Difference In Thermal Expansion Coefficients Research Articles

Investigation of the influence of process parameters on crack formation and mechanisms in Ti-48Al-2Cr-2Nb alloy via laser directed energy deposition

The Ti-48Al-2Cr-2Nb alloy was fabricated using laser directed energy deposition(LDED), and the impact of various process parameters on the macroscopic crack morphology was analyzed. The mechanism of crack formation was investigated through the analysis of crack microstructure, phase composition, crystal orientation, and elemental composition. The process parameters were optimized by response surface methodology(RSM) and the laser remelting method was used to suppress the crack formation. The results showed that the cracks were mainly caused by lack of fusion, residual stress during LDED and stress between different phases of TiAl alloy. The mismatch of process parameters results in insufficient energy for powder melting, ultimately leading to lack of fusion occurrence. To minimize crack formation, the response surface method was employed to optimize process parameters and reduce crack generation. The higher temperature gradient led to the existence of residual stress in the sample, and the higher stress between α 2 phase and B2 phase formed in the deposition process due to the difference of thermal expansion coefficients. The region where the two phases converge was the region with the highest crack sensitivity, and cracks occured in the region where α 2 phase and B2 phase converge in the form of excellent transgranular fracture. The samples prepared by using the optimized parameters can effectively restrain the cracks caused by lack of fusion, but can not restrain the cracks caused by the stress between phases. Laser remelting after LDED can not only reduce the temperature gradient and residual stress, but also remelt the unmelted powder on the surface of the as-deposited samples, effectively inhibiting the generation of cracks, and preparing crack-free samples.

Releasing the residual stress of Al2O3/304 stainless steel joint by introducing BN into Ag‐Cu‐Ti filler metal

AbstractOwing to the native character of the coefficient of thermal expansion (CTE) mismatch between Al2O3 and 304 stainless steel, obtaining high strength for Al2O3/304 stainless steel joints remains challenging. In this study, the residual stress of an Al2O3/304 stainless steel brazed joint was relieved using the Ag‐Cu‐Ti + boron nitride (BN) composite filler. The results indicated that the elements between the filler and base metals on both sides were mutually diffused. Moreover, reaction layers formed at the interface, connecting the Al2O3 ceramic and 304 stainless steel. The microstructure of the joint was as follows: Al2O3/Cu3Ti3O + TiCu/Ag (s, s) + Cu (s, s) + TiN + TiB2 + TiCu/Cu0.8Fe0.2Ti/TiFe2/Fe‐Cr/304 stainless steel. When the .1 wt% BN was added, the average strength of the joint reached 151 ± 10 MPa, 15% higher than the average strength of the joint only using Ag‐Cu‐Ti filler (131 ± 9 MPa). The improvement in the shear strength and reduction in the residual stress was attributed to the formation of TiN and TiB2, reducing the CTE difference and refining the microstructure.

Thermomechanic behavior of epitaxial GeTe ferroelectric films

A key development toward new electronic devices integrating memory and processing capabilities could be based on the electric control of the spin texture of charge carriers in semiconductors. In that respect, GeTe has been recently recognized as a promising ferroelectric Rashba semiconductor, with giant spin splitting of the band structure, due to the inversion symmetry breaking arising from ferroelectric polarization. Here, we address the temperature dependence of the ferroelectric structure of GeTe thin films grown on Si(111). We demonstrate the hysteretic behavior of the ferroelectric domain density upon heating/cooling cycles by low energy electron microscopy. This behavior is associated with an abnormal evolution of the GeTe lattice parameter as shown by x-ray diffraction. We explain these thermomechanical phenomena by a large difference of thermal expansion coefficients between the film and the substrate and to the pinning of the GeTe/Si interface. The accumulated elastic energy by the GeTe thin film during sample cooling is released by the formation of a-nanodomains with in-plane ferroelectric polarization components.

