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Catastrophic brittle failure of ductile materials by liquid metal embrittlement (LME) is a widely documented phenomenon but the fundamentals of its initiation mechanism are poorly understood. The widespread use of Zn-coated advanced high strength steels in the automotive industry has been plagued by Zn-induced LME which is frequently observed in high-temperature forming and welding applications. In this study, numerical modeling and an atomic-scale experimental investigation are used in order to gain insight into the atomistic events that lead to the onset of LME cracking. The results showed that the formation of a stress-induced diffusion wedge (SIDW) at the exposed grain boundary (GB) due to the interdiffusion of Zn-embrittler atoms was the trigger for LME. The formation of the SIDW facilitated the diffusion of the Zn-embrittler atoms into the GBs, which compromised their mechanical integrity. The results show that LME initiation entails several steps: (i) solid-state GB diffusion, (ii) formation of the SIDW, (iii) eventual melting of the SIDW, and (iv) opening of the liquid wedge due to interdiffusion and the application of externally applied stresses. [ABSTRACT FROM AUTHOR]

Dissimilar pulsed laser welding of 400-μm NiTi and Cu wires was used to investigate the effect of laser positioning on NiTi-Cu joint performance. Laser positioning was found to have a significant role in the microstructure due to the difference in mixing patterns and composition distributions. Homogenous element distribution was observed in both the NiTi offset and the centerline joints, while complex mixing patterns were observed in the Cu offset joints. Changing the laser position resulted in a change in alloy content in the weld pool that affected the mixing patterns, and the phases that were formed in the weld zone of each sample. By moving the laser beam from NiTi toward Cu, NiTi and NiTiCu in the weld zone were replaced by pure Cu which resulted in a decreased average hardness from ~ 512 to ~ 158 HV. However, high hardness values were found inside the weld zone of Cu offset welds due to the presence of hard and brittle ternary intermetallic compounds. The 100-μm Cu offset joint displayed the worst mechanical response due to its inhomogeneous microstructure and the presence of cracks and pores. It was shown that positioning the laser beam on the NiTi or on the centerline resulted in a better transition in microstructure that led to acceptable mechanical properties without optimizing laser parameters or inserting any interlayers. [ABSTRACT FROM AUTHOR]

Abstract The microstructure and tensile properties of a vacuum-assist high-pressure die casting (HPDC) Mg-3.0Nd-0.3Zn-0.6 Zr alloy and the effect of runner design on the externally solidified crystals (ESCs) were investigated. The microstructure of the alloy mainly comprised ESCs (large primary α-Mg grains) formed in the chamber, small primary α-Mg grains crystalized in the cavity, divorced eutectic Mg12Nd and a small amount of Mg12(NdxZn1-x) or Mg12(NdxZnyZr1-x-y) phases in the region near the shrinkage. The average ESCs grain size decreased as the distance from the center increased remarkably. The employment of ESCs collector contributed to the depression of ESCs grain size, particularly in center region. On the contrary, the grain size of small primary α-Mg has overall slightly downward trend from center to skin region, and the size difference was small. The employment of an ESCs collector effectively decreased the number and size of the ESCs, and accordingly both strength and elongation of the alloy were improved. This specially design ESCs collector improve the runner system and optimize the mold design. The elongation of the alloy approached 10%. [ABSTRACT FROM AUTHOR]

The key to improve the strength of Al/steel dissimilar joint is to toughen the hard and brittle interfacial intermetallics. Introducing soft and tough dispersions into the interfacial intermetallics would be a promising method. In the present study, Zn-Al alloy (Zn-22Al) was used as the filler metal to generate dispersions in the interfacial intermetallics matrix of dissimilar laser Al/steel joint. The results have shown that soft and tough dispersions, i.e., FeZn10, Al-rich amorphous phase, and Zn solid solution, were successfully introduced into the hard and brittle Fe2Al5−xZnx matrix. The formation of these dispersions was resulted from diffusion of elements Fe and Al and infiltration of liquid Zn along the grain boundaries of Fe2Al5−xZnx matrix and the subsequent chemical reaction and solidification. With this kind of interfacial microstructure, the joint exhibited a stable peak fracture load of 1200 N even though the interfacial layer thickness increased from 20 to 45 μm. This was because the interfacial layer was toughened by the soft and tough dispersions. Using this example, introduction of soft and tough dispersions into hard and brittle intermetallics matrix to toughen interfacial layer and then strengthening of dissimilar joints will be highlighted. [ABSTRACT FROM AUTHOR]

