Nanostructured Shape Memory Alloy Research Articles

Shape memory alloy nanostructures with coupled dynamic thermo-mechanical effects

Employing the Ginzburg–Landau phase-field theory, a new coupled dynamic thermo-mechanical 3D model has been proposed for modeling the cubic-to-tetragonal martensitic transformations in shape memory alloy (SMA) nanostructures. The stress-induced phase transformations and thermo-mechanical behavior of nanostructured SMAs have been investigated. The mechanical and thermal hysteresis phenomena, local non-uniform phase transformations and corresponding non-uniform temperatures and deformations’ distributions are captured successfully using the developed model. The predicted microstructure evolution qualitatively matches with the experimental observations. The developed coupled dynamic model has provided a better understanding of underlying martensitic transformation mechanisms in SMAs, as well as their effect on the thermo-mechanical behavior of nanostructures.

Structural transformations in NiTi shape memory alloy nanowires

Martensitic phase transformation in bulk Nickle-Titanium (NiTi)—the most widely used shape memory alloy—has been extensively studied in the past. However, the structures and properties of nanostructured NiTi remain poorly understood. Here, we perform molecular dynamics simulations to study structural transformations in NiTi nanowires. We find that the tendency to reduce the surface energy in NiTi nanowires can lead to a new phase transformation mechanism from the austenitic B2 to the martensitic B19 phase. We further show that the NiTi nanowires exhibit the pseudoelastic effects during thermo-mechanical cycling of loading and unloading via the B2 and B19 transformations. Our simulations also reveal the unique formation of compound twins, which are expected to dominate the patterning of the nanostructured NiTi alloys at high loads. This work provides the novel mechanistic insights into the martensitic phase transformations in nanostructured shape memory alloy systems.

Dynamic multi-axial behavior of shape memory alloy nanowires with coupled thermo-mechanical phase-field models

The objective of this paper is to provide new insight into the dynamic thermo-mechanical properties of shape memory alloy (SMA) nanowires subjected to multi-axial loadings. The phase-field model with Ginzburg–Landau energy, having appropriate strain based order parameter and strain gradient energy contributions, is used to study the martensitic transformations in the representative 2D square-to-rectangular phase transformations for FePd SMA nanowires. The microstructure and mechanical behavior of martensitic transformations in SMA nanostructures have been studied extensively in the literature for uniaxial loading, usually under isothermal assumptions. The developed model describes the martensitic transformations in SMAs based on the equations for momentum and energy with bi-directional coupling via strain, strain rate and temperature. These governing equations of the thermo-mechanical model are numerically solved simultaneously for different external loadings starting with the evolved twinned and austenitic phases. We observed a strong influence of multi-axial loading on dynamic thermo-mechanical properties of SMA nanowires. Notably, the multi-axial loadings are quite distinct as compared to the uniaxial loading case, and the particular axial stress level is reached at a lower strain. The SMA behaviors predicted by the model are in qualitative agreements with experimental and numerical results published in the literature. The new results reported here on the nanowire response to multi-axial loadings provide new physical insight into underlying phenomena and are important, for example, in developing better SMA-based MEMS and NEMS devices

Atomistic characterization of pseudoelasticity and shape memory in NiTi nanopillars

Molecular dynamics simulations are performed to study the atomistic mechanisms governing the pseudoelasticity and shape memory in nickel–titanium (NiTi) nanostructures. For a 〈110〉 – oriented nanopillar subjected to compressive loading–unloading, we observe either a pseudoelastic or shape memory response, depending on the applied strain and temperature that control the reversibility of phase transformation and deformation twinning. We show that irreversible twinning arises owing to the dislocation pinning of twin boundaries, while hierarchically twinned microstructures facilitate the reversible twinning. The nanoscale size effects are manifested as the load serration, stress plateau and large hysteresis loop in stress–strain curves that result from the high stresses required to drive the nucleation-controlled phase transformation and deformation twinning in nanosized volumes. Our results underscore the importance of atomistically resolved modeling for understanding the phase and deformation reversibilities that dictate the pseudoelasticity and shape memory behavior in nanostructured shape memory alloys.