Experiment and numerical simulation study of thermal fatigue of an aluminum alloy piston

AbstractIn this study, a magneto‐thermo‐structural coupling finite element method was proposed to calculate the electromagnetic field, temperature field, stress field, and strain field of an aluminum alloy piston during thermal shock process. The computed temperature results at the measurement points of T3 and T4 agree well with the measured data. The concept of tensile strain energy density was proposed as a thermal fatigue damage parameter, and the thermal fatigue life of the piston was predicted on the basis of the calculated results by magneto‐thermo‐structural coupling analysis and tensile strain energy density‐life equation; the discrepancy between the predicted life and the experimental one is 16%. The thermal fatigue failure mechanism of the piston was also clarified. High cyclic stresses occur on the edge of Si due to the difference in thermal expansion coefficient of Al and Si; micro cracks first originate on the edge of Si, then grow through combining other micro cracks, and eventually form macro cracks.

Design of surface with integrated properties of sterilization and desorption

As might be one of the hottest topics in public health research, the war against bacteria has been going on some thousands of years. Unfortunately, we too are threatened by bacterial adhesion and contamination as the ancients even though the health and well-being of populations has been dramatically improved owing to the advances in human medicine and environmental sanitation. In this study, we designed a double-layer coating that consisted of polymethyl methacrylate (PMMA) and carbon nanotube (CNT)-loaded polydimethylsiloxane (PDMS) and introduced antibacterial molecule chains (AMCs), which can repeatedly release and replenish reactive oxygen species (ROS), on the coating surface. Benefiting from the characteristics of releasing ROS after the AMCs were irradiated by NIR and wrinkle structure originated from the difference in thermal expansion coefficients between PMMA and PDMS, as revealed by Uv-visible absorption spectrum and morphological characterization, the performance of sterilization and desorption would be integrated into the coating surface by irradiating the coating surface with near-infrared (NIR) repeatedly. The results of antimicrobial experiments confirmed that all of the bacteria on the coating surface were terminated and deported after the coating was irradiated six times by NIR. Additionally, after bacteria were killed and desorbed, the coating would return to its original form and AMC could be re-endowed with the ability to release ROS through treatment, showing potential of being applied in the modification of biomaterials.

Expansivity of Fused Quartz Glass Measured with 6 x 10-10/K

Thermal expansion sometimes dominates uncertainty in a precision measurement. A cell-based refractometer has been designed at NIST which targets 10−6 relative uncertainty in the measurement of helium refractivity; in terms of absolute refractive index at ambient conditions, the accuracy goal is 3 × 10−11. To achieve this level of accuracy, the length of a 0.5 m gas cell would need to be known within 100 nm. This is achievable when cell length is measured by coordinate-measuring machine at 20 °C. However, the refractometer will operate at the thermodynamically known fixed-points of water and gallium, near 0 °C and 30 °C, respectively. The cell is made from fused quartz glass, which has a nominal thermal expansion coefficient of 0.4 (μm/m)/K. Therefore, to scale the accuracy of the dimensional metrology across 20 °C to the triple-point of water requires that the thermal expansion coefficient of fused quartz glass is known within 10 (nm/m)/K, or 2.5 %. A method is described to measure the thermal expansion coefficient of fused quartz glass. The measurement principle is to monitor the change in resonance frequency of a Fabry–Perot cavity as its temperature changes; the Fabry–Perot cavity is made from fused quartz glass. The standard uncertainty in the measurement was less than 0.6 (nm/m)/K, or 0.15 %. The limit on performance is arguably uncertainty in the reflection phase-shift temperature dependence, because neither thermooptic nor thermal expansion coefficients of thinfilm coatings are reliably known. However, several other uncertainty contributors are at the same level of magnitude, and so any improvement in performance would entail significant effort. Furthermore, measurements of three different samples revealed that material inhomogeneity leads to differences in the effective thermal expansion coefficient of fused quartz; inhomogeneity in thermal expansion among samples is 24 times larger than the measurement uncertainty in a single sample.

