Inelastic current-voltage spectroscopy of atomic wires

A tight-binding model is developed to describe the electron-phonon coupling in atomic wires under an applied voltage and to model, their inelastic current-voltage spectroscopy. Particular longitudinal phonons are found to have greatly enhanced coupling to the electronic states of the system. This leads to a large drop in differential conductance at threshold energies associated with these phonons. It is found that with increasing tension these energies decrease, while the size of the conductance drops increases, in agreement with experiment.

Current-induced embrittlement of atomic wires

Recent experiments suggest that gold single-atom contacts and atomic chains break at applied voltages of 1 to 2 V. In order to understand why current flow affects these defect-free conductors, we have calculated the current-induced forces on atoms in a Au chain between two Au electrodes. These forces are not by themselves sufficient to rupture the chain. However, the current reduces the work to break the chain, which results in a dramatic increase in the probability of thermally activated spontaneous fracture of the chain. This current-induced embrittlement poses a fundamental limit to the current-carrying capacity of atomic wires.

Non-linear conductance of disordered quantum wires

The self-consistent electron potential in a current-carrying disordered quantum wire is spatially inhomogeneous due to the formation of resistivity dipoles across scattering centres. In this paper it is argued that these inhomogeneities in the potential result in a suppression of the differential conductance of such a wire at finite applied voltage. A semi-classical argument allows this suppression, quadratic in the voltage, to be related directly to the amount of intrinsic defect scattering in the wire. This result is then tested against numerical calculations.

Newtonian origin of the spin motive force in ferromagnetic atomic wires

We demonstrate numerically the existence of a spin-motive force acting on spin carriers when moving in a time and space dependent internal ?eld. This is the case for electrons in a one-dimensional wire with a precessing domain wall. The effect can be explained solely by adiabatic dynamics and is shown to exist for both classical and quantum systems.

Fatigue behavior of laser-welded NiTi wires in small-strain cyclic bending

NiTi wires and their weldments are commonly used in micro-electro-mechanical systems (MEMS), and in such applications, cyclic loading are commonly encountered. In this paper, the bending-rotation fatigue (BRF) test was used to study the bending fatigue behavior of NiTi wire laser weldment in the small-strain regime. The fracture mechanism, which includes crack initiation, crack growth and propagation of the weldment in the BRF test, was investigated with the aid of SEM fractography and discussed in terms of the microstructure. It was found that crack initiation was primarily surface-condition dependent. The cracks were found to initiate at the surface defects at the weld zone (WZ) surface, and the crack propagation was assisted by the gas inclusions in the WZ. The weldment was finally fractured in a ductile manner. The fatigue life was found to decrease with increasing surface strain and also with increasing bending frequency (controlled by the rotational speed in the BRF test). In comparison, the fatigue life of the unwelded NiTi wires was higher than their welded counterparts at all strain levels and bending frequencies. The decrease in fatigue resistance of the weldment could be attributed to the surface and microstructural defects introduced during laser welding.

Constitutive model for localized Lüders-like stress-induced martensitic transformation and super-elastic behaviors of laser-welded NiTi wires

The application of the shape memory alloy NiTi in micro-electro-mechanical-systems (MEMSs) is extensive nowadays. In MEMS, complex while precise motion control is always vital. This makes the degradation of the functional properties of NiTi during cycling loading such as the appearance of residual strain become a serious problem to study, in particular for laser micro-welded NiTi in real applications. Although many experimental efforts have been put to study the mechanical properties of laser welded NiTi, surprisingly, up to the best of our understanding, there has not been attempts to quantitatively model the laser-welded NiTi under mechanical cycling in spite of the accurate prediction required in applications and the large number of constitutive models to quantify the thermo-mechanical behavior of shape memory alloys. As the first attempt to fill the gap, we employ a recent constitutive model, which describes the localized SIMT in NiTi under cyclic deformation; with suitable modifications to model the mechanical behavior of the laser welded NiTi under cyclic tension. The simulation of the model on a range of tensile cyclic deformation is consistent with the results of a series of experiments. From this, we conclude that the plastic deformation localized in the welded regions (WZ and HAZs) of the NiTi weldment can explain most of the extra amount of residual strain appearing in welded NiTi compared to the bare one. Meanwhile, contrary to common belief, we find that the ability of the weldment to memorize its transformation history, sometimes known as ‘return point memory’, still remains unchanged basically though the effective working limit of this ability reduces to within 6% deformation.

