Papers by Author: Tadeusz Wierzchoń

Abstract: Shape memory effects, the course and characteristic temperatures of phase transitions and mechanical properties and surface properties of NiTi alloys strongly depend on the chemical composition, the production process used and the plastic working and thermomechanical treatment as well as surface treatment. The test alloy was obtained by vacuum metallurgy by melting the components in a graphite crucible and casting into a graphite ingot mould. In order to obtain the rods in the process of hot plastic working, hot forging was applied using a smith hammer and rotary hot forging on a swaging machine. The resulting rods were subjected to an appropriate heat treatment and thermo mechanical treatment to obtain, at room temperature, a parent phase structure B2.The paper presents the results of the research of NiTi rods after hot rotary forging. The phase composition of the samples of the tested alloy after different heat treatments were determined by X-ray powder diffraction technique. It was found that at room temperature, depending upon the processing the samples had a structure parent phase with a small amount of martensite. The courses of phase transitions and the changes of the temperature characteristic were determined on the basis of the recorded DSC curves. It was found that the test rate after aging in the temperature range of 400-500 °C transitions take place involving the rhombohedral R phase. The temperature ranges of shape recovery of samples after various heat treatments were determined by recording the recovery of the shape during heating, in tests performed according to the standard ASTM 2082-06.

Abstract: NiTi alloy is being increasingly used in medicine due to its unique properties, i.e. shape memory and superelasticity. As a self-passivating material it is characterized by relatively high biocompatibility, however its use for long-term medical implants is questionable due to the nickel content of ≥ 50%. Therefore, the investigations on the surface modification of NiTi alloy are carried out to improve its corrosion resistance and thus reduce the metalosis effect, i.e. the migration of the alloy constituents, especially nickel, into the surrounding tissue.In this paper, the surface topography and corrosion resistance of NiTi alloy (50,8%Ni) both before and after low-temperature nitriding and oxynitriding processes under glow discharge conditions, are presented.The study of surface topography showed a slight increase in roughness parameters after nitriding process and a significant increase in these parameters after the oxynitriding process. A similar trend was observed in the study of corrosion resistance. Both processes increase the corrosion resistance of NiTi alloy, as shown by both the impedance spectroscopy results, the values of corrosion potential (-65 mV for the alloy in the initial state, - 45 mV for the alloy with the nitrided layer, + 18 mV for the alloy with the oxynitrided layer) and the values corrosion current (respectively 0.047 μA/cm², 0.043 μA/cm², 0.015 μA/cm²).These comparative studies present an improvement of corrosion resistance of NiTi after the processes under glow discharge. The best results were obtained for the oxynitrided layer.

Abstract: The paper presents results of structural studies of hot extruded NiTi shape memory alloy that is in the B2 phase at room temperature. Texture of the alloy was determined from the X-ray diffraction measurements. It was found that in result of 60 % sample reduction (at a cross-section of a bar formed by hot extrusion) weak axial texture - type <110>_B2 was formed. The volume of the grains oriented in this way was approx. 20 %. Basing on metallographic observations it was also found that the size of the grains formed as a result of the thermomechanical treatment was uniform with the average area of 1700 μm². This information is significant from the point of view of functional properties. Hot extruded alloy revealed presence of the reversible martensitic transformation. Its characteristic temperatures were slight higher than in as-cast alloy. Moreover, the extruded NiTi alloy showed 100 % of the shape recovery.

Abstract: In order to increase corrosion resistivity of the NiTi alloy the surface is covered by layers. Layers can be made from such as titanium nitrides and/or oxides as well as their mixture. Recently, a glow discharge technique has been applied for coatings formation. However, the deposition process requires to be done at elevated temperature. Therefore, it may have a negative effect on the structure, which is responsible for the shape memory phenomena. The results obtained from studies, done over the influence of the glow discharge nitriding and combination of nitriding and oxidizing process on the structure, the kinetics of martensitic transformation, the one-way shape memory effect and the superelasticity effect of the NiTi alloys are reported. The results showed that during deposition process, curried out at temperatures above 250°C and for time up to 30 minutes, the precipitation of dispersive particles of Ni4Ti3 phase already starts and has a positive effect on the superelasticity phenomena. The applied deposition technique does not affect also negatively the shape memory effect.

