Jozef Veselý

Effect of Thermomechical Pretreatment on Mechanical Properties of Modified Al-Mn-Fe-Si Based Alloys

Fins in automotive heat exchangers are manufactured from Al-Mn-Fe-Si foils. Since continuous twin-roll casting (TRC) reduces energy and work consumption, it is the preferred manufacturing technology. The precipitation processes resulting from annealing of TRC Al-Mn-Fe- Si based alloys were studied by resistometric measurements during linear heating from room temperature up to 620 °C, hardness measurements and transmission electron microscopy examinations. Primary particles and precipitates of the cubic α-Al15(Mn,Fe)3Si2, hexagonal Al8Fe2Si and orthorhombic Al6(Fe,Mn) phases were identified. Phases of different types prevail in different alloys depending on composition. The increase of cold rolling prestrain prior to annealing induces a significant shift to lower temperatures of the start of precipitation. Prestrain accelerates precipitation kinetics by redistributing solute atoms and favoring their segregation on nucleation sites such as dislocations, subgrain and grain boundaries.

Twin domain imaging in topological insulator Bi2Te3 and Bi2Se3 epitaxial thin films by scanning X-ray nanobeam microscopy and electron backscatter diffraction

Imaging with surface- and bulk-sensitive electron and X-ray diffraction based microscopic techniques enabled identification of the twin domain distribution of Bi2Te3 and Bi2Se3 thin films. Correlations between the surface topography and crystal orientation are established.

Crystallization dynamics and interface stability of strontium titanate thin films on silicon

Nonstoichiometric SrTiO3 thin films were fabricated by different thin-film deposition methods. The impact on the oxide/silcon interface stability as well as the crystallization onset temperature is investigated.

Nanocrystalline Al7075 + 1 wt % Zr Alloy Prepared Using Mechanical Milling and Spark Plasma Sintering

The microstructure, phase composition, and microhardness of both gas-atomized and mechanically milled powders of the Al7075 + 1 wt % Zr alloy were investigated. The gas-atomized powder exhibited a cellular microstructure (grain size of a few µm) with layers of intermetallic phases along the cell boundaries. Mechanical milling (400 revolutions per minute (RPM)/8 h) resulted in a grain size reduction to the nanocrystalline range (20 to 100 nm) along with the dissolution of the intermetallic phases. Milling led to an increase in the powder’s microhardness from 97 to 343 HV. Compacts prepared by spark plasma sintering (SPS) exhibited negligible porosity. The grain size of the originally gas-atomized material was retained, but the continuous layers of intermetallic phases were replaced by individual particles. Recrystallization led to a grain size increase to 365 nm in the SPS compact prepared from the originally milled powder. Small precipitates of the Al3Zr phase were observed in the SPS compacts, and they are believed to be responsible for the retainment of the sub-microcrystalline microstructure during SPS. A more intensive precipitation in this SPS compact can be attributed to a faster diffusion due to a high density of dislocations and grain boundaries in the milled powder.

Evolution of Microstructure and Microhardness in Ti-15Mo β-Ti Alloy Prepared by High Pressure Torsion

The main aim of this study is to analyze the effect of the severe plastic deformation (SPD) on the mechanical properties and defect structure of metastable beta Ti alloys. Experiments were performed on two different β-Ti alloys: Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al which were subjected to severe plastic deformation (SPD) by high pressure torsion (HPT). The increase of hardness with increasing equivalent strain was determined by microhardness mapping. Dislocation density was studied by advanced techniques of positron annihilation spectroscopy (PAS). Microhardness and dislocation density increases with increasing equivalent strain inserted by severe plastic deformation.

Quantitative numerical method for analysing slip traces observed by AFM

Atomic force microscopy (AFM) is used more and more routinely to study, at the nanometre scale, the slip traces produced on the surface of deformed crystalline materials. Taking full advantage of the quantitative height data of the slip traces, which can be extracted from these observations, requires however an adequate and robust processing of the images. In this paper an original method is presented, which allows the fitting of AFM scan-lines with a specific parameterized step function without any averaging treatment of the original data. This yields a quantitative and full description of the changes in step shape along the slip trace. The strength of the proposed method is established on several typical examples met in plasticity by analysing nano-scale structures formed on the sample surface by emerging dislocations.

Evaluation of anisotropic small-angle neutron scattering data from metastable β-Ti alloy

ABSTRACT Small-angle neutron scattering (SANS) data from single-crystal metastable β-Ti alloys exhibit an anisotropic character with interparticle interference maxima due to ordering of the dense system of ω particles. For an evaluation of such data, the program NOC (previously used for evaluation of data from dense ordered γ′ precipitate system in single-crystal nickel-based superalloys) was well suited. Nevertheless, an improvement of this evaluation program was necessary in its model-forming part. A further change of the evaluation program concerned the mode in which the size distribution was calculated. This upgrade is presented. The improved NOC program was employed for the evaluation of SANS data of annealed metastable single-crystal β-Ti alloy containing ω particles. 3D fit in reciprocal space was successfully used. A model of ellipsoids did not lead to a better fit than a model of particles with the spherical shape. The microstructural parameters of ω particles were determined.

The Influence of Milling and Spark Plasma Sintering on the Microstructure and Properties of the Al7075 Alloy

The compact samples of an Al7075 alloy were prepared by a combination of gas atomization, high energy milling, and spark plasma sintering. The predominantly cellular morphology observed in gas atomized powder particles was completely changed by mechanical milling. The continuous-like intermetallic phases present along intercellular boundaries were destroyed; nevertheless, a small amount of Mg(Zn,Cu,Al)2 phase was observed also in the milled powder. Milling resulted in a severe plastic deformation of the material and led to a reduction of grain size from several µm into the nanocrystalline region. The combination of these microstructural characteristics resulted in abnormally high microhardness values exceeding 300 HV. Consolidation through spark plasma sintering (SPS) resulted in bulk samples with negligible porosity. The heat exposition during SPS led to precipitation of intermetallic phases from the non-equilibrium microstructure of both gas atomized and milled powders. SPS of the milled powder resulted in a recrystallization of the severely deformed structure. An ultra-fine grained structure (grain size close to 500 nm) with grains divided primarily by high-angle boundaries was formed. A simultaneous release of stored deformation energy and an increase in the grain size caused a drop of microhardness to values close to 150 HV. This value was retained even after annealing at 425 °C.

Observations of Dislocations and Other Results

The main part of this chapter are the reports of AFM observations of the slip traces and TEM observations of corresponding dislocation structures. First, however, parameters and results of sample deformation as well as TEM characterisation of order, antiphase domains and carbide particles are presented.

Iron Rich Iron-Aluminides

Over the 80 years of research on iron-aluminides immense amount of literature was produced. This chapter summarizes some of the relevant prior work.