The inexpensive cost and high manufacture efficient metal wire and arc additive manufacturing (WAAM) system was designed in this work. Its application potential was evaluated from the three aspects of formability, microstructures, and mechanical properties. By using compulsory cooling solution implemented in the open-source WAAM system, the complex-shaped metal parts were deposited completely with no obvious defects, such as cracks, pores, or incomplete fusion. The properties of the WAAM part were evaluated by optical microscopy (OM), scanning electron microscopy (SEM), microhardness, and microtensile test. The results indicate that the formability of metal parts fabricated by the open-source WAAM system was improved by using compulsory cooling solution. The microstructures of the WAAM part are exhibited as granular structure which consisted of the granular ferrite and the residual austenite interspersed with a little pearlite in the intermediate zone. And the average ferrite grain size of non-overlapping layer is relatively smaller than that of overlapping layer. The specimen perpendicular to the building direction exhibits a better mechanical property. [ABSTRACT FROM AUTHOR]

Highlights: [•] The 98.5% relative density W–5wt% Cr alloy was fabricated by SPS method. [•] The Vickers hardness of the annealed samples can reach 760Hv. [•] Obtained nano-scaled microstructure may be beneficial for irradiation resistance of W-based alloy. [Copyright &y& Elsevier]

The bulk specific heat of fully dense nanocrystalline Ni-P electrodeposits with essentially constant P content (about 4 at%) and varying average grain sizes from 6.9 to 28.9 nm was investigated using modulated differential scanning calorimetry. In the lower temperature range from room temperature to 120 °C, at which the as-deposited sample microstructure was thermally stable, the bulk specific heat varied only within ~2 % despite the substantial variation of interface volume fractions from 0.11 to 0.39 for this series of samples. Moreover, the measured bulk specific heat values of the Ni-P samples were all located within the reported specific heat value range for conventional polycrystalline Ni. Evidently, the contribution due to grain size-related interface excess free volume is negligible and the bulk specific heat for the materials can be characterized as a structure-insensitive property. In the elevated temperature range from 150 °C to the Curie temperature of 357 °C, the magnetic contribution to the specific heat was significantly influenced by the chemical environment of P in the Ni-P samples. When P atoms were in the form of supersaturated solution in the nickel matrix, a complete suppression of the characteristic λ peak of the magnetic contribution in the specific heat curves was observed for all materials. The λ peak re-appeared in the specific heat curve after the Ni-P sample underwent a transformation to a two-phase microstructure consisting of Ni and NiP grains. It can be concluded that at a given P content, paramagnetic phosphorus atoms in the form of solutes are more effective in reducing the magnetic contributions to the specific heat than the form of paramagnetic NiP second-phase particles for the nanocrystalline Ni-P alloy. [ABSTRACT FROM AUTHOR]

This work reports the oxidation and crack healing behavior of a fine-grained (~2 μm) Cr2 AlC MAX phase ceramic. The oxidation behavior was investigated in the temperature range 900°C-1200°C for times up to 100 h. The material showed a good oxidation resistance, owing to the formation of a dense and thin α- Al2 O3 layer. The microstructure, composition and thickness of the oxide scale were characterized. Its oxidative crack healing behavior as a function of temperature, healing time, and initial crack size was studied systematically. The material showed excellent healing behavior. The main crack healing mechanism is the filling of the crack by oxides well adhering to the crack faces. The crack geometry before and after healing was characterized by X-ray tomography. Three-point bend tests showed the dependence of strength recovery at 1100°C as a function of initial crack length and healing time. [ABSTRACT FROM AUTHOR]

