Surface characterization and wear behavior of ion implanted NiTi shape memory alloy

The aim of this study was investigation of changes in the modified near-surface layer on the NiTi shape memory alloy, caused by ion implantation as well as their influence on the mechanical and shape memory properties of this material. Surface of NiTi has been modified by nitrogen ion beam at several fluences 1 × 10¹⁷ cm^?2, 1 × 10¹⁸ cm^?2 and 2 × 10¹⁸ cm^?2 at the energy 50 keV. The effect of implantation parameters on surface characteristics and wear properties was investigated using dry-sliding-wear test, depth sensing indentation test and scanning profilometry method. The experimental results have shown how the ion implantation treatment can change the original surface: reducing Ni content in the surface, increasing the surface hardness (furthermore, the hardness improvement extended to the regions much deeper than the implanted layer), and improving the sliding wear resistance. The experimental results of surface treatment conditions and mechanical properties of the modified NiTi alloys are compared, analyzed and discussed in this paper.     

Investigating microstructural evolution during homogenization of the equiatomic NiTi shape memory alloy produced by vacuum arc remelting

The vacuum arc remelting process is widely used for the commercial production of NiTi shape memory alloys. Due to the absence of electromagnetic forces in this method, several remelting and long-time homogenizing are required. In this work, a Ti-50 at.% Ni alloy was prepared using the non-consumable vacuum arc melting technique in a water-cooled copper crucible. After four times of remelting process, specimens were subjected to homogenization at 1000 °C. Micro/macrostructural changes during homogenization were investigated by optical microscope and SEM equipped with EDS analyzer. The results showed that the as-cast specimen consisted of mostly Ni₃Ti, Ti₂Ni and monoclinic (B1^9′) phases with high segregation. By increasing the holding time during the homogenization process at 1000 °C, the amount of austenite (B₂) phase was increased, while segregation and unfavorable phases, and accordingly, hardness were decreased. After 4 h of homogenization, austenite (B₂) was the only phase maintained in the microstructure of Ti-50 at.% Ni. In addition, macrostructure of the alloy was turned into polygonal structure after such a homogenization treatment.     

Deformation and strain rate sensitivity of a Zr-Cu-Fe-Al metallic glass

The deformation behaviour of Zr₆₅Cu₂₀Fe₅Al₁₀ bulk metallic glass has been studied at room temperature under uniaxial compression conditions at the strain rate of 5 × 10⁻⁴ s⁻¹ and performing jump tests for the strain rates (SR) ranging between 5 × 10⁻⁶ s⁻¹ and 5 × 10⁻² s⁻¹. The alloy always shows the formation of shear bands and exhibits serrated flow. In the SR range of 5 × 10⁻⁶ to 5 × 10⁻³ s⁻¹ absence of strain rate sensitivity within the experimental error is observed. However, when the SR changes from 5 × 10⁻³ s⁻¹ to 5 × 10⁻² s⁻¹ the alloy exhibits a negative strain rate sensitivity of −0.0026. The number of shear bands on the side view appears to be correlated with the range of stress softening from the maximum stress to the stress at which the sample fails.     

Hot tensile properties and microstructural evolution of as cast NiTi and NiTiCu shape memory alloys

Hot tensile properties of as cast NiTi and NiTiCu shape memory alloys were investigated by hot tensile test at temperature range of 700–1100 °C using the strain rate of 0.1 s⁻¹. The NiTi alloy exhibited a maximum hot ductility at temperature range of 750–1000 °C, while the NiTiCu alloy showed it at temperature range of 800–1000 °C. It was found that at temperatures less than 750 °C, diffusion-assisted deformation mechanism was inactive leading to semi-brittle type of failure and limited ductility in both alloys. Also it was found that at temperature range of 800–1000 °C, dynamic recrystallization is dominant leading to high ductility. Likewise, the fracture surface of the specimens presenting the maximum hot ductility showed an ideal type of ductile rupture in which they gradually pulled out to a fine point. On the other hand, the decline in ductility occurred at the temperatures above 1000 °C was attributed to the liquid phase formation leading to interdendritic and intergranular type of fracture.     

Fabrication and characteristics of porous NiTi shape memory alloy synthesized by microwave sintering

Porous nickel titanium (NiTi) shape memory alloy (SMA) was successfully fabricated by microwave sintering method. This method allows formation of porous structures without using any pore-forming agents. Moreover, microwave sintering of NiTi SMA can be successfully performed at a relatively low sintering temperature of 850 °C and a short sintering time of 15 min. The pore characteristics, microstructure, phase transformation and stress-strain behavior of the porous NiTi SMA were investigated. The porous NiTi SMA exhibited porosity ratios from 27% to 48% and pore sizes range from 50 to 200 μm when using different sintering temperatures and holding times. The predominant B2 (NiTi) and B19′ (NiTi) phases were identified in the porous NiTi SMA. A multi-step phase transformation took place on heating and a two-step phase transformation took place on cooling of the porous NiTi SMA. The irrecoverable strains decreased with increasing sintering temperature, but the holding time had little effect on the stress-strain behavior at 60 °C.     

