Multiple Thermal Cycles Research Articles

Underwater additive manufacturing of Ti-6Al-4V alloy by laser metal deposition: Formability, gran growth and microstructure evolution

No underwater laser additive manufacturing technology has been reported. In this paper, the underwater thin-walled Ti-6Al-4V part was built firstly by underwater laser metal deposition (ULMD), and the formability, grain growth and microstructure transformation were investigated. The stable local dry cavity and the shielding-gas flow layer surrounding the deposited layer formed during the ULMD process utilizing an improved ULMD nozzle, generating the uniform thin-walled part without cracks and pores. Columnar β-grains, growing epitaxially opposite to the heat flow direction, were observed in the whole ULMD part, and the grain size increased firstly and then reduced with increasing the deposition layer, attributed to the comparative growth and decreasing cooling rate. The water cooling effect suppressed the formation of equiaxed β grains that only formed in the top region of ULMD part. For the microstructures of ULMD sample, only acicular martensites α' with different size scales formed in the bottom region, and martensites α' were replaced gradually by α phase with lamellar, lathy and blocky shapes with increasing the building height, except the top region because the absence of multiple thermal cycles. The larger cooling rate promoted the formation of α' martensite and decreased the width of α lath of ULMD part.

Enhanced Thermoelectric Transport and Stability in Atomic Layer Deposited-HfO2/ZnO and TiO2/ZnO-Sandwiched Multilayer Thin Films.

Herein, enhancements in thermoelectric (TE) performance, both the power factor (PF) and thermal stability, are exhibited by sandwiching HfO2 and TiO2 layers onto atomic layer deposited-ZnO thin films. High-temperature TE measurements from 300 to 450 K revealed an almost two-fold improvement in electrical conductivity for TiO2/ZnO (TZO) samples, primarily owing to an increase in carrier concentration by Ti doping. On the other hand, HfO2/ZnO (HZO) achieved the highest PF values owing to maintaining Seebeck coefficients comparable to pure ZnO. HZO also exhibited excellent stability after multiple thermal cycles, which has not been previously observed for pure or doped ZnO thin films. Such improvement in both TE properties and thermal stability of HZO can be attributed to a shift in crystalline orientation from the a axis to c axis, as well as the high bond dissociation energy of Hf-O, stabilizing the ZnO structure. These unique properties exhibited by HZO and TZO thin films synthesized by atomic layer deposition pave the way for next-generation transparent TE devices.

Development of thermally stable coarse water-in-oil emulsions as potential DNA bioreactors

Conventional polymerase chain reaction (PCR) performed in a continuous aqueous phase often shows an inability to detect ultra-low DNA concentration. Droplet digital PCR, where DNA polymerization happens in isolated water droplets of water-in-oil (W/O) emulsions, could significantly improve the detection limit. However, water droplet breakups under multiple thermal cycles necessary for DNA polymerization is preventing the wide-spread application of such technology. The present research aims to develop thermally stable coarse W/O emulsions that could act as DNA microreactors. Span 80, polyglycerol polyricinoleate (PGPR), or sodium bis(2-ethylhexyl) sulfosuccinate (AOT) were dissolved in mixtures of light and heavy mineral oils at different concentrations. The aqueous phase was selected based on PCR protocol, without the presence of enzymes and DNA. Emulsions prepared without bovine serum albumin (BSA) in the aqueous phase was used as a control to understand its effect on stability. Emulsions were formed using a vortex mixer for 2 minutes at 3200 rpm. Only the emulsions without visible aqueous phase separation were exposed to PCR thermal cycling. The volume mean diameters of water droplets were calculated by image analysis. Span 80-stabilized emulsions destabilized upon thermal cycling, while PGPR-stabilized emulsions remained stable with an increase in water droplet size. AOT was able to form thermally stable emulsions, with an average droplet size around 119 µm without any significant change. The influence of emulsifiers concentration, surface saturation, molecular interaction with BSA at the interface, and continuous oil phase viscosity was considered to explain the mechanism of emulsion formation and thermal stability.

