Elina Huttunen-Saarivirta

Microstructure, fabrication and properties of quasicrystalline Al–Cu–Fe alloys: a review

Abstract Quasicrystalline materials constitute a new materials group with certain crystalline structural characteristics, such as the generation of Bragg peaks in the X-ray data and points in the electron diffraction pattern, but translational symmetry is forbidden for crystalline materials. Thus, there exists aperiodicity in the structure of quasicrystalline materials. Besides being theoretically interesting due to their complicated atomic structure, the unique properties of quasicrystalline materials—low electrical and thermal conductivity, unusual optical properties, low surface energy and coefficient of friction, oxidation resistance, biocompatibility and high hardness, to name a few—also make them interesting for many practical purposes. Quasicrystalline phases are today encountered in over 100 alloy systems, of which the majority are aluminium based. Few of the alloying elements used to form aluminium-based quasicrystals are reasonable in price, easily available and non-toxic. However, quasicrystalline Al–Cu–Fe ternary alloys fulfill all these alloying-element criteria. In this paper, the microstructure, fabrication and properties of quasicrystalline Al–Cu–Fe alloys are reviewed from the perspective of materials engineer. The paper discusses the microstructure and metallurgy of quasicrystalline Al–Cu–Fe alloys. The preparation methods of quasicrystals in general and their application to the fabrication of Al–Cu–Fe quasicrystalline alloys are considered. The characteristics of different production methods, including both conventional methods yielding stable phases and more advanced methods introducing metastable phases, are compared in this paper. The properties of Al–Cu–Fe quasicrystals are also reviewed, aiming at a better understanding of the basic differences between crystalline and quasicrystalline materials with respect to structure and properties. Finally current and possible future applications of Al–Cu–Fe quasicrystals are discussed in the light of their properties.

Observations on the uniformity of immersion tin coatings on copper

Abstract The development of chemical tin coatings by a replacement reaction Cu+Sn 2+ ⇒Cu 2+ +Sn from hydrochloric acid based and methanesulphonic acid based baths on the inner surface of a copper tube was studied by transmission electron microscopy and scanning electron microscopy in order to explore the factors influencing the uniformity of tin coatings. Despite the fact that tin coatings develop in different ways in studied baths, uniform coatings are equally plated from both these baths when the substrate surface is clean and smooth. However, differences in the surface-sensitivity, i.e. the dependence of coating uniformity on the substrate surface condition, of plating baths materialise when plated on irregular or improperly cleaned substrate. A hydrochloric acid based bath may yield non-uniform coatings on irregular or improperly cleaned substrate, since large-sized grains develop immediately after coating initiation. These large grains are suggested to orientate to follow the contours of impurity particles or surface irregularities introducing misalignment in grain columns and, thus, non-uniformity into coatings. In contrast, a methanesulphonic acid based bath is still able to produce uniform tin coatings on contaminated or rough surfaces, as the coating is proposed to accommodate to substrate surface irregularities by the build-up of a nanocrystalline zone at the early stages of tin coating deposition. Sulfur-containing elements in this methanesulphonic acid based plating bath play the central role in the nanocrystalline layer formation.

Influence of Cr alloying on the microstructure of thermally sprayed quasicrystalline Al-Cu-Fe coatings

Abstract The present work reports the structural development of Al–Cu–Fe and Al–Cu–Fe–Cr coatings deposited by the high velocity oxy-fuel thermal spraying process and the influence of Cr alloying on the phase selection of Al–Cu–Fe coatings at various deposition temperatures. The porosity levels of the Al–Cu–Fe and Al–Cu–Fe–Cr coatings of the study are demonstrated to be lower than those reported for corresponding plasma-sprayed coatings. The results show that high velocity oxy-fuel spraying technique produces Al–Cu–Fe coatings that are phase structurally similar to plasma-sprayed Al–Cu–Fe coatings reported in literature. Al–Cu–Fe coatings are composed of a crystalline β-AlFe phase and a quasicrystalline i-Al65Cu20Fe15 phase as well as an oxidised form of either or both of these phases. Addition of Cr to Al–Cu–Fe alloys introduces coatings that are made up of the crystalline θ-Al2Cu phase and two quasicrystalline phases, the i1-Al80Cr13.5Fe6.5 and i2-Al13Cr3Cu4 phases. The formation of these icosahedral phases in Al–Cu–Fe–Cr alloys has not been reported before, although the occurrence of quasicrystal approximants with compositions close to those of the i1-Al80Cr13.5Fe6.5 and i2-Al13Cr3Cu4 phases has been demonstrated. On the basis of our results we propose that the icosahedral phase structure is greatly stabilised by the Cr addition to Al–Cu–Fe alloys.