Shape memory effect in Fe-Mn-Ni-Si-C alloys with low Mn contents

An attempt was made to develop a new Fe-Mn-Si-based shape memory alloy from a Fe-17Mn-6Si-0.3C (mass%) shape memory alloy, which was previously reported to show a superior shape memory effect without any costly training treatment, by lowering its Mn content. The shape memory effect and the phase transformation behavior were investigated for the as-solution treated Fe-(17-2x)Mn-6Si-0.3C-xNi (x = 0, 1, 2, 3, 4) polycrystalline alloys. The shape recovery strain exceeded 2% in the alloys with x = 0-2, which is sufficient for an industrially applicable shape memory effect; however, it suddenly decreased in the alloys between x = 2 and 3 although the significant shape recovery strain still exceeded 1%. In the alloys with x = 3 and 4, X-ray diffraction analysis and transmission electron microscope observation revealed the existence of α′ martensite, which forms at the intersection of the ? martensite plates and suppresses the crystallographic reversibility of the γ austenite to ? martensitic transformation.     

Vacuum brazing of NiTi alloy by AgCu eutectic filler

Abstract

Joining of NiTi alloy to itself has been realised by vacuum brazing process using AgCu28 eutectic as filler metal. Microstructures, mechanical and shape memory behaviour have been investigated. The shearing strength of the brazed joint exceeds 100 MPa, and rupture occurs at the diffusion layer of parent metal beside brazing metal. The brazed joint will be stronger than parent metal on condition of the specimen with a joint of lap length 10 times of plate thickness. The brazed specimen shows a good shape memory behaviour. From the point of view of practice, the brazing joint design principle and brazing quality improvement have been discussed.     

Influence of aluminum ions implanted on oxidation behavior of ZIRLO alloy at 500 °C

The beneficial effect of aluminum ion implantation on the oxidation behavior of ZIRLO alloy at 500 °C has been studied. ZIRLO alloy specimens were implanted with aluminum ions with fluence range from 1×10¹⁶ to 1×10¹⁸ ion/cm², using a MEVVA source at an extraction voltage of 40 kV at maximum temperature of 380 °C. The weight gain curves were measured after being oxidized in the air at 500 °C for 120 min, which showed that a significant improvement was achieved in the oxidation behavior of ZIRLO alloy implanted with aluminum compared with that of the virgin ZIRLO alloy. It has been obviously found that when the fluence is 1×10¹⁸ ion/cm², the oxidization of the implanted ZIRLO alloy is reduced into 30% of the virgin ZIRLO alloy.     

Relevant aspects in the clinical applications of NiTi shape memory alloys

NiTi shape memory alloys showing pseudoelastic behaviour have great potential in dental and orthopaedic applications where constant correcting loads may be required. In most of the clinical applications the device may have been heat treated and during its life in service it will be cyclically deformed. It is therefore important to investigate the effect of cyclic straining and heat treatments upon the transformation stresses and temperatures of the material. The aim of this work is to study the thermal and mechanical ageing of a pseudoelastic NiTi shape memory alloy, as well as the environmental in vitro degradation of the alloy due to the effect of artificial saliva.This paper was accepted for publication after the 1995 Conference of the European Society of Biomaterials, Oporto, Portugal, 10–13 September.     

Superplasticity of Mg-Zn-Y alloy containing quasicrystal phase processed by equal channel angular pressing

Equal channel angular pressing (ECAP) has been conducted on as-cast Mg-4.3 wt.%Zn-0.7 wt.%Y Mg alloy containing quasicrystal phase at a temperature of 623 K. After 8 ECAP passes, the grain size of the as-cast alloy is decreased from ∼ 120 to ∼ 3.5 μm, and the coarse eutectic quasicrystal phases are broken and dispersed in the alloy. Tensile testing has been performed on the ECAPed Mg-Zn-Y alloy at temperatures of 523 K and 623 K with initial strain rates from 1.5 × 10^− 3 to 1.5 × 10^− 4 s^− 1. The ECAPed alloy exhibits a maximum elongation of about 600% when testing at 623 K using an initial strain rate of 1.5 × 10^− 4 s^− 1. Grain boundary sliding is considered to be the dominant deformation mechanism of the Mg-Zn-Y alloy in the temperature and strain-rate range investigated.     

