Katarína Sülleiová

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 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.

Grain Size Estimation in Anisotropic Materials

The estimation of grain size (volume) by using computer database of tessellations is explained and demonstrated. As model material was selected an anisotropic material formed by compression-moulded pellets of PVC. The pellets were first covered with carbon paste to highlight borders of grains in final specimen and then moulded; their volumes were exactly known. Standard profile and intercept counts were carried out on the planar sections of the specimen and used to estimate the grain size by means of computer database. The obtained estimates were then compared with the known pellet characteristics. The estimation of grain volume and of other suitable characteristics by inspection of section planes is necessary in opaque materials (e.g. metal alloys, crystalline polymers, ceramics), where their values are inaccessible by direct measurements. The aim of this work is show that the standard approaches can be improved with the help of a suitably prepared computer database.

Influence of technological factors on dispersion strengthened materials deformation mechanism studied by ‘in-situ tensile test in SEM’

Direct observation of deformation process until formation of fracture on extreme small samples of composite in SEM is interesting from practical and interpretation aspects. In the present work the effects of composite composition, volume fractions of secondary phases as well as the matrix grain sizes (micro and nano size) on deformation and failure processes of Al and Cu-based composites were evaluated. Experimental materials were prepared by the different powder metallurgy methods. Interpretation of results was based on the quantification of the physical parameters of material phases depending on the preparation methods and on the analysis of deformation processes using the ‘in-situ tensile test in SEM’. The mechanism of deformation and fracture for each composite system was estimated and models of fracture were proposed.

Numerical simulation, formation of microstructure and mechanical properties of nanocopper prepared by severe plastic deformation

The development of the nanostructure in commercial pure copper, the strength and ductility as well as fracture micromechanisms after severe plastic deformation (SPD) with the technology of equal channel angular pressing (ECAP) are analysed. Experimental results and analyses showed that both strength and ductility can be increased simultaneously by SPD. The final grain size decreased from the initial 50 μm by SPD to 100–300 nm after ten passes. An increase of the ductility together with an increase of strength caused by SPD is explained by a strong grain refinement and by a dynamic equilibrium of weakening and strengthening and it is visible on the final static tensile test stress-strain charts. The fracture micromechanism is influenced by the number of ECAP passes. Transcrystalline ductile fracture took place in the range from one to 14 passes, whereas mixed fracture (transcrystalline ductile fracture with intercrystalline facets) occurred after 15 and 16 ECAP passes. Probable reasons of different fracture mechanisms are explained. Mathematical simulations of the ECAP were realised in software product DEFORM based on the finite element method (FEM).

Observation of Anisotropy of Creep Fracture Using Small Punch Test for Al-Al4C3 System Produced by Equal Channel Angular Pressing

Abstract The anisotropy of the creep properties and fracture using small punch tests for the Al-Al4C3 system produced by ECAP were analysed in this work. Small punch creep tests under constant force were performed at the temperature of 623 K. It was shown that the fracture results, i.e. time to fracture and deflection at fracture were different in specimens with different orientation with respect to the axis of ECAP deformation. Fracture surface analysis of the tested small punch specimens was conducted. Fractures have transcrystalline ductile character. The fracture dimples are equiaxial as well as elongated in the dependence on strain direction. Fracture dimples are of two categories, the small ones sized from 0.1 to 0.5 μm and large ones ranging from 3 to 6 μm.

Microstructure, Mechanical Properties and Fracture of Pt -Y2O3 Composites

The composite prepared by mechanical alloying of ine Pt powder obtained by waste recovery from linings >f glass furnaces with Y 2 0 3 dispersed particles (0.5 nass %) has shown better mechanical properties at room temperature as well as at higher temperatures in comparison with those of Pt alloys commercially produced. The qualitative factor, which is known to be a function of starting powders parameters, technology of preparation and compacting operations, shows some correlation particularly with size and space distribution of Y 2 0 3 phase in the Pt matrix. K e y w o r d s : Pt-system, Fracture mechanism, Mechanical properties, Qualitative factor

Micromechanisms of Fracture of Magnesium Based Composite After Superplastic Deformation

Abstract The micromechanisms of fracture of AZ61 + 1 wt. % Al2O3 composite in the zone of superplastic deformation was analysed and quantified in this work. The specimens were tested at temperature of 200°C at different strain rates. Changing the strain rate, from 1x10-2 s-1 to 1x10-4 s-1, a significant growth of ductility was observed. At maximum value of superplasticity the fracture was transcrystalline ductile with dimples of two size categories. Based on the statistical analysis of fracture micromechanisms at the elevated temperature and strain rates of 10-0- 1x10-4 s-1 hyperbolic dependency was depicted according to Gurland - Plateau theory.

Fracture Description of AZ61 Mg-Al2O3 Materials Studied by “In Situ Tensile Test in SEM“

In situ observation of AZ61 Mg alloy with 5 wt. % of Al2O3 in the SEM was performed to study influence of the weight fraction of Al2O3 particles on the deformation and fracture description during the tensile test. Structure of the experimental materials was also analysed; microstructures were heterogeneous, with randomly distributed globular Al2O3 particles (average diameter of 25 nm) and Mg17Al12 intermetallic phase (average diameter of 0.4 mm). It was shown that during the tensile deformation the failure of Mg17Al12 particles and decohesion of the matrix-Al2O3 particles interphase boundary started simultaneously. Decohesion resulted from the different physical properties of matrix and Al2O3 particles. The influence of the Al2O3 weight fraction on the final fracture was evident; for the material with 5 wt. % of Al2O3, the fracture surface was approximately perpendicular to the loading direction. The fracture surface had transcrystalline ductile character.