Dynamic thermo-mechanical coupling and size effects in finite shape memory alloy nanostructures

In this paper, the dynamics of martensitic transformations in shape memory alloy (SMA) constrained finite nanostructures is studied using a phase field model with the Ginzburg–Landau free energy. The nonlinear coupled thermo-mechanical properties in SMAs have been extensively studied in the bulk case. However at the nanoscale the thermal physics has been usually neglected in a study of SMA properties, and most of the model developments have been carried out under the assumption of isothermal phase transformations. The main aim of this paper is to develop a model that couples the thermal physics and the mechanical dynamics to study the influence of such coupling on the mechanical properties of SMA nanostructures. The developed model is solved using the finite element method. Analyzing FePd alloy nanowires, we observe a steeper slope of the stress–strain curve in the coupled thermo-mechanical case due to temperature evolution during the loading–unloading cycle of such nanostructures. We also observe the martensitic suppression phenomenon in constrained nanowires and nanograins on cooling. We have developed a semi-analytical model to predict a critical size at the onset of martensitic suppression. The semi-analytical model predicts a critical size which is in a good agreement with the numerical results for FePd nanograins.

Melt-spun thin ribbons of shape memory TiNiCu alloy for micromechanical applications

The development of micromechanical devices (MEMS and NEMS) on the basis of nanostructured shape memory alloys is reported. A Ti50Ni25Cu25 (at. %) alloy fabricated by the melt spinning technique in the form of a ribbon with a thickness around 40 μm and a width about 1.5 mm was chosen as a starting material. Technological parameters were optimized to produce the alloy in an amorphous state. The thickness of the ribbon was reduced to 5–14 μm by means of electrochemical polishing. A nanostructural state of the thin ribbons was obtained via the dynamic crystallization of the amorphous alloy by application of a single electric pulse with duration in the range of 300–900 μs. A microtweezers prototype with a composite cantilever of 0.8 μm thick and 8 μm long was developed and produced on the basis of the obtained nanostructured thin ribbons by means of the focused ion beam technique. Controlled deformation of the micromanipulator was achieved by heating using semiconductor laser radiation in a vacuum chamber of scanning ion-probe microscope.

Nanostructured thin ribbons of a shape memory TiNiCu alloy

The present work deals with the investigation of the nanostructured shape memory alloys for the purpose of development of micromechanical devices. The Ti 50Ni 25Cu 25 (at.%) alloy fabricated by the melt spinning technique in the form of a ribbon with a thickness around 40 μm and a width about 1.5 mm was chosen as a starting material. Technological parameters were optimized to produce the alloy in an amorphous state. The thickness of the ribbon was reduced to 5–10 μm by means of electrochemical polishing. A special nanostructurization of the thin ribbons was obtained via the dynamic crystallization of the amorphous alloy by application of a single electric pulse with duration near 500 μs. A composite micromanipulator with overall dimensions of 20 × 5 × 0.9 μm was developed and produced on the basis of the obtained nanostructured thin ribbons by means of the focused ion beam (FIB) technique.

Nanostructured shape memory alloys of the TiNi-TiCu system

We study the shape memory Ti50Ni25Cu25 (at %) alloy fabricated by melt spinning. The techno-logical parameters were optimized to obtain the alloy in the amorphous state. The dynamic crystallization of the alloy by a single electric pulse with durations of 1 to 100 ms was used to form the nanostructural state. Transmission electron microscopy and differential scanning calorimetry study show that reducing the pulse duration to 2 ms resulted in considerable refinement of the alloy with the formation of nanosized martensitic plates (20–60 nm). It was established that nanostructurization of the alloy can lead to an increase in the value of the recovery strain when the shape memory effect is manifested.

Electron beam lithography of Free-Standing Ni–Mn–Ga films

We present a process to fabricate freely movable shape memory alloy (SMA) structures by electron-beam lithography (EBL) and sacrificial layer technology starting from free-standing SMA films. Functional films such as SMA and ferromagnetic SMA films may not be structured on the same substrate used for deposition due to process incompatibilities. In addition, EBL and sacrificial layer technologies entail various constraints and requirements on substrate material and properties. These issues can be solved by introducing a substrate inversion technology that includes deposition of sacrificial structures followed by electroplating of a new metal substrate onto the functional film and release of the initial substrate. This process is demonstrated for a SMA film of NiMn-Ga that has been sputter-deposited on a polymer substrate allowing the fabrication of large arrays of mechanically active SMA nanostructures.

Nanostructured Shape Memory Alloys for Biomedical Applications

Application of TiNi shape memory alloy in biomedical field is rapidly expanding. Some of the applications calls for non-conventional properties, which may require new methods of thermomechanical treatment and surface modification. In the present study, the effect of nanocrystallization/amorphization by various method of severe plastic deformation, such as, shot peening, cold rolling and high pressure torsion, was investigated on properties of TiNi shape memory alloys. Shot peening using iron based metallic glass media was found to be an effective method to obtain the amorphous surface. Surface amorphization improved the corrosion resistance. Nanocrystalline TiNi exhibited peculiar superelastic properties. Correlation between the microstructure and phase transformation in nanostructured TiNi was discussed.