Characterization of high-purity germanium detectors with amorphous germanium contacts in cryogenic liquids

For the first time, planar high-purity germanium detectors with thin amorphous germanium contacts were successfully operated directly in liquid nitrogen and liquid argon in a cryostat at the Max-Planck-Institut für Physics in Munich. The detectors were fabricated at the Lawrence Berkeley National Laboratory and the University of South Dakota, using crystals grown at the University of South Dakota. They survived long-distance transportation and multiple thermal cycles in both cryogenic liquids and showed reasonable leakage currents and spectroscopic performance. Also discussed are the pros and cons of using thin amorphous semiconductor materials as an alternative contact technology in large-scale germanium experiments searching for physics beyond the Standard Model.

Fabrication and characterization of electrospun fatty acid form‐stable phase change materials in the presence of copper nanoparticles

Latent heat storage system using phase change materials (PCMs) has been recognized as one of the most useful technologies for energy conservation. In this study, a novel type of fatty acid eutectic of methyl palmitate (MP) and lauric acid (LA)/polyacrylonitrile (PAN) composite phase change fiber is prepared by single electrospinning method. Additionally, copper nanoparticles (CNPs) with different mass ratio are combined for improving the thermal conductivity of the PCM. The structure and morphology of the fabricated composite PCMs are observed by scanning electron microscopy (SEM), and the thermal properties and performance are also characterized. SEM results show that the liquid fatty acid has been fully stabled by the three-dimensional structure of the fibers. Good compatibility among the components of the composites is also demonstrated. Besides, the addition of nanoparticles leads to an improved thermal conductivity by over 115.2% and a phase transition temperature 21.24 °C as well as a high latent heat of 85.07 J/g. Moreover, excellent thermal reliability of the phase change fiber is confirmed by multiple thermal cycles. Hence, the composite PCM prepared in this study shows a promising potential for thermal energy system such as building insulating and thermal mass regulating textiles.

A two-surface thermomechanical plasticity model considering thermal cyclic behavior

In thermal-related engineering such as thermal energy structures and nuclear waste disposal, it is essential to well understand volume change and excess pore water pressure buildup of soils under thermal cycles. However, most existing thermo-mechanical models can merely simulate one heating–cooling cycle and fail in capturing accumulation phenomenon due to multiple thermal cycles. In this study, a two-surface elasto-plastic model considering thermal cyclic behavior is proposed. This model is based on the bounding surface plasticity and progressive plasticity by introducing two yield surfaces and two loading yield limits. A dependency law is proposed by linking two loading yield limits with a thermal accumulation parameter nc, allowing the thermal cyclic behavior to be taken into account. Parameter nc controls the evolution rate of the inner loading yield limit approaching the loading yield limit following a thermal loading path. By extending the thermo-hydro-mechanical equations into the elastic–plastic state, the excess pore water pressure buildup of soil due to thermal cycles is also accounted. Then, thermal cycle tests on four fine-grained soils (natural Boom clay, Geneva clay, Bonny silt, and reconstituted Pontida clay) under different OCRs and stresses are simulated and compared. The results show that the proposed model can well describe both strain accumulation phenomenon and excess pore water pressure buildup of fine-grained soils under the effect of thermal cycles.

Effect of the additional heat input on the microstructure and property in NiCrMoV welded joint simulated by multiple thermal cycles

Abstract The effect of additional heat input on the NiCrMoV weldments via thermal cycles was comparatively investigated. After thermal effect with one to three cycles and post-weld heat treatment (PWHT), no obvious changes in the microhardness and microstructure were detected in the weldments except a hardness-increased zone formed within the original weld metal (WM), which was designated as the present heat-affected zone (pre-HAZ). Compared with the tempered martensite in the as-welded WM, the tempered lath martensite with dispersed second phase particles was revealed in the pre-HAZ rooted in the original WM. Though the pre-HAZ showed larger hardness owing to microstructure change by rapid cooling during thermal simulation, yet the microhardness in this zone still did not exceed previous hardness range for each specimen. It could be concluded that the microstructure in WM remained stable and the reliable property was anticipated for this weldment even experienced further reheating e.g. repair welding process.

