Volume 110, Issue 1

The superalloys are usually used as structural material in jet engines due to their high-temperature stability and good endurance. Many components found in the hot-part of jet engines are of complex shape, therefore casting is utilized for to their production. One of the problems associated with the casting process of highly alloyed alloys, such as superalloys, is partitioning of alloying elements upon solidification. This segregation might lead to the formation of low melting temperature eutectics. Their presence in the material microstructure will have a negative effect on the weldability. Negative impact on weldability is one of the reasons why secondary phases should be avoided as microstructural elements in the welded materials. To eliminate such hazardous phases, the material should be subjected to a heat treatment with the aim of homogenizing the microstructure and chemical composition, which should enhance the weldability. The aim of this work was the application of analytical electron microscopy in combination with energy dispersive X-ray spectroscopy as well as tomographic techniques for qualitative and quantitative characterization of structural elements in as-cast Allvac 718Plus Ni-based superalloy subjected to later heat treatment. Alloy 718Plus is a newly developed superalloy in which the secondary Laves phase forms as a low-melting eutectic upon casting. The development of innovative materials for aeronautics and clean energy systems requires the use of modern research methods for structure characterization on the level from micro- to the nanoscale. The performed analysis, allowed identification of phases (Laves and η) occurring in interdendritic regions of as-cast Allvac 718Plus superalloy and analyzing their microstructure down to the atomic scale, which revealed their complex nature. The experiments and investigation show that advanced microscopic techniques and test methods in conjunction with tomographic techniques enable complementary information about solidified structures of the alloy to be obtained that can be useful for the understanding process of casting and welding of the Allvac 718Plus.

The influence of Q-switched pulsed Nd:YAG laser interference heating, using 120 mJ of pulse energy and a variable number of consecutive pulses, on the microstructure and magnetic properties of amorphous Fe 80 Si 11 B 9 alloy was examined. Microstructural analysis, using light, scanning and transmission electron microscopy, was complemented by results of Mössbauer spectroscopy and measurement of magnetic properties (vibrating sample magnetometer). Periodically distributed crystallized micro-areas, ∼10 μm in diameter, in an amorphous matrix were produced by the treatments. Magnetization measurements showed that the as-cast ribbon and laser light irradiated samples are magnetically soft materials. The results lead to the conclusion that the dots corresponding to the laser modified regions exhibit a perpendicular magnetic anisotropy.

In this work Ni-rich NiTi alloy subjected to 35 % cold-rolling in the martensitic state followed by annealing at 450 °C for 15 min has been studied by transmission electron microscopy observations. It has been found that the material after thermo-mechanical treatment has a non-uniform nanostructure with canal-like shaped amorphous areas containing crystallites of R and B19′ phases. Zones surrounding these regions contain grains elongated in different directions in different areas. They are identified to be a mixture of B2 parent, R rhombohedral and B19′ martensitic phases. Selected area electron diffraction as well as transmission electron microscope orientation maps taken from these zones exhibit differences in their crystallographic orientations. It is proposed that the amorphous areas form along grain boundaries during severe plastic deformation.

Two MgLiAl alloys consisting either of α (hcp) + β (bcc) phases or only β phase were subjected to twist channel angular pressing TCAP (with helical component) or cyclic compression to a total strain of ∊ = 5 in order to study the effectiveness of various deformation modes on grain refinement. After the first TCAP pass grains of α phase were refined from 30 μm down to about 6 μm and of β phase from initial 200 μm down to 8 μm. MAXStrain cycling led to much finer grains of α and β phases in the range 200 – 300 nm causing higher hardening and indicating higher effectiveness of the process. The Li 2 MgAl precipitates were refined during severe plastic deformation (SPD) processes and in addition fine particles of hexagonal α phase of size below 100 nm were observed within the β phase showing orientation relationship (0001) α ‖ (011) β. Two-phase material after SPD showed deviation of about 1.5° from the ideal Burgers orientation relationship (0001) α ‖ (011) β.

This work presents the results concerning microstructural, mechanical property and electrical resistance investigations of La 1- x Sr x CoO 3 thin films produced by the laser ablation (PLD – Pulsed Laser Deposition) method on Si and MgO single crystal substrates with (001) orientation. The microstructure and chemical/phase composition analysis of the deposited films was carried out using scanning/transmission electron microscopy, X-ray diffraction and atomic force microscopy. Additionally, adhesion and nanohardness tests were performed. The investigations allowed verification of the quality of the prepared La 1- x Sr x CoO 3 thin films and show that implementing the PLD technique makes it possible to obtain smooth and dense films with nanocrystalline structures, while preserving the composition of the bulk target. Research studies showed that by properly selecting the parameters for the PLD process it is possible to deposit films with a significantly lower amount and smaller size of undesirable nanoparticles. A specialized measuring station was used to determine the response of thin films to an oxidizing gas atmosphere. The observed decrease in resistance of the thin films in the presence of 50 ppm of NO 2 oxidizing gas is typical for p-type semiconductors. This indicates that all of the produced films exhibit a sensitivity to this gas and could be used as NO 2 sensors.

