B. Formanek

Influence of coatings obtained by PVD on the properties of aircraft compressor blades

Abstract In this paper, the results of investigations to increase aircraft compressor blade durability were presented. The blades were made of the martensitic steels. The coatings were obtained by the physical vapour deposition (PVD) method. TiN and (Ti, Al)N coatings on a titanium nitride base and CrN and CrCrN multilayer coatings on a chromium nitride base were used in the tests. Low corrosion resistance of the TiN coatings was shown in laboratory tests. The (Ti,Al)N coatings, particularly CrN and CrN modifications, behaved better. Coating density and chemical interaction of the coating and steel in the corrosive medium were shown to have an influence on the blade properties. A significant increase in the corrosion resistance was achieved by using the diffusion and chemical treatment of coatings obtained by the PVD method. Investigations of coatings in a humid atmosphere containing SO2 showed very low resistance of the steel. The resistance of the coatings was increased significantly by additional chemical treatment.

Erosion- and corrosion-resistant coatings for aircraft compressor blades

Abstract In this paper the comparative test results of protective properties of several coatings for aircraft engine compressor blades are presented. Erosion and corrosion tests for the base metals and coatings were carried out. Corrosion test for base materials and anodic coating was made in neutral salt spray. For cathodic coatings a modified corrosion test was performed. It has been determined that the coatings obtained by physical vapour deposition (PVD), particularly (Ti, Al) N and TiN(Pt) are the best in the erosion and corrosion test conditions. The electroplated Ni-Cd coatings do not show satisfactory erosion resistance, but they have good corrosion resistance. It seems that PVD coatings fulfil the requirements for the protection of aero engine compressor blades.

Mathematical Modeling of Solid-State Diffusion during Mechanical Alloying

It is known experimentally that solid-state interdiffusion is substantially enhanced during plastic deformation. This is especially noticeable in Mechanical Alloying (MA) which is used for producing a wide range of metastable materials (supersaturated solid solutions, amorphous phases, nanostructures) with unique properties. However, a physical mechanism of enhanced diffusion during MA is not clearly understood yet, and a comprehensive model of this complex phenomenon has not been developed so far. Moreover, the role of the diffusion process in MA is hotly debated in literature. In this work a new, self-consistent mathematical model of solid-state interdiffusion in a binary substitutional system A-B during periodic plastic deformation is developed. The model includes basic physical factors that affect diffusion, such as generation of non-equilibrium point defects by gliding screw dislocations during deformation and their relaxation in periods between impacts. The cross-link terms are considered, and interaction of point defects with edge dislocations and incoherent phase boundary A/B is taken into account. Computer simulation is performed using realistic data (e.g., quasi-equilibrium self-diffusion coefficients known in literature) and the process parameters typical of MA in a vibratory mill. A repeated “deformation-rest” cycle is considered. The results of modeling reveal the physical mechanism of the enhancement of solid-state diffusion by periodic plastic deformation during MA and demonstrate that within the frame of this approach supersaturated solid solutions can form within a reasonably short processing time.

Influence of Al–Al2O3 composite powder on the matrix microstructure in composite casts

Abstract The structural characterization is presented of composite casts with an AlMg4 alloy matrix, with the composite powder (Al–Al2O3)cp reinforcement obtained in a rotary-vibration mill. On the cross-sections of ingots, metallographic specimens have been made and, by means of quantitative metallographic methods, the grain size distributions have been determined. Statistical tests of the results obtained from the grain plane section area measurements have been performed. An influence has been found of the (Al–Al2O3)cp composite powder on the matrix grain size distribution, depending on the distribution of the composite powder particles in the ingot, whereas no influence of (Al–Al2O3)cp on the mean grain size of the AlMg4 alloy has been found.

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.