Phase change materials based on comb-like polynorbornenes and octadecylamine-functionalized graphene oxide nanosheets for thermal energy storage

Organic phase change materials (PCMs) have been proved as one of the most promising materials for alleviating the energy crisis and environmental pollution. However, the “leakage” of PCMs during the phase change process has hampered their advanced applications in thermal energy storage. Here, we deliver a novel shape-stabilization strategy to fabricate a series of new form-stable PCMs (FSPCMs) by employing comb-like polynorbornenes along with the octadecylamine-functionalized graphene oxide (C18-rGO) nanosheets as the supporting networks and paraffin as the latent heat material. The graphene oxide (GO) nanosheets crosslinked by long-chain carbon alkyl group prevented the graphene derivate materials from forming the serious issue of agglomeration, and endowed excellent structural stability to the composite FSPCMs. Meanwhile, the toughness of the composites was further improved with the addition of the comb-like polynorbornenes. Paraffin was tightly imprisoned in the frameworks formed by the C18-rGO nanosheets and comb-like polynorbornenes, resulting in no leakage even above their phase change temperature. The multiple thermal cycling tests proved that these FSPCMs had excellent thermal cycling reliability. In particular, they were also capable of absorbing or releasing thermal energy at a high rate, offering a significant potential to be used as the effective thermal management materials in electronic devices or solar energy storage systems. Notably, comb-like polynorbornenes demonstrated to be capable of being employed as the supporting networks of paraffin, opening a new door for fabricating novel types of FSPCMs as well as broadening the practical application of the comb-like polymers.

Sealing Performance and Mechanism of a High-Temperature Near-Neutral Sealing Adhesive in the Ternary System of ZrO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>

A high-temperature sealing adhesive was prepared using kyanite, andalusite, and magnesia fully stabilized zirconia (Mg-FSZ) as raw materials and nanozirconia (ZrO2) sol as binder. The adhesive is chemically near-neutral and suitable for bonding acid, neutral, or alkaline materials at 1560 °C. The mineral composition, volume expansion, and air permeability of multiple thermal cycles were analyzed. Results showed that the volume expansion caused by the mullitization of kyanite and andalusite can compensate for the sintering shrinkage high temperature. MgO could gradually desolvatize from the Mg-FSZ particles and react with SiO2 and Al2O3 to produce a stable forsterite and magnesium aluminate spinel. Hence, Mg-FSZ became unstable, and cubic ZrO2 transformed into monoclinic ZrO2 when cooled, leading to a volume expansion of the adhesive, which ensured the sealing effect. A larger critical particle size of Mg-FSZ can provide the adhesive with a more persistent volume expansion during thermal cycles due to the more durable desolvatization of MgO. The nanoZrO2 sol binder can improve the sintering of the adhesive and bonding of the joint, resulting in a low gas permeability.

Thermo-mechanical-metallurgical modeling and validation for ferritic steel weldments

This work explained the effects of solid-state phase transformation (SSPT) on welding residual stress (WRS) evolution in SA508 Gr.3 C1.1(SA508) ferritic steel single-pass and multi-pass weldments. A thermo-mechanical-metallurgical (TMM) model incorporating thermo-elastic-plastic stains and SSPT-related strains was presented. The SSPT kinetics and transformation-induced plasticity (TRIP) models of SA508 steel were established by employing user-defined subroutines developed for ABAQUS. The simulated WRS distribution in the single-pass weldment was compared with previously published neutron diffraction measurement data, and a good agreement was achieved and thus validating the developed TMM model. The microstructural and WRS distribution characteristics of the multi-pass weldment were further investigated based on the validated TMM model. Numerical simulations illustrated how the transformation and heat transfer/cumulative behavior associated with different welding process conditions resulted in the generation of triaxial compressive and tensile stresses in the upper and underlying weld beads at the multi-pass welded joint, respectively. Moreover, an additional simulation model with SSPT but without TRIP was developed to reveal the potential effects of TRIP on WRS distribution. This study numerically provided valuable understanding of microstructure and WRS evolution in air cooling bainitic steel weldments under multiple thermal cycles and constituted a fundamental basis for developing new welding materials to minimize the formation of deleterious tensile WRS by simultaneously combining bainitic and martensitic transformations.