Using spark plasma sintering, Ti3 AlC2/ W composites were prepared at 1300°C. They contained 'core-shell' microstructures in which a Ti xW1− x 'shell' surrounded a W 'core', in a Ti3 AlC2 matrix. The composite hardness increased with W addition, and the hardening effect is likely achieved by the Ti x W1− x interfacial layer providing strong bonding between Ti3 AlC2 and W, and by the presence of hard W. Microstructural development during high-temperature oxidation of Ti3 AlC2/ W composites involves α- Al2 O3 and rutile ( TiO2) formation ≥1000°C and Al2 TiO5 formation at ~1400°C while tungsten oxides appear to have volatilized above 800°C. Likely due to exaggerated, secondary grain growth of TiO2-doped alumina and the effect of W addition, fine (<1 μm) Al2 O3 grains formed dense, anisomorphic laths on Ti3 AlC2/5 wt% W surfaces ≥1200°C and coarsened to large (>5 μm), dense, TiO2-doped Al2 O3 clusters on Ti3 AlC2/10 wt% W surfaces ≥1400°C. W potentially affects the oxidation behavior of Ti3 AlC2/ W composites beneficially by causing formation of Ti x W1− x thus altering the defect structure of Ti3 AlC2, resulting in Al having a higher activity and by changing the scale morphology by forming dense Al2 O3 laths in a thinner oxide coating, and detrimentally through release of volatile tungsten oxides generating cavities in the oxide scale. For Ti3 AlC2/5 wt% W oxidation, the former beneficial effects appear to dominate over the latter detrimental effect. [ABSTRACT FROM AUTHOR]

Abstract: The W–15wt.% Cu composite powders were synthesized by a sol–gel method combining with hydrogen reduction. X-ray diffraction patterns show that pure W and Cu phases were obtained at a reduction temperature of 750°C for 2h. The particle size of such powder is in the range from 100 to 400nm and in each particle the W and Cu phases are seemingly mixed homogeneously in the nanometer scale. The powders were microwave sintered for 25min at different temperatures of 1050, 1150 and 1200°C, respectively. The results show that W–15wt.% Cu samples microwave sintered at 1200°C exhibit a relative density higher than 97% and an average particle size of W as small as 600nm. The thermal conductivity and the Vickers microhardness of the sintered W–15wt.% Cu samples are in the range from 140 to 187W/mK and from 235 to 347HV, respectively. [Copyright &y& Elsevier]

Abstract: The W–Ni alloys with varying amount of Ni (0.1, 0.25, 0.5 and 1.0wt.%) were microwave sintered at 1450°C for different holding time 5, 15 and 30min, and their microstructure, grain size, relative density, thermal conductivity, and Vickers microhardness were characterized. Comparing to the addition of Fe and Cr, the Ni addition can greatly improve the relative density and maintain the high thermal conductivity of W at the same time. It was shown that the addition of 1.0wt.% Ni into W and microwave sintering at 1450°C for 5min would be the best conditions to obtain W–Ni alloys with a relative density close to 100% and an average grain size as small as 15μm. The Vickers microhardness and thermal conductivity of the sintered W–Ni samples range from 370 to 440 and from 90 to 130W/mK, respectively. [Copyright &y& Elsevier]

Abstract: The high-cycle fatigue behavior of Ti46Al8Nb cast alloy with fully lamellar microstructure has been studied at room temperature, 400°C, 600°C and 750°C. The result shows that temperature has great influence on the fatigue strength of Ti46Al8Nb alloy. Compared with that at room temperature, fatigue strength is significantly reduced at 400°C. However, at 600°C the fatigue strength is slightly improved. A further increasing of temperature up to 750°C gives rise to larger scatter of fatigue life and produces a more flat S-N (stress amplitude versus number of cycles to failure) curve. Fractographic observations indicate that fatigue cracks initiate at the interior casting defects and the lamellar colonies near the surface. Mixed modes of delamination, translamellar fracture and stepwise fracture are observed on the fracture surfaces at all temperatures. [Copyright &y& Elsevier]

