H. Morawiec

Two-stage martensitic transformation in a deformed and annealed NiTi alloy

Deformation and subsequent annealing of NiTi alloys offers great possibilities of controlling characteristic temperatures, sequences of transformations and shape recovery. Todoroki and Tamura showed that depending on annealing temperatures, changes in transformation sequences in deformed alloys occur during both heating and cooling. They also observed that the cooling DSC (Differential Scanning Calorimetry) curves exhibit two peaks in the range of the martensitic transformation for specimens annealed at 500 C. Referring to the paper of Monasevich and Pascal they explained this effect as the formation of two types of martensite. In this paper the authors try to prove that the appearance of two peaks in the martensitic range on the DTA (Differential Thermal Analysis) cooling curves may be due to changes of dislocation configuration caused by the low-temperature annealing.

Effect of early stages of precipitation and recovery on the multi-step transformation in deformed and annealed near-equiatomic NiTi alloy

Abstract Multi-step martensitic transformation in Ni50.6at.%–Ti alloy was studied by means of differential scanning calorimetry and electron microscopy methods. The alloy was subjected to 10% deformation after solution treatment and then annealed at 673 K for different times. The time evolution of the multi-step transformation is explained in terms of the complex alloy structure and non-homogeneity of the precipitate distribution in the matrix.

Structure and properties of melt-spun Cu–Al–Ni shape memory alloys

Abstract Increase of the wheel speed causes the grain refinement, rise of a columnar morphology of grains and the DO 3 fibre texture in the melt-spun ribbons of the studied alloys. This influences their tensile strength, yield stress and shape recovery under load. Additional possibilities of influencing the structure and hence the properties of the ribbons open up for the alloys containing Ti+Mn by presence of particles of the X -phase as well as by recrystallisation. The ribbons of the studied alloys melt-spun with the wheel speed up to 20 m/s exhibit a relatively high transformation temperature.

TEM study of the interface in ceramic-reinforced aluminum-based composites

Abstract The microstructure and interfaces of aluminum matrix composites were studied using conventional and high-resolution transmission electron microscopy (HREM). Also convergent-beam electron diffraction (CBED) was used. Two kinds of Al-based ceramic-reinforced composites (Al–TiC and Al–TiB 2 ) were studied. Transmission electron microscopy (TEM) revealed the presence of a relatively high dislocation density in the matrix of both kinds of investigated composites. The dislocation density is higher near the interface region than inside the matrix. The local strength of the matrix was determined near the reinforcing particles, using TEM at CBED. Analysis of the higher-order Laue zone (HOLZ) lines allowed a determination of the changes in the lattice constant versus the distance from the interface.

The effect of Ni, Co and Cr on the primary particle structure in Cu–Al–Nb–X shape memory alloys

Abstract Niobium, cobalt and chromium have a very low solubility in a Cu–Al alloy. Therefore, primary particles were observed in the martensitic matrix of this alloy. The kind, size and number of the primary particles depend on the alloying element. The main particles were identified as Nb(Cu, Al)2. The characteristic feature of this phase is the presence of stacking faults. The number of stacking faults depends on the chemical composition of these particles. This phase includes up to 7 Ni, 11 Co or 10–22 Cr (at.%). Moreover, small oval particles of AlNi, AlCo and Cr phases were identified. Changes in the chemical composition of the Nb(Cu, Al)2 phase as well as the presence of AlNi, AlCo and Cr particles cause changes in the mechanical and physical properties of the alloys studied.

Structure of Low Temperature Nitrided/Oxidized Layer Formed on NiTi Shape Memory Alloy

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.

Surface Structure of NiTi Alloy Passivated by Autoclaving

Sterilization of the NiTi alloy in boiling water or steam causes passivation, which results in an amorphous 3.5 nm thick TiO2 layer on the surface. Between the surface and the matrix a transition layer of Ni2O3 and NiO was observed, using the X-ray photoelectron spectroscopy. Differences in sterilization conditions affect the amount of metallic nickel on the surface.

Texture Analysis of Hot Rolled Ni-Mn-Ga Alloys

The studies were carried out on three Ni-Mn-Ga polycrystalline alloys with the 10M martensite and the L21 parent phase. The specimens were cut along the [001] and [110] directions of the columnar grains from the ingots of either round or flat shape. The samples were heated up to 10000C and rolled in one direction in several steps. The final reduction of the samples thickness was: 28%, 36%, 57% and 69%. The texture of the initial [001] orientation rolled with the highest deformation 60% shows the following orientation components: {111}<112>, {112}<011>, {111}<011> which are typical for deformed and recrystallized metals of the A2 structure. The texture of the specimen with the initial [110] orientation rolled with 69% can be described as a fibre texture scattered around the <110> direction.

Extruded Rods with Axial Texture of Polycrystalline Ni-Mn-Ga Alloys

The Ni-Mn-Ga alloys extruded at high temperature with a relatively large reduction rate were the subject of the studies. The microscopic observations of the transient zone in the extruded rods revealed rotation of the columnar grains in the ingot along the rod axis - giving the preferred <001> orientations. Comparison of the texture, magnetization and magneto-crystalline anisotropy constant for samples cut out from the columnar grain zone and these cut from the extruded rods was carried out for two alloys which differ in chemical composition and Ms temperature. Both alloys have the 5M martensitic structure at room temperature. The samples cut out of the ingot and the extruded rods have shown the axial <001> orientation, similar magnetization curves and magnetocrystalline anisotropy constant Ku = 1.5-1.6 J/m3•105.

Superelasticity of NiTi Ring-Shaped Springs Induced by Aging for Cranioplasty Applications

This paper concerns the application of titanium-nickel rings in modeling the cranium. After being fixed to the osseous margins, the ring’s expansion at the same time broadens and shortens the cranium vault. The rings formed from a straight superelastic wire, flattened to an ellipse, do not show the presence of a typical force plateau but rather a pseudoelastic loop during loading-unloading in the relationship between the force and the deflection. Based on the idea that superelasticity in more complex shape-springs may be induced by the precipitation hardening process, the further studies were carried out on alloys with higher nickel contents (51.06 at.% Ni). The rings that had been formed were welded and aged at an optimal temperature and time. The improved superelastic behavior during compression and unloading the rings was obtained by introducing small deformation by drawing the quenched wires before forming the rings and aging. Very positive clinical reshaping by long-term distraction with the superelastic ring-shaped springs was achieved in young children under one year and a less spectacular effect was observed in the group of older children.