1-D constitutive model for evolution of stress-induced R-phase and localized Lüders-like stress-induced martensitic transformation of super-elastic NiTi wires

NiTi alloys have been widely used in the applications for micro-electro-mechanical-systems (MEMS), which often involve some precise and complex motion control. However, when using the NiTi alloys in MEMS application, the main problem to be considered is the degradation of functional property during cycling loading. This also stresses the importance of accurate prediction of the functional behavior of NiTi alloys. In the last two decades, a large number of constitutive models have been proposed to achieve the task. A portion of them focused on the deformation behavior of NiTi alloys under cyclic loading, which is a practical and non-negligible situation. Despite of the scale of modeling studies of the field in NiTi alloys, two experimental observations under uniaxial tension loading have not received proper attentions. First, a deviation from linearity well before the stress-induced martensitic transformation (SIMT) has not been modeled. Recent experiments confirmed that it is caused by the formation of stress-induced R phase. Second, the influence of the well-known localized Lüders-like SIMT on the macroscopic behavior of NiTi alloys, in particular the residual strain during cyclic loading, has not been addressed. In response, we develop a 1-D phenomenological constitutive model for NiTi alloys with two novel features: the formation of stress-induced R phase and the explicit modeling of the localized Lüders-like SIMT. The derived constitutive relations are simple and at the same time sufficient to describe the behavior of NiTi alloys. The accumulation of residual strain caused by R phase under different loading schemes is accurately described by the proposed model. Also, the residual strain caused by irreversible SIMT at different maximum loading strain under cyclic tension loading in individual samples can be explained by and fitted into a single equation in the proposed model. These results show that the proposed model successfully captures the behavior of R phase and the essence of localized SIMT.

Susceptibility to environmentally induced cracking of laser-welded NiTi wires in Hanks’ solution at open-circuit potential

In the present study the tensile and super-elastic behaviours of laser-welded NiTi wires in Hanks’ solution at open-circuit potential (OCP) were investigated using tensile and cyclic slow-strain-rate tests (SSRT). In comparison with NiTi weldment tested in oil (non-corrosive environment), the weldment in Hanks’ solution suffered from obvious degradation in the tensile properties as evidenced by lower tensile strength, reduced maximum elongation, and a brittle fracture mode. Moreover, a larger residual strain was observed in the weldment after stress–strain cycles in Hanks’ solution. In addition to the microstructural defects resulting from the welding process, the inferior tensile and super-elastic behaviours of the NiTi weldment in Hanks’ solution could be attributed to the trapping of a large amount of hydrogen in the weld zone and heat-affected zone.

Effect of post-weld heat-treatment on the oxide film and corrosion behaviour of laser-welded shape memory NiTi wires

Post-weld heat-treatment (PWHT) was applied to NiTi weldments to improve the corrosion behaviour by modifying the microstructure and surface composition. The surface oxide film on the weldments is principally TiO2, together with some Ti, TiO, and Ti2O3. The surface Ti/Ni ratio of the weldments after PWHT is increased. The oxide film formed in Hanks’ solution is thicker on the weldments after PWHT. The pitting resistance of the weldments is increased by PWHT. The galvanic effect in the weldments is very small. The weldment with PWHT at 350 °C shows the best corrosion resistance among other heat-treated weldments in this study.