Thermal strain relaxation of GaAs overgrown on nanovoid based Ge/Si substrate

The integration of high quality III-V materials-based devices on the Si platform is considered as an enabling path towards achieving the long-standing goal of creating low-cost/high-performance devices. However, the heteroepitaxy of III-V compounds on a Si substrate generates high stress due to the difference in thermal expansion coefficient leading notably to crack generation. Here, we report on the reduction of the thermal strain of GaAs and Ge deposited on a Si substrate. The method is based on the formation of buried high density nanovoids by electrochemical etching of Ge/Si virtual substrate followed by direct epitaxial growth of Ge and GaAs. The thermal strain has been investigated for GaAs epilayer by High-Resolution X-Ray Diffraction and Raman spectroscopy showing a 30% decrease in strain. The strain reduction relies on the nanovoids-mediated thermal strain accommodation. Furthermore, photoluminescence measurements showed a 10-fold increase of the intensity, resulting in an improved optical and crystalline quality, making this virtual substrate promising for III-V based devices on Silicon.

Design and analysis of an optical planar waveguide using Invar as core and silicon nitride as cladding layer to control birefringence

Abstract The main problem with optical planar waveguide is thermal stress which arises when layers inside the planar waveguide get fused together with different coefficient of thermal expansion (CTE) and due to variation in thermal properties, the problem of thermally induced strain and birefringence occurs inside the waveguide glass layer. In proposed work, the mathematical analysis of thermal induced stress and the thermal strain in waveguide glass layer is calculated. ‘C’ software is used for analysis of an optical planar waveguide to calculate the thermal stress. By using material properties of Invar (64 % iron-36 % nickel alloy) and silicon nitride (Si3N4) an optical planar waveguide is designed. Here when silicon nitride (Si3N4) is used as a cladding layer and Invar (64 % iron-36 % nickel alloy) as a core layer in an optical planar waveguide, the problem of birefringence get eradicated as zero thermal stress was achieved.

In-Situ Observation and Analysis of the Evolution of Copper Aluminum Composite Interface

To study the micromorphology and dynamic evolution law of copper aluminum composite interface evolution, ultra-high temperature laser Confocal microscopy (CLSM) was used to observe and analyze the evolution of copper aluminum interface in situ, and then SEM, EDS and other advanced material analysis methods were used to observe the micromorphology of the composite layer, and study the composition of the interface layer and the formation process of the copper aluminum composite interface. The results indicate that the formation of the copper aluminum composite interface layer is mainly related to the mutual diffusion of copper aluminum atoms and the interface reaction between copper and aluminum. The bonding of the copper aluminum composite interface is mainly related to the melting of the metal surface of the interface layer and the mutual diffusion of copper aluminum atoms, which is the main mechanism of the copper aluminum composite interface bonding. The intermetallic compound is mainly Al2Cu. In situ, observation of copper aluminum composite interface shows that there is a clear and relatively flat boundary between copper and the interface layer, while the boundary between aluminum and the interface layer is not straight, which is caused by the difference in thermal expansion coefficient, Lattice constant and hardness between intermetallic compounds and matrix and between intermetallic compounds. At the same time, it was found that there is a certain relationship between the visual changes of the copper aluminum composite interface image and reaction-diffusion migration during in-situ observation using a confocal laser scanning high-temperature microscope. Moreover, under no pressure, the oxide layer and interface inclusions can seriously affect the interface bonding.

Finite element analysis and experimental verification of residual stress in brazed diamond with Ni-Cr filler alloy

Residual stress is the main factor affecting the processing performance and service life of brazed diamond tools. Due to the high melting point of Ni-Cr filler alloy, the difference in thermal expansion coefficient between diamond and brazing layer is large during high brazing temperature, and excessive residual stress tends to be generated at the brazed joint. The existing process parameters are difficult to meet the demand for the lower residual stress under high brazing temperature. Suppressing effectively excessive residual stress has become the focus of current researches for process optimization of brazed diamond. In this paper, the conventional spherical finite element model of diamond is replaced by a six-octahedral finite element model of diamond, and a 3D finite element model of brazed diamond is established. The effects of brazing temperature, brazing layer thickness and holding time on the residual stresses in brazed diamond are analyzed. The results show that higher brazing temperatures result in larger differences in thermal expansion coefficients between the diamond and brazing layer, resulting in larger residual stress. The higher brazing layer thickness will enlarge the stress affected area inside diamond and thus generate higher stress. The residual stress decreases with rising holding time, however, the subsequent thermal damage to the diamond increases. Experimental measured tendency of residual stress for brazed diamond through Raman spectroscopy is well consistent with that of calculated results. After observing of the surface morphology for brazed diamond through super-depth of field microscope, the brazed diamond presents the low residual stress, weak graphitization and excellent processing performance at the condition of brazing temperature of 1010 °C, brazing layer thickness of 0.3 mm and holding time of 5 min.