Abstract: A novel thermoreversible hydrogel based on poly(2-ethyl-2-oxazoline)-derived amphiphilic triblock copolymer, poly(2-ethyl-2-oxazoline)–poly(d,l-lactide)–poly(2-ethyl-2-oxazoline) (PEOz–PLA–PEOz), was developed. The synthesis of PEOz–PLA–PEOz was carried out by coupling monohydroxylated PEOz–PLA diblocks with adipoyl chloride as coupling agent and dimethylamino pyridine as catalyst. The tube inverting and rheological tests showed that triblock copolymers had sol–gel–sol transition behavior with increasing temperature, and the gelation was found to be thermoreversible. The critical gelation concentration, the sol–gel transition temperature at a given concentration depended on the EOz/LA ratio and the molecular weight of PEOz. Scanning electron microscopy observation revealed that the resultant bulky gel exhibited an interconnected porous three-dimensional (3D) microstructure after freeze-drying. In addition, the hydrogels showed good cytocompatibility in vitro. MTT assays revealed that the human skin fibroblast cells encapsulated within the hydrogels were viable and proliferated inside the 3D scaffold. This newly described thermoreversible hydrogel demonstrated attractive properties to serve as cell matrix for a variety of tissue engineering applications or pharmaceutical delivery vehicles. [Copyright &y& Elsevier]

Abstract: The influences of parameters of tungsten inert gas arc welding on the morphology, microstructure, tensile property and fracture of welded joints of Ni-base superalloy have been studied. Results show that the increase of welding current and the decrease of welding speed bring about the large amount of heat input in the welding pool and the enlargement of width and deepness of the welding pool. The increase of impulse frequency has the same effect on the microstructure compared with the increase of welding current. The effect of welding parameters on the tensile strength and fracture was analyzed. It is found that the root of welding joint is unwelded when the welding current is lower, so that the strength and elongation of welded joint are inferior. And the more welding defects in the welding zone and the more hard and brittle phase precipitates in the overheated zone when the welding current is too high. Consequently, the strength and plasticity go up first and then go down, i.e. they have a peak value with welding current increasing. In addition, the decrease of impulse frequency is beneficial to the strength of the welded joint. [Copyright &y& Elsevier]

Abstract: The growth of the intermetallic layers at the interface of Ti–Ni diffusion couples was investigated under the co-effect of heat and direct current. Isothermal diffusion treatments for Ti–Ni couples were conducted at 500, 600 and 700°C for 5, 10 and 15h with and without the passage of DC current of 10A intensity. It was found that both Ti2Ni and TiNi3 layer form at the Ti–Ni interface in all couples treated by different process, but TiNi layer forms in the couples annealed above 600°C without current or at 500°C with current. The growth of the whole interfacial layer shows a parabolic relationship with time. The apparent activation energy of growth for the whole interfacial layer is 83.76kJ/mol in the couple treated by heating without a current, and it decreases to 42.11kJ/mol in the couple treated with a direct current of 10A during heating. The effect of the current on the growth of different intermetallic layers varies with its direction. [ABSTRACT FROM AUTHOR]

series of Ni-P alloys (Ni-rich) have been synthesized and solidified. It is found that the solidification microstructure of the Ni-19.0 at.%P alloy that has long been regarded as the eutectic composition contains many primary phase dendrites besides rod eutectics. In contrast, only rod eutectic microstructure appears in the samples of Ni-19.6 at.%P, and this alloy shows the lowest end temperature of melting among all the compositions investigated. The results indicate that the actual eutectic composition at the Ni-rich part of the Ni-P system is Ni-19.6 at.%P. The eutectic temperature is determined to be about 1163 K by the differential scanning calorimetry analysis. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this study was to evaluate the effect of laser welding on fatigue properties of DP600 and DP980 steels in relation to the microstructural change and softening in the heat-affected zone. The degree of softening was found to be stronger in the DP980 welded joints than in the the DP600 welded joints. While the severity of the soft zone in the welded DP980 joints led to a significant decrease in the fatigue limit, the mild soft zone present in the DP600 welded joints had little or only a minor effect on the fatigue resistance. Despite the strong effect of the soft zone, the DP980 welded joints showed a similar or even longer fatigue life than that of both the DP600 welded joints and DP600 base metal at higher stress amplitudes. Fatigue crack initiation was observed to occur from the specimen surface, and crack propagation was characterized by the characteristic fatigue striations coupled with secondary cracks. [Copyright &y& Elsevier]

The effect of Nb content on the microstructure and mechanical properties of Zr-based bulk metallic glass (BMG) were investigated. The addition of Nb led to the formation of the Zr-based metallic glass composites with a ductile dentritic phase by in situ precipitation. The presence of the in situ precipitated phase enhanced significantly the plasticity of the composite under uniaxial compressive test. The interactions between the precipitated phase and the shear band affect the deformation mechanism and fracture mode of the BMG by enhancing the affecting level of the normal stress on the shear surface, and the constant α in the Mohr–Coulomb criterion can reflect the extent of the interactions among particles and the amorphous matrix. [ABSTRACT FROM AUTHOR]