Palm oil-based bio-PCM for energy efficient building applications: Multipurpose thermal investigation and life cycle assessment

This study aims at investigating the potential use of a bio-based phase change material, i.e. expired palm oil from the food industry, as a more sustainable alternative to petrochemical-based organic PCMs. To this purpose, thermogravimetric analysis (TGA) and isoconversional methods (Starink and Miura-Maki methods) are applied and the main thermo-physical properties of the blend are investigated by means of differential scanning calorimetry (DSC) and extensive thermal monitoring in a controlled realistic environment. Finally, a life cycle assessment is used to evaluate the environmental impact of the bio-based material in comparison to the more common petrochemical-based application. Kinetic analysis results indicate the two dimensional phase boundary reaction model as the most reliable scheme for describing the oxidation of palm oil, with an activation energy of about 73 kJ · mol−1. The DSC and the thermal monitoring procedure, showed two separate melting peaks in the ambient temperature range, which globally guarantee a melting enthalpy of about 50 kJ · kg−1, i.e. of the same order of magnitude of the first developed PCMs. Results from the life cycle analysis reveal that the expired palm oil can be considered a promising material for bio-based latent applications.Globally, the palm oil has proved itself as a promising, low cost, and environmentally friendly alternative for passive thermal storage solutions (e.g. building envelope applications) where stability across multiple thermal cycles, low health risks, and low leakage are crucial parameters to be addressed.

Multiple welding simulated microstructure and corrosion resistance of super austenitic stainless steel 254SMo welding heat affected zone

This paper studies the effects of thermal cycle and heat input on microstructure, grain size, precipitated phase and corrosion properties in the simulated heat-affected zone of super austenitic stainless steel 254SMo. The results show that the microstructure grains in the heat affected zone gradually grow up, the pitting corrosion resistance decreases continuously and intergranular corrosion resistance firstly decreases and then increases with the thermal cycle and heat input increase. The precipitated phase appears in the heat-affected zone at the multiple thermal cycles and high heat input.

Multiple welding simulated microstructure and corrosion resistance of super austenitic stainless steel 254SMo welding heat affected zone

This paper studies the effects of thermal cycle and heat input on microstructure, grain size, precipitated phase and corrosion properties in the simulated heat-affected zone of super austenitic stainless steel 254SMo. The results show that the microstructure grains in the heat affected zone gradually grow up, the pitting corrosion resistance decreases continuously and intergranular corrosion resistance firstly decreases and then increases with the thermal cycle and heat input increase. The precipitated phase appears in the heat-affected zone at the multiple thermal cycles and high heat input.

Grain Size-Effect on Intergranular Cracking in Shape Memory Zirconia During Cyclic Martensitic Transformations

Bulk polycrystalline zirconia-based shape memory ceramics (SMCs) generally crack as they undergo martensitic transformations, largely due to the transformation mismatch stresses. Polycrystalline SMCs subjected to cyclic transformations bodily lose individual grains and progressively comminute, but it is not yet clear how the grain size of a polycrystal affects such cyclic degradation. We explore this issue in 1.5 mol% yttria-doped zirconia SMCs by varying the grain size from 0.6 to 7.9 µm in pellets of fixed composition and sample size and subjecting them to multiple thermal cycles through the transformation. A smaller grain size is found to increase the number of cycles required to disaggregate the pellet because of the larger amount of grain boundary area that must crack. Calorimetry analysis shows that the energy relieved through cracking decreases with increasing grain size and reveals an apparent material length scale of ~2 micrometers for the stress relief zone. For grain diameters below this critical length scale, complete stress relief is suggested, while at larger grain sizes, intergranular cracking apparently does not fully relieve the transformation mismatch stresses. Alternate accommodation mechanisms are required including the formation of multiple variants and even some transgranular fracture.

