Papers by Author: Józef Lelątko

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: 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: The structure, point defect and ordering parameter of Fe25Al samples is examined with the Mössbauer spectroscopy Positron Annihilation Lifetime Spectroscopy XRD and SEM. The studies are carried out for samples in as-cast state and after heat treatments: annealing for 24 hours at 900°C (or 1050°C) and either slow cooling with furnace or quenching to oil. Among the research method used, Mössbauer spectroscopy for determination of hyperfine structure parameters was adopted. These parameters, sensitive to changes in spin and charge electron densities in the nearest neighbourhood of a Mössbauer isotope nucleus, caused by specific configurations of atoms, are directly connected with the degree of ordering of a compound. Spectral analysis has been carried out using an authors’ software developed based on a theoretical model relating the shape of a Mössbauer spectrum to the sample microstructure. It has been shown that Mössbauer spectroscopy enables quantitative evaluation of the degree of ordering of phases occurring in samples characterised by large graining, in the case of which it is not possible to determine the long-range order parameter by X-ray diffraction. The PALS method only one type of defects is detected. The positron lifetime in these defects (V) suggests that they are quenched-in Fe-monovacancies (VFe). The vacancy concentration strongly depends on the rate of cooling.

Abstract: The NiTi alloy (50.6 at.% Ni) passivated for 30 min at 130°C by autoclaving has been studied towards corrosion resistance in aqueous solutions of 3% NaCl, 0.1 M H₂SO₄, 1 M H₂SO₄ and HBSS. Structure and thickness of the passive layer (TiO2, rutile) were examined by X-ray reflectivity method and high resolution electron microscopy. Corrosion behavior of this oxide layer was investigated by open circuit potential method and polarization curves. It was found that the corrosion resistance of the passivated NiTi alloy is strongly dependent on the type of corrosive environment. The higher corrosion resistance of the tested samples was revealed in sulfate solutions as compared to chloride ones. The highest resistance to electrochemical corrosion of the NiTi alloy was observed in 0.1 M H2SO4 solution. Susceptibility to pitting corrosion of the tested samples was observed which increased with the concentration rise of chlorine anions in solution. Electrochemical tests for 316L stainless steel carried out under the same experimental conditions revealed a weaker corrosion resistance in all solutions as compared to the highly corrosion resistant NiTi alloy.

Abstract: Two Ni-based (Ni-Mn-Ga and Ni-Mn-Co-In) ferromagnetic nonstoichiometric shape memory alloys were studied in order to determine the influence of hot extrusion process on macro, microstructure and texture of the studied alloys. The microstructure of the alloys in the as cast state and after extrusion was analyzed by electron backscatter diffraction technique. Typical microstructure of the as cast alloys consisted of radially oriented columnar grains elongated perpendicularly to the casting axis. For the alloys Ni-Mn-Ga and Ni-Mn-Co-In alloys the 10M and 14M modulated martensite were observed, respectively. After extrusion Ni-Mn-Ga samples revealed high density of fibre texture parallel to the extrusion axis. In spite of applying different extrusion parameters it was not possible to avoid cracks and overcome the brittleness of the Ni-Mn-Co-In alloys.

Abstract: Nickel-based single-crystal superalloys are widely used for production of high pressure turbine blades. The studied blades were obtained in an ALD Vacuum Technologies furnace by the Bridgman technique. Crystallization process was carried out with drawing rates of 3 mm/min and 5 mm/min. The dendrite microstructure based on the γ/γ’ phases and their defects was characterized using X-Ray topography, Laue diffraction, transmission electron microscopy. The defect structure samples were examined with the positron annihilation lifetime spectroscopy. It was found that crystal orientation, lattice parameter of γ’ phase and concentration of defects are correlated. The defect concentration increases in some areas which was the result of deviation of the primary dendrite arm from crystallization direction . Only one type of point defects was detected.

Abstract: In order to improve a biocompatibility of a NiTi shape memory alloys used as a long-term implant, surface was covered by protective multi-layers coating. First, alloy was passivated in autoclave, and then hydroxyapatite was deposited using electrophoresis at room temperature. In result of that multi-layer was formed on the top of the surface consisted of titanium oxide and hydroxyapatite. Phase identification done with use of grazing incidence beam X-ray diffraction proved sequence of obtained coatings. Applied procedure for multi-layer deposition allowed avoiding of the B2 parent phase decomposition. Moreover, the martensitic transformation occurs in two-steps.

Abstract: In present paper two ribbons of the Ni₄₄Co₆Mn₃₆In₁₄ (at.%) were prepared under different melt-spinning technique conditions. Microstructure of the ribbons was studied by scanning electron microscopy (SEM). Depending on the liquid ejection overpressure two types of ribbons microstructures were observed. Ribbon T1 for which ejection overpressure was 1.5 bar showed typical melt-spun ribbon microstructure consisting of a top layer of small equi-axial grains and columnar grains below. For T2 ribbon (ejection overpressure 0.2 bar) only a small fraction of the columnar grains were observed. Structure analysis of the ribbons performed by XRD showed that at room temperature both ribbons have B2 parent phase superstructure. No gamma phase precipitates were observed. In order to determine the orientation of the grains the EBSD technique was applied.

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 ternary Ni47Ti50Co3 alloy was produced using twin roll technique and traditional casting. The bulk alloy was hot rolled and thermally treated. Both alloys showed presence of the reversible martensitic transformation, in which the parent phase transformed to the B19’monoclinic martensite via the R-phase. However, they significantly differed in grains orientation. Thermally treated alloy showed weak fibre texture (35% of total amount), whereas the strip revealed fibre and {100} sheet texture components. In the strip more than 70% of total amount of grains were preferentially oriented. Presence of the texture affected the shape memory effect. In the strip, the two-way shape memory effect was induced after 30 training cycles. In thermally treated bulk, 52% of shape recovery was reached after 100 training cycles.