Michal Besterci

Influence of Al2O3 particles volume fraction on fracture mechanism in the Cu–Al2O3 system

Abstract The influence of Al 2 O 3 particles volume fraction on the fracture mechanism in the Cu–Al 2 O 3 system is analyzed by “in situ tensile test in SEM”. It is shown that tensile strain first produces microcavities on matrix–particle interfaces due to decohesion of particles from the matrix. The cavities grow and coalesce first in the direction of rows of particles with growing strain. Cracks develop in both directions: in the rows, and in the direction perpendicular to the rows, leading to final fracture. The deformation value at fracture is dependent on Al 2 O 3 particle volume fraction. A model explaining the fracture mechanism is introduced.

Dispersion-strengthened aluminium prepared by mechanical alloying

Mechanical alloying was developed for the production of Ni and Fe superalloys. The advantage of preparing amorphous materials by mechanical alloying in comparison with the technology of quick solidification is represented by the fact that it makes it possible to produce larger quantities of materials, and extends the possibilities for alloying. The technology of mechanical alloying is used for metals, ceramic and polymers, and considerable attention is paid to it throughout the world.

Creep behaviour of MoSi2–SiC and MoSi2–HfO2

Abstract Creep resistance of two MoSi 2 -based materials containing SiC and HfO 2 particles, respectively, in ambient atmosphere was studied in the temperature range 1100–1400°C under a load of 100 MPa. The microstructure and its response to high-temperature load were investigated by TEM using the thin foil technique. Comparison of the creep resistance of both materials at each particular testing temperature shows that the performance of MoSi 2 –HfO 2 is about one order of magnitude better than the other one.

Analysis of spatial arrangement of particles in thin foil of Al-Al4C3 material

TEM images of thin foils with quasi-globular particles are examined by means of two methods of spatial statistics. The spatial arrangement of particle reference points is described by means of quadrat count statistics and by polygonal method (the analysis of the Voronoi mosaic generated by patterns of particle reference points). A good agreement between the both approaches is found, the polygonal method is more sensitive and its results are more conclusive.

Influence of strain rate on fracture of dispersion strengthened Al-Al4C3 systems

Abstract In the presented work the change in fracture for the Al-Al 4 C 3 system was investigated and analyzed at temperatures from 20 to 450°C and strain rates from 2.5 10 −5 to 10 −1 s −1 . At room temperatures during tensile testing the strain is controlled by dislocation movement and reactions. The first part of the strain is characterized by work hardening expressed by the exponent “n”, the second part by the local strain in the neck. For high temperatures in the investigated region the principal mechanism keying the strain is the presence of dynamic recovery processes. Strain rate influences on fracture are analyzed.

Influence of SPD by ECAP on Cu Properties

Equal channel angular pressing (ECAP) is a material processing method for developing an ultrafine-grained (UFG) structure by introducing severe plastic deformation (SPD) in a bulk material with no changes in its cross-section. Numerous analytical and numerical studies on equal channel angular pressing have been performed in recent years. The present work focuses on the effects of die geometry width is defined by the angle between two channels Φ, angle on outer corner of die Ψ (or radius R) and angle within internal corner (or radius r) of die on average effective strain after one pass route. Next, there are analyses of strength properties, plastic properties, fracture mechanism, as well as analyses of Cu structure evolution after SPD by ECAP technology, in the paper. The sixteen passes through the ECAP matrix were realized using route C. The following experimental results and their analyses, the biggest increase of strength and microhardness was proved already after 4th pass. Valuation of fracture surfaces shows that after 12th pass plastic fracture is transformed from transcrystalline ductile mixed fracture. After 4th pass, the avarage grain size decreased from initial approximate size 7 µm to 200 nm, whereby the average grain size was changeless after subsequent deformations. Possible mechanism of high-angle boundary nanograins evolution consists of formation of cell structure, subgrains that transform with the increase of deformation into nanograins with big-angle misorientation.