Fabrication of thick YBa2Cu3Oy (YBCO) films with high critical current densities (Jc) has remained one of the major challenges in the coated conductor development process. In this work, we report successful fabrication of several multi-layer YBCO films with thicknesses ranging from 0.1 micrometers to 1.2 micrometers. The films were deposited on a single crystal substrate by using a multi-layer TFA-MOD technique. The effect of variation in the thickness of an individual layer in a multi-layer film on the overall properties of the film was studied by processing multi-layer films with individual layers of 0.1 micrometers, 0.2 micrometers and 0.5 micrometers. The Jc, value for the 0.1 micrometers thick single-layer film was as high as 7 MA/cm2, and the Jc for 6-layer 1.2 micrometers thick film was 1 MA/cm2 at 77 K and self field. XRD, SEM and TEM were used to study the dependence of Jc on the orientation, texture and microstructure of the film. It was found that the texture, connections between the two successive layers, porosity, and the crack formation affect the Jc of the film. In addition, we report a multi layer fabrication route with a shorter duration of the pyrolysis reaction according to the number of coats. [ABSTRACT FROM AUTHOR]

Many efforts have been made to improve the current carrying capability of Y1Ba2Cu3O7 (YBCO) films by introducing pinning centers using a variety of techniques. In this study, we have succeeded in producing Y2O3, BaZrO3 and Sm doped YBCO films for the development of coated conductors by a metal-organic deposition method using trifluoroacetates (TFA-MOD). Jc exceeding 5 MA/cm2 at 77 K and self field was obtained in Y2O3 doped YBCO film. Also it was found that YBCO films with Sm substituting for Y have shown an improved critical current density characteristic over a wide range of magnetic fields. A Jc of 0.25 MA/cm2 at 3 T and 77 K was obtained which is more than 10% of the zero-field Jc. Similar results were obtained in BaZrO3 doped YBCO films. Additionally, TEM analysis was performed to study the microstructures and the pinning mechanisms. The results indicate that significant enhancement of self-field and in-field Jc of Y2O3, BaZrO3 and Sm doped YBCO films may be due to introducing nano-scaled Y203particles, nano-scaled 90° rotated Y1/3Sm2/3Ba2Cu3O7 domains and BaZrO3 nanodots by different chemical doping, respectively. This indicates that chemical doping is a promising approach for enhancement of current carrying capability of YBCO coated conductors. [ABSTRACT FROM AUTHOR]

This work studied the effects of oxygen contamination in the argon shielding gases on weld microstructures and properties during laser welding of commercially pure titanium thin sheets. The experimental results, mainly analyzed by optical and scanning electron microscopy and mechanical testing, have indicated correlations between weld surface colour, weld microstructure and mechanical properties (strength, ductility, hardness). As the oxygen content increased, the weld surface colour changed from silver, straw to blue while the surface hardness continued to increase. On the other hand, with the increasing of oxygen content, the weld strength increased first and then decreased because the microstructure changed from mainly serrated alpha in welds made with pure argon shielding gas to mainly acicular and platelet alpha. Practical guidelines are also discussed, based on the study, to deal with shielding deficiencies in laser welding of titanium. [ABSTRACT FROM AUTHOR]

A new tungsten matrix composite containing 30 vol% titanium carbide particles (W-TiC) produced by sintering under 20 MPa pressure at 2000°C in a vacuum has been developed in order to improve the elevated temperature strength of tungsten. Flexural strength tests of the W-TiC composite in the temperature range 20–1200°C showed that the strength was significantly increased by the presence of TiC particles. The flexural strength at 1000°C was 1155 MPa, which was much higher than that at 20°C (770 MPa). Microstructural observations showed that a interdiffusion zone was produced at the W matrix-TiC particle interface, and a strong bond was formed between TiC and W, which was very beneficial to the elevated temperature mechanical properties. The mechanisms of fracture at 20°C and 1000°C were investigated. The fracture at 20°C was brittle. There was a growth-coalescence process for the initial cracks during the fracture process of the W-TiC composite at 1000°C, and the W matrix exhibited ductile tearing. The excellent elevated temperature strength of W-TiC composite was attributed to the brittle-ductile transition in the W matrix, which allows more effective strengthening from TiC particles. [ABSTRACT FROM AUTHOR]

