Volume 101, Issue 5

In order to clarify grain refinement mechanisms and optimize possible potent refiners in Mg and TiAl alloys, a simulation method for heterogeneous nucleation based on the free growth model by Greer is applied. The predictions of the model were improved by modifications taking into account violations of a perfect model system: Imperfections of inoculants and breakdown of stable spherical grain growth. The model predictions are compared with results from casting experiments and show a distinct improvement.

Polycrystalline platinum-based alloys show very good creep properties at high temperatures. Small amounts of boron (less than 1 at.%) considerably increase the creep strength. Transmission electron microscopy measurements were conducted to localize the element boron in the samples. With image electron energy loss spectroscopy spectra were extracted revealing an enrichment of B at grain boundaries. The boron distribution was plotted with the three window method.

Convergent beam electron diffraction is a well known technique for measuring local lattice parameters with high accuracy. In this work we investigate the possibility of applying convergent beam electron diffraction to a Pt-based superalloy. The measurements were made at room temperature and at – 170 °C along the [9 2 27] zone axis. It is shown that despite strong dynamic effects and inelastic scattering, the lattice parameter in this orientation can be determined with acceptable accuracy of 0.1 – 0.3 pm.

A Pt–Al–Cr–Ni–Re alloy is investigated in terms of microstructure and mechanical properties. This Pt-base alloy possesses a matrix/γ′-microstructure known from the nickel-base superalloys. Mechanical properties of the individual phases were investigated by means of nanoindentation. Hardness and modulus of elasticity for γ′ precipitates of this Pt-base alloy are of the same order of magnitude as for the single crystal Ni-base superalloys PWA 1484 and CMSX-4.

The shape of X-ray reflections from the cuboidal /′-microstructure was investigated. The measurements were performed on the 4 th generation single-crystal nickel-base superalloy TMS138. It is shown that reflections from non-cubic crystallographic planes split not only in d-scale due to the different spacing of the - and ′-lattices but also in the pole figure due to the tilting of -lattice planes. This tilting results from the elastic distortion of the -lattice caused by the /′-misfit. The results obtained are discussed under the methodical aspect of misfit measurement.

The influence of alloying elements, including rhenium and ruthenium, on the physicochemical and structural parameters of single crystal nickel-base superalloys has been analyzed. The results were used in computer designing of a single crystal nickel-base superalloy containing rhenium and ruthenium. The optimized alloy composition, providing the best physicochemical properties and longest creep lifetime, contains 6 wt.% rhenium and 4 wt.% ruthenium. <001> single crystals of the designed superalloy were directionally solidified and investigated in as-cast, heat treated and creep deformed conditions. The investigations included: characterization of the superalloy microstructure, dendritic segregation of alloying elements, their partition between the γ- and γ′-phases etc. Creep rupture tests were performed in the temperature interval of 900 – 1 100 °C and included tests longer 1 000 h.

Knowledge of the crystallization sequence of metallic glasses can yield a deep insight into the origin of their thermal stability. Crystallization of metallic glasses often leads to the simultaneous formation of different metastable crystalline phases that makes their identification difficult. In order to separate formation of crystalline phases from each other, long term isothermal pre-annealing below the glass transition temperature of Zr 46 .8Ti 8 .2Cu 7 .5Ni 10 Be 27 .5 bulk metallic glass prior to crystallization was performed. It is found that this pre-annealing strongly influences the crystallization sequence and depending on the pre-annealing time leads to preferential formation of different phases. In addition to the well known intermetallic Be 2 Zr phase and a quasicrystalline phase, three further previously unknown crystalline phases are identified and described in terms of their structure and composition. The formation of the crystalline phases is discussed with respect to the glass forming ability and thermal stability of the glass phase.

Diffusion brazing is a widely-used technology for the repair of cracks in hot section turbine components, mostly fabricated from nickel-based superalloys. However, the filling of wide cracks in the range of 100–300 μm is difficult since the precipitation of brittle secondary phases, which are formed by the conventionally used melting point depressants B and Si, leads to deteriorating mechanical properties. Therefore, new Ni – Mn-based braze alloys were developed which allow a very fast epitaxial healing of particularly wide cracks in single-crystalline components. As B and Si are replaced by Mn, the repair process can be significantly shortened since the epitaxial solidification is not completely controlled by diffusion but can also be controlled by cooling. Ni – Mn-based systems enhanced by Al, Cr and Ti were investigated. In this work an improved brazing cycle for the minimization of porosity within the braze gap as well as an enhanced heat treatment, which produces a γ/γ′ microstructure very similar to the parent material, are presented. Results from tensile tests at room temperature and at 900 °C conducted on 300 μm gap width samples are discussed.

