Jacek Krawczyk

Growth morphology of single-crystal grains obtained by directional crystallisation of an Al–Cu–Fe alloy

Quasicrystalline as well as crystalline faceted single grains of four phases were obtained during directional crystallisation of an Al–Cu–Fe alloy by the Bridgman technique. The monoclinic λ phase, Al13(Cu, Fe)4, dominating at high temperatures formed single-crystal lamellae 0.5 mm to 1 mm thick. A second type of attractive morphological form exhibiting flux dissolution terraces was observed on spherical single crystals of β phase Al(Fe, Cu). Rectangular, hexagonal and octagonal shaped dissolution terraces were revealed at the positions of two-, three- and four-fold symmetry axes, respectively. A single quasicrystalline ψ phase, Al6Cu2Fe, exhibited icosahedral symmetry with growth forms of a dodecahedron with pentagonal facets. The flux dissolution of the β phase apparently plays an essential role in a peritectic reaction leading to quasicrystalline ψ phase formation. Polygonal single grains of ω phase Al7Cu2Fe exhibiting tetragonal symmetry formed the fourth type of thermodynamically stable growth forms. Single grains of the ω phase crystallised in the form of pellets with an octagonal cross-section. The growth morphology of the stable phases was investigated by scanning electron microscopy. The chemical composition of the growth forms described was confirmed by X-ray microanalysis using a scanning electron microscope, whereas the phase composition was determined using electron selected area diffraction and X-ray powder diffraction.

Microstructure of a composite with a quasicrystalline phase fraction obtained by directional solidification of Al61Cu27Fe12 alloy

The microstructure of a composite containing a quasicrystal phase, i.e. so-called crystal–quasicrystal (CQ) composite, was studied. The CQ composite was obtained by the Bridgman method via solidification of Al61Cu27Fe12 alloy (numbers indicate at%). The process was conducted at a pull out rate of v = 0.07 mm/min. The average temperature gradient in the heating zone was 43 K/cm. The composite matrix consisted of cubic β phase Al(Fe, Cu), with reinforcement of λ-phase rod-shaped fibres surrounded by a quasicrystal icosahedral ψ phase, which also existed in the fibre core. The fibres were rhomboidal in cross-section. The composite was studied using X-ray and electron diffraction, light-optical and scanning electron microscopy (SEM), X-ray topography and Laue diffraction.

Structural Defects of Initial Crystallization Areas in Single-Crystalline Turbine Blades

The as-cast single-crystalline turbine blades made of CMSX-4 superalloy were studied. The blades were obtained by the Bridgman technique at a withdrawal rate of 5mm/min. The as-cast samples were prepared by cutting the blade root with a fragment of selector. The dendritic structure of obtained samples was studied by Scanning Electron Microscopy. The crystal orientation and lattice parameters were analyzed by Ω-scan mapping method. Additionally, the X-ray diffraction topography was applied. It was found that the most structural defects are created in the areas where a change in the shape and dimensions of the blades occur. Even minor changes in their geometry and the unevenness of mold walls may also affect the formation of defects.

X Ray Topography Study of Deformed Crystal-Quasicrystal Composites of Al Cu Fe Alloy

Crystal-quasicrystal columnar composites (CQ composites) of Al Cu Fe alloy subject to tensile strain at room temperature were studied by the reflective X ray topography. In those composites singlecrystalline β phase occurred as the matrix and rods consisting of quasicrystalline ψ phase and crystalline λ phase as the reinforcement. X-ray topograms of plate like composite specimens were obtained using reflexes of singlecrystalline matrix. It has been found that at strains ε up to 0.33% in a linear range of stress strain σ(ε) relationship the topograms remain unchanged. Instead, at ε higher than 0.33% significant contract changes have been observed in some areas of topograms. It has been determined that at ε higher than 0.33% the components of misorientation angle of those areas increase with increasing strain. Methods of powder phase analysis, metallographic examinations by means of optical microscopy methods and X-ray diffraction topography methods were used.

Crystallization of CoSi2-Si Eutectic Composites

The subject of the study is the composite material of CoSi2-Si eutectic composition. The influence of crystallization speed on microstructure was examined. The ingots were obtained using Bridgman method. Three different pulling down speeds of crucibles with melted charges were applied. The prepared samples were subjected to metallographic, X-ray, TEM techniques and mechanical examinations in order to characterize the properties of the studied composite material. It was shown that the favourable crucible pulling down speed to obtain CoSi2-Si fibrous eutectic microstructure was 3mm/h.

