discussion on the Causes of Cracking and Failure of Anticorrosion Layer of a Pipeline

The cause of a rare cracking failure accident of the polyethylene (PE) anti-corrosion layers of a pipeline is analyzed and determined. The properties of the PE raw materials and the anti-corrosion layers were detected and analyzed according to the standard, and the micro-morphology of the anti-corrosion layers was characterized by the scanning electron microscope (SEM). The storage modulus and loss of the anti-corrosion layers at the range of -100~40 °C were tested by the dynamic mechanical analysis (DMA), and the stress of the anti-corrosion layers at low temperature (-40 °C) was calculated. The average thermal expansion coefficient of the anti-corrosion layers in the range of -40~60 °C was tested by the thermomechanical analysis (TMA), and the value at room temperature was obtained, which was compared with the value of the steel materials. The failure accidents caused by raw materials and anti-corrosion layers due to the quality problems, as well as aging and damage of anti-corrosion layers are excluded. The SEM results confirmed that the anti-corrosion layers with no obvious large-scale defects, there were microscopic defects less than 1 μm. At the low temperature (-40 °C), the storage modulus of the anti-corrosion layers is 2.58 GPa, and the corresponding stress is 23.99 MPa, which is higher than the tensile strength test value (22.21 MPa). The average thermal expansion coefficient of the anti-corrosion layers is 14.3×10-5/°C, which is higher than that of the steel material (1.2×10-5/°C). The high stress at low temperature (-40 °C) is one of the reasons for induced defects to cause cracks, and the huge difference in thermal expansion coefficient between the anti-corrosion layers and the steel material is another reason for that.

Biaxial stress in La2Ce2O7/YSZ thermal barrier coatings for oxidation evolution

High temperature oxidation could cause the spalling and failure of thermal barrier coatings (TBCs). La2Ce2O7(LCO)/YSZ double-layer TBCs were investigated with high temperature oxidation at 1100 °C, 1150 °C and 1200 °C, respectively. Interfacial crack propagation and TGO growth behavior were comparatively investigated, and the failure of double-layer TBCs is the result of the joint action of interface transverse crack propagation and TGO growth. Due to the difference in thermal expansion coefficients, the YSZ layer is subjected to biaxial stress, which contains the TGO growth stress and the thermal mismatch stress during the oxidation process. The transverse crack propagation was developed at the LCO/YSZ interface and the YSZ/BC interface, eventually. The thermal barrier coatings TGO thickened abnormally at 1000 °C and 1150 °C, while delamination at the LCO/YSZ interface resulted in coating failure at 1200°C. The thermal mismatch stress at the LCO/YSZ interface increased mainly due to the thermal expansion coefficient between LCO and YSZ. Transverse microcracks at the LCO/YSZ interface gradually propagated, resulting in delamination failure to LCO/YSZ. Therefore, the failure mechanism of biaxial internal stress has important guiding significance for the application of double-layer TBCs.

Corrosion Behavior of Spherical Chromium Carbide Reinforced NiCrBSi Hardmetal Coatings in Sulphuric Acid Solution

In the present work, four groups of spherical chromium carbide reinforced NiCrBSi hardmetal coatings were prepared on AISI 4145 steel by plasma transferred arc (PTA) technique. The corrosion behavior of the four as-received hardmetal coatings in 0.5 mol/L H2SO4 solution was investigated by polarization curve and electrochemical impedance spectroscopy (EIS). The results revealed that more Cr-rich carbides (Cr3C2, Cr7C3 and M23(C, B)6) are formed in the chromium carbide reinforced coatings, while for the NiCrBSi hardmetal coating only Cr7C3 carbide was detected by XRD. The polarization results show that the chromium carbide reinforced NiCrBSi hardmetal coatings have positive corrosion potential and lower corrosion current, providing a better protective effect to the substrate metal. The combined effects of Cr-rich carbide ceramic phases and a more stable passive film of Cr2O3 greatly improved the corrosion resistances of the chromium carbide reinforced NiCrBSi hardmetal coatings. The coating with the highest spherical chromium carbide addition has more pores because of the thermal stress due to the difference of thermal expansion coefficient between the NiCrBSi bonding phase and chromium carbide reinforced phase. The negative effects of the pores weaken the corrosion resistance, and the coating with the 30% chromium carbide content shows the best corrosion resistance. For NiCrBSi hardmetal coatings with higher reinforced chromium carbide content, the repeatability of the corrosion current obtained by polarization fitting is not as good as that of coatings with lower chromium carbide content. The repeatability of polarization results becomes worse when the specimens keep in a more stable passive state.

Electro-thermally driven biaxial bending artificial muscle based on oriented graphite nanoplate nanocomposite/polyimide complex structure

In recent years, artificial muscle is of great research interest due to its promising application. However, low deformation, complicated fabrication process, and high cost hinder their development. Herein, electro-thermally driven biaxial bending artificial muscle based on oriented graphite nanoplate nanocomposite (GN)/polyimide (PI) complex structure is successfully fabricated by cost-effective process utilizing coefficient of thermal expansion difference between them. GN/PI bi-layer films were further assembled and packaged into multi-unit biaxial bending actuator, which could extend and contract like artificial muscle. The unique characteristics of large deformation, easy control, low cost and simple fabrication process distinguish the GN/PI bi-layer based artificial muscle from others. Driven by the voltage of 10 V, the artificial muscle could reversibly reach a shrinkage rate of 11% and a maximum lifting height of 2.5 cm with the object weight of 5.0 g. Moreover, a walking robot has also been designed to achieve a large displacement.

