Paweł Ostachowski

Superplastic Flow of Metals Extruded by KoBo Method

The extrusion of metals by the KoBo method, which is a new technological solution, allows to perform the process at low temperature with severe plastic deformation (SPD) under cyclic change of deformation path conditions. Apart from the localized plastic flow in shear bands intensive generation of point defects (self interstitials) takes place. It is important that excessive concentration of interstitial atoms is preserved in extruded products and often takes the shape of nanoclusters. Their presence causes that the effect of superplasticity (e.g. about 800% elongation of aluminum alloy 7075) does not require nanometric sizes of the grains and appears in metals extruded by the KoBo method with grains of over dozen or even several dozen micrometers. The results suggest the necessity to verify the mechanisms currently considered as decisive of the superplastic flow of metals.

A new constitutive approach to large strain plastic deformation

Abstract This paper presents the results of studies regarding the effect of nonconventional plastic deformation on the mechanism of plastic flow of metals. The applied method, called KOBO extrusion, in which monotonic large deformation is accompanied by cyclic reversible plastic twisting, falls into the category of severe plastic deformation. The cyclic reversible plastic twisting, which proved to be redundant in terms of changes in the materials geometry, appears to be a particularly efficient source of point defects. The experimentally proven Newtonian viscosity of metals at low temperatures under KOBO extrusion conditions implies that it is necessary to conduct a study on the crystal plasticity from the point of view of the constitutive law in which a crystals viscosity totally controls the kinetics of the process. The well known dependency of the viscosity parameter on the diffusion coefficient, which, in turn, is a function of point defect concentration, dictates the link between the frequency of reversible plastic twist and the crystals viscosity and, consequently, determines the mode of plastic flow. The results of this work provide experimental verification which justifies this new approach to crystal plasticity.

Mechanical properties of aluminum extruded via the KOBO method with direct and lateral outflow

Abstract Aluminum was extruded via the KOBO method with direct and lateral outflow at ambient temperature with an extrusion ratio of λ = 100, keeping constant extrusion force all the time. Despite the far different metal flow geometries, in both cases products were obtained with very good quality (die shape reflection and surface state) and a slight variation in mechanical properties. It was also found that higher torsion angle of extruded metal lowers their levels. The results show both the real possibilities of conducting the extrusion process with lateral outflow, as well as a successive argument supporting the concept of superplastic flow of metal in the KOBO process.

Effect of heat treatment on the precipitation hardening in FeNiCoAlTaB shape memory alloys

Abstract In order to obtain optimal mechanical properties, the effect of heat treatment on the precipitation hardening in multicomponent Fe-based shape memory alloys (containing Ni, Co, Al, Ta, B) was studied. The polycrystalline material was investigated after application of three different processing schemas: slowly cooled, quenched and subsequently annealed with various aging conditions. The study was carried out using synchrotron X-ray diffraction along with mechanical tests, revealing the evolution of strengthening phases. As a result an optimum heat treatment for 10 h at 700°C was established yielding an optimal mechanical response.

Effect of Temperature and Strain Rate on the Mechanical Properties of 99.5 Aluminium Rods Extruded by KOBO

In this study, aluminium rods were cold extruded in a direct process by KOBO method in two variants: variant I with varying (decreasing) frequency of die oscillations necessary to maintain a constant extrusion force, and variant II with constant frequency of die oscillations, leading to a decrease in the extrusion force. The tensile test of rods was carried out in a temperature range of 20 - 200°C and at a strain rate from 8xE10-5 to 8xE10-1 s-1. Significant differences in the elongation of the tested rods were observed. It was found that rods extruded at variable die oscillations and stretched at room temperature had similar elongation, independent of the strain rate. With the increase of temperature, the elongation of samples stretched at a low speed was growing from a value of about 8% at room temperature up to 40% at 200°C. At high strain rates, despite the increasing temperature, the elongation remained at the same level, i.e. 5-6%. In rods extruded at constant die oscillations, the elongation at a low strain rate was growing with the temperature from 10% at room temperature up to 29% at 200°C. At high strain rates, the elongation decreased from 28% at room temperature to 11% at 200°C. The results were interrelated with examinations of the structure of rods and fractures of tensile specimens. In the material extruded by KOBO method with constant die oscillations, the beginnings of the recrystallization process were observed, absent in the material extruded at variable die oscillations.