Local and high‐strength joining of transparent glass and glass‐ceramics by femtosecond Bessel beam

AbstractThe creation of robust joint among different materials with notably different coefficients of thermal expansion (CTE) is an urgent task for the development of the engineering, laser technology, and aerospace industry. In this regard, femtosecond laser welding is a promising technique for efficient adhesion‐free bonding of transparent materials. This paper reports on the laser welding of laser phosphate glass (CTE = 110 × 10−7 K−1) and spinel‐based glass–ceramic (CTE = 50 × 10−7 K−1) by the Bessel beam. Dependencies of weld geometric parameters on the pulse energy and scanning speed were investigated, and the laser exposure conditions were optimized for durable welding. The weld joints were tested on shear strength, which was shown to be as record high as up to 147 MPa. Atomic force microscopy, Raman spectroscopy, and elemental analysis were used to investigate the structure and morphology of the weld after its fracture, which revealed the laser‐induced amorphization of glass–ceramics and the formation of strong interconnection by laser‐driven mutual diffusion of two glassy phases. The results demonstrate the prospects for using femtosecond lasers for efficient industrial welding of a wide range of materials.

A review on new cobalt-free cathode materials for reversible solid oxide fuel cells

The exponential growth in the requirement of fuel cells and batteries leads to increased demand for cobalt due to its common use in high-performance Li-ion batteries and high-temperature fuel cells/electrolyzers. This sharp increment in demand raises concern about the availability of limited reserves of cobalt which can impact the price of cobalt. Moreover, the geographic limitations of cobalt resources may endanger the whole supply chain. In addition to all those, huge moral issues of cobalt mining are also another problem. Hence, leading battery, fuel cells and electrolyzer manufacturers are looking for sustainable alternatives to reduce cobalt dependency. A more specific limitation is shown in Solid Oxide Fuel Cells (SOFCs) cathode materials that contain cobalt. Incompatibilities have already been observed between the cathode materials containing cobalt and the electrolytes in terms of the thermal expansion coefficient mismatch during the transition of the operating temperature from high to low. An advantage of low operating temperatures is the reduction of material costs compared to high temperature. Increasing the electrochemical performance of the cell and eliminating thermal expansion coefficient difference problems are in concert aimed at the development of cobalt-free cathode materials. Therefore, cobalt-free cathode materials are vital for the sustainability of SOFCs and green transition of the energy sector since they can be used as cathode and anode material in symmetrical SOFCs which is also known as reversible SOFC (RSOFC). In this review, we comprehensively summarize the recent advances of cobalt-free perovskite cathode materials for intermediate temperature RSOFCs.

Toward soft, stretchable and resilient high filled composite gels for potential application as thermal interface materials

Due to the different coefficients of thermal expansion between the heat generation and heat dissipation units, the applications of soft high filled composite gels as thermal interface materials require both high stretchability and excellent resilience. However, high stretchable composite gels reported far usually contain sacrificial species to dissipate energy, thus compromising their resilience. Herein, a soft (0.13 MPa), stretchable (172%), and resilient (>70%) composite gel with ultrahigh filler loading (90 wt%) is developed. The stretchability and resilience are realized by lengthening free polymer strands to form appropriate entanglements, in which the loose entanglements transfer the applied force into a large area and the stiff crosslinks prevent the entanglements from disentangling. In additional, the reversible elasticity of the filler network also dictates the resilience, so delaying its deconstruction allows for excellent resilience. The ultrahigh filler loading also endows the composite gel with high thermal conductivity (4.04 W·m−1·k−1), and the potential of the as-prepared composite gel for a TIM is discovered.