The microstructure of a scale formed on the ATI 718Plus superalloy during oxidation at 850 °C for up to 1 000 h in air was characterised using various electron microscopy techniques taking advantage of recent development and capabilities of energy dispersive X-ray spectrometry. The study shows that a protective multilayered Cr 2 O 3 scale was formed on this alloy; the outer Cr 2 O 3 layer was coarse-grained, while the inner one was fine-grained. Underneath, a thin layer composed of TiNbO 4 as well as an internal oxidation zone that was a dozen or so micrometres thick and consisted of individual Al 2 O 3 precipitates had formed. The growth of the Cr 2 O 3 layer caused the appearance of a chromium depletion zone, in which the γ′ particles were not observed.

Cold sprayed Ni coatings offer new possibilities for repairing damaged and mis-machined components. The morphology of the powder particles and grain-size distribution have a significant influence on the particles' behaviour during compaction and on the properties of the coating. In this study, two Ni powders with different morphology and grain-size distribution were cold sprayed on a 7075 Al alloy substrate. The first powder, Ni HR, had an irregular spherical shape, whereas the second, Ni E, possessed a dendritic shape. SEM and TEM investigations revealed differences in the microstructure and properties of the Ni cold-sprayed coatings. The coating sprayed using the dendritic powder revealed enhanced mechanical properties and the surface had half the roughness of the Ni coatings deposited using the irregular spheroidal powder.

Magnetron sputtered Ti/Al freestanding multilayer foils were treated with an electric current pulse of 0.5 A or 4 A for ∼1 s in order to initiate a self-sustaining high-temperature synthesis. Scanning electron microscopy observations showed that the sample treated with 0.5 A is characterized by the presence of periodic rippled bands, while the 4 A one has a nearly unaffected shape. Transmission electron microscopy experiments revealed that the microstructure of Ti/Al foil ignited with 0.5 A consists of large equiaxed grains of γ-TiAl phase and alternating plates of γ-TiAl + α 2 -Ti 3 Al phases, carrying vestiges of their initial multilayer character, while the 4 A treatment resulted in the formation of a duplex microstructure.

The change of γ′ phase morphology in quenched Ni-based superalloy strengthened by γ′ precipitates was investigated by means of scanning electron microscopy and X-ray diffraction. Lattice parameters of γ and γ′ phases and subsequently the δ parameter were calculated. Relatively high content of gamma prime formers resulted in a substantially high volume fraction of precipitates in the dendrite cores. The volume fraction of γ′ phase was significantly decreased due to dissolution of precipitates in the surrounding matrix, as a result of short-term subsolidus exposure. The lattice parameter of γ′ phase was higher than that of γ phase, which results in a positive δ parameter in fully heat treated (FHT) and also FHT+ Quench conditions. For (200) and (220) reflections the γ/γ′ misfit was lower in the FHT + Q sample.

In order to obtain optimal mechanical properties, the effect of heat treatment on the precipitation hardening in multicomponent Fe-based shape memory alloys (containing Ni, Co, Al, Ta, B) was studied. The polycrystalline material was investigated after application of three different processing schemas: slowly cooled, quenched and subsequently annealed with various aging conditions. The study was carried out using synchrotron X-ray diffraction along with mechanical tests, revealing the evolution of strengthening phases. As a result an optimum heat treatment for 10 h at 700°C was established yielding an optimal mechanical response.

Synthesis of a nanocomposite containing iron sulfides and sulfur was carried out in ethylene glycol. Spherical-shaped Fe 3 O 4 nanoparticles were used as the precursor. The structure of the FeS 2 /Fe 3 S 4 /S 8 nanocomposite, as well as the mechanism of formation, are described with X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy. Strong interaction between sulfur and oxygen was confirmed. Formation of the FeS 2 /Fe 3 S 4 /S 8 nanocomposite was associated with the reaction between Fe 3 O 4 and H 2 S, and the reaction between greigite and H 2 S produced by the decomposition of thioacetamide. Highly crystalline pyrite was formed in these reactions, while the sulfur and greigite appearing on the edges formed a highly disordered structure.

In this study the microstructure of nickel alloy (Inconel 625) surface layer after laser shock processing (LSP) was investigated by means of light microscopy, scanning electron microscopy and transmission electron microscopy. The LSP treatment was performed using a Q switched Nd:YAG laser. Optical and SEM images of the Inconel 625 after LSP showed that on the surface as well as on the cross-sections of the treated samples slip bands were clearly visible. Moreover, TEM micrographs revealed that under the thin melting layer of the treated material a high density of dislocations was present. The changes in microstructure come down even to 100 μm below the surface. The new and the most important finding of this study is that, in addition to slip bands, numerous cracks of hard carbide precipitates in the area affected by the laser process were visible both on the treated surface and on the cross-section of the material.