J. Van Humbeeck

Critical overview of Nitinol surfaces and their modifications for medical applications

Nitinol, a group of nearly equiatomic shape memory and superelastic NiTi alloys, is being extensively explored for medical applications. Release of Ni in the human body, a potential problem with Nitinol implant devices, has stimulated a great deal of research on its surface modifications and coatings. In order to use any of the developed surfaces in implant designs, it is important to understand whether they really have advantages over bare Nitinol. This paper overviews the current situation, discusses the advantages and disadvantages of new surfaces as well as the limitations of the studies performed. It presents a comprehensive analysis of surface topography, chemistry, corrosion behavior, nickel release and biological responses to Nitinol surfaces modified mechanically or using such methods as etching in acids and alkaline solutions, electropolishing, heat and ion beam treatments, boiling in water and autoclaving, conventional and ion plasma implantations, laser melting and bioactive coating deposition. The analysis demonstrates that the presently developed surfaces vary in thickness from a few nanometers to micrometers, and that they can effectively prevent Ni release if the surface integrity is maintained under strain and if no Ni-enriched sub-layers are present. Whether it is appropriate to use various low temperature pre-treatment protocols (< or = 160 degrees C) developed originally for pure titanium for Nitinol surface modifications and coatings is also discussed. The importance of selection of original Nitinol surfaces with regard to the performance of coatings and comparative performance of controls in the studies is emphasized. Considering the obvious advantages of bare Nitinol surfaces for superelastic implants, details of their preparation are also outlined.

Surface oxidation of NiTi shape memory alloy

Mechanically polished NiTi alloy (50 at% Ni) was subjected to heat treatment in air in the temperature range 300-800 degrees C and characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Thermogravimetry measurements were carried out to investigate the kinetics of oxidation. The results of thermodynamic calculations were compared to the experimental observations. It was found that NiTi alloy exhibits different oxidation behaviour at temperatures below and above 500 degrees C. A Ni-free zone was found in the oxide layer for oxidation temperatures of 500 degrees C and 600 degrees C. The oxidation at 500 degrees C produces a smooth protective nickel-free oxide layer with a relatively small amount of Ni species at the air/oxide interface, which is in favour of good biocompatibility of NiTi implants. The oxidation mechanism for the NiTi shape memory alloy is discussed.

Asymmetry of stress–strain curves under tension and compression for NiTi shape memory alloys

Abstract The stress–strain curves of polycrystalline martensitic NiTi shape memory alloys are often different for loading under tension and compression. Under tension, a flat stress-plateau occurs, while under compression, the material is quickly strain hardened and no flat stress-plateau is observed. Cyclic deformation under tension–compression also shows that it is more difficult to deform the material during compression than during tension, where an asymmetric stress–strain loop is obtained. TEM observations show that, under tension to 4% strain, martensite variants are partially reoriented via migration of variant interfaces with formation of dislocation networks mainly along the junction plane areas, and no significantly plastic deformation has been observed inside the martensite twin bands. While under compression to 4% strain, a high density of dislocations has been generated in both the martensite twin bands and the variant accommodation area, and no significant martensite reorientation via variant interfacial migration has been observed. This shows that the deformation mechanism of martensitic polycrystalline NiTi SMAs under tension is different from that under compression.

Damping capacity of thermoelastic martensite in shape memory alloys

Shape memory alloys attract increasing interest as materials that can be used for passive as well as active damping applications. The passive high damping capacity finds its origin in the thermoelastic martensitic phase due to the hysteretic mobility of martensite variants or different phase interfaces. The damping capacity increases with increasing amplitude of the applied vibration. Special interest exists moreover for damping extremely large displacements. This is realised by applying the mechanical hysteresis occurring during pseudoelastic loading. This aspect is nowadays very strongly studied as a tool for protecting buildings against earthquakes in seismic active regions. Active damping can be obtained in hybrid composites by controlling the recovery stresses or strains of embedded shape memory alloy wires. This controls the internal energy of a structure which allows controlled modal modification and tuning of the dynamic properties of structural elements. But also impact damage, acoustic radiation, dynamic shape control can be actively controlled. As a consequence, improved fatigue-resistance, better performance and a longer lifetime of the structural elements can be obtained. This paper overviews the specific damping properties and damping functional behaviour of shape memory alloys, with special emphasis on NiTi. It is illustrated by actual applications and applications under development.

Shape memory alloys: A material and a technology

Shape memory alloys offer attractive potentials such as: superelastic behaviour, reversible strains of several percent during heating or cooling over a limited temperature range, generation of high recovery stresses, and a work output with a high power/weight ratio. This paper describes the origin of those properties and gives an overview of the shape memory functions and shape memory applications.