Ambient effect on damping peak of NiTi shape memory alloy

We report the effects of ambient condition and loading rate on the damping capacity of a superelastic nickel-titanium shape memory alloy during stress-induced martensitic phase transformation with release and absorption of latent heat. The damping capacity was measured via a tensile loading-unloading cycle in the strain-rate range of 10^− 5-10^− 1/s and three ambient conditions: still air and flowing air with velocities of 2 m/s and 17 m/s. It is found that, for each ambient condition, the maximum damping capacity (damping peak) is achieved at the strain rate whose loading time (t_T) is close to the characteristic heat-transfer time (t_h) of the ambient condition.     

Superelastic behavior of aged and thermomechanical treated NiTi alloy at Af + 10 °C

The phase transformation in NiTi shape memory alloys is very sensitive to aging and thermomechanical treatment (cold work and annealing). In this research, a Ni-rich alloy has been cold-rolled and then subjected to various annealing treatments and also aging treatments at different temperatures and times. To investigate the superelastic behavior, mechanical testing was carried out at A_f + 10 °C to eliminate the effect of testing temperature on the superelasticity parameters including loading/unloading plateau and mechanical hysteresis. The effect of dislocations and precipitates resulted from thermomechanical and aging treatments on the transformation behavior was studied by Differential Scanning Calorimetry (DSC) and three point bending (TPB) tests. The results showed that these treatments are effective on the thermal and mechanical-induced phase transformation characteristics. Transformation temperatures increase and loading/unloading plateau and thermal and mechanical hystereses decrease by increasing aging and annealing time. Increasing of aging and annealing temperatures results in decreasing of loading/unloading plateau and thermal and mechanical hysteresis.     

Electron energy loss spectroscopy study of the effect of low-energy Ar-ion bombardment on the surface structure and composition of Pt80Co20 (1 1 1) alloy

Electron energy loss spectroscopy has been employed for investigation of the effect of 600 eV Ar⁺-ion irradiation in the dose range 7×10¹⁶-4×10¹⁷ ions/cm² on the atomic structure and surface composition of alloy Pt₈₀Co₂₀(1 1 1). A method of the layer-by-layer reconstruction of the lattice interplanar distance changes based on the analysis of the plasmon spectra excitation was proposed. The ion bombardment was shown to result in a non-monotonic variation of the lattice interplanar distance due to formation of the stable defects, with the topmost layer being in the state of compression. Using the ionization energy loss spectra, a layer-by-layer concentration profile of the alloy components was reconstructed for different doses of ion irradiation of the surface. The Ar⁺-ion bombardment of the alloy was found to result in the preferential sputtering of Co and in the enrichment of the near-surface region by Pt atoms with formation of an altered layer, which is characterized by a non-monotonic concentration profile dependent on the irradiation dose. The results obtained are discussed in the framework of the models of preferential sputtering and radiation-induced segregation.     

Role of aluminum ion implantation on microstructure, microhardness and corrosion properties of titanium alloy

Ti-Al-Zr alloy was implanted with Al at cumulative doses between 1 × 10¹⁷ and 1 × 10¹⁸ ions/cm². The results indicate that the Al-implanted layers are ∼0.1 μm thick and are composed almost entirely of an amorphous layer. Implanted layer hardness is dose dependent and is increased by more than a factor of 4 for the high-dose implanted specimen when compared with that of the substrate material. The corrosion resistance of the sample was markedly improved after aluminum implantation.     

Effect of Titanium ion implantation on the oxidation behaviour of zircalloy-4 at 500 °C

The beneficial effect of titanium ion implantation on the oxidation behaviour of zircalloy-4 at 500 °C was investigated. Titanium ions were implanted by a MEVVA source at an energy of 40 keV with dose 5×10¹⁶, 1×10¹⁷, and 2×10¹⁷ ion/cm² at the maximum temperature 130 °C. Weight gain curves of the as-received and implanted zircalloy-4 were measured after oxidation in air at 500 °C for 100 min. It was found that improvement was achieved in the oxidation behaviour of titanium ion implanted samples compared with that of the as-received one. The valence of the oxides in the scale was analyzed by X-ray photoemission spectroscopy. Glancing angle X-ray diffraction was used to examine the phase transformation in the oxide films and is showed that the addition of titanium transformed the phase from monoclinic zirconia to hexagonal zirconia. Finally, the mechanism of improvements oxidation behaviour is discussed.     