Microstructural characterisation of thermally sprayed quasicrystalline Al–Co–Fe–Cr coatings

Abstract A microstructural characterisation was carried out for Al–Co–Fe–Cr feed powder and the coatings sprayed with a high velocity oxy-fuel method using different operation conditions. The aims of the study were to explore the structural development of thick Al–Co–Fe–Cr coatings and the influence of the spraying parameters on the microstructure of produced Al–Co–Fe–Cr coatings. X-ray diffractometry, scanning electron microscopy and analytical transmission electron microscopy were the techniques used in the phase identification and in the microstructural exploration of the study. The results show that Al–Co–Fe–Cr feed powder and the coatings sprayed with low and high operation temperature are composed of a dodecagonal quasicrystalline phase. The composition of this new dodecagonal phase approximately corresponds to that of the feed powder, being A1 70.6 Co 12.5 Fe 9.4 Cr 7.5 . The dodecagonal phase does not decompose during the spraying process. Instead, it orientates to form a lamellar coating structure. When a lower spraying temperature is used, the incomplete melting of powder particles introduces a partly orientated coating structure. Due to this incomplete melting of powder particles, porosity is also involved in these coatings. Higher spraying temperature, in turn, promotes oxidation, leading to the incorporation of an oxygen-containing film on the splat boundaries. While the feed powder and the coating deposited with a lower spraying temperature are one-phase quasicrystalline structures, the coating sprayed with a higher operation temperature is comprised of a dodecagonal phase and an oxygen-containing phase. This oxygen-containing phase is not pure aluminium oxide but contains all the elements present in the alloy.

Microstructural study of the initiation and formation of immersion tin coating on copper

The initiation and growth of immersion tin coating from hydrochloric acid based and methanesulphonic acid based baths on the inner surface of copper tube was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The formation of immersion tin coating crystallites from hydrochloric acid based bath occurs semi-coherently with copper substrate. Tin crystallites grow actively both in vertical and horizontal directions yielding an equiaxial grain structure. The immersion tin deposition from methanesulphonic acid based bath proceeds by the formation of tin nanocrystals. These nanocrystals transform into columnar grains during the grain growth process. Differences in the coating formation characteristics between studied baths are reflected to the microstructural details and properties of coatings, such as surface morphology, porosity and adhesion.

Influence of Chlorination and Choice of Materials on Fouling in Cooling Water System under Brackish Seawater Conditions

Cooling systems remove heat from components and industrial equipment. Water cooling, employing natural waters, is typically used for cooling large industrial facilities, such as power plants, factories or refineries. Due to moderate temperatures, cooling water cycles are susceptible to biofouling, inorganic fouling and scaling, which may reduce heat transfer and enhance corrosion. Hypochlorite treatment or antifouling coatings are used to prevent biological fouling in these systems. In this research, we examine biofouling and materials’ degradation in a brackish seawater environment using a range of test materials, both uncoated and coated. The fouling and corrosion resistance of titanium alloy (Ti-6Al-4V), super austenitic stainless steel (254SMO) and epoxy-coated carbon steel (Intershield Inerta160) were studied in the absence and presence of hypochlorite. Our results demonstrate that biological fouling is intensive in cooling systems using brackish seawater in sub-arctic areas. The microfouling comprised a vast diversity of bacteria, archaea, fungi, algae and protozoa. Chlorination was effective against biological fouling: up to a 10–1000-fold decrease in bacterial and archaeal numbers was detected. Chlorination also changed the diversity of the biofilm-forming community. Nevertheless, our results also suggest that chlorination enhances cracking of the epoxy coating.

