Search Results

You are looking at 1 - 5 of 5 items for

  • Author: K. Pieła x
Clear All Modify Search
Open access

K. Pieła

Anomaly of the Work Hardening of Zn-Cu Single Crystals Oriented for Slip in Secondary Systems

The copper alloyed (up to 1.5%) zinc single crystals oriented for slip in non-basal systems (orientation close to < 1120 >) were subjected to compression test within a range of temperatures of 77-293K. It has been stated, that Zn-Cu crystals exhibit characteristic anomalies of the thermal dependence of yield stress and of the strain hardening exponent. Both of them are related to the change in type and sequence of active non-basal slip systems: pyramidal of the 1st order {1011} < 1123 > (Py-1) and pyramidal of the 2nd order {1122} < 1123 > (Py-2). The temperature anomaly of the yield stress results from the change of the slip from Py-2 systems to simultaneous slip in the Py-2 and Py-1 (Py-2 + Py-1) systems, occurring in the preyielding stage. On the other hand, sequential activation of pyramidal systems taking place in advanced plastic stage (i.e. the first Py-2 and next Py-2 + Py-1 systems) is responsible for temperature anomaly of strain hardening exponent. Increase in copper addition favors the activity of Py-2 systems at the expense of Py-1 slip, what leads to a drastic differences in plastic behavior of zinc single crystals.

Open access

M. Jaskowski, K. Pieła and L. Błaz

Abstract

The influence of the number of extrusion steps in KoBo method (at the same total extrusion ratio of λ = 100) on structure, mechanical properties and work hardening characteristics of AlMg4.5 and AlMg4.5Mn (AA5083) alloys was investigated. It was found that one-step extrusion leads to the formation of recrystallised structure of the material, while the use of two-step extrusion yields a fibrous structure of a “mixed” type, i.e. containing areas where the intensive recovery effects are associated with partially recrystallised structure. As a consequence, the strength properties of the latter extrudate are much higher in both as extruded state and after the subsequent cold rolling. In all cases, the tensile stress-strain curves of the extrudates show the flow stress serrations that are typical for the Portevin - LeChatelier (P-L) effect. In a few tensile tests, the P-L effect was preceded by the plastic flow instability being typical for the occurrence of Lüders bands. Both AlMg4.5 and AlMg4.5Mn extruded wires show a monotonic increase of the work hardening that results from the following cold deformation in the groove rolling.

Open access

K. Pieła, L. Błaz and M. Jaskowski

Abstract

Commercial purity aluminum was extruded by means of KoBo method at varied processing parameters. Received extrudates, with different mechanical and electrical properties and work hardening behavior, were obtained. It was found, that some conditions of KoBo extrusion process such as low initial billet temperature, low extrusion rate and low frequency of oscillating die lead to extremely high strength and high electrical resistivity of the material. The absence of work hardening (up to 40% strain) during subsequent groove rolling is also a specific feature of received materials. It was suggested that mentioned features are related to the development of overbalance concentration of point defects (clusters) generated during the extrusion process. During following cold rolling of the extrudate, mentioned defects annihilate at gliding dislocations and make the dislocation climbing and their rearrangement easier. Therefore, until the exhaustion of this mechanism, the hardening of material during cold rolling is very limited. Following increase of the material strengthening at higher rolling strains point to the return of the material to its typical behavior observed for cold deformed aluminum produced by conventional hot extrusion.

Open access

K. Pieła, L. Błaz, Z. Sierpinski and T. Forys

Calorimetric and dilatometric tests were performed on AA7075 aluminum alloy annealed at constant heating rate of 15°C/min in temperature range 20-470°C and discussed in relation to the hardness test and structure observation results. The samples were machined from furnace cooled material (FC), furnace cooled and deformed (FCD) material, solution treated (ST) material and solution treated and deformed (STD) material. It was found that the nucleation and growth of transition η’ and stable η (MgZn2) phases caused remarkable reduction of thermal expansion coefficient t , whereas both the dissolution of η phase and formation of GP zones were accompanied by an increase of t value. While αi phase started to dissolve at 250°C, a widespread endothermic effect was observed on DSC curves. Dissolution of η particles at high annealing temperatures was accompanied by the solid solution hardening of the alloy. Mentioned hardening process was overlapped by expected material softening that was ascribed to recovery and recrystallization processes. Because of superposition the solution hardening and recrystallization softening, recrystallization temperature could not be precisely defined on the basis of simple hardness measurements.

Open access

M. Bieda, S. Boczkal, P. Koprowski, K. Sztwiertnia and K. Pieła

Pure aluminium (6N) and commercially pure aluminium (99.7) was deformed by KOBO method. Microstructure and texture of both materials after deformation was analyzed by means of scanning and transmission electron microscopy. Advanced methods of crystallographic orientations measurements like Electron Backscatter Diffraction - EBSD (SEM) and microdiffraction (TEM) was used. Grain size distribution and misorientation between grains in cross and longitudinal sections of the samples were analyzed. Differences in size and homogeneity of the grains were observed in both materials. Pure aluminium was characterized by larger grain size in both sections of extruded material. Whereas commercially pure aluminium reveals smaller grain size and more homogeneous and stable microstructure.