Effect of postweld heat treatment on the microstructure and cyclic deformation behavior of laser-welded NiTi-shape memory wires

NiTi wires of 0.5 mm diameter were laser welded using a CW 100-W fiber laser in an argon shielding environment with or without postweld heat-treatment (PWHT). The microstructure and the phases present were studied by scanning-electron microscopy (SEM), transmission-electron microscopy (TEM), and X-ray diffractometry (XRD). The phase transformation behavior and the cyclic stress–strain behavior of the NiTi weldments were studied using differential scanning calorimetry (DSC) and cyclic tensile testing. TEM and XRD analyses reveal the presence of Ni4Ti3 particles after PWHT at or above 623 K (350 °C). In the cyclic tensile test, PWHT at 623 K (350 °C) improves the cyclic deformation behavior of the weldment by reducing the accumulated residual strain, whereas PWHT at 723 K (450 °C) provides no benefit to the cyclic deformation behavior. Welding also reduces the tensile strength and fracture elongation of NiTi wires, but the deterioration could be alleviated by PWHT.

Parameter Optimization on Laser Welding of NiTi Wires by Taguchi Method

In this research, a preliminary study was done to find out the initial parameter window to obtain the full-penetrated NiTi weldment. A L27 Taguchi experiment was then carried out to statistically study the effects of the welding parameters and their possible interactions on the weld bead aspect ratio (or penetration over fuse-zone width ratio), and to determine the optimized parameter settings to produce the full-penetrated weldment with desirable aspect ratio. From the statistical results in the Taguchi experiment, the laser mode was found to be the most important factor that substantially affects the aspect ratio. Strong interaction between the power and focus position was found in the Taguchi experiment. The optimized weldment was mainly of columnar dendritic structure in the weld zone (WZ), while the HAZ exhibited equiaxed grain structure. The XRD and DSC results showed that the WZ remained the B2 austenite structure without any precipitates, but with a significant decrease of phase transformation temperatures. The results in the micro-hardness and tensile tests indicated that the mechanical properties of NiTi were decreased to a certain extent after fibre laser welding.

Reduction of environmentally induced cracking of laser-welded shape memory NiTi wires via post-weld heat-treatment

In this study, the environmentally induced cracking behaviour of the NiTi weldment with and without post-weld heat-treatment (PWHT) in Hanks’ solution at 37.5 °C at OCP were studied by tensile and cyclic slow-strain-rate tests (SSRT), and compared with those tested in oil (an inert environment). Our previous results in the tensile and cyclic SSRT showed that the weldment without PWHT showed high susceptibility to the hydrogen cracking, as evidenced by the degradation of tensile and super-elastic properties when testing in Hanks' solution. The weldment after PWHT was much less susceptible to hydrogen attack in Hanks' solution as no obvious degradation in the tensile and super-elastic properties was observed, and only a very small amount of micro-cracks were found in the fracture surface. The susceptibility to hydrogen cracking of the NiTi weldment could be alleviated by applying PWHT at the optimized temperature of 350 °C after laser welding.

Effect of Post-weld Heat-treatment on the Stress Corrosion Cracking Behaviour of Laser-welded Shape Memory NiTi Wires in Hanks’ Solution

In this study, the stress-corrosion cracking (SCC) behaviour of laser-welded NiTi wires before and after post-weld heat-treatment (PWHT) was investigated. The samples were subjected to slow strain rate testing (SSRT) under tensile loading in Hanks’ solution at 37.5 °C (or 310.5 K) at a constant anodic potential (200 mVSCE). The current density of the samples during the SSRT was captured by a potentiostat, and used as an indicator to determine the susceptibility to SCC. Fractography was analyzed using scanning-electron microscopy (SEM). The experimental results showed that the laser-welded sample after PWHT was immune to the SCC as evidenced by the stable current density throughout the SSRT. This is attributed to the precipitation of fine and coherent nano-sized Ni4Ti3 precipitates in the welded regions (weld zone, WZ and heat-affected zone, HAZ) after PWHT, resulting in (i) enrichment of TiO2 content in the passive film and (ii) higher resistance against the local plastic deformation in the welded regions.