Room temperature tensile properties of polycrystal Ti–47Al–2Mn–2Nb alloy with near lamellar (NL) microstructures were investigated at the strain rates between 10−5 and 1000 s−1 using a self-designed Split-Hopkinson tensile bar setup with a rotating disk and conventional testing machine. It was found that tensile ductility varies within a narrow range with the strain rate, while dynamic strengths (σd) of the alloy are obviously higher than static strengths (σs). There exists linear relationship between σs and the logarithm of the strain rate (ln έ), and between σd and the strain rate itself (έ). Fractography analysis indicates that the alloy fractures in a mixed mode of predominant transgranular cleavage and minor intergranular cracking under static and dynamic strain rates. Environmental effect is excluded from the main cause for the room temperature brittleness of the investigated alloy. [ABSTRACT FROM AUTHOR]

Abstract: The use of dual phase (DP) steels in the automobile industry unavoidably involves welding and dynamic loading. The aim of this investigation was to evaluate the microstructural change and mechanical properties of laser welded dissimilar DP600/DP980 steel joints. The dissimilar joints showed a significant microstructural change from nearly full martensite in the fusion zone (FZ) to the unchanged ferrite–martensite dual-phase microstructure in the base metal. The welding resulted in a significant hardness increase in the FZ but the formation of a soft zone in the heat-affected zone (HAZ). The dissimilar welded joints were observed to exhibit a distinctive unsymmetrical hardness profile, yield-point-like phenomenon, and single-stage work hardening characteristic, with yield strength and work hardening rate lying in-between those of DP600 and DP980 base metals, and ultimate tensile strength equivalent to that of DP600 base metal. Although the welded joints showed a lower fatigue limit than the base metals, the fatigue life of the welded joints at higher stress amplitudes was almost the same as that of the DP600 base metal. The welded joints failed in the soft zone at the DP600 side under tensile loading and fatigue loading at the higher stress amplitudes. Fatigue crack initiation occurred from the specimen surface and crack propagation was characterized by typical fatigue striation together with secondary cracks. [ABSTRACT FROM AUTHOR]

Abstract: Reliable Si3N4/Si3N4 joints were obtained using Cuph name="sbnd" />Ti as filler alloy. The effect of holding time on interfacial microstructure and bonding strength of the joints was investigated. All the joints consist of continuous reaction layer (fine-grain TiN), discontinuous reaction layer (coarse-grain TiN), Cu-based solid solution and reaction products. Prolonged exposure to the elevated temperature causes the thickness of the discontinuous reaction layer to increase, due to the coarsening of TiN grains. And the amount of Pdincreases instead of Ti With the increase of holding time, the bonding strength of the joints decreases. The formation of a reaction layer with a suitable thickness promotes the improvement of bonding strength of joints. [Copyright &y& Elsevier]

Abstract: A 30μm thick nanocrystalline coating (NC) with a composition identical to that of conventional coarse-grained 304 stainless steel (SS) was prepared via direct current magnetron sputtering (DCMS). The DCMS NC exhibited a typical columnar microstructure, with a mean grain size of ∼12nm in the plane normal to the growth direction. Comparison of the oxidation of the 304 NC with 304 SS (∼20μm in an average grain size) was investigated at temperatures (700–900°C) in flowing air with 0–40% H2O vapor. Catastrophic breakaway oxidation started from the upstream edge of the coarse-grained sample in wet air after an incubation period, due to the conversion of protective Cr-rich scale into non-protective Fe-rich one as a result of the water-vapor-promoted evaporation of chromia from scale. Increasing temperature or water-vapor concentration accelerated the appearance of breakaway oxidation. In contrast, breakaway oxidation did not occur in wet air for the DCMS NC, where abundant grain boundaries greatly enhanced the Cr diffusion to guarantee the stable growth of the Cr-rich scale. [Copyright &y& Elsevier]