Microstructure characteristics and failure mechanisms of Ti-48Al-2Nb-2Cr titanium aluminide intermetallic alloy fabricated by directed energy deposition technique

The high-energy input and thermal history during additive manufacturing lead to complex phase transformations in titanium aluminide alloy. This study mostly focuses on determining the solid-state phase transformation mechanisms during laser deposition and the failure mechanisms of alloys using molecular dynamics simulations. Because of the directional temperature gradient, columnar grains with fully lamellar microstructures are formed first after solidification. A narrow region just below the melting pool is reheated to high temperatures, thus enhancing the precipitation of new equiaxed grains. Multiple thermal cycles in the α + γ phase region promote the formation of massive γ phases (γm) at the grain boundaries. Finally, a nearly lamellar microstructure of alternating columnar and equiaxed grains with γm phases is formed. The deposited titanium aluminide alloy has good room and high-temperature (760 °C) tensile properties of 545 ± 9 and 471 ± 37 MPa, with elongations of 1.50 % ± 0.47 % and 1.50 % ± 0.45 %, respectively. The room and high-temperature samples both fail in the columnar grain region. The molecular dynamics simulations suggest that the interface between α2 and γm is the weakest, especially in the case of semicoherent interfaces (6° angle in the [1–10] direction), which provides good nucleation sites for cracks. Although the equiaxed grain regions contain several γm–α2 interfaces, the samples still fail in the columnar grain regions due to the increase in the cracking distance in the equiaxed regions caused by randomly oriented α2 + γ lamellae and the comparably good plasticity of the γm phases.

Additive manufacturing of ultrafine-grained high-strength titanium alloys

Additive manufacturing, often known as three-dimensional (3D) printing, is a process in which a part is built layer-by-layer and is a promising approach for creating components close to their final (net) shape. This process is challenging the dominance of conventional manufacturing processes for products with high complexity and low material waste1. Titanium alloys made by additive manufacturing have been used in applications in various industries. However, the intrinsic high cooling rates and high thermal gradient of the fusion-based metal additive manufacturing process often leads to a very fine microstructure and a tendency towards almost exclusively columnar grains, particularly in titanium-based alloys1. (Columnar grains in additively manufactured titanium components can result in anisotropic mechanical properties and are therefore undesirable2.) Attempts to optimize the processing parameters of additive manufacturing have shown that it is difficult to alter the conditions to promote equiaxed growth of titanium grains3. In contrast with other common engineering alloys such as aluminium, there is no commercial grain refiner for titanium that is able to effectively refine the microstructure. To address this challenge, here we report on the development of titanium-copper alloys that have a high constitutional supercooling capacity as a result of partitioning of the alloying element during solidification, which can override the negative effect of a high thermal gradient in the laser-melted region during additive manufacturing. Without any special process control or additional treatment, our as-printed titanium-copper alloy specimens have a fully equiaxed fine-grained microstructure. They also display promising mechanical properties, such as high yield strength and uniform elongation, compared to conventional alloys under similar processing conditions, owing to the formation of an ultrafine eutectoid microstructure that appears as a result of exploiting the high cooling rates and multiple thermal cycles of the manufacturing process. We anticipate that this approach will be applicable to other eutectoid-forming alloy systems, and that it will have applications in the aerospace and biomedical industries.

Effect of thermal cycling and hydro‐thermal cycling on physical and mechanical properties of sandstone

AbstractThermal storage of sandstone is widely distributed in sedimentary basins in China with a very high degree of exploitation and utilization, while the low recharge rate is the bottleneck of its sustainable development. A new method is proposed to discover the physical and mechanical behaviors of sandstone after thermal cycling and hydro‐thermal cycling treatments in this study. The stress‐strain curves, strength, failure behavior, acoustic emission (AE), P‐wave velocity, density features, X‐ray, and microscope images of specimens are analyzed in detail to investigate and describe the differences of the sandstone after the two different experimental treatments. The results indicate that multiple thermal cycling and hydro‐thermal cycling have an obvious effect on the compaction stage of sandstone at the temperature of 100°C. Meanwhile, strengths and Young's modulus of specimens after hydro‐thermal cycling treatment decrease with cycling times and the reduce degree is greater than that of the thermal cycling treatment on sandstone. The brittleness of the sandstone decreases as the hydro‐thermal cycling number increases from 0 to 15, and the influence of hydro‐thermal cycling is greater than that of thermal cycling on sandstone. In addition, based on the limited cycling experimental results, the action of the thermal cycling leaded the thermal stress of the crystal to nonuniform, which leaded to micronarrow and sharp cracks in specimens. Furthermore, the action of the hydro‐thermal cycling generated thermal stress and ablation coupling effect, which leaded wider and rounder cracks than that of the specimen under thermal cycling condition. Finally, it indicates that hydro‐thermal cycling has a greater effect than that of the thermal cycling treatment on the physical and mechanical properties of sandstone on the macro‐ and microlevels.