Corrosion damage and regeneration of aluminide coatings on aircraft turbine blades

Abstract The corrosion-induced microstructure changes associated with the successive stages of damage of the protective layers on nickel-base alloys have been studied up to complete layer penetration. The microanalytical investigations performed on the distribution of the alloying elements show increased contents of chromium and sulphur in the corrosion attack front zone. However, the microstructure of the diffusion zone, enriched with molybdenum, tungsten and chromium, remains practically unchanged. Attempts at regeneration of the corrosion-worn protective layers of the blades by removing the corrosion products and remnants of the coating with a subsequent new layer deposition have been also described. Two methods of blade recoating are presented in the paper: 1. (a) chemical-vapour-deposited chromoaluminizing either in low- or high-activity processes with the analysis of the effect of processing parameters on the layer microstructure and properties and 2. (b) diffusion annealing of the sprayed silicoaluminizing suspension. The purpose of regeneration is to regain the initial properties of the blades after a period of service. Some of blades formerly rejected for their inadequate structure or defects in the protective layer would be also regenerated in this way. The criteria of surface quality assessment and verification of the blades in successive stages of surface coating regeneration have been worked out.

Microstructure wear resistance and erosion resistance of plasma-sprayed boride coatings

Abstract The paper presents both material and technological concepts of coatings containing selected borides (FeB, CrB2, (TiCr)B2) and carbides (B4C, B4C−CrB). The selected borides and powder compositions were prepared by diffusion synthesis (chemically) and then were electrolytically covered with a nickel layer. The plasma spray parameters were determined. X-ray analysis was carried out to investigated the chemical compositions of the coatings. From the X-ray analysis results, we determined the phase transformations in the coatings after plasma spraying. Both the wear and erosion resistances of the coatings obtained were determined and compared with those of WC–Co, NiCrSiB and 45 VFNS (Metco) coatings. The results obtained in the tribological tests were applied to a selection of highly wear-resistant boride coatings.

On the Physicochemical Mechanism of the Influence of Preliminary Mechanical Activation on Self-Propagating High-Temperature Synthesis

The analysis of physicochemical mechanism of the influence of mechanical activation (MA) of a charge mixture on the subsequent self-propagating high-temperature synthesis (SHS) of intermetallic compounds is performed. Numerical estimates have revealed an insignificant role of the energy stored in solid reactants due to cold work during MA. The characteristic time of relaxation of non-equilibrium vacancies, which were generated in metals by MA, during heating in the SHS wave is estimated, and their insignificant influence on the reaction kinetics at high temperatures is demonstrated. It is shown that a strong effect of preliminary MA on SHS can be attributed only to deformation-enhanced solid-state diffusion during MA, which can lead to the formation of a supersaturated solid solution and thus affect the conditions for nucleation of a product phase (intermetallic compound) upon heating.

Mössbauer and XRD Studies on Composite Powder with Phases from Fe-Al System Obtained by Mechanically Activated SHS Mehtod

The systematic studies of non-stoichiometric phases from Fe-Al system obtained by selfpropagating high-temperature synthesis (SHS) were the aim of the present work. Investigations were performed using 57Fe transmission Mössbauer spectroscopy. The presence of nonsteichiomertic FeAl, FeAl2 and Fe2Al5 phases was found. It is can be concluded that the studied powder material is in non-equilibrium state. As the result of milling process the contents of FeAl phase increase whereas of FeAl2 and Fe2Al5 ones decrease. The determined crystal structure parameters and the hyperfine interaction parameters are presented and discussed.

Microstructure and resistance to cracking of modified Al-Si and Al-Cr diffusion coatings on ŻS6K nickel-based superalloys

Abstract The subject of the investigations was a superalloy with the following chemical composition: 0.14 wt.% C, 10.2 wt.% Cr, 5.3 wt.% Al, 2.8 wt.% Ti, 3.7 wt.% Mo, 5.2 wt.% W, 4.7 wt.% Co, 1.5 wt.% Fe and the balance nickel. A pack cementation technique of coating was applied. Acoustic emission signals were recorded as a function of specimen elongation (Σ N - e ). The values of elongation ( e 0 ) at the moment of the first crack appearance and at the beginning of rapid fracture were measured. The crack resistances of the aluminide, silicoatuminide and chromoaluminide diffusion coatings were the main subject of the studies performed. If we take the e 0 parameter as a crack resistance criterion it was found that, when compared with the aluminide coatings, Al-Si and Al-Cr coatings are less resistant. The crack morphologies of aluminium and Al-Cr coatings were similar in character. Among the typical features were cracks that did not spread through the entire layer thickness and discontinuous cracks with branches. In the depleted zone of the coatings, cracks developed along the NiAl phase grain boundaries.