Antonín Dlouhý

Advanced scanning transmission stereo electron microscopy of structural and functional engineering materials.

Stereo transmission electron microscopy (TEM) provides a 3D impression of the microstructure in a thin TEM foil. It allows to perform depth and TEM foil thickness measurements and to decide whether a microstructural feature lies inside of a thin foil or on its surface. It allows appreciating the true three-dimensional nature of dislocation configurations. In the present study we first review some basic elements of classical stereo TEM. We then show how the method can be extended by working in the scanning transmission electron microscope (STEM) mode of a modern analytical 200 kV TEM equipped with a field emission gun (FEG TEM) and a high angle annular dark field (HAADF) detector. We combine two micrographs of a stereo pair into one anaglyph. When viewed with special colored glasses the anaglyph provides a direct and realistic 3D impression of the microstructure. Three examples are provided which demonstrate the potential of this extended stereo TEM technique: a single crystal Ni-base superalloy, a 9% Chromium tempered martensite ferritic steel and a NiTi shape memory alloy. We consider the effect of camera length, show how foil thicknesses can be measured, and discuss the depth of focus and surface effects.

On the determination of the volume fraction of Ni4Ti3 precipitates in binary Ni-rich NiTi shape memory alloys

Abstract The size distributions and volume fractions of Ni4Ti3 precipitates in Ni-rich Ni–Ti shape memory alloys affect their mechanical and functional properties. In the present work precipitate sizes and volume fractions were measured after various thermo-mechanical treatments using transmission electron microscopy (TEM) in combination with quantitative metallography. TEM tilting experiments revealed that the Ni4Ti3 particles have a disk-like shape with a disk diameter D and a disk thickness t. These two parameters together with the number density of particles per unit volume were determined after stress-free and stress-assisted aging at 400, 500 and 530 °C. Volume fraction results are presented after different thermo-mechanical treatments and our quantitative metallographic procedure is documented.

Dislocation slip and deformation twinning interplay during high temperature deformation in γ-TiAl base intermetallics

Abstract Interrupted compression creep tests were performed at 973 K and 350 MPa to study strain dependent microstructural changes in near-γ equiaxed Ti–48Al–2Cr–2Nb–B alloy. Dislocation densities and volume fraction of deformation twins were measured in the range of strain where sharp creep rate minima occur. The steep decrease of creep rate during initial 1% of strain is associated with a considerable increase in density of ordinary dislocations and superdislocations. Twinning processes are rather limited during this primary stage. On the other hand, in the strain range of minimum creep rate the volume fraction of deformation twins grows due to the increasing number of twinned grains and decreasing spacing between individual twins. Based on these results it is suggested that, in the pure- and near-γ TiAl equiaxed grain microstructures, the increase of creep rate after minimum and its new gradual saturation reflects a contribution of deformation twins to the creep strain accumulation kinetics. The contribution is twofold: (i) deformation twins relax incompatibility stresses set in during primary creep; and (ii) twinning supplies up to 30% of the overall strain in advanced stages of creep.

Interactions between particles and low-angle dislocation boundaries during high-temperature deformation

Abstract Stability and motion of low-angle dislocation boundaries in an array of particles is investigated. The 2D model considers discrete dislocations and circular precipitates with and without coherency stress fields. Dislocation – dislocation interactions, superimposed with precipitate stress fields and an externally applied stress, drive glide and climb of the dislocations. Results show that, in the case of a simple external shear loading, a single-valued critical applied shear stress exists which separates stable and unstable low-angle boundary configurations. This critical stress can differ considerably from the Orowan stress. More complicated applied stress states result in less clearly defined transitions between stable and unstable boundary regimes. Dislocation – dislocation interactions that stabilize low-angle dislocation boundaries thus may influence the high-temperature strength of particle-strengthened alloys.

3D Discrete Dislocation Modelling of High Temperature Plasticity

A 3D model is presented that addresses an evolution of flexible dislocation lines at high temperatures. The model is based on the linear theory of elasticity. A smooth dislocation line is approximated by a piecewise curve composed of short straight dislocation segments. Each dislocation segment is acted upon by a Peach-Koehler force due to a local stress field. All segment-segment interactions as well as an externally applied stress are considered. A segment mobility is proportional to the Peach-Koehler force, temperature-dependent factors control climb and glide motion of the segments. The potential of the model is demonstrated in simulations of simple high temperature processes including interactions of dislocations with secondary particles.

