Search

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]

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]

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: 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]

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]

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]