Libor Pantělejev

Microstructure and Mechanical Properties of Ultrafine-Grained Magnesium AZ91 Alloy

Microstructure, mechanical properties, cyclic plastic deformation behavior and fatigue strength of ultrafine-grained (UFG) magnesium alloy AZ91 processed by equal channel angular pressing (ECAP) were investigated. ECAP of originally cast alloy results in development of bimodal structure, improved yield stress, tensile strength and ductility when compared to the as-cast state. Endurance limit based on 107 cycles is also improved, however exhibits large scatter. Initiation of fatigue cracks takes place in regions of large grains in the bimodal structure, where the content of Mg17Al12 particles is low.

Strain Mapping by Scanning Low Energy Electron Microscopy

The use of the scanning low energy electron microscopy (SLEEM) has been slowly making its way into the field of materials science, hampered not by limitations in the technique but rather by relative scarcity of these instruments in research institutes and laboratories. This paper reports the results obtained from an investigation of the microstructure of ultra fine-grained (UFG) copper fabricated using equal channel angular pressing (ECAP) method, namely in the as-pressed state and after annealing. SLEEM is very sensitive to the perfection of crystal lattice and using SLEEM, local strain can be effectively imaged.

Mechanical and Microstructural Properties of 2618 Al-Alloy Processed by SLM Remelting Strategy

Paper deals with the comparison of mechanical properties and microstructure of aluminium alloy 2618 fabricated by Selective Laser Melting (SLM) and material of the same grade manufactured by standard extrusion process. The SLM specimens were fabricated with different processing strategies (meander and remelting). Presence of cracks was found in both cases of used strategies, but in case of meander strategy, crack are of shorter character and distributed rather within individual welds. In case of remelting strategy, cracks are oriented mostly parallel to building direction and transcend fusion boundaries (FB) across several layers. It was found that defects present in microstructure of SLM material significantly affect its mechanical properties. Ultimate tensile strength (UTS) for extruded material reached 392 MPa, while for SLM material produced with meander strategy UTS was 273 MPa and for remelting strategy it was 24 MPa only.

Selective Laser Melting Strategy for Fabrication of Thin Struts Usable in Lattice Structures

This paper deals with the selective laser melting (SLM) processing strategy for strut-lattice structure production which uses only contour lines and allows the porosity and roughness level to be managed based on combination of the input and linear energy parameters. To evaluate the influence of a laser scanning strategy on material properties and surface roughness a set of experiments was performed. The single welds test was used to find the appropriate processing parameters to achieve continuous welds with known width. Strut samples were used to find a suitable value of weld overlapping and to clarify the influence of input and linear laser energy on the strut porosity and surface roughness. The samples of inclined hollow struts were used to compare the wall thickness with single welds width; the results showed about 25% wider welds in the case of a hollow strut. Using the proposed SLM strategy it is possible to reach a significantly lower porosity and surface roughness of the struts. The best results for struts with an inclination of 35.26° were achieved with 25% track overlapping, input energy in the range from 9 J to 10.5 J and linear energy Elin from 0.25 to 0.4 J/mm; in particular, the relative density of 99.83% and the surface roughness on the side of the strut of Ra 14.6 μm in an as-built state was achieved.

Microstructural changes of ECAP-processed magnesium alloy AZ91 during cyclic loading at different stress-amplitude levels

Microstructural changes of magnesium alloy AZ91 after fatigue loading in the EX-ECAP state were evaluated using EBSD. It was found that both the number fraction of low-angle boundaries and parameter KAM decreased after the testing at a stress amplitude of 160 MPa but started to increase with the increasing stress amplitude. This behaviour can be explained with a mutual influence of dislocation accumulation (which is stronger with a higher stress amplitude) and dynamic softening (which is weaker with a decreasing number of cycles/cycles to failure). The average grain size remained almost unchanged except at a stress amplitude of 180 MPa, which could have been caused by certain conditions allowing an ideal development of both mentioned phenomena.

Mechanical Properties of Extruded and ECAP Processed Magnesium Alloy AZ91 at Elevated Temperature

In this paper tensile properties at elevated temperature of extruded AZ91 magnesium alloy and the same alloy further processed by ECAP (exECAP) are compared. The tensile tests were performed at room temperature and for the temperature range of 100 to 300 °C. Loading speed 2 mm/min was used for the tests. At room temperature mechanical properties except elongation were slightly higher for extruded material yet still very similar to properties of exECAPed material. Overall trend of properties evolution with increasing temperature was also similar but the decrease of strength or the increase of elongation and reduction of area respectively is more intensive for exECAPed material. Elongation of exECAPed material exceeded elongation of extruded material more than twice at 300 °C and with value of ~260% this alloy exhibited pseudosuperplastic behavior.

Geometrical Shielding Produced by Intergranular Crack-Tip Branching in Fe–V–P Alloy

The geometrical shielding produced by intergranular crack-tip branching in the fracture toughness tests of the Fe–V–P alloy is quantitatively assessed particularly with respect to the contribution of crack splitting. This process was evaluated by an identification of secondary intergranular cracks visualized on metalographical samples perpendicular to the fracture surface. The analysis of mixed trans/intergranular fracture revealed no special influence of triple-point branching (splitting) on the total crack tip shielding in cases of such highly spatially tortuous crack fronts. Thus, the previously reported results taking only the effect of crack tip kinking and meandering into account were proved to be correct.