Thermal expansion and mechanical properties of urethane-modified epoxy bonded CFRP/steel joints at low and high temperatures for automotive

Long-term reliability is critical in automobiles owing to their frequent exposure to the external environment and lengthy operational durations. Using dissimilar materials as adhesives to reduce the weight of vehicles results in a mismatch of coefficients of thermal expansion (CTE). Difference in CTE causes constant internal stress at the adhesive interface when the temperature changes, leading to delamination of the adhesive at the interfaces, microcrack initiations, and crack propagations. Therefore, a blend of linear urethane prepolymer and epoxy was tested to alleviate stress concentration. Dynamic mechanical analysis (DMA) was performed to determine the modulus according to the urethane content, while tensile strength was measured to evaluate changes in the physical properties. It was observed that increasing the urethane content decreased the modulus but progressively enhanced the tensile strength elongation rate. The lap shear strength specimen was thermally shocked to validate the decrease relative to the initial value based on the cycles. Consequently, combining urethane and epoxy successfully enhanced the elongation and reduced the internal stress caused by temperature changes.

The formation mechanism of metal-ceramic interlayer interface during laser powder bed fusion

ABSTRACT Experiments on Laser powder bed fusion (LPBF) of powdered Ti on Al2O3 substrate were conducted and the interface formation was studied using a multi-material fluid dynamics model. Results show that the melt pool is relatively shallow, with relatively flat interlayer interface under LPBF’s conduction mode. In this condition, a thin sheath of molten Al2O3 forms and acts as a lubricating film for the molten Ti, leading to Rayleigh instability due to high flow inertia. Keyhole formation penetrates the Al2O3 substrate, resulting in a wavy interlayer interface. The recoil pressure from the keyhole and overall melt inertia are suppressed by the highly viscous molten Al2O3, thereby improving single-track melt pool stability. However, the thermal expansion coefficient difference between Ti and Al2O3 led to the formation of transverse cracks. Achieving a defect-free metal-on-ceramic single track remains a challenge, despite this study serving as a guide for melt track and interface control.

Nine Point Bending Test Technique for Understanding of Sintered Silver Die Bonding Failure Mechanism

Abstract The Sintered silver (s–Ag) die degradation is commonly evaluated by thermal shocked test (TST), which evaluates the material’s durability against a heating/cooling cycle. Materials with different coefficient of thermal expansion (CTE) give rise to thermal out-of-plane deformation surrounding the bonding part, which deteriorates s–Ag die part by repeated thermal and mechanical stress during TST. For the safe and reliable design of s–Ag die toward long-term durability, the contribution of thermal and mechanical stresses to degradation should be understood separately. Clarify the overall s–Ag die degradation mechanism during TST compared to the new mechanical bending test that can apply out-of-plane deformation. The authors propose a new mechanical bending test technique, called the nine-point bending (NBT) test, which can provide out-of-plane deformation with a s–Ag die-attached specimen as TST like. By comparing NBT and TST, the degradation mechanism of the s–Ag die-attach element can be understood from both thermal and mechanical aspects. In scanning acoustic tomography (SAT) analysis, a similar degradation ratio between NBT and TST is obtained, which indicates that mechanical stress plays a significant role in deteriorating s–Ag die layer in TST. After 1000 cycles, however, cracking and s–Ag material aging coexist in TST only, destabilizing s–Ag die fracture. s–Ag main degradation cause in TST is clarified with mechanical stress by comparing NBT. In addition, thermally material aging destabilized the s–Ag degradation during TST.

Bio‐Inspired Homologous Magnesium‐Based Reflective Filters with Angular Heterochromatic Structural Colors

AbstractThe existing color filter implementation schemes are prone to introduce prestress during the preparation process due to the large difference in thermal expansion coefficients of different layers of materials, resulting in poor mechanical stability. Inspired by beetle forewings’ structure, it is proposed to construct asymmetric Fabry–Perot (F‐P) angular heterochromatic structural colors utilizing homologous materials through a continuous deposition method of magnetron sputtering technology. The fabricated large‐area F‐P‐type reflective filters not only exhibit high color saturation but also achieve good bonding ability between the layers with a steady‐state friction coefficient of ≈0.1. The large‐area reflective filter can achieve different colors such as blue, yellow, violet, and green, which is in agreement with the simulation results. This work not only provides a new design method for advanced decorative applications but also provides protection to the substrate.