Two-way shape memory effect developed by martensite deformation in NiTi

Abstract The effect of tensile deformation in the martensitic state on the transformation behaviour of a polycrystalline near-equiatomic NiTi alloy was investigated by differential scanning calorimetry and thermal mechanical analysis. The experimental results indicated that the process of martensite reorientation proceeded in a Luders manner over a stress plateau and continued into the next stage of uniform deformation with an increasing stress. The transition of the martensite reorientation process from a localized manner to a uniform manner during tensile deformation of a polycrystalline matrix is yet to be explained. The results also demonstrated that the reorientation deformation was effective in developing a two-way memory effect. The maximum two-way memory effect developed was comparable in magnitude to that developed by conventional training procedures in similar alloys. The deformation also caused a thermal stabilization to the deformed martensite. The stabilization effect was a one-time effect, which vanished once the deformed martensite reverted back to austenite on heating.

Impulse excitation apparatus to measure resonant frequencies, elastic moduli, and internal friction at room and high temperature

This paper presents a new apparatus to measure elastic properties and internal friction of materials. The apparatus excites the test specimen by a light mechanical impact (impulse excitation) and performs a software-based analysis of the resulting vibration. The resonant frequencies fr of the test object are determined and, in the case of isotropic and regular shaped specimens, the elastic moduli are calculated. The internal friction value (Q−1) is determined for each fr as Q−1=k/(πfr) with k the exponential decay parameter of the vibration component of frequency fr. A furnace was designed and equipped with automated impulse excitation and vibration detection devices, thus allowing computer-controlled measurements at temperatures up to 1750 °C. The precision of the measured fr depends on the size and stiffness of the specimen, and varies from the order of 10−3 (that is ±1 Hz at 1 kHz) in soft, high damping materials or light specimens, to values as precise as 10−5 (that is ±0.1 Hz at 10 kHz) in larger or ...

Metallographic methods for revealing the multiphase microstructure of TRIP-assisted steels

Classical etching techniques used for the investigation of steel microstructures allow the simultaneous observation of only a restricted number of phases. So far, this limitation has not been too detrimental, because most low-carbon steel grades possess a quite simple microstructure. The recent interest in the so-called TRIP-assisted multiphase steels characterized by complex microstructures requires new developments in metallographic methods. This paper proposes an extension of already known techniques to allow the study of four kinds of TRIP-aided steels. The actual restrictions justifying the development of an improved method are emphasized. In spite of its simplicity, the procedure has the advantage of allowing the simultaneous observation of the four phases that generally compose the microstructure of TRIP-assisted steels; that is, ferrite, bainite, austenite, and martensite. Light and electron microscopy as well as diffraction techniques are used to demonstrate the interest of the method.

EFFECT OF TEXTURE ORIENTATION ON THE MARTENSITE DEFORMATION OF NiTi SHAPE MEMORY ALLOY SHEET

Abstract For a cold-rolled NiTi sheet, the tensile stress–strain curves show a flat stress-plateau during tension along the rolling direction, while under tension along the transverse direction the specimens are quickly strain-hardened and no flat stress-plateau occurred. This shows that the deformation mechanisms of martensite twins are different when loading along different directions. TEM observations show that, in the as-annealed condition, the major type of twins in the martensite phase is 〈011〉 type II twins in the present material. Also present are (001) compound twins and a small amount of (11 1 ) type I twins. Deformation details of these three types of twins are different along rolling and transverse directions. After deformation along the rolling direction to 6% strain, reorientation and de-twinning of the 〈011〉 type II twins have occurred, while after deformation along the transverse direction to 6% strain, no significant reorientation and de-twinning of 〈011〉 type II twins have been observed. Instead, a high density of dislocations has been generated inside the 〈011〉 type II twins and de-twinning of the (001) compound twins has been observed. A further crystallographic analysis shows that the shear direction of each type of martensite twins relative to the loading direction is different, which may explain the different deformation behaviour of the twins. This may also account for the macroscopical deformation behaviour of the material.

Microstructure of NiTi shape memory alloy due to tension–compression cyclic deformation

Abstract Experimental results have shown that, during mechanical cycling under tension–compression load within ±4% strains, the NiTi shape memory alloy is cyclic strain-hardened. The maximum stresses under both tension and compression increase with increasing number of cycles and tend to stabilize with further cycling. The present work is focused on the martensite microstructure developed as a result of mechanical cycling. TEM observations show that, before cycling, the martensite variants are well self-accommodated to each other with the 〈011〉 type II twinning as the main lattice invariant shear. After mechanical cycling, the martensite plates are still self-accommodated and the (111) type I twinning is most frequently observed. In addition to the stress-induced re-orientation of martensite and twin boundary movement within the martensite plate, various lattice defects have been developed both in the junction plane areas of martensite plates and within the martensite twins.