Pb(Zr0.52Ti0.48)O3/TiNi multilayered heterostructures on Si substrates for smart systems

Ferroelectric/shape memory alloy thin film multilayered heterostructures possess both sensing and actuating functions and are considered to be smart. In this article, Pb(Zr_0.52Ti_0.48)O₃ (PZT) ferroelectric thin films and Ti-riched TiNi shape memory alloy thin films have been deposited on Si and SiO₂/Si substrates in the 400-600 °C temperature range by pulsed laser deposition technique. Deposition processing, microstructure and surface morphology of these films are described. The TiNi films deposited at 500 °C had an austenitic B2 structure with preferred (110) orientation. The surfaces of the films were very smooth with the root-mean-square roughness on a unit cell level. The structure of the TiNi films had a significant influence on that of the subsequently deposited PZT films. The single B2 austenite phase of the TiNi favored the growth of perovskite PZT films. The PZT/TiNi heterostructures with the PZT and TiNi films respectively deposited at 600 and 500 °C exhibited a polarization-electric field hysteresis behavior with a leakage current of about 2 × 10^− 6 A/cm².     

Inhomogeneous structure of near-surface layers in the ion-implanted NiTi alloy

This paper reports the application of nitrogen ion implantation for modification of a shape memory alloy. It is known that the problem of creating a protective surface coating for the shape memory alloy is the most acute for potential applications of this material. Thus, the problem of increasing surface protective properties and, at the same time, simultaneous preservation of functional properties of shape memory materials is a subject of research and development [Pelletier H, Muller D, Mille P, Grob J. Surf Coat Technol 2002;158:309.]. The surface characterization of nitrogen implanted (fluence 10¹⁸ cm^?2 and energy 50 keV) equiatomic commercial NiTi alloy samples was performed with the assistance of high resolution transmission electron microscopy (HTEM) techniques and modifications of phase composition before and after irradiation are studied at room and martensitic transformation temperatures by X-ray diffraction methods. Differential scanning calorimetry (DSC, TA Instruments) was used to characterize the transformation sequence and transformation temperatures for the initial and surface-modified materials. Experimental results of an inhomogeneous structure of near-surface layers in the ion-implanted NiTi alloy are discussed in this paper.     

FEM analysis of localized deformation behaviour in pseudoelastic NiTi shape memory alloys

This paper takes into account the localized deformation behaviour of a pseudoelastic NiTi shape memory alloy with finite element method. A three‐dimensional micromechanical model has been developed, in which the difference between the elastic properties of austenite and martensite is considered. The model is implemented as User MATerial subroutine (UMAT) into ABAQUS. Then, a polycrystalline NiTi shape memory alloy with [1 1 1] texture under tensile loading is simulated. The main features of the propagation of the deformation band reported in literatures are captured in the simulation. It is also shown that the initiation and propagation of the deformation bands are strongly affected by the geometry of the specimens.     

Properties of implanted PET by W ion using MEVVA implantation

Polyethylene terephthalane (PET) has been modified with W ions from a metal vapour arc source (MEVVA). W ions were implanted at 136 keV to doses ranging from 5×10¹⁵ to 2×10¹⁷/cm². The surface of the implanted PET darkened with increasing ion dose, when the metal ion dose is greater than 1×10¹⁷ cm⁻² the colour changed to metallic bright. The surface resistance obviously decreases with increasing dose. The resistivity is stable after long-term storage. TEM photos revealed the presence of W nanometer particles on the surface resulting from the high does implantation. The depth of implanted layer is approximately between 180 and 100 nm for W-implanted PET to doses of 2×10¹⁷/cm² and 5×10¹⁶/cm², respectively. The conductivity and wear resistance have been improved significantly due to W ion implantation. It can be seen that nanometer particles of W precipitation, and carbides have been formed in the implanted layer. The nano-hardness of the implanted PET has been measured by a nano-indenter. The results show that the surface hardness, modulus and wear resistance could be increased.     

Mechanical properties of additive laser-welded NiTi alloy

Laser welding would be a suitable joining technique for NiTi shape memory alloy if the mechanical properties of laser weld were improved. With this purpose, effects of additive on mechanical properties of laser-welded NiTi alloy have been experimentally studied. Welding specimens used in this study were 2 mm thick hot-rolled plates with a chemical composition of Ni50.9Ti49.1. (Ni50.9Ti49.1)-Ce2 (at.%) alloy foil or Ni47Ti44Nb9 plate was used as filler metal to add Ce or Nb element into NiTi laser weld metal. Both tensile strength and the toughness of additive-welding specimens were improved significantly compared with non-additive-welding specimen. The mechanical property improvement was attributed to the fine solidification NiTi grains and good grain-linking in weld center. The microstructure control mechanisms of these two additive welds were discussed.