The thermophysical properties and microstructure of n-octadecane with crystalline and amorphous were investigated by employing the molecular dynamics (MD) simulation. The distribution of the end to end distance and bond torsion angle of the n-octadecane molecular chain and the mean square displacement and thermal conductivity before and after phase transition were also examined. MD simulation results indicates that the molecular chain conformation of amorphous n-octadecane in solid state is gradually changed from stretching to torsion by increasing temperature, and the chains will stretch out as the temperature rises in the liquid state. Compared with amorphous paraffin, the diffusion coefficient and the phase transition temperature of crystalline paraffin is lower than that of amorphous paraffin. The thermal conductivity of crystalline paraffin is much higher than that of amorphous paraffin. It is shown that improving the order degree of PCMs is an effective method to enhance their thermophysical properties. [ABSTRACT FROM AUTHOR]

Cracks repairing of superalloy is very important to the aerospace industry. Most of the crack repairing studies have the same “crack width” which is quite different from the taper-shaped crack in reality. In this study tapered slots with maximum 200 μm width were artificially made by femtosecond laser in small plates of IN738LC superalloy to imitate service cracks. The “cracks” were repaired by diffusion brazing with a mixed filler alloy at 1100 °C for 24 h and followed by homogenization at temperatures of 1160 °C (HT-1), 1180 °C (HT-2) and 1190 °C (HT-3). With increasing homogenization temperature and time, the volume faction of borides in diffusion affected zone (DAZ) decreased. During the homogenization with HT-3, most of the borides in the DAZ of the joint dissolved, merely a small quantity of others coarsened by Ostwald ripening. In order of the processes as-bonded, HT-1, HT-2 and HT-3, the chemical composition of the isothermal solidification zones (ISZs) gradually came close to that of the base metal and the hardness of the ISZs increased, while the hardness of the DAZs decreased. A uniform bonded joint could be obtained with HT-3, and the shear strength of the joint reached up to approximately 95% of the base metal. The corresponding fracture path and fractography of the samples were observed to explain the mechanism of strength evolution. [ABSTRACT FROM AUTHOR]

Joining NiTi to Ti6Al4V is of great interest for applications in the biomedical and aerospace fields. Despite the importance, no joining techniques have been developed that avoid the formation of brittle intermetallics to produce high strength joints. In this work, Niobium was used as an interlayer to prevent the formation of these brittle phases when joining NiTi to Ti6Al4V. The presence of this interlayer ensured that crack free welds were obtained and no brittle intermetallic compounds were observed. The Niobium interlayer was of a much higher melting temperature than the base materials so the bulk Niobium did not melt during the joining process, acting as a diffusion barrier between the NiTi and Ti6Al4V. The laser was focused on the Ti6Al4V side of the joint, which joined the Ti6Al4V and Niobium by fusion welding. At this interface a (Ti, Nb) region was formed due to dilution of the Niobium and mixing with the Ti6Al4V. At the NiTi–Nb interface a eutectic reaction was responsible for joining. Mechanical testing of the joints revealed that the minimum tensile strength matched the ultimate tensile strength of the weakest material, Niobium. These results highlight new possibilities for the use of high melting point filler materials when joining NiTi to dissimilar materials, so that the formation of undesired phases can be avoided. [ABSTRACT FROM AUTHOR]

Abstract: An infrared emissivity coating material containing γ-Al2O3 was prepared on 2024 aluminium alloy surface by the microarc oxidation (MAO) method. The microstructure of the coatings was analysed by SEM, XRD and EDS techniques. The infrared emissivity properties tested at 500°C were investigated by an infrared radiometer based on a Fourier transform infrared spectrometer. The results show that the infrared emissivity values of coated Al samples depend on the phase composition and surface roughness of the coatings. Corresponding to increasing coatings thickness, the gradually increasing γ-Al2O3 content and some oxide compounds containing Si and P contribute to the higher infrared emissivity value (about 0.85) in the wavelength range of 8–20μm. The increasing surface roughness leads to an obvious increase in emissivity from 0.2 to 0.4 at wavelength 3–5μm. [Copyright &y& Elsevier]