Diagnostic qPCR Assay to Detect Fusarium brasiliense, a Causal Agent of Soybean Sudden Death Syndrome and Root Rot of Dry Bean.

Species within clade 2 of the Fusarium solani species complex (FSSC) are significant pathogens of dry bean (Phaseolus vulgaris) and soybean (Glycine max), causing root rot and/or sudden death syndrome (SDS). These species are morphologically difficult to distinguish and often require molecular tools for proper diagnosis to a species level. Here, a TaqMan probe-based quantitative PCR (qPCR) assay was developed to distinguish Fusarium brasiliense from other closely related species within clade 2 of the FSSC. The assay displays high specificity against close relatives and high sensitivity, with a detection limit of 100 fg. This assay was able to detect F. brasiliense from purified mycelia, infected dry bean roots, and soil samples throughout Michigan. When multiplexed with an existing qPCR assay specific to Fusarium virguliforme, accurate quantification of both F. brasiliense and F. virguliforme was obtained, which can facilitate accurate diagnoses and identify coinfections with a single reaction. The assay is compatible with multiple qPCR thermal cycling platforms and will be helpful in providing accurate detection of F. brasiliense. Management of root rot and SDS pathogens in clade 2 of the FSSC is challenging and must be done proactively, because no midseason management strategies currently exist. However, accurate detection can facilitate management decisions for subsequent growing seasons to successfully manage these pathogens.

Cobalt–free La0.5Sr0.5Fe0.9Mo0.1O3–δ electrode for symmetrical SOFC running on H2 and CO fuels

Cobalt–free perovskite La0.5Sr0.5Fe0.9Mo0.1O3–δ (LSFMo) was evaluated as the electrode of symmetrical SOFC (S–SOFC) using H2 and CO as fuels. XRD results demonstrate that the LSFMo electrode is redox stable at a temperature below 800 °C. Furthermore, the chemical compatibility of LSFMo electrode with both Sm0.2Ce0.8O1.95 (SDC) and La0.9Sr0.1Ga0.8Mg0.2O3–δ (LSGM) electrolytes was assessed. The thermal expansion coefficients (TECs) of the LSFMo at 30–850 °C are 13.4 × 10−6 K−1 in air and 15.1 × 10−6 K−1 in 5% H2/Ar. However, for the pre–reduced LSFMo, the TEC in 5% H2/Ar decreases to 11.8 × 10−6 K−1. Using LSFMo as symmetrical electrode of S–SOFCs, the maximum power densities (Pmax) of 562 mW cm−2 in dry H2 and 474 mW cm−2 in dry CO at 750 °C are attained based on 300 μm thick SDC electrolyte, while based on the LSGM electrolyte with same thickness, the Pmax attain 508 mW cm−2 in dry H2 and 379 mW cm−2 in dry CO. For the LSGM supported S–SOFC, the total polarization resistances (Rp) attain 0.460 Ω cm2 in dry H2 and 1.099 Ω cm2 in dry CO at 750 °C. Additionally, the S–SOFCs with LSFMo electrode exhibit good power cycling stability, long–term stability and multiple redox and thermal cycling stability. Above results indicate that the LSFMo material is a very potential symmetrical electrode for S–SOFCs.

The effect of thermal cycling process between high and low temperatures on the microstructure and properties of Mg-10Li-3Al-3Zn-0.25Si alloy

The microstructure evolution and mechanical properties of the Mg-10Li-3Al-3Zn-0.25Si alloy undergoing multiple thermal cycles between high and low temperatures were explored in this study. The results showed that the thermal cycling process between high and low temperatures can lead to β-Li grain refinement and solid solution transformation of MgLiZn in the Mg-10Li-3Al-3Zn-0.25Si alloy. Furthermore, the strength of the Mg-10Li-3Al-3Zn-0.25Si alloy was reduced by 6–8%, while the elongation of the alloy was enhanced by approximately 65% after the thermal cycling process.