The influence of microstructure on the stress–strain behavior of an Ni-rich NiTi shape memory alloy is examined. Specimens cut from a large-diameter bar of Ni 50.7 Ti 49.3 shape memory alloy were analyzed in two states: (i) annealed and (ii) annealed and aged. The annealed state shows a fully austenitic structure with no precipitates and no distortions caused by residual stresses. The annealed and aged state has coherent Ni 4 Ti 3 particles precipitated in the proximity of the austenite grain boundaries. The size of the precipitates increases moving away from the grain boundaries toward the grain interiors. The evolution of the two states in the stress–strain–temperature space has been analyzed using tensile specimens with special geometry. Due to the complex effects of the coherent precipitates, the specimens in the aged state exhibited lower stress plateaus in the tensile loading–unloading curves, which enabled the occurrence of transformation pseudoelasticity from room temperature to 333 K.

The microstructural changes of a Hastelloy alloy under 10 MeV electron irradiation at 650 °C for 700 h to a total fluence of 2 × 10 – 3 dpa (displacements per atom) were investigated by transmission electron microscopy and by 3-dimensional atom-probe tomography utilizing a local-electrode atom-probe tomograph. Transmission electron microscopy analysis before irradiation reveals two types of precipitates: (i) spheroidal with a mean diameter of 22 nm; and (ii) ones with an irregular morphology of a few nm in diameter. The spheroidal precipitates (i) are coherent with the fcc matrix and their number density is ∼3 × 10 21 m – 3 . Electron diffraction patterns from these precipitates exhibit superlattice reflections corresponding to the L1 2 ordered structure. The chemical composition as measured by atom-probe tomography, is about 75 at.% Ni with additions of Al, Ti and Mo. The other type of precipitate (ii) is mainly enriched in Ni and Mo and its number density is ca. 3.6 × 10 22 m – 3 . The diffuse diffraction intensity maxima arising from these precipitates are observed at <1 1/2 0> fcc , which are attributed to compositional short-range order. After electron irradiation, the Ni – Al-rich precipitates (i) exhibiting L1 2 superlattice reflections have disappeared completely, while the diffuse intensity maxima from the Ni – Mo rich clusters (ii) are still visible. The results are discussed with respect to the influence of the electron irradiation on the morphology and structural changes of the ordered precipitates.

TiAl-based intermetallic layers with additions of Ag, Cr, Y and Hf were deposited on γ-TiAl specimens using magnetron sputtering. The oxidation behaviour of the coated Ti-45Al-8Nb (at.%) alloy was investigated at 850 °C and 900 °C under cyclic oxidation conditions in air. The ternary Ti-50Al-2Ag (at.%) coating did not provide oxidation protection to the substrate material at 850 °C. Two-phase γ-TiAl + Ti(Cr, Al) 2 Laves coatings exhibited high oxidation resistance at 900 °C for up to 1000 cycles of 1 h dwell time at high temperature. During thermal exposure phase dissolution and transformation occurred. The slow oxide growth rates observed for the Ti – Al – Cr intermetallic layers were associated with the formation of a thin continuous alumina top scale established by the Laves, B2 and Z-phases. Small additions of hafnium and yttrium improved the oxidation resistance. In contrast, Ti – Al – Cr – Ag coatings exhibited poor oxidation behaviour at 900 °C.

CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl substrate material using high power impulse magnetron sputtering technology. The nitride coating provided effective oxidation protection to γ-TiAl at 850 °C for exposure time periods exceeding 2 000 cycles of 1 h dwell time at high temperature in air. High oxidation resistance was also observed at 900 °C. After exposure to air at 850 °C for 300 h, coated tension specimens exhibited a reduction in fatigue strength of about 70 MPa compared to the bare γ-TiAl alloy. Zirconia topcoats produced by electron-beam physical vapour deposition were well adherent to the CrAlYN/CrN coating with an oxy-nitride overcoat. When thermally cycled at 900 °C, the lifetime of this thermal barrier coating system on γ-TiAl exceeded 1 000 1 h cycles. Below cracks in the nitride coating, the substrate was oxidised and protrusions of an outer oxide scale with columnar structure formed.