Influence of Heat Treatment on Defect Structures in Single-Crystalline Blade Roots Studied by X-ray Topography and Positron Annihilation Lifetime Spectroscopy

Single-crystalline superalloy CMSX-4 is studied in the as-cast state and after heat treatment, with material being taken from turbine blade castings. The effect of the heat treatment on the defect structure of the root area near the selector/root connection is emphasized. Multiscale analysis is performed to correlate results obtained by X-ray topography and positron annihilation lifetime spectroscopy (PALS). Electron microscopy observations were also carried out to characterize the inhomogeneity in dendritic structure. The X-ray topography was used to compare defects of the misorientation nature, occurring in as-cast and treated states. The type and concentration of defects before and after heat treatment in different root areas were determined using the PALS method, which enables voids, mono-vacancies, and dislocations to be taken into account. In this way, differences in the concentration of defects caused by heat treatment are rationalized.

Morphology of Tungsten Growth Forms in Quasicrystalline Al65Cu20Fe14W Alloys

The growth morphology and arrangement of tungsten and β-phase (Al (Fe, Cu)) in Al‑Cu‑Fe‑W alloy were analyzed. The composition of Al65Cu20Fe14W1 (at.%) was used for preparation of ingots containing the icosahedral quasicrystalline phase.The ingots were obtained in a two-stage process. At the first stage the induction melting of the elements and preliminary homogenization by mechanical mixing were carried out. The second stage was realized by vertical Bridgman technique of directional crystallization. The X-ray phase analysis, optical and scanning electron microscopy (SEM) observations and chemical analysis were performed. It was stated that the whiskers of tungsten were irregularly distributed in the volume of ingots. They often form a clump-like agglomerations. The whiskers have different diameters and length. Minimal diameter of the whiskers was about 10 – 100 nm and maximal – several dozen micrometers. The whiskers of tungsten formed a clump-like frames on which the membrane form of β-phase was stretched out or a non-planar oval forms.

The Microstructure of Quasicrystalline Al-Cu-Fe Alloys Doped with Tungsten

The microstructure of Al65Cu20Fe14 (numbers indicate at.%) alloy doped with 1 at.% of W was studied. The selected alloy composition should allow to obtain the quasicrystalline icosahedral phase after solidification process. The bulk samples were obtained in two stages. At first, the synthesis of alloy through premelting of component elements in induction furnace and then, the directional solidification by the Bridgman method were performed. The morphology of selected areas of the samples were studied using Scanning Electron Microscope equipped with energy dispersive X-ray spectroscope, which was used to examine chemical compositions of each analysed areas. Additionally the X-ray powder diffraction was used to identify the phases present in the alloys. It was stated that the filaments of tungsten were present in the alloys. The filaments have thickness ranged from 0.01 to 2.5 μm. As a result of investigation, the arrangement of filaments in the material was determined.

Solidification Rate Influence on Microstructure of Composites with Quasicrystal Phase Fraction

The composites of Al61Cu27Fe12 alloys containing quasicrystalline and crystalline phases (CQ composites) were obtained by the Bridgman method. The morphology of composites crystallized with different solidification rates was studied. The solidification rate influence on fibrous reinforcement morphology was analyzed. The microsections for analysis were prepared parallel and perpendicular to the direction of crystallization. The optical and the scanning electron microscope were used for metallographic observation. Obtained composites were examined by X-ray powder diffraction and reflective X-ray topography. The Laue method enabled to conclude that the matrix is singlecrystalline. The different level of structural perfection of reinforcement fibres was presented at various solidification rates.

The Fracture Morphology of Composites with Quasicrystalline Phase

The composites of Al-Cu-Fe alloys with fibrous reinforcement, containing quasicrystals, e.i. crystal-quasicrystal composites (CQ composites) were studied. The composites were obtained in situ by the Bridgman method. The plate-like samples were studied by tensile test performed up to rupture. Tensile fracture surface of composite and reinforcement fibers were investigated. The role of voids on the fracture morphology and the reason of its formation were discussed.