Vacuum Induction Melting and Investment Casting Technologies Tailored to Near-Gamma TiAl Alloys

The present study focuses on vacuum induction melting and investment casting of neargamma TiAl intermetallic alloys. The attention is mainly given to a cost-effective melting process in which a primary alloy ingot is re-melted in a ceramic crucible and cast into a ceramic shell mould. Two types of crucibles (based on Al2O3 and Y2O3) are considered. The most detrimental reactions that govern the contamination of the molten alloy with ceramic particles were determined. Results suggest that the crucible wall attack can be considerably limited by using either the Y2O3 (with no SiO2-type binder) or Al2O3 crucibles with a suitable coating. After pouring, a mechanical interaction associated with different thermal expansions of TiAl casts and ceramic shell moulds can result in serious product damage. A simple 1D-1D model of the cooling process was formulated and the heat flow as well as stress states in the cast-mould system were numerically solved. Process parameters (melt superheat, initial mould temperature, cooling kinetics and mould composition) were optimized in order to reduce the stress in the casts. The optimized parameters delimited a processing window in which complex-shaped TiAl castings like turbocharger wheels can be fabricated.

Evolution of microstructure during creep in gamma Ti–52Al at 1100 K and high applied stresses

Abstract Microstructural changes due to compression creep in gamma Ti–52Al phase at 1100 K and 250 MPa have been investigated using a conventional metallography and transmission electron microscopy. Three distinct microstructural types, the 3D dislocation network, subgrain boundaries and twinning, were found in individual grains after interrupted creep tests. The microstructures always consisted of three fundamental deformation modes: (i) ordinary dislocations, (ii) superdislocations, and (iii) deformation twins that contributed to the overall creep strain from the very beginning of creep. This microstructural evolution and the corresponding metal-type creep behaviour contrasts with the sharp creep rate minima and a lack of superdislocations and deformation twins reported earlier for specimens deformed at 1100 K and applied stresses below 200 MPa. The partitioning of the total strain among fundamental deformation modes is estimated and the link between sharp minima of creep rate and the insufficient activ...

Monotonous and cyclic creep in a heat-resistant 16Cr10W4MoTiAl nickel-based alloy—transmission electron microscopy study of microstructure

Abstract A transmission electron microscopy (TEM) study of the dislocation structure developed in a nickel-based 16Cr10W4MoTiAl heat-resistant alloy was performed. Specimens subjected to monotonic or cyclic creep at 1023 K were investigated. Two distinctly different arrangements of dislocations were found after monotonic creep. At low applied stresses, numerous dislocation loops around γ′ particles indicate that the motion of dislocations is strongly influenced by γ′ precipitates. In the range of high applied stresses, pronounced slip bands form which are composed of several closely spaced {111 slip planes. The cutting of γ′ particles by dislocations is observed frequently and the dislocation motion does not seem to be considerably hindered by precipitates. The distribution of dislocations after cyclic creep can be characterized as a mixture of the dislocation structures representative of low and high applied stress regions in monotonic creep. Two different dislocation arrangements are discussed in terms of the creep mechanism and the response of the alloy to cyclic creep loading.

Fatigue properties of high Nb TiAl alloy

Fatigue properties of the new generation of TiAl alloys with high Nb content are studied. Comparison with the previous alloy with 2at% Nb shows that the new alloy resists better to cyclic deformation at high temperatures. The microstructural observation proved that at 750 °C, the easiest deformation mode is the glide of ordinary dislocations followed by superdislocation glide and twinning. Nevertheless, all three modes seem to be active.

Stability and Motion of Low Angle Dislocation Boundaries in Precipitation Hardened Crystals

The present study investigates stability and motion of low angle dislocation boundaries in an array of precipitates. The model considers discrete dislocations and precipitates that are treated as impenetrable particles. Peach-Koehler forces, which originate due to the combined effect of dislocation-dislocation interactions and the applied stress, act the individual dislocations on. Both, the dislocation glide and the dislocation climb at elevated temperatures are taken into account. Results of the numerical study suggest that a critical applied shear stress (CASS) always exists which separates stable and unstable low angle boundary configurations. Varying particle size, interparticle spacing and density of dislocations in the boundary cause changes of the CASS that are systematically investigated. It is shown that the CASSs can considerably differ from the standard Orowan stress controlling the equilibrium of an isolated dislocation in a given microstructure. This result underlines the importance of long-range dislocation interactions that influence the high temperature strength of the precipitation-hardened alloys.