Volume 112, Issue 8

A key application of CUSTOM 450 alloy is in the construction of gas turbine compressor blades. The study of pitting corrosion can prevent the failure of many gas turbine compressor blades. In this study, a reflective digital holography microscopy method was employed to investigate the growth of pitting corrosion in depth. To this end, a constant potential of 350 mV SCE in a 3.5 wt.% NaCl solution was applied to the specimen. The generated pits were simulated in three dimensions, and it was indicated that pitting corrosion rate was decreased as time passed. Comparing the obtained experimental data with the data gathered from the real industrial environment surrounding a compressor installation, an accelerated test was proposed. By the proposed accelerated test, it is possible to produce a pit similar to the one that will be initiated and propagated at any time in the future in real conditions.

The intergranular corrosion behavior of 6061 aluminum alloy welded joints produced by metal inert gas welding and friction stir welding was studied. The microstructure of the welded joints and the intergranular corrosion morphology of the cross-section were analyzed by optical microscopy. The results show that the most sensitive area of intergranular corrosion is the partially melted zone of the metal inert gas welding, and the maximum corrosion depth is about seven times that of the base metal, followed by the unmixed zone. The heat affected zone has the lowest sensitivity. Although the welding seam corroded seriously, general corrosion played a leading role. With the high heat input of metal inert gas welding, the sensitivity to intergranular corrosion in the partially melted zone increased significantly, while other zones had little change. For friction stir welding joints, the heat affected zone suffered from the most severe corrosion, and the nugget zone the least. However, the difference is not apparent. The susceptibility to intergranular corrosion of friction stir welding joints is weaker than that of metal inert gas welding joints but more severe than the base metal.

One of the main manufacturing challenges is to obtain dried cellulose nanofibrils (CNFs) so that they can be cost effectively transported to customers. This work presents a study on using two methods of drying: freeze drying and spray drying; these dried CNFs were then characterized. The dried CNFs from either freeze drying or spray drying could not recover their original state after simple re-dispersion in water. Compared to spray dried CNFs, the microstructure of the freeze dried CNFs remained in a better shape. This was because the packing of nanofibrils as a result of freeze drying was not as tight as that from spray drying. It was demonstrated by the lower final mass residue and crystallinity of the freeze-dried CNFs, which led to better re-dispersion in water. X-ray diffractometry proved the occurrence of aggregation/hornification of the dried CNFs with increased crystallinity. Time-dependent sedimentation confirmed that the dried CNFs were incapable of forming stable water-re-dispersible suspensions.

In this study, self-lubricating porous bronze plain bearing samples were manufactured differently from conventional plain bearings and the wear behaviours were investigated. Plain bearing samples were manufactured by sintering of pre-alloyed spherical CuSn11 bronze powders with grain size of 100– 200 μm. Then, special polymeric composite mixtures were prepared as PTFE (polytetrafluoroethylene) and graphite (GR) additive PTFE mixtures. The GR additive PTFE mixtures were prepared by addition of the GR powder with an average particle size of 200 μm at ratios of 10 wt.% and 20 wt.% into the PTFE solution. Next, the polymeric composite mixtures were impregnated into the porous structure of samples by the spray pulverization coating method. The purpose of the impregnation process was to minimise wear at a longer sliding distance by the gain of self-lubricating property to the samples. The plain bearing samples were produced as three different types (PTFE, PTFE + 10% GR and PTFE + 20% GR). Wear tests were carried out using a plain bearing test rig at different sliding speeds (0.5, 1.0 and 1.5 m s –1 ) and under applied loads (30, 50 and 70 N). Wear values were determined as weight loss of the samples. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used for the wear analysis of the samples. The results show that the mixtures of PTFE with GR significantly reduced the wear loses of plain bearing surfaces under dry sliding conditions. The PTFE + 10% GR sample had the lowest wear loss under 0.5 m s –1 sliding speed and 30 N applied load conditions after 2.5 hour in comparison with other samples. In other words, the PTFE + 10% GR sample showed lower wear than (31.25%) PTFE sample (without GR additive) and (2.65%) PTFE + 20% GR sample. Also, the wear loss of the PTFE + 20% GR sample (3.67 mg) was slightly increased (difference 1.27 mg/53%) compared to the PTFE + 10% GR sample (2.40 mg) under average load and sliding speed conditions.

In this work, using the first-principles method, the alloying stability, electronic structure, and elastic properties of Al-based intermetallics were investigated. It was found that these alloys have a strong alloying ability and structural stability due to the negative formation energies and the cohesive energies. The valence bonds of these intermetallic compounds are attributed to the valence electrons of Cu 3δ states for AlCu 3 , Cu 3δ and Zr 4δ states for AlCu 2 Zr, and Al 3s, Zr 5s and 4δ states for AlZr 3 , respectively. Furthermore, the correlation between elastic properties of these intermetallic compounds and their electronic structures was revealed. The results show that structural parameters and elastic properties such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and anisotropy agreed well with experimental results.

As any largescale cast material, specific Compositionally Complex Alloys or High Entropy Superalloys contain segregations, leading to unideal, inhomogeneous properties. This work presents the effects of a homogenization heat treatment at 1 150°C for 6 h of a large-scale cast Al 4.4 Co 26 Cr 18 Fe 18 Ni 26 Ti 5.5 alloy. In order to reveal these effects, homogenized specimens were analyzed and compared to the as-cast state with regard to chemical homogeneity as well as the homogeneity of elemental solution by means of scanning electron microscopy, energy dispersive X-ray spectroscopy as well as X-ray diffraction and hardness measurements. Despite the increased Al content, intermetallic phases and segregations, observable in the as-cast state, dissolve during homogenization. Improved, but not full homogeneity of elemental distribution after annealing can be determined. The improved state of solution and homogeneity agrees with the increasing lattice parameter from 3.572 Å to 3.594 Å and the decreasing hardness from 320.3 HV10 to 245.2 HV10 during homogenization.

Al7075–Ti–Y natural composites were prepared by using a combination of ultrasonic vibration and casting. The effects of titanium (Ti) and yttrium (Y) on the nucleation of primary α-Al were studied and the influencing mechanism of Y on the TiAl 3 formation was analysed. Furthermore, a reaction kinetics model of TiAl 3 under ultrasonic vibration was established. The results showed that the uniformly distributed TiAl 3 and Al 3 Y nano-particles resulted in grain refinement. Y changed the morphology of TiAl 3 and the appearance of corrosion pits was related to the fact that Y was dissolved within the TiAl 3 structure to form Ti(Al,Y) 3 . The established model revealed that ultrasonic vibration significantly promoted the formation of TiAl 3 , and that ultrasonic time was the main factor affecting its growth.

The ferroelectric barium strontium titanate (Ba 1-x Sr x TiO 3 ) is a homogeneous solid solution prepared from the mixture of barium titanate (BaTiO 3 ), strontium titanate (SrTiO 3 ) and titanium (IV) isopropoxide. Barium strontium titanate (BST) nanomaterials with improved permittivity and dielectric properties due to their nano-properties have attracted great interest for extensive and versatile applications as super capacitors, dielectrics, ceramics and catalysts. Introduction of metal ion dopants into the parent system of BST significantly alters its structural, morphological, electrical, optical and dielectric characteristics. This review is aimed at addressing synthesis, characterization methods, photocatalytic and electrical applications of metal ions doped BST nanomaterials. The effect of doping BST, through metal ions, on its properties and application with most probable reasons have been thoroughly discussed.