Kinetics of Mechanical Alloying, Mechanical Properties of Micro and Nanostructural Al-C Systems

Abstract A method of mechanical alloying process is described. Carbon transformation to Al4C3 is characterized within the different heat treatment schedules and nine commercial carbon powders are tested. The micromechanism of carbon incorporation into the metallic powder, and its compacting are described. The influence of dispersed carbides on mechanical properties is evaluated together with the influence of deformation on microstructure and properties. It was proved that the transformation efficiency of carbon to Al4C3 by heat treatment of aluminium with the porous furnace black and electrographite is higher than that of the hard cracked graphite. Microstructure changes consisted of the fracture processes and welding of the particles with incorporation of C phase and forming of final granules. The dispersed phase Al4C3 particle size was measured on thin foil structures, and it was constant and as small as 30 nm. The particle size was influenced neither by the carbon type nor by the heat treatment technology applied. Subgrain size measured in the range of 100 grains in thin foils depended on the carbon type, as well. It ranged from 0.3 to 0.7 µm. Using a DSI (depth sensing indentation technique), the Martens hardness, indentation modulus E and deformation work W for Al matrix and Al4C3 particles have been measured. The temperature dependence of ductility, and reduction of area in the temperature range of 623–723 K and strain rate of 10−1 s−1, indicated a considerable increase of these properties. In a case when the volume fraction of Al4C3 changes from lower to higher, the grain rotation mechanism dominates instead of the grain boundary sliding. The comparison of the tensile test results and changes in fracture for the Al-Al4C3 system at two temperatures and two strain rates is summarized. The dependence of the minimum deflection rate on the applied force as well as the dependence of the time to fracture on the applied force for two temperature levels (623 and 723 K) by small punch testing are depicted. The composite was tested in two different states: a) as received by mechanical alloying with hot extrusion (HE) as the final operation and b) ECAPed (mean grain size of 100–200 nm). The dependence of the minimum deflection rate on the applied force as well as the dependence of the time to fracture on the applied force for two temperature levels are evaluated. The anisotropy of the creep properties and fracture using small punch tests for the Al-Al4C3 system produced by ECAP were analysed.

Shear Testing of Al and Al-Al4C3 Materials at Elevated Temperatures

The shear creep properties of the A1-A14C3 composite material were investigated for the first time at different temperatures ranging from 523 Κ to 773 Κ and different shear stress levels ranging from 25 to 40 MPa in comparison to pure aluminum. The specimens were loaded in pure shear using a specially designed patented specimen geometry and a prototype shear-testing machine. The experimental results indicate that the AI4C3 composite aluminum exhibits shear creep resistance more than four orders of magnitude higher than that of the unalloyed aluminum and can be loaded at shear stress levels exceeding two times those of the pure aluminum material. K e y w o r d s : Shear creep testing, Al, AI-AI4C3 composite, elevated temperature, F.E. analysis

Microstructure and Mechanical Properties of Al-Al4C3 Materials

Dispersion strengthened aluminum compacts have been prepared by powder metallurgy. The base microstructure is aluminum matrix strengthened with dispersed ceramic particles. The strengthening is direct by dislocation movement retardation, and indirect by deformation induced microstructure modification in the next technological steps. The method of mechanical alloying process is described. Carbon transformation to carbide Al4C3 is characterised for different heat treatment schedules and nine commercial carbon powders tested. The micromechanism of carbon incorporation into the metallic powder, and the compacting of it are described. The influence of dispersed carbides on mechanical properties is evaluated together with the influence of deformation on microstructure and properties. Ductility anomalies up to a type of superplasticity were observed at certain tensile testing strain rates.

Superplastic deformation of Al-Al4C3 composites prepared by powder metallurgy

The deformation of Al–Al4C3 composites with different volume fraction of Al4C3 phase was tested at different temperatures and different strain rates. It is shown that at temperatures between 400 and 450 °C and the highest strain rate applied 10−1 s−1, a significant ductility growth was observed. According to the results of TEM analysis, this behaviour is supposed to be the result of dynamic grain polygonization, grain slip and rotation, partial recrystallization and dislocation creep in the tested system, known as strain induced dynamic recovery. The increase of the volume fraction of secondary phase in the investigated composite changed the deformation mechanism from more slip on grain boundaries to more grain rotation.