Abstract: Following laser hole sealing the hardness of CP1 Ti welds was found to be directly related to the microstructure. It was observed that welds made with electrolyte present had a microstructure which consisted mostly of the acicular alpha phase compared to welds made without electrolyte which had a mixed structure of serrated alpha, platelet alpha and acicular alpha phases. The hardest regions of welds made in the presence of electrolyte consisted of a fine colonized acicular alpha phase. Results from energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis further suggested that an increase in the composition of interstitial elements was the primary mechanism responsible for increased hardness when welding with electrolyte present. [Copyright &y& Elsevier]

Abstract: We use phase-field models to characterize the microstructure present in magnesium AZ31 alloy solidified under welding conditions. We focus our attention on the study of the conditions under which a columnar-to-equiaxed transition (CET) is observed in resistance spot welds. Our simulations show how the size and shape of the columnar and equiaxed regions depend on factors such as cooling rate, temperature gradient and the nature of inoculant particles. Our results are compared with experimental observations. In addition, we contrast our findings with predictions from a previously developed steady-state model for the CET. [Copyright &y& Elsevier]

Abstract: Bilayered CoFe2O4/0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 nanocomposite films are successfully prepared on Pt/Ti/SiO2/Si substrate via simple sol–gel process. X-ray diffraction result reveals that there exists no chemical reaction or phase diffusion between the CoFe2O4 and 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 phases. The microstructure is characterized by scanning/transmission electron microscopy (STEM). The composite thin films exhibit both strong ferroelectric and ferromagnetic responses at room temperature. The maximal magnetoelectric coupling coefficient of the nanocomposite films reaches up to 25mV/cmOe, occurs at a lower bias magnetic field (H dc) of 550Oe. [Copyright &y& Elsevier]

Abstract: The influence of additives on the reaction kinetics and microstructure refinement in LiBH4–MgH2 composites is investigated in detail. Indications of the rate-limiting processes during the reactions are obtained by comparison of the measured reaction kinetics with simulations with one specific rate-limiting process. The kinetics of the sorption reactions are derived from volumetric measurements as well as from in situ X-ray diffraction measurements. During desorption, the hydrogen is released at a constant rate, which is possibly correlated with the one-dimensional growth of MgB2 platelets. In contrast, the kinetic curves of the absorption reactions exhibit the typical shape of contracting-volume controlled kinetics. The microscopical interpretation of kinetic measurements are supported by transmission electron microscopy images confirming the formation of additive-nanostructures in the grain boundaries upon cycling. The present investigations underline the importance of the additives as nucleation substrates and the influence of microstructure on the reaction kinetics. [Copyright &y& Elsevier]

Abstract: Si3N4 ceramic was self-jointed using a filler alloy of Cu–Pd–Ti, and the microstructure of the joint was analyzed. By using a filler alloy of Cu76.5Pd8.5Ti15 (at.%), a high quality Si3N4/Si3N4 joint was obtained by brazing at 1100–1200°C for 30min under a pressure of 2×10−3 MPa. The microstructure of the Si3N4/Si3N4 joint which was observed by EPMA, XRD and TEM, and the results indicated that a reaction layer of TiN existed at the interface between Si3N4 ceramic and filler alloy. The center of the joint was Cu base solid solution containing Pd, and some reaction phases of TiN, PdTiSi and Pd2Si found in the Cu [Pd] solid solution. [Copyright &y& Elsevier]

Abstract: Si3N4 ceramic has been joined successfully to itself by brazing in a vacuum at 1373–1573K holding for 1.8ks using a Cu76.5Pd8.5Ti15 alloy filler. By observation and analysis of the microstructure of the joints, it is found that the joints comprise three parts: (1) a continuous reaction layer of TiN between Si3N4 and filler alloy, (2) a discontinuous reaction layer containing TiN, Pd2Si and Ti5Si3 connecting the continuous reaction layer, and (3) the Cu-based solid solution in the mid of the joint and some reaction products (TiN, Pd2Si, PdTiSi and Ti5Si3) in the solution. With increasing brazing temperature, both the thickness of the continuous reaction layer and amount of reaction phases increase, which improves the bending strength of the joints. When the brazing temperature surpasses 1473K, voids are formed in the joint because of the decomposition of the Si3N4 ceramic, leading to a decrease of the bond strength. The room-temperature bending strength of the joint brazed at 1423K reaches a maximum value of 155.8MPa. [Copyright &y& Elsevier]