Abstract: The present work summarises the results, which were obtained from studies carried out on the structure of the nitride and nitride-oxide surface layers with use of the electron transmission microscopy. The layers were formed using glow discharge technique at relatively low temperature (300°C). It has been shown that low temperature nitriding or nitriding/oxiding process produced a thin layer ~30 nm thick. They were formed from titanium nitride as well as titanium oxides. The structure revealed that nanoparticles were surrounded by high amount of amorphous phase. Especially, electron microscopy was useful method for studying the phase boundary between the layer and the NiTi matrix. During deposition process, which was carried out at temperature above 300°C, the intermediate layer of Ni3Ti intermetallic phase appeared between titanium oxides and/or nitrides. Lowering deposition temperature down to 300°C or below resulted in absence of such sublayer. Moreover, thickness, structure of layers, absence of sublayer formed during glow discharge process, can significantly influence deformation during inducing of the shape memory or superelasticity effect.

Abstract: Aluminium alloys are the materials of choice when high-strenght-to-weight rations are required in structural components, and used widely in the automotive and aerospace industries. As an example, the use of an aluminium components in the automobile industry has greatly increased due to weight savings and resultant fuel economy improvements. There are many methods of surface consolidation of an aluminium alloys. This work presents the hybrid creation method of the newly layers type (Mn-P) on the AlSi13Mg1CuNi alloy, its microstructure, hardness, chemical and phase compositions as well as wear and corrosion resistance. Growth the wear resistance of an aluminium alloy coated with the layer type (Mn-P) is visable. The corrosion characteristics of these layers are also considered.

Abstract: The paper analyzes the magnetic properties and stability of austenite, athermal martensite, and deformation martensite that form in Ni27Ti2AlMoNb steel subjected to glow discharge assisted nitriding, and also of nitrogen austenite (S phase with various lattice constants) which occurs when the nitriding process is conducted below the temperature A_s. The analysis of these structural components and their morphology was performed using a magnetic force microscope (MFM), whereas the phase composition of the nitrided layers produced on this steel was determined by X-ray diffraction.

Abstract: Structure of the nitride-oxide surface layers, formed using glow discharge technique at low temperature (between 200 and 380°C) changing the parameters of the process, was examined applying X-ray diffraction as well as transmission electron microscopy. The phase analysis was supported by results obtained from XPS measurement. Obtained results have shown that low temperature nitriding/oxiding produced the thin layers (18 ÷ 30 nm in thickness), which consist of titanium nitride and oxide phase. Low amount of the nickel-titanium oxide was also identified in the layer produced at higher temperature. Decrease of nitriding/oxidizing temperature below 300oC cause that between nitride-oxide surface layer and NiTi alloy did not create the intermediate layer of Ni3Ti phase. The results were verified and confirmed by observation carried out using high resolution electron microscopy. The corrosion properties of such coatings were tested in the physiological Tyrod’s solution using the cyclic potentiodynamic polarization method.

Abstract: The nearly-equiatomic NiTi (50.6 at.%Ni, Af = 10°C) alloy was nitrided at 1073 K using glow discharge technique. Additionally, at the end of the nitriding some amount of oxygen was added. Phase identification and thickness of the nitrided/oxided layer was done using electron microscopy method and the X-ray reflectivity measurements. The chemical composition of the layers was determined using energy dispersive spectrometer. Combination of nitriding and oxidation of the NiTi surface produces nitride/oxide complex layer of a nanocrystalline structure. The average thickness of the obtained layer was about 0,5 μm. The potentiodynamical studies in Tyrod’s solution reveal good corrosion resistance of obtained nitrided/oxidized NiTi surface proving its high quality.

Abstract: In order to increase corrosion resistance, the NiTi shape memory alloy was covered by protection layers using glow discharge method. Samples were nitrided with some oxygen addition at temperatures: 350, 380, 400 and 800°C. Microstructure and sequence of obtained layers were studied applying X-ray diffraction technique as well as electron microscopy. Low temperature process produces a layers which consist of Ti3O5, TiN and Ni2Ti4O. Intermediate Ni3Ti layer was between Ni2Ti4O and NiTi matrix. High temperature process forms TiO2 and TiN with interlayer Ti2Ni. Surface does not contain phase with Ni, which is considered as toxic element.