Characterization of residual stresses in fiber metal laminate interfaces – A combined approach applying hole-drilling method and energy-dispersive X-ray diffraction

Fiber metal laminates (FMLs) can be manufactured by direct bonding of fiber reinforced plastics (FRP) and metals using advanced manufacturing approaches, e.g., the prepreg-press-technology. Such materials gain high interest in lightweight applications as they benefit from the superior characteristics of both materials. However, residual stresses are induced in the manufacturing process mainly due to the difference in thermal expansion coefficients between FRP and metal. These residual stresses can lead to the delamination at the transition zone/ interface between FRP and metal and, thus, detrimentally affect the structural integrity and the durability of the FML structure.The objective of the present work is to introduce reliable experimental approaches to accurately measure the distribution of residual stresses across the thickness of the FML, with a special focus on the interface between metal and FRP. Symmetric FMLs with a specific stacking sequence of FRP/steel/FRP are designed and fabricated for residual stress analysis using different approaches, i.e., relaxation and diffraction methods, as well as for results comparison and validation purposes. The incremental hole drilling method (IHD) is used to determine the residual stresses across the thickness of the FMLs. By choosing FRPs with a relatively small thickness, a reliable measurement using IHD can even be achieved across the transition zone between FRP and metal. The energy-dispersive X-ray diffraction method (ED-XRD), as an advanced non-destructive approach, is employed for in-depth residual stress measurement in the metallic part of the FMLs. Each measurement method is characterized by its own advantages, intrinsic limitations as well as uncertainty sources. The obtained results give confidence that both methods are able to assess the qualitative distribution as well as the absolute magnitudes of residual stresses in the FMLs. Based on the approach detailed the reliability of the two measurement methods is cross-validated.

Dependence of surface residual stress on the coefficient of thermal expansion for materials subjected to laser peening without coating

We applied single laser pulse irradiation and laser peening without coating (LPwC) to materials that had different coefficients of thermal expansion (CTEs) to confirm the thermal effects on the surface residual stress (SRS) of the materials. First, 1000 MPa-grade high-strength steel (HT1000) and silicon nitride ceramics (Si3N4) were irradiated underwater with one pulse of a frequency-doubled Q-switched Nd:YAG laser. Si3N4 was also subjected to multiple-pulse laser irradiation of 2, 4, and 10 pulses on the same spot. The SRS within the laser-irradiated spots was measured by X-ray diffraction (XRD); the results showed that the SRS on HT1000 ranged from 600 to 700 MPa in tension, whereas the SRS on Si3N4 with a lower CTE was approximately 300 MPa in compression. Next, Fe-Ni low-expansion alloys with different CTEs were fabricated and subjected to LPwC followed by XRD; their characteristics were compared with SUS304, Alloy600, and Si3N4. The results showed that the SRS was closely correlated with the CTE of the materials. The SRS of Fe-Ni low-expansion alloys tended to be compressive, even under LPwC conditions where the SRS would not be sufficiently compressive for materials that had a high CTE, such as SUS304 and Alloy600, suggesting that CTE is an important index that is closely related to SRS status after LPwC. The SRS of Si3N4 with a low CTE was always approximately 300 MPa in compression, regardless of the pulse density, including single- and multiple-pulse laser irradiation. These results indicated that the SRS of Si3N4 became saturated in a compressive state after single pulse irradiation.

Control of multi-scale cracking for improvement of the reliability of carbon/carbon composites via design of interlaminar stress

Controlled cracking at micro/mesoscale is crucial for decreasing the strength scatter and improving the reliability of carbon/carbon composites (C/Cs). In this work, the controlled cracking is realized by a designed interlaminar residual thermal stress (RTS), which is introduced inside the C/Cs by the alternate stacking of two types of C/C plies with different coefficients of thermal expansion. Firstly, multiscale finite element (FE) modeling is performed to capture the interlaminar RTS induced by the alternate stacking and evaluate the risk of delamination due to the free edge effect. Secondly, mechanisms responsible for the controlled cracking are elaborated with the aid of the FE modeling and experimental characterizations, and the controlled deflections and bifurcations of the crack strongly shield the influence of multiscale flaws. Finally, by adopting this strategy, the flexural strength of the C/Cs increases by 66% while the statistical Weibull modulus also increases from 4.9 to 8.3, verifying the simultaneously improved strength and reliability of the C/Cs.

Microstructures and mechanical properties of YG18 cemented carbide/40Cr steel joints vacuum brazed using Ag–Cu–Ti filler metal

YG18 cemented carbide and 40Cr steel were brazed using Ag–Cu–Ti filler metal under vacuum, and the microstructural evolution of the YG18/Ag–Cu–Ti filler metal/40Cr steel brazed-joint interface was investigated to determine how brazing temperature affected the brazed-joint microstructure, bonding strength, and fracture behavior. At the YG18 cemented carbide/filler metal interface, Ti atoms in the filler metal and C atoms in the YG18 cemented carbide formed a continuous TiC layer, reducing the difference in thermal expansion coefficients between the YG18 cemented carbide and filler metal. At the 40Cr steel/filler metal interface, Ti atoms in the filler metal and C atoms in the 40Cr steel formed a continuous TiC layer, preventing the diffusion of Fe atoms to the cemented carbide side and formation of a hard, brittle phase. YG18 cemented carbide/TiC/Cu2Ti/Ag–Cu eutectic + TiCu granules/TiC/40Cr steel-brazed joints were obtained. With increasing brazing temperature from 860 to 920 °C, the bonding strength of the YG18 cemented carbide/40Cr steel joint initially increased and then decreased. The joint brazed at 900 °C for 10 min failed in the middle of the brazing seam, and its average bonding strength was 235 MPa.