Papers by Author: Mariusz Kulczyk

Abstract: Metals with a body-centered cubic structure such as iron exhibit a ductile-brittle transition which results in a brittleness below a particular temperature. This temperature depends on many factors such as strain rate, size and geometry of investigated samples or the purity of the material. Another important parameter influencing fracture mechanism is grain size. It is known that grain refinement can be an efficient way to change fracture mechanism.The goal of this study was to investigate fracture mechanisms of ultrafine-grained iron processed by hydrostatic extrusion (HE). Materials subjected to various total strain levels were tested. The average grain size of the HE-processed iron was below 350 nm. The mechanical tests were carried out at various temperatures, ranging from room temperature to liquid nitrogen temperature. It was found that the fracture mechanism depends on a density of dislocation and the loading direction. It was found that materials with the dislocation density above a certain critical value break in a ductile manner even at impact tensile test in liquid nitrogen. However, bending tests of miniature beams have shown that ductile fracture occurs only when the crack propagates along the radial direction of the extruded material, whereas, on the direction parallel to the axial direction, cleavage fracture was observed. A theoretical model explaining this phenomenon was proposed. This model is based on the Rice model and it considers the competition between two phenomena - dislocation slip in the stress field of the crack front and Griffith cleavage.

Abstract: The study is aimed at comparing the changes which occur in the microstructure and thermo-physical properties of pure copper (99.9%) and when copper alloyed with chromium and zirconium subjected to severe plastic deformation (SPD). The plastic deformation techniques employed were hydrostatic extrusion (HE), equal channel angular pressing (ECAP), and a combination of these two processes. The materials thus obtained had an ultra-fine-grained structure with the thermo-physical properties differing from those of the untreated materials. It appeared that there is a correlation between the deformation method employed and the thermo-physical properties of the materials, such as diffusivity and specific heat.

Abstract: In this work, an attempt has been made to improve the mechanical strength of 6063 aluminium alloy and thus its lightness via combination of severe plastic deformation (grain size refinement) and heat treatment (precipitation hardening). 6063 aluminium alloy was chosen as the best material for lightweight structures, where mass reduction is important, because of its high extrudability. Samples were hydrostatically extruded (HE) in supersaturated condition and subsequently subjected to an aging process. HE brings about significant grain size refinement well below 1 micron. The influence of aging parameters such as time and temperature on mechanical properties evolution of extruded material was determined. The microstructure of ultrafine grained (UFG) alloy was investigated using transmission electron microscopy. The average grain diameter and grain boundary misorientation angles (using Kikuchi lines) were measured. Mechanical properties were examined in microhardness and tensile tests. The results have shown that it is possible to combine grain boundary and precipitation strengthening and obtain ultrahigh strength in 6xxx series alloys. Additionally, heat treatment of UFG samples causes an increase in ductility measured in tensile tests, which is rather poor in severely deformed materials. To prove advantages of UFG aged samples for lightweight applications, finite element modelling was performed to compare the mass of chair elements made of coarse and ultrafine grained material. Simulations were made for the same stresses applied. It has been shown that if the chair was made of UFG aluminium alloy the mass reduction would be approximately 30 %.

Abstract: Grain size refinement is an efficient way to improve mechanical strength and thus make light metals even lighter in terms of specific strength. However, the strength improvement is at the expense of ductility. Therefore, a better understanding of microstructural factors influencing both parameters is of prime importance for further development of ultrafine grained materials. In this work, we report results obtained for 5483 aluminium alloy which was subjected to several severe plastic deformation (SPD) methods, i.e. equal channel angular pressing (ECAP), Hydrostatic Extrusion (HE) and the combination of the two. Detailed microstructural analysis revealed significant difference in the grain size and grain boundary characteristics between samples obtained following different routes. It was found that although the grain size is a prime microstructural parameter determining mechanical strength, second order factors such as grain size distribution and distribution of grain boundary misorientation angles also play a significant role.

Abstract: The aim of the present work was to compare microstructures and mechanical properties of nano-Al alloys fabricated by two different methods: (i) SPD induced grain refinement, (ii) plastic consolidation of nano-powders or nano-crystalline ribbons. SPD grain refinement has been implemented by hydrostatic extrusion, HE. The ribbons were rapidly solidified using a melt spinning methods. Plastic consolidation of powder and ribbons was conducted by warm extrusion. The results of the studies show that by applying various fabrication routes for a given chemical composition, diverse nano-structures can be obtained, which differ in terms of grain size and shape, grain boundary character and dislocation density. As a result, the alloys also differ significantly in the mechanical properties. The findings are discussed in terms of the possibilities for optimizing properties of the bulk-nano-metals.

Abstract: Current and prospective trends in application of metallic nanomaterials have been studied. The study has been conducted within the Nanoroad SME European project – as the first step for a roadmap for industrial application of nanomaterials. The web page of the project is http://www.nanoroad.net/. The present report presents an analysis of patents, papers, national and European projects in the field of nano-metals, and also an analysis of the present state of research and expected trends in this domain. Based on the performed analysis a data base of nanomaterials has been developed as well as roadmaps with expected time to applications. It can be found under http://bourgogne.arist.tm.fr/nanoroadsme/home/. The roadmap is mainly addressed to SMEs to help them to decide about applications or production of nanomaterials.

Abstract: Hydrostatic extrusion can be viewed as one of the methods of Severe Plastic Deformation, SPD, for the fabrication of ultra-fine grained alloys which causes a significant increase in the mechanical properties such as tensile strength and hardness. In the present study the microstructure of 6082 aluminium alloy after hydrostatic extrusion was investigated. Hydroextrusion was performed in three steps with accumulated true strains of 1.34, 2.73 and 3.74 respectively. Microstructural observations were carried out using SEM, TEM and light microscopy. Grain and inclusion sizes, shapes and distribution were investigated in the HE processed samples. The study has shown that the hydrostatic extrusion process results in a profound refinement of both the grain size and the inclusions in 6082 aluminium alloy.

Abstract: The paper presents the results of investigations into the mechanical properties and tribological characteristics of 316 LVM processed by hydrostatic extrusion (HE). The mechanical properties were characterized by microhardness measurements and compression tests. The wear properties were investigated using a pin-on-disc tribometer under dry and lubricated conditions. The friction coefficient was measured as a function of the time of the wear test. The results indicate that the hydrostatic extrusion process significantly improves the mechanical properties and the wear resistance of 316 LVM stainless steel. The results are discussed in terms of the microstructural changes induced during processing by hydrostatic extrusion.

Abstract: An ultra-fine grained microstructure was obtained in high purity nickel by a combination of (a) equal-channel angular pressing (ECAP) and (b) hydrostatic extrusion (HE) with a cumulative true strain of ~11.2. The resulting microstructure was examined by light and TEM microscopy. Mechanical properties have been measured by tensile and hardness tests. It was found that HE of ECAP-ed samples leads to a significant grain size refinement (from 330 to 160nm) and to an increase in microstructural homogeneity. SPD nickel, made by a combination of the ECAP and hydrostatic extrusion methods, has high strength and ductility (i.e.: YS=1120MPa and εf = 11%). The microstructure transformation was accompanied by a strength increase of 78% compared to ECAP alone. The results obtained fit well with the Hall-Petch relationship. A combination of ECAP and HE has achieved much better properties than either single process and show it to be a promising procedure for manufacturing bulk UFG nickel.