The stress corrosion cracking behaviour of 6061 and 6013 sheet materials was investigated in naturally aged and peak-aged heat treatment conditions. Constant deformation, constant load and constant extension rate tests were carried out using aqueous solutions of 0.6 M NaCl and 0.6 M NaCl + 0.06 M NaHCO 3 . The microstructure of the alloys was examined using transmission electron microscopy. Potentiodynamic polarisation measurements were performed in aerated and deaerated aqueous chloride and/or bicarbonate solutions to determine corrosion and breakdown potentials. Under constant deformation and constant load conditions, the alloys 6061 and 6013 stressed in transverse direction were resistant to stress corrosion cracking in both T4 and T6 tempers. The constant extension rate testing technique indicated environment-induced cracking with alloy 6061-T4, whereas deterioration in ductility of the other alloys was caused by pitting and intergranular corrosion. Sensitivity to intergranular corrosion did not cause stress corrosion cracking in 6013-T6 sheet.

The unique nanostructure formed during severe as well as moderate braking on the surface of brake discs was investigated by conventional and analytical Transmission Electron Microscopy. In both cases nanocrystalline magnetite mixed with carbon nanoinclusions and minor amounts of other pad constituents were identified. On the basis of these observations the friction performance of a single micro-contact was simulated with the method of Movable Cellular Automata. Inspite of a simplified nanostructure which was examined in two dimensions only, the calculated mean coefficient of friction fitted well to the value usually demanded for automotive braking. Furthermore, the model predicts that oxide films without soft nanoinclusions are not capable of providing smooth velocity accommodation at the pad–disc interface and thus lead to unstable friction behaviour.

Al-rich Ti – Al alloys, as compared to Ti-rich -TiAl-based alloys, offer an additional reduction in density of 20 %, better oxidation resistance and sufficient strength at high temperatures. High temperature creep of a binary Al 62 Ti 28 alloy was studied in compression in the temperature range between 1 173 K and 1 323 K in air. It is shown that the alloy exhibits quite reasonable creep resistance at 1 173 K, especially in view of its low density of around 3.8 g cm – 3 . Stress exponents calculated as the slope n = log (strain rate)/ log (stress) = 4 were found to be relatively constant for the temperature and stress regime investigated. This indicates that dislocation climb may be the rate controlling creep mechanism. The values of the activation energies for creep for the as-cast and the annealed Al 62 Ti 38 material coincides well with those found in the literature for interdiffusion of Al in -TiAl.

Edge dislocation segments bowing out of dislocation walls of persistent slip bands (PSBs) during cycling are experimentally well documented. Instead of dealing with the problem in its complexity a simplified computer simulation of this process is presented. The bowing out segment interacts with the wall of the PSB. The walls are represented by the elastic field of a cluster of edge dipolar loops. Mutual interactions between the loops in the cluster and an interaction between the loop cluster and the bowing out dislocation segments are incorporated in the simulation procedure. The dislocation segments deposited at the walls are trapped in the elastic potential valleys produced by the dipole loops. Our aim is to determine the local shear stress needed to bow out fully the edge dislocation segment from the PSB wall. The obtained result is in reasonable agreement with the local shear stress at the wall deduced from measurements.

Hot rolled AISI-4012 sheet steels have been heat treated to obtain a ductile dual phase microstructure. The specimens were heat treated in the ferrite (α) and austenite (γ) phase region at temperatures of 740, 760, 790 and 820 °C and corresponding time intervals of 15, 30, 60 and 90 min and then quenched into water. Hardness measurements, tensile, bending, and impact tests were performed in order to determine the mechanical properties of the dual phase steel. These were examined with respect to the martensite volume fraction. Both the flow and tensile stresses increased whereas elongation decreased with increasing dual phase treatment temperature and martensite volume fraction. The optimum mechanical properties and the appropriate dual-phase microstructure are achieved at 740 and 760 °C annealing with subsequent water quenching. From examinations it was deduced that ductile fracture occurred in the tested samples.

Friction stir welding (FSW) can be used to join dissimilar Al alloys which have different mechanical properties without causing any weld zone defects under a wide range of welding conditions. In this study, AA2024 and AA5052 aluminum alloys were successfully joined together using FSW. FSW process parameters such as tool rotational speed and traverse speed were varied to explore their effects on the microstructural and mechanical properties of welded couples. Tension and fatigue tests were carried out and hardness measurements were conducted on cross-sections of the weld zones. Fatigue tests were performed using a fatigue test machine and the fatigue strength was analyzed by drawing S–N curves and critically observing fatigue fracture surfaces of the tested samples. The microstructures of the alloys and fracture surfaces were examined by optical and scanning electron microscopy. Experimental results indicate that microstructural and mechanical properties are significantly affected by changing welding parameters within the chosen range of conditions.