Ennoblement, corrosion, and biofouling in brackish seawater: Comparison between six stainless steel grades

In this work, six common stainless steel grades were compared with respect to ennoblement characteristics, corrosion performance and tendency to biofouling in brackish sea water in a pilot-scale cooling water circuit. Two tests were performed, each employing three test materials, until differences between the materials were detected. Open circuit potential (OCP) was measured continuously in situ. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements were conducted before and after the tests. Exposed specimens were further subjected to examinations by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), and the biofouling was studied using epifluorescence microscopy, quantitative polymerase chain reaction (qPCR) and high-throughput sequencing (HTP sequencing). The results revealed dissimilarities between the stainless steel grades in corrosion behaviour and biofouling tendency. The test material that differed from the most of the other studied alloys was grade EN 1.4162. It experienced fastest and most efficient ennoblement of OCP, its passive area shrank to the greatest extent and the cathodic reaction was accelerated to a significant degree by the development of biofilm. Furthermore, microbiological analyses revealed that bacterial community on EN 1.4162 was dominated by Actinobacteria, whereas on the other five test materials Proteobacteria was the main bacterial phylum.

Effect of temperature and concentration of precursors on morphology and photocatalytic activity of zinc oxide thin films prepared by hydrothermal route

Zinc oxide (ZnO) is an important semiconductive material due to its potential applications, such as conductive gas sensors, transparent conductive electrodes, solar cells, and photocatalysts. Photocatalytic activity can be exploited in the decomposition of hazardous pollutants from environment. In this study, we produced zinc oxide thin films on stainless steel plates by hydrothermal method varying the precursor concentration (from 0.029 M to 0.16 M) and the synthesis temperature (from 70 °C to 90 °C). Morphology of the synthesized films was examined using field-emission scanning electron microscopy (FESEM) and photocatalytic activity of the films was characterized using methylene blue decomposition tests. It was found that the morphology of the nanostructures was strongly affected by the precursor concentration and the temperature of the synthesis. At lower concentrations zinc oxide grew as thin needlelike nanorods of uniform length and shape and aligned perpendicular to the stainless steel substrate surface. At higher concentrations the shape of the rods transformed towards hexagon shaped units and further on towards flaky platelets. Temperature changes caused variations in the coating thickness and the orientation of the crystal units. It was also observed, that the photocatalytic activity of the prepared films was clearly dependent on the morphology of the surfaces.

Corrosion of Aluminized and Uncoated 9–12% Cr Boiler Steels in Simulated Biomass andWaste Combustion Conditions

Abstract Coatings are seen a promising way to improve the corrosion resistance of relatively cheap power plant steels to enable higher steam temperatures than currently in use. In this research, 9–12% Cr steels P91 and HCM12A are coated with aluminium diffusion coating by a slurry method and exposed for 336 hours at 833 K and 883 K to atmospheres containing varying amounts of O2, H2O, HCl and SO2. Corrosion behaviour of the coated steels is compared to that of those steels in an uncoated condition. Characterization is performed by weighing, SEM + EDS and XRD. The results show that corrosion resistance of P91 and HCM12A is significantly improved by the aluminium diffusion coating at high temperatures in atmospheres containing HCl and SO2. The corrosion rate of the aluminized specimens slightly increases with increase in test temperature but remains virtually the same irrespective of the composition of the atmosphere. On the other hand, the corrosion rate of the uncoated specimens is dependent on both the atmosphere and the temperature. The steels undergo active oxidation that results in formation of non-protective, thick and layered scales in HCl containing atmospheres. SO2 addition slightly decreases the corrosion rate although it is anyway higher than that in SO2 containing atmosphere without HCl.

Microstructural study on the inhibition of immersion tin coating formation by the substrate surface contamination

The formation of an immersion tin coating from a hydrochloric acid based bath on the inner surface of clean and contaminated copper tubes was studied by scanning electron microscopy and transmission electron microscopy in order to explore the role of substrate surface contamination in the immersion tin coating build-up. The mechanism of inhibition by a drawing lubricant layer was the major attribute under exploration. The substrate surface contamination was found to influence both the coating nucleation and growth processes and the overall appearance of the immersion tin coating. A discontinuous, thin, nanocrystalline and irregular tin coating forms on a contaminated copper substrate instead of a continuous, crystalline, regular tin coating of a reasonable thickness, which builds up on a clean copper surface. However, the nucleation mechanism of immersion tin deposit is equivalent on clean and contaminated substrate surfaces. The oily contamination layer is, thus, only suggested to block the accessibility of aqueous plating solution onto the substrate surface, thereby hindering the coating nucleation at the areas of poor cleaning. Besides, no grain growth was encountered in tin coatings deposited on the poorly cleaned copper.