Relationship between microstructure, and residual strain and stress in stainless steels in-situ alloyed by double-wire arc additive manufacturing (D-WAAM) process

In the field of welding, assembly of thick parts requires the base metal to be chamfered, followed by multipass welding. The process generates thermal, mechanical, and metallurgical phenomena resulting in the formation of residual strains and stresses. Strains must be limited, as they can cause misalignments during welding and leads to geometrical defects. Residual stresses are especially problematic in structural assemblies, as they add to the external stresses applied to the part during service, often reducing its in-service lifetime and increasing the risk of failure.The objective of this study is to use two stainless steel filler metals with different coefficients of thermal expansion (austenitic 304 L and ferritic 430) to reduce residual stresses and strains in the context of deposits produced by the Double-Wire Arc Additive Manufacturing process on 304 L baseplates. To achieve such a goal, the two filler are mixed in various proportions. After deposition and cooling, residual stresses are determined using neutron diffraction and contour method, strains are studied with profilometry. Microstructure is analyzed via EBSD, free dilatometry tests and XRD.We were able to reduce strains and stresses with a three-phase microstructure (austenite, ferrite, and martensite). The part that offers the best compromise (50% 430 - 50% 304 L filler on 304 L baseplate) exhibits a 67% reduction in strain, and stresses are reduced down to 40%, as compared to the 100% 304 L filler on 304 L baseplate part. The results obtained from neutron diffraction and contour method exhibit a close agreement, with a discrepancy standing below 100 MPa.

Structural, thermal, and mechanical characterisation of PEEK-based composites in cryogenic temperature

Thermal, thermo-mechanical and mechanical properties of four different commercially available polyetheretherketones (PEEK) based materials were investigated. PEEK matrix was either modified and/or reinforced with carbon fibres, graphite and/or PTFE. Impact strength was measured at three different temperatures: 25 °C, −100 °C, and −195 °C. At 25 °C, thermal stability and mechanical properties, including the elastic modulus, compression, and impact strength, were enhanced with the addition of carbon fibres. Matrix modification had a minor impact on thermal stability, while the mechanical properties decreased, except for impact strength. At −100 °C, the mechanical properties of the neat polymers were improved, including increased impact strength by 20% compared to values at 25 °C. Addition of fillers hindered the rise of impact strength due to complex mechanisms caused by different coefficients of thermal expansion of reinforcements and matrix. At −195 °C, the significant increase of impact strength was revealed for unmodified PEEK reaching 30 times higher values than at 25 °C, while matrix modification suppressed the rise of impact strength. The scratch test indicated the superior behaviour of unfilled PEEK during the tested load range (up to 15 N), while the effect of the fillers was observed at much lower load threshold of 7 N.

FEM Analysis of Buckled Dielectric Thin-Film Packaging Based on 3D Direct Numerical Simulation.

This paper presents a 3D direct numerical simulation of buckled thin-film packaging based on transferred elastic thin-film wrinkling bonded on a compliant polymer ring. The mode change of the fabricated thin-film cap is found by measuring the thin-film cap shape at different times after Si substrate debonding. The conventional linear and nonlinear buckling simulations are not adequate to understand the behavior of the thin-film buckling mechanism creating such packaging cap mode change. Direct buckling simulation is recently reported as an easy and useful numerical wrinkling simulation method. A novel 3D FEM model of a thin-film package suitable for direct 3D buckling simulation is built to reduce the mode mixture between different buckling modes. Buckling modes of the packaging cap are investigated in terms of elastic moduli of package materials and applied strain due to thermal expansion coefficient difference. Based on the simulation results, it is found that there are two main modes in the fabricated thin-film buckling package determining the shape of the transferred thin-film packaging cover depending on the elasticity ratio between the cap and sealing ring materials. The mode shift from wrinkling cap mode to out-of-plane cap mode due